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Sample records for exploding white-dwarf model

  1. The Fate of Exploding White Dwarfs

    Science.gov (United States)

    Fisher, Robert

    2018-01-01

    Type Ia supernovae play an important role as standardizable candles for cosmology, providing one of the most important probes into the nature of dark energy. Yet, the nature of the stellar progenitors which give rise to Type Ia supernovae remains elusive. For decades, the leading model explaining Type Ia supernovae properties consisted of a white dwarf accreting to near the Chandrasekhar mass, in the single-degenerate channel. More recently, a variety of lines of evidence point instead towards merging binary white dwarfs, in the double-degenerate channel, as the progenitors of most Type Ia supernovae. In this talk, I will focus upon recent advances at the interface between observation and theory which will help crack the Type Ia progenitor problem. In particular, I will present new insights obtained from recent multidimensional numerical simulations of both the double-degenerate and single-degenerate channels which I have undertaken with my students and collaborators. I will discuss how new models and observations will help elucidate the long-standing mystery of Type supernovae.

  2. Modelling the formation of double white dwarfs

    NARCIS (Netherlands)

    van der Sluijs, M.V.; Verbunt, F.W.M.|info:eu-repo/dai/nl/068970374; Pols, O.|info:eu-repo/dai/nl/111811155

    2006-01-01

    We investigate the formation of the ten double-lined double white dwarfs that have been observed so far. A detailed stellar evolution code is used to calculate grids of single-star and binary models and we use these to reconstruct possible evolutionary scenarios. We apply various criteria to select

  3. Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star.

    Science.gov (United States)

    Nugent, Peter E; Sullivan, Mark; Cenko, S Bradley; Thomas, Rollin C; Kasen, Daniel; Howell, D Andrew; Bersier, David; Bloom, Joshua S; Kulkarni, S R; Kandrashoff, Michael T; Filippenko, Alexei V; Silverman, Jeffrey M; Marcy, Geoffrey W; Howard, Andrew W; Isaacson, Howard T; Maguire, Kate; Suzuki, Nao; Tarlton, James E; Pan, Yen-Chen; Bildsten, Lars; Fulton, Benjamin J; Parrent, Jerod T; Sand, David; Podsiadlowski, Philipp; Bianco, Federica B; Dilday, Benjamin; Graham, Melissa L; Lyman, Joe; James, Phil; Kasliwal, Mansi M; Law, Nicholas M; Quimby, Robert M; Hook, Isobel M; Walker, Emma S; Mazzali, Paolo; Pian, Elena; Ofek, Eran O; Gal-Yam, Avishay; Poznanski, Dovi

    2011-12-14

    Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor.

  4. A white dwarf explodes inside a dense circumstellar disk peeking at a puzzling supernova with spectropolarimetry

    CERN Multimedia

    2004-01-01

    "By measuring polarized light from an unusual exploding star, an international team of astrophysicists and astronomers has worked out the first detailed picture of a Type Ia supernova and the distinctive star system in which it exploded" (2 pages)

  5. A Model for the Galactic Population of Symbiotic Stars with White Dwarf Accretors

    Science.gov (United States)

    Yungelson, L.; Livio, M.; Tutukov, A.; Kenyon, S. J.

    1995-07-01

    By means of a population synthesis code, we investigate the formation of symbiotic systems in which the hot components are assumed to be white dwarfs which are either burning hydrogen steadily or are in a post-nova plateau" phase, in the evolution of exploding white dwarfs. Our estimate for the total number of symbiotic systems in the Galaxy, ˜3000-30,000 (depending on different model assumptions), is compatible with observational estimates. The crucial parameter for the determination of the birthrate and number of symbiotic stars is the mass of the hydrogen layer which the white dwarf can accumulate prior to hydrogen ignition. We model the distributions of symbiotic stars over orbital periods, masses of the components, mass-loss rates by the cool components, and brightness of components, and we obtain a reasonable agreement with observations. We show that in systems which are the most efficient in producing the symbiotic phenomenon, the accretors have to capture up to ˜30% of the matter lost by the cool component via a stellar wind. If the fraction of captured matter is significantly lower, it becomes impossible to explain even the lowest observational estimates of the number of symbiotic stars. The theoretical estimate of the average rate of symbiotic novae is ˜0.1 yr-1, compatible with the observed one. The apparent normal chemical composition of symbiotic novae can be explained if the white dwarfs in these systems, which have systematically lower masses than in cataclysmic binaries, manage to preserve "buffer" helium layers between their CO cores and the accreted hydrogen envelopes. Mass exchange in symbiotic systems does not lead to SN Ia's via the accumulation of a Chandrasekhar mass. However, if sub-Chandrasekhar-mass, double-detonation models indeed produce SN Ia's, then symbiotic systems can be the progenitors of ≲⅓ of the events. According to the model, SN Ia's in symbiotic binaries belong to young and intermediate-age populations (t ≲ 6 × 109

  6. Modeling and Analysis of CTIO 1.5m White Dwarf Spectra

    Science.gov (United States)

    Gulledge, Deborah Jean; Tucker, Douglas; Smith, John Allyn; Wester, William; Robertson, Jacob; Mueller, Jack H.; Fix, Mees; Narayan, Gautham; Tremblay, P.-E.

    2018-01-01

    We present results to date on spectroscopic reductions of white dwarf stars from the CTIO 1.5m telescope in support of calibrations for the Dark Energy Survey, which is based in the Southern hemisphere to map galaxies and gather information on dark energy.Science requirements for the survey require a 0.5\\% uncertainty in color, driven by supernova science. The Dark Energy Survey relies on a calibration technique that uses white dwarf stars to set zero points. These white dwarf spectra are fit to model spectra at the same temperatures and surface gravities. Fits are done both by a WD modeling expert, Pier Emmanuel Tremblay, and by WD model fitting software created by Gautham Narayan. Fits from both are comparable and give similar results, which are then used to generate synthetic photometry. These synthetic photometry values are compared to the measured values from the survey to verify that the zero points are correct.

  7. The potential of 3D radiation-hydrodynamics models for white dwarf asteroseismology

    Directory of Open Access Journals (Sweden)

    Tremblay P.-E.

    2013-03-01

    Full Text Available White dwarfs with hydrogen-rich atmospheres (DA are the most abundant of all degenerate objects. In recent years work has been dedicated to increase the accuracy of their model atmospheres. Most notably, convective motions are now treated with 3D radiation-hydrodynamics instead of the standard mixing-length theory. We present and describe selected 3D model atmospheres close and within the instability strip of the pulsating ZZ Ceti white dwarfs. Our 3D simulations depend only weakly on numerical parameters and compared to 1D models, they provide more realistic determinations of the depth of the convective zone. The 3D structures can then be adopted as input for asteroseismology.

  8. Left Behind: A Bound Remnant from a White Dwarf Supernova?

    Science.gov (United States)

    Jha, Saurabh

    2017-08-01

    Type Ia supernovae (SN Ia) have enormous importance to cosmology and astrophysics, but their progenitors and explosion mechanisms are not understood in detail. Recently, observations and theoretical models have suggested that not all thermonuclear white-dwarf supernova explosions are normal SN Ia. In particular, type Iax supernovae (peculiar cousins to SN Ia), are thought to be exploding white dwarfs that are not completely disrupted, leaving behind a bound remnant. In deep and serendipitous HST pre-explosion data, we have discovered a luminous, blue progenitor system for the type Iax SN 2012Z in NGC 1309, which we interpret as a helium-star donor to the exploding white dwarf. HST observations of SN 2012Z in 2016, when the supernova light was expected to have faded away, still show a source at the location, as expected in our model where the pre-explosion flux was coming from the companion. However, the 2016 data also show a surprise: an excess flux compared to the progenitor system. Our proposed observations here will help unravel the mystery of that excess flux: is it from the bound ex-white dwarf remnant? Or is it from the shocked companion star that has been bombarded by supernova ejecta? Either of these possibilities would provide key new evidence as to the nature of these white dwarf supernovae.

  9. The Dusty Accretion of Polluted White Dwarfs

    Science.gov (United States)

    Bonsor, A.; Farihi, J.; Wyatt, M. C.; van Lieshout, R.

    2017-03-01

    Infrared observations of polluted white dwarfs provide key insights into the accretion processes in action. The standard model for the observed infrared excesses is a flat, opaque, dust disc. The infrared observations are inconsistent with the presence of such a disc around all polluted white dwarfs. We discuss potential explanations for the absence of an infrared excess for many polluted white dwarfs.

  10. Throwing Icebergs at White Dwarfs

    Science.gov (United States)

    Kohler, Susanna

    2017-08-01

    Where do the metals come from that pollute the atmospheres of many white dwarfs? Close-in asteroids may not be the only culprits! A new study shows that distant planet-size and icy objects could share some of the blame.Pollution ProblemsArtists impression of rocky debris lying close around a white dwarf star. [NASA/ESA/STScI/G. Bacon]When a low- to intermediate-mass star reaches the end of its life, its outer layers are blown off, leaving behind its compact core. The strong gravity of this white dwarf causes elements heavier than hydrogen and helium to rapidly sink to its center in a process known as sedimentation, leaving an atmosphere that should be free of metallic elements.Therefore its perhaps surprising that roughly 2550% of all white dwarfs are observed to have atmospheric pollution by heavy elements. The short timescales for sedimentation suggest that these elements were added to the white dwarf recently but how did they get there?Bringing Ice InwardIn the generally accepted theory, pre-existing rocky bodies or an orbiting asteroid belt survive the stars evolution, later accreting onto the final white dwarf. But this scenario doesnt explain a few observations that suggest white dwarfs might be accreting larger planetary-size bodies and bodies with ices and volatile materials.Dynamical evolution of a Neptune-like planet (a) and a Kuiper belt analog object (b) in wide binary star systems. Both have large eccentricity excitations during the white dwarf phase. [Stephan et al. 2017]How might you get large or icy objects which would begin on very wide orbits close enough to a white dwarf to become disrupted and accrete? Led by Alexander Stephan, a team of scientists at UCLA now suggest that the key is for the white dwarf to be in a binary system.Influence of a CompanionIn the authors model, the white-dwarf progenitor is orbited by both a distant stellar companion (a common occurrence) and a number of large potential polluters, which could have masses between that

  11. Hot DA white dwarf model atmosphere calculations: including improved Ni PI cross-sections

    Science.gov (United States)

    Preval, S. P.; Barstow, M. A.; Badnell, N. R.; Hubeny, I.; Holberg, J. B.

    2017-02-01

    To calculate realistic models of objects with Ni in their atmospheres, accurate atomic data for the relevant ionization stages need to be included in model atmosphere calculations. In the context of white dwarf stars, we investigate the effect of changing the Ni IV-VI bound-bound and bound-free atomic data on model atmosphere calculations. Models including photoionization cross-section (PICS) calculated with AUTOSTRUCTURE show significant flux attenuation of up to ˜80 per cent shortward of 180 Å in the extreme ultraviolet (EUV) region compared to a model using hydrogenic PICS. Comparatively, models including a larger set of Ni transitions left the EUV, UV, and optical continua unaffected. We use models calculated with permutations of these atomic data to test for potential changes to measured metal abundances of the hot DA white dwarf G191-B2B. Models including AUTOSTRUCTURE PICS were found to change the abundances of N and O by as much as ˜22 per cent compared to models using hydrogenic PICS, but heavier species were relatively unaffected. Models including AUTOSTRUCTURE PICS caused the abundances of N/O IV and V to diverge. This is because the increased opacity in the AUTOSTRUCTURE PICS model causes these charge states to form higher in the atmosphere, more so for N/O V. Models using an extended line list caused significant changes to the Ni IV-V abundances. While both PICS and an extended line list cause changes in both synthetic spectra and measured abundances, the biggest changes are caused by using AUTOSTRUCTURE PICS for Ni.

  12. Singing and dancing white dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Mukadam, Anjum S; Szkody, Paula [Department of Astronomy, University of Washington, Seattle, WA 98195 (United States); Gaensicke, Boris T [Department of Physics, University of Warwick, Coventry, CV4 7AL (United Kingdom); Nitta, Atsuko, E-mail: anjum@astro.washington.ed [Gemini Observatory, Hilo, HI 96720 (United States)

    2009-06-01

    Accreting white dwarfs have recently been shown to exhibit non-radial pulsations similar to their non-interacting counterparts. This allows us to probe the interior of the accreting white dwarf using seismology, and may be the only way to determine masses for non-eclipsing cataclysmic variables. Improving our understanding of accreting white dwarfs will have implications for models of supernovae Type Ia. Pulsating white dwarfs in cataclysmic variables are also useful in establishing the effects of accretion on pulsations. A search for nonradial pulsations among suitable candidates has led to the discovery of twelve such systems known to date. With the goal of establishing an instability strip (or strips) for these pulsating accretors, we acquired HST ultra-violet time-series spectroscopy of six pulsating white dwarfs in cataclysmic variables in 2007 and 2008. This approach enables us to measure the effective temperature of the white dwarf using the co-added spectrum, and to simultaneously characterize the pulsations. We also intended to constrain the pulsation mode identification by comparing the ultra-violet amplitudes to those from near-simultaneous ground-based photometry. Our preliminary results indicate a broad instability strip in the temperature range of 10500-15400 K.

  13. White dwarf cooling sequences and cosmochronology

    Science.gov (United States)

    Isern, J.; Artigas, A.; García-Berro, E.

    2013-03-01

    The evolution of white dwarfs is a simple gravothermal process. This means that their luminosity function, i.e. the number of white dwarfs per unit bolometric magnitude and unit volume as a function of bolometric magnitude, is a monotonically increasing function that decreases abruptly as a consequence of the finite age of the Galaxy. The precision and the accuracy of the white dwarf luminosity functions obtained with the recent large surveys together with the improved quality of the theoretical models of evolution of white dwarfs allow to feed the hope that in a near future it will be possible to reconstruct the history of the different Galactic populations.

  14. 3D MODEL ATMOSPHERES FOR EXTREMELY LOW-MASS WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Tremblay, P.-E. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218 (United States); Gianninas, A.; Kilic, M. [Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK, 73019 (United States); Ludwig, H.-G. [Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, D-69117 Heidelberg (Germany); Steffen, M. [Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, D-14482 Potsdam (Germany); Freytag, B. [Department of Physics and Astronomy at Uppsala University, Regementsvägen 1, Box 516, SE-75120 Uppsala (Sweden); Hermes, J. J., E-mail: tremblay@stsci.edu [Department of Physics, University of Warwick, Coventry CV4 7AL (United Kingdom)

    2015-08-20

    We present an extended grid of mean three-dimensional (3D) spectra for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD radiation-hydrodynamics 3D simulations covering T{sub eff} = 6000–11,500 K and log g = 5–6.5 (g in cm s{sup −2}) to derive analytical functions to convert spectroscopically determined 1D temperatures and surface gravities to 3D atmospheric parameters. Along with the previously published 3D models, the 1D to 3D corrections are now available for essentially all known convective DA WDs (i.e., log g = 5–9). For low-mass WDs, the correction in temperature is relatively small (a few percent at the most), but the surface gravities measured from the 3D models are lower by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the discrepancies seen in the radius and mass measurements for relatively cool ELM WDs in eclipsing double WD and WD + millisecond pulsar binary systems. We also use the 3D corrections to revise the boundaries of the ZZ Ceti instability strip, including the recently found ELM pulsators.

  15. MULTI-DIMENSIONAL MODELS FOR DOUBLE DETONATION IN SUB-CHANDRASEKHAR MASS WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Moll, R.; Woosley, S. E. [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)

    2013-09-10

    Using two-dimensional and three-dimensional simulations, we study the ''robustness'' of the double detonation scenario for Type Ia supernovae, in which a detonation in the helium shell of a carbon-oxygen white dwarf induces a secondary detonation in the underlying core. We find that a helium detonation cannot easily descend into the core unless it commences (artificially) well above the hottest layer calculated for the helium shell in current presupernova models. Compressional waves induced by the sliding helium detonation, however, robustly generate hot spots which trigger a detonation in the core. Our simulations show that this is true even for non-axisymmetric initial conditions. If the helium is ignited at multiple points, then the internal waves can pass through one another or be reflected, but this added complexity does not defeat the generation of the hot spot. The ignition of very low-mass helium shells depends on whether a thermonuclear runaway can simultaneously commence in a sufficiently large region.

  16. Multi-dimensional Models for Double Detonation in Sub-Chandrasekhar Mass White Dwarfs

    Science.gov (United States)

    Moll, R.; Woosley, S. E.

    2013-09-01

    Using two-dimensional and three-dimensional simulations, we study the "robustness" of the double detonation scenario for Type Ia supernovae, in which a detonation in the helium shell of a carbon-oxygen white dwarf induces a secondary detonation in the underlying core. We find that a helium detonation cannot easily descend into the core unless it commences (artificially) well above the hottest layer calculated for the helium shell in current presupernova models. Compressional waves induced by the sliding helium detonation, however, robustly generate hot spots which trigger a detonation in the core. Our simulations show that this is true even for non-axisymmetric initial conditions. If the helium is ignited at multiple points, then the internal waves can pass through one another or be reflected, but this added complexity does not defeat the generation of the hot spot. The ignition of very low-mass helium shells depends on whether a thermonuclear runaway can simultaneously commence in a sufficiently large region.

  17. Astrometric Binaries: White Dwarfs?

    Science.gov (United States)

    Oliversen, Nancy A.

    We propose to observe a selection of astrometric or spectroscopicastrometric binaries nearer than about 20 pc with unseen low mass companions. Systems of this type are important for determining the luminosity function of low mass stars (white dwarfs and very late main sequence M stars), and their contribution to the total mass of the galaxy. Systems of this type are also important because the low mass, invisible companions are potential candidates in the search for planets. Our target list is selected primarily from the list of 31 astrometric binaries near the sun by Lippincott (1978, Space Sci. Rev., 22, 153), with additional candidates from recent observations by Kamper. The elimination of stars with previous IUE observations, red companions resolved by infrared speckle interferometry, or primaries later than M1 (because if white dwarf companions are present they should have been detected in the visible region) reduces the list to 5 targets which need further information. IUE SWP low dispersion observations of these targets will show clearly whether the remaining unseen companions are white dwarfs, thus eliminating very cool main sequence stars or planets. This is also important in providing complete statistical information about the nearest stars. The discovery of a white dwarf in such a nearby system would provide important additional information about the masses of white dwarfs. Recent results by Greenstein (1986, A. J., 92, 859) from binary systems containing white dwarfs imply that 80% of such systems are as yet undetected. The preference of binaries for companions of approximately equal mass makes the Lippincott-Kamper list of A through K primaries with unseen companions a good one to use to search for white dwarfs. The mass and light dominance of the current primary over the white dwarf in the visible makes ultraviolet observations essential to obtain an accurate census of white dwarf binaries.

  18. Non-LTE line-blanketed model atmospheres of hot stars. 2: Hot, metal-rich white dwarfs

    Science.gov (United States)

    Lanz, T.; Hubeny, I.

    1995-01-01

    We present several model atmospheres for a typical hot metal-rich DA white dwarf, T(sub eff) = 60,000 K, log g = 7.5. We consider pure hydrogen models, as well as models with various abundances of two typical 'trace' elements-carbon and iron. We calculte a number of Local Thermodynamic Equilibrium (LTE) and non-LTE models, taking into account the effect of numerous lines of these elements on the atmospheric structure. We demostrate that while the non-LTE effects are notvery significant for pure hydrogen models, except for describing correctly the central emission in H-alpha they are essential for predicting correctly the ionization balance of metals, such as carbon and iron. Previously reported discrepancies in LTE abundances determinations using C III and C IV lines are easily explained by non-LTE effects. We show that if the iron abundance is larger than 10(exp -5), the iron line opacity has to be considered not only for the spectrum synthesis, but also in the model construction itself. For such metal abundances, non-LTE metal line-blanketed models are needed for detailed abundance studies of hot, metal-rich white dwarfs. We also discuss the predicted Extreme Ultraviolet (EUV) spectrum and show that it is very sensitive to metal abundances, as well as to non-LTE effects.

  19. Monitoring and modelling of white dwarfs with extremely weak magnetic fields. WD 2047+372 and WD 2359-434

    Science.gov (United States)

    Landstreet, J. D.; Bagnulo, S.; Valyavin, G.; Valeev, A. F.

    2017-11-01

    Magnetic fields are detected in a few percent of white dwarfs. The number of such magnetic white dwarfs known is now some hundreds. Fields range in strength from a few kG to several hundred MG. Almost all the known magnetic white dwarfs have a mean field modulus ≥1 MG. We are trying to fill a major gap in observational knowledge at the low field limit (≤200 kG) using circular spectro-polarimetry. In this paper we report the discovery and monitoring of strong, periodic magnetic variability in two previously discovered "super-weak field" magnetic white dwarfs, WD 2047+372 and WD 2359-434. WD 2047+372 has a mean longitudinal field that reverses between about -12 and + 15 kG, with a period of 0.243 d, while its mean field modulus appears nearly constant at 60 kG. The observations can be interpreted in terms of a dipolar field tilted with respect to the stellar rotation axis. WD 2359-434 always shows a weak positive longitudinal field with values between about 0 and + 12 kG, varying only weakly with stellar rotation, while the mean field modulus varies between about 50 and 100 kG. The rotation period is found to be 0.112 d using the variable shape of the Hα line core, consistent with available photometry. The field of this star appears to be much more complex than a dipole, and is probably not axisymmetric. Available photometry shows that WD 2359-434 is a light variable with an amplitude of only 0.005 mag; our own photometry shows that if WD 2047+372 is photometrically variable, the amplitude is below about 0.01 mag. These are the first models for magnetic white dwarfs with fields below about 100 kG based on magnetic measurements through the full stellar rotation. They reveal two very different magnetic surface configurations, and that, contrary to simple ohmic decay theory, WD 2359-434 has a much more complex surface field than the much younger WD 2047+372. Based, in part, on observations collected at the European Organisation for Astronomical Research in the

  20. ON THE EVOLUTION OF MAGNETIC WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Tremblay, P.-E. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Fontaine, G.; Brassard, P. [Département de Physique, Université de Montréal, C. P. 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7 (Canada); Freytag, B. [Department of Physics and Astronomy at Uppsala University, Regementsvägen 1, Box 516, SE-75120 Uppsala (Sweden); Steiner, O. [Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, D-79104 Freiburg (Germany); Ludwig, H.-G. [Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, D-69117 Heidelberg (Germany); Steffen, M. [Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, D-14482 Potsdam (Germany); Wedemeyer, S., E-mail: tremblay@stsci.edu [Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo (Norway)

    2015-10-10

    We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-β parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1–50 kG, which is much smaller than the typical 1–1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B ≳ 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (T{sub eff}) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection due to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, the effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with T{sub eff} ≲ 10,000 K cool significantly slower than non-magnetic degenerates.

  1. CSS 41177: an eclipsing double white dwarf binary

    Science.gov (United States)

    Bours, Madelon

    2013-10-01

    The overwhelming majority of stellar remnants are white dwarfs. Despite their abundance and importance to, amongst others, Galactic age determinations and our understanding of type Ia supernovae fewer than a dozen white dwarfs have model-independent measurements of fundamental parameters like mass and radius. A major limitation on the observational side is that such parameters are extremely difficult to determine in a model-independant way for single white dwarfs. Close white dwarf binaries can provide these important tests.The largest class of white dwarf binaries in the Galaxy are the detached double white dwarfs, which are becoming increasingly popular as the progenitor systems of Type Ia supernovae. In recent years four eclipsing double white dwarfs have been found, creating the opportunity for precision mass and radius measurements of two white dwarfs at once. Our target, CSS 41177, contains two extremely low-gravity white dwarfs with very different temperatures, presenting us with a unique chance to test the existing mass-radius relation at its extremes.Here we propose a 2 orbit HST/COS FUV observation of CSS 41177, to accurately determine the temperature and surface gravity of the hot white dwarf. Through the flux ratio from the light curve this will at the same time constrain those of the cool white dwarf. Therefore it will allow us to add two more white dwarfs with accurate parameters to the short list of white dwarfs for which precise masses and radii are known.Note: The proposed observations are part of the doctoral thesis of Ms. Madelon C.P. Bours.

  2. The Tübingen Model-Atom Database: A Revised Aluminum Model Atom and its Application for the Spectral Analysis of White Dwarfs

    Science.gov (United States)

    Löbling, L.

    2017-03-01

    Aluminum (Al) nucleosynthesis takes place during the asymptotic-giant-branch (AGB) phase of stellar evolution. Al abundance determinations in hot white dwarf stars provide constraints to understand this process. Precise abundance measurements require advanced non-local thermodynamic stellar-atmosphere models and reliable atomic data. In the framework of the German Astrophysical Virtual Observatory (GAVO), the Tübingen Model-Atom Database (TMAD) contains ready-to- use model atoms for elements from hydrogen to barium. A revised, elaborated Al model atom has recently been added. We present preliminary stellar-atmosphere models and emergent Al line spectra for the hot white dwarfs G191-B2B and RE 0503-289.

  3. Building Magnetic Fields in White Dwarfs

    Science.gov (United States)

    Kohler, Susanna

    2017-03-01

    white dwarf forms a solid, oxygen-rich core with a liquid, carbon-rich mantle thats Rayleigh-Taylor unstable: as crystallization continues, the solids continue to sink out of the mantle.By analytically modeling this process, Isern and collaborators demonstrate that the Rayleigh-Taylor instabilities in the convective mantle can drive a dynamo large enough to generate the magnetic field strengths weve observed in white dwarfs.Magnetic field density as a function of the dynamo energy density. The plots show Earth and Jupiter (black dots), T Tauri stars (cyan), M dwarf stars (magenta), and two types of white dwarfs (blue and red). Do these lie on the same scaling relation? [Isern et al. 2017]A Universal Process?This setup the solid core with an unstable liquid mantle on top is exactly the structure expected to occur in planets such as Earth and Jupiter. These planets magnetic fields are similarly thought to be generated by convective dynamos powered by the cooling and chemical separation of their interiors and the process can also be scaled up to account for the magnetic fields of fully convective objects like T Tauri stars, as well.If white-dwarf magnetic fields are generated by the same type of dynamo, this may be a universal process for creating magnetic fields in astrophysical objects though other processes may well be at work too.CitationJordi Isern et al 2017 ApJL 836 L28. doi:10.3847/2041-8213/aa5eae

  4. White dwarf planets

    Directory of Open Access Journals (Sweden)

    Bonsor Amy

    2013-04-01

    Full Text Available The recognition that planets may survive the late stages of stellar evolution, and the prospects for finding them around White Dwarfs, are growing. We discuss two aspects governing planetary survival through stellar evolution to the White Dwarf stage. First we discuss the case of a single planet, and its survival under the effects of stellar mass loss, radius expansion, and tidal orbital decay as the star evolves along the Asymptotic Giant Branch. We show that, for stars initially of 1 − 5 M⊙, any planets within about 1 − 5 AU will be engulfed, this distance depending on the stellar and planet masses and the planet's eccentricity. Planets engulfed by the star's envelope are unlikely to survive. Hence, planets surviving the Asymptotic Giant Branch phase will probably be found beyond ∼ 2 AU for a 1  M⊙ progenitor and ∼ 10 AU for a 5 M⊙ progenitor. We then discuss the evolution of two-planet systems around evolving stars. As stars lose mass, planet–planet interactions become stronger, and many systems stable on the Main Sequence become destabilised following evolution of the primary. The outcome of such instabilities is typically the ejection of one planet, with the survivor being left on an eccentric orbit. These eccentric planets could in turn be responsible for feeding planetesimals into the neighbourhood of White Dwarfs, causing observed pollution and circumstellar discs.

  5. White dwarf dynamical interactions

    OpenAIRE

    Aznar Siguan, Gabriela

    2015-01-01

    Premi Extraordinari de Doctorat, promoció 2014-2015. Àmbit de Ciències Merging white dwarfs is a promising channel to trigger Type Ia supernovae, known as the double degenerate scenario. Supernovae are stellar explosions that radiate as much energy as any ordinary star is expected to emit over its entire life span, outshining briefly the whole hosting galaxy. They enrich the interstellar medium with higher mass elements and trigger the formation of new stars by the produced expanding shock...

  6. White Dwarf Mergers on Adaptive Meshes

    Science.gov (United States)

    Katz, Maximilian Peter

    The mergers of binary white dwarf systems are potential progenitors of astrophysical explosions such as Type Ia supernovae. These white dwarfs can merge either by orbital decay through the emission of gravitational waves or by direct collisions as a result of orbital perturbations. The coalescence of the stars may ignite nuclear fusion, resulting in the destruction of both stars through a thermonuclear runaway and ensuing detonation. The goal of this dissertation is to simulate binary white dwarf systems using the techniques of computational fluid dynamics and therefore to understand what numerical techniques are necessary to obtain accurate dynamical evolution of the system, as well as to learn what conditions are necessary to enable a realistic detonation. For this purpose I have used software that solves the relevant fluid equations, the Poisson equation for self-gravity, and the systems governing nuclear reactions between atomic species. These equations are modeled on a computational domain that uses the technique of adaptive mesh refinement to have the highest spatial resolution in the areas of the domain that are most sensitive to the need for accurate numerical evolution. I have identified that the most important obstacles to accurate evolution are the numerical violation of conservation of energy and angular momentum in the system, and the development of numerically seeded thermonuclear detonations that do not bear resemblance to physically correct detonations. I then developed methods for ameliorating these problems, and determined what metrics can be used for judging whether a given white dwarf merger simulation is trustworthy. This involved the development of a number of algorithmic improvements to the simulation software, which I describe. Finally, I performed high-resolution simulations of typical cases of white dwarf mergers and head-on collisions to demonstrate the impacts of these choices. The results of these simulations and the corresponding

  7. Novel modelling of ultracompact X-ray binary evolution - stable mass transfer from white dwarfs to neutron stars

    Science.gov (United States)

    Sengar, Rahul; Tauris, Thomas M.; Langer, Norbert; Istrate, Alina G.

    2017-09-01

    Tight binaries of helium white dwarfs (He WDs) orbiting millisecond pulsars (MSPs) will eventually 'merge' due to gravitational damping of the orbit. The outcome has been predicted to be the production of long-lived ultracompact X-ray binaries (UCXBs), in which the WD transfers material to the accreting neutron star (NS). Here we present complete numerical computations, for the first time, of such stable mass transfer from a He WD to a NS. We have calculated a number of complete binary stellar evolution tracks, starting from pre-low-mass X-ray binary systems, and evolved these to detached MSP+WD systems and further on to UCXBs. The minimum orbital period is found to be as short as 5.6 min. We followed the subsequent widening of the systems until the donor stars become planets with a mass of ˜0.005 M⊙ after roughly a Hubble time. Our models are able to explain the properties of observed UCXBs with high helium abundances and we can identify these sources on the ascending or descending branch in a diagram displaying mass-transfer rate versus orbital period.

  8. Axion cooling of white dwarfs

    OpenAIRE

    Isern, J.; Catalan, S.; Garcia--Berro, E.; Salaris, M.; Torres, S.

    2013-01-01

    The evolution of white dwarfs is a simple gravothermal process. This process can be tested in two ways, through the luminosity function of these stars and through the secular variation of the period of pulsation of those stars that are variable. Here we show how the mass of the axion can be constrained using the white dwarf luminosity function.

  9. Astrophysics of white dwarf binaries

    NARCIS (Netherlands)

    Nelemans, G.A.

    2006-01-01

    White dwarf binaries are the most common compact binaries in the Universe and are especially important for low-frequency gravitational wave detectors such as LISA. There are a number of open questions about binary evolution and the Galactic population of white dwarf binaries that can be solved using

  10. Probing LSST's Ability to Detect Planets Around White Dwarfs

    Science.gov (United States)

    Cortes, Jorge; Kipping, David

    2018-01-01

    Over the last four years more than 2,000 planets outside our solar system have been discovered, motivating us to search for and characterize potentially habitable worlds. Most planets orbit Sun-like stars, but more exotic stars can also host planets. Debris disks and disintegrating planetary bodies have been detected around white dwarf stars, the inert, Earth-sized cores of once-thriving stars like our Sun. These detections are clues that planets may exist around white dwarfs. Due to the faintness of white dwarfs and the potential rarity of planets around them, a vast survey is required to have a chance at detecting these planetary systems. The Large Synoptic Survey Telescope (LSST), scheduled to commence operations in 2023, will image the entire southern sky every few nights for 10 years, providing our first real opportunity to detect planets around white dwarfs. We characterized LSST’s ability to detect planets around white dwarfs through simulations that incorporate realistic models for LSST’s observing strategy and the white dwarf distribution within the Milky Way galaxy. This was done through the use of LSST's Operations Simulator (OpSim) and Catalog Simulator (CatSim). Our preliminary results indicate that, if all white dwarfs were to possess a planet, LSST would yield a detection for every 100 observed white dwarfs. In the future, a larger set of ongoing simulations will help us quantify the number of planets LSST could potentially find.

  11. Lessons for Asteroseismology from White Dwarf Stars

    Indian Academy of Sciences (India)

    The interpretation of pulsation data for sun-like stars is currently facing challenges quite similar to those faced by white dwarf modelers ten years ago. The observational requirements for uninterrupted long-term monitoring are beginning to be satisfied by successful multi-site campaigns and dedicated satellite missions.

  12. Detailed analysis of carbon atmosphere white dwarfs

    Science.gov (United States)

    Dufour, Patrick

    2009-07-01

    We propose to obtain UV spectra for the newly discovered white dwarf stars with a carbon-dominated atmosphere. Model calculations show that these stars emit most of their light in the UV part of the electromagnetic spectrum and that an accurate determination of the flux in this region is crucial for an accurate determination of the atmospheric parameters. It will also provide a unique opportunity to test the atomic data and broadening theory in stellar conditions never met before. This will play a primordial role in our path to understand the origin of these objects as well to obtain a better understanding of the evolution of stars in general. The principal objective we hope to achieve with these observations are 1} obtain accurate surface gravity/mass for these stars, 2} constrain/determine the abundance of other elements {O, He, Mg, Ne etc.}, especially oxygen, 3} verify the accuracy of the various theoretical atomic data used in the model calculations, 4} understand the origin and evolution of carbon atmosphere white dwarfs, in particular whether progenitor stars as massive as 10.5 solar masses can produce white dwarfs, rather than supernovae. We propose to observe 5 objects chosen carefully to cover the range of observed properties among carbon atmosphere white dwarfs {effective temperature, surface gravity, abundance of hydrogen/helium and magnetic field}.

  13. Theoretical Study of White Dwarf Double Stars

    Science.gov (United States)

    Hira, Ajit; Koetter, Ted; Rivera, Ruben; Diaz, Juan

    2015-04-01

    We continue our interest in the computational simulation of the astrophysical phenomena with a study of gravitationally-bound binary stars, composed of at least one white dwarf star. Of particular interest to astrophysicists are the conditions inside a white dwarf star in the time frame leading up to its explosive end as a Type Ia supernova, for an understanding of the massive stellar explosions. In addition, the studies of the evolution of white dwarfs could serve as promising probes of theories of gravitation. We developed FORTRAN computer programs to implement our models for white dwarfs and other stars. These codes allow for different sizes and masses of stars. Simulations were done in the mass interval from 0.1 to 2.0 solar masses. Our goal was to obtain both atmospheric and orbital parameters. The computational results thus obtained are compared with relevant observational data. The data are further analyzed to identify trends in terms of sizes and masses of stars. We hope to extend our computational studies to blue giant stars in the future. Research Supported by National Science Foundation.

  14. Computational Study of White Dwarf Stars

    Science.gov (United States)

    Pacheco, Jose; Hira, Ajit; Jaramillo, Danelle

    2014-03-01

    We begin our interest in the computational simulation of the astrophysical phenomena with a study of white dwarf stars. Of particular interest to astrophysicists are the conditions inside a white dwarf star in the time frame leading up to its explosive end as a Type Ia supernova, for an understanding of the massive stellar explosions. In addition, the studies of the evolution of white dwarfs could serve as promising probes of theories of gravitation. First, we set up the equations of equilibrium for the star of interest. Then we derived the appropriate equation of state. Next, a FORTRAN computer program was developed to implement our model for white dwarfs. This code allows for different sizes and masses of stars. Simulations were done in the mass interval from 0.4 to 0.8 solar masses. Our goal was to obtain both atmospheric and orbital parameters. The computational results thus obtained are compared with relevant observational data. The data are further analyzed to identify trends in terms of sizes and masses of stars. We hope to extend our computational studies to red giant stars in the future.

  15. Line Broadening in White Dwarf Photospheres

    Science.gov (United States)

    Winget, D. E.

    2012-06-01

    White dwarfs are the simplest stars with the simplest surface chemical compositions known. Spectroscopically we detect only hydrogen in surfaces of the vast majority of these stars. The remainders are of various types, including stars with surfaces of nearly pure helium and some apparently massive stars with carbon and oxygen at the photosphere. We will examine the potential offered by the white dwarf stars in the context of both astrophysics and physics. This potential includes studying cosmochronology--establishing the age and evolutionary history of our galaxy and an independent lower limit on the age of the universe, constraining the properties of axions and WIMPS in the context of dark matter models, constraining dark energy by establishing the properties of the massive progenitors of type Ia supernovae, studying nucleosynthesis from their internal composition structure, and crystallization in dense Coulomb plasmas, among many others. Realizing this tremendous scientific potential depends on the determination of two boundary conditions for each star: the surface gravity and effective temperature. To do this, we must establish the photospheric plasma conditions, density and temperature, using observations of the stellar absorption spectra. Our understanding of line broadening appears to be an obstacle, at present. We will discuss the evidence for past theoretical inadequacies in line broadening theory and the hope for recent and future calculations. We will discuss how the experiments underway on the Z-facility at Sandia National Laboratories --where we can create macroscopic uniform plasmas under white dwarf photospheric conditions--will provide the benchmarks for improving our understanding of line broadening under white dwarf photospheric plasma conditions. These experiments will guide future theory and improve our understanding of the white dwarf stars and, through them, the contents and evolution of the cosmos.

  16. Throwing Icebergs at White Dwarfs

    OpenAIRE

    Stephan, Alexander P.; Naoz, Smadar; Zuckerman, B.

    2017-01-01

    White dwarfs have atmospheres that are expected to consist nearly entirely of hydrogen and helium, since heavier elements will sink out of sight on short timescales. However, observations have revealed atmospheric pollution by heavier elements in about a quarter to a half of all white dwarfs. While most of the pollution can be accounted for with asteroidal or dwarf planetary material, recent observations indicate that larger planetary bodies, as well as icy and volatile material from Kuiper b...

  17. The white dwarf population of NGC 6397

    Science.gov (United States)

    Torres, Santiago; García-Berro, Enrique; Althaus, Leandro G.; Camisassa, María E.

    2015-09-01

    Context. NGC 6397 is one of the most interesting, well-observed, and most thoroughly theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low-metallicity progenitors, to determine the age of NGC 6397 and the percentage of unresolved binaries. We also assess other important characteristics of the cluster, such as the slope of the initial mass function or the fraction of white dwarfs with hydrogen-deficient atmospheres. Aims: We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color-magnitude diagram and the corresponding magnitude and color distributions. Methods: To do this, we used an advanced Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. Results: Our theoretical models and the observed data agree well. In particular, we find that this agreement is best for those cooling sequences that take into account residual hydrogen burning. This result has important consequences for the evolution of progenitor stars during the thermally pulsing asymptotic giant branch phase, since it implies that appreciable third dredge-up in low-mass, low-metallicity progenitors is not expected to occur. Using a standard burst duration of 1.0 Gyr, we obtain that the age of the cluster is 12.8+0.50-0.75 Gyr. Greater ages are also compatible with the observed data, but then unrealistic longer durations of the initial burst of star formation are needed to fit the luminosity function. Conclusions: We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main-sequence stars, provides a unique tool for modeling the properties of globular clusters.

  18. Binary white dwarfs in the halo of the Milky Way

    Science.gov (United States)

    van Oirschot, Pim; Nelemans, Gijs; Toonen, Silvia; Pols, Onno; Brown, Anthony G. A.; Helmi, Amina; Portegies Zwart, Simon

    2014-09-01

    Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolution of low-metallicity halo stars at distances up to ~3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite. Results: By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference with a bottom-heavy IMF is small. A burst of star formation 13 Gyr ago fits slightly better than a star formation burst 10 Gyr ago and also slightly better than continuous star formation 10-13 Gyr ago. Gaia will be the first instument to constrain the bright end of the field halo WDLF, where contributions from binary WDs are considerable. Many of these will have He cores, of which a handful have atypical surface gravities (log g 0 in our standard model for WD cooling. These so called pre-WDs, if observed, can help us to constrain white dwarf cooling models and might teach us something about the fraction of halo stars that reside in binaries. Appendices are available in electronic form at http://www.aanda.org

  19. Models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing

    Science.gov (United States)

    Istrate, A. G.; Marchant, P.; Tauris, T. M.; Langer, N.; Stancliffe, R. J.; Grassitelli, L.

    2016-10-01

    A large number of extremely low-mass helium white dwarfs (ELM WDs) have been discovered in recent years. The majority of them are found in close binary systems suggesting they are formed either through a common-envelope phase or via stable mass transfer in a low-mass X-ray binary (LMXB) or a cataclysmic variable (CV) system. Here, we investigate the formation of these objects through the LMXB channel with emphasis on the proto-WD evolution in environments with different metallicities. We study for the first time the combined effects of rotational mixing and element diffusion (e.g. gravitational settling, thermal and chemical diffusion) on the evolution of proto-WDs and on the cooling properties of the resulting WDs. We present state-of-the-art binary stellar evolution models computed with MESA for metallicities of Z = 0.02, 0.01, 0.001 and 0.0002, producing WDs with masses between 0.16-0.45 M⊙. Our results confirm that element diffusion plays a significant role in the evolution of proto-WDs that experience hydrogen shell flashes. The occurrence of these flashes produces a clear dichotomy in the cooling timescales of ELM WDs, which has important consequences e.g. for the age determination of binary millisecond pulsars. In addition, we confirm that the threshold mass at which this dichotomy occurs depends on metallicity. Rotational mixing is found to counteract the effect of gravitational settling in the surface layers of young, bloated ELM proto-WDs and therefore plays a key role in determining their surface chemical abundances, I.e. the observed presence of metals in their atmospheres. We predict that these proto-WDs have helium-rich envelopes through a significant part of their lifetime. This is of great importance as helium is a crucial ingredient in the driving of the κ-mechanism suggested for the newly observed ELM proto-WD pulsators. However, we find that the number of hydrogen shell flashes and, as a result, the hydrogen envelope mass at the beginning of

  20. A black hole-white dwarf compact binary model for long gamma-ray bursts without supernova association

    Science.gov (United States)

    Dong, Yi-Ze; Gu, Wei-Min; Liu, Tong; Wang, Junfeng

    2018-03-01

    Gamma-ray bursts (GRBs) are luminous and violent phenomena in the Universe. Traditionally, long GRBs are expected to be produced by the collapse of massive stars and associated with supernovae. However, some low-redshift long GRBs have no detection of supernova association, such as GRBs 060505, 060614, and 111005A. It is hard to classify these events convincingly according to usual classifications, and the lack of the supernova implies a non-massive star origin. We propose a new path to produce long GRBs without supernova association, the unstable and extremely violent accretion in a contact binary system consisting of a stellar-mass black hole and a white dwarf, which fills an important gap in compact binary evolution.

  1. A Black Hole - White Dwarf Compact Binary Model for Long Gamma-ray Bursts without Supernova Association

    Science.gov (United States)

    Dong, Yi-Ze; Gu, Wei-Min; Liu, Tong; Wang, Junfeng

    2018-01-01

    Gamma-ray bursts (GRBs) are luminous and violent phenomena in the universe. Traditionally, long GRBs are expected to be produced by the collapse of massive stars and associated with supernovae. However, some low-redshift long GRBs have no detection of supernova association, such as GRBs 060505, 060614 and 111005A. It is hard to classify these events convincingly according to usual classifications, and the lack of the supernova implies a non-massive star origin. We propose a new path to produce long GRBs without supernova association, the unstable and extremely violent accretion in a contact binary system consisting of a stellar-mass black hole and a white dwarf, which fills an important gap in compact binary evolution.

  2. Branes constrictions with White Dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    García-Aspeitia, Miguel A., E-mail: aspeitia@fisica.uaz.edu.mx [Consejo Nacional de Ciencia y Tecnología, Av, Insurgentes Sur 1582, Colonia Crédito Constructor, Del. Benito Juárez, C.P. 03940, Mexico, D.F. (Mexico); Unidad Académica de Física, Universidad Autónoma de Zacatecas, Calzada Solidaridad esquina con Paseo a la Bufa S/N, C.P. 98060, Zacatecas (Mexico)

    2015-11-06

    We consider here a robust study of stellar dynamics for white dwarf stars with polytropic matter in the weak-field approximation using the Lane–Emden equation from the brane-world scenario. We also derive an analytical solution to the nonlocal energy density and show the behavior and sensitivity of these stars to the presence of extra dimensions. Similarly, we analyze stability and compactness, in order to show whether it is possible to agree with the conventional wisdom of white dwarfs dynamics. Our results predict an average value of the brane tension of <λ>≳84.818 MeV{sup 4}, with a standard deviation σ≃82.021 MeV{sup 4}, which comes from a sample of dwarf stars, being weaker than other astrophysical observations but remaining higher than cosmological results provided by nucleosynthesis among others.

  3. Branes constrictions with White Dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Garcia-Aspeitia, Miguel A. [Consejo Nacional de Ciencia y Tecnologia, Mexico (Mexico); Unidad Academica de Fisica, Universidad Autonoma de Zacatecas (Mexico)

    2015-11-15

    We consider here a robust study of stellar dynamics for white dwarf stars with polytropic matter in the weak-field approximation using the Lane-Emden equation from the brane-world scenario. We also derive an analytical solution to the nonlocal energy density and show the behavior and sensitivity of these stars to the presence of extra dimensions. Similarly, we analyze stability and compactness, in order to show whether it is possible to agree with the conventional wisdom of white dwarfs dynamics. Our results predict an average value of the brane tension of left angle λ right angle >or similar 84.818 MeV{sup 4}, with a standard deviation σ ≅ 82.021 MeV{sup 4}, which comes from a sample of dwarf stars, being weaker than other astrophysical observations but remaining higher than cosmological results provided by nucleosynthesis among others. (orig.)

  4. Branes constrictions with White Dwarfs

    Science.gov (United States)

    García-Aspeitia, Miguel A.

    2015-11-01

    We consider here a robust study of stellar dynamics for white dwarf stars with polytropic matter in the weak-field approximation using the Lane-Emden equation from the brane-world scenario. We also derive an analytical solution to the nonlocal energy density and show the behavior and sensitivity of these stars to the presence of extra dimensions. Similarly, we analyze stability and compactness, in order to show whether it is possible to agree with the conventional wisdom of white dwarfs dynamics. Our results predict an average value of the brane tension of < λ rangle ≳ 84.818MeV^4, with a standard deviation σ ˜eq 82.021MeV^4, which comes from a sample of dwarf stars, being weaker than other astrophysical observations but remaining higher than cosmological results provided by nucleosynthesis among others.

  5. The population of single and binary white dwarfs of the Galactic bulge

    Science.gov (United States)

    Torres, S.; García-Berro, E.; Cojocaru, R.; Calamida, A.

    2018-02-01

    Recent Hubble Space Telescope observations have unveiled the white dwarf cooling sequence of the Galactic bulge. Although the degenerate sequence can be well fitted employing the most up-to-date theoretical cooling sequences, observations show a systematic excess of red objects that cannot be explained by the theoretical models of single carbon-oxygen white dwarfs of the appropriate masses. Here we present a population synthesis study of the white dwarf cooling sequence of the Galactic bulge that takes into account the populations of both single white dwarfs and binary systems containing at least one white dwarf. These calculations incorporate state-of-the-art cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, for both white dwarfs with carbon-oxygen and helium cores, and also take into account detailed prescriptions of the evolutionary history of binary systems. Our Monte Carlo simulator also incorporates all the known observational biases. This allows us to model with a high degree of realism the white dwarf population of the Galactic bulge. We find that the observed excess of red stars can be partially attributed to white dwarf plus main sequence binaries, and to cataclysmic variables or dwarf novae. Our best fit is obtained with a higher binary fraction and an initial mass function slope steeper than standard values, as well as with the inclusion of differential reddening and blending. Our results also show that the possible contribution of double degenerate systems or young and thick-disk bulge stars is negligible.

  6. Disk Accretion of Tidally Disrupted Rocky Bodies onto White Dwarfs

    Science.gov (United States)

    Feng, W.; Desch, S.

    2017-03-01

    The prevailing model for the pollution of white dwarf photospheres invokes accretion from a disk of gas and solid particles, fed by tidal disruption of rocky bodies inside the Roche radius. Current models can successfully explain the accretion rates of metals onto white dwarfs, provided the gaseous disks viscously spread at rates consistent with a partially suppressed magnetorotational instability (Metzger et al. 2012); however, these models do not explore the extent of the magnetorotational instability in disks by calculating the degree of ionization. We present ionization fractions for thermal and non-thermal processes to assess the extent of the magnetorotational instability in white dwarf disks. We determine that the disk viscosity parameter α can be as high as 0.1 in white disks, implying that the magnetorotational instability must be carefully modeled.

  7. Double White Dwarf Merger Rates

    Science.gov (United States)

    Toonen, Silvia; Nelemans, Gijs; Portegies Zwart, Simon

    2013-01-01

    Type Ia supernovae (SNe Ia) are very successfully used as standard candles on cosmological distance scales, but so far the nature of the progenitor(s) is unclear. A possible scenario for SNe Ia are merging carbon/oxygen white dwarfs with a combined mass exceeding the Chandrasekhar mass. We determine the theoretical rates and delay time distribution of these mergers for two different common envelope prescriptions and metallicities. The shape of the delay time distributions is rather insensitive to the assumptions. The normalization is a factor ~3-13 too low compared to observations.

  8. Transmission of Thermonuclear Detonations through Layers of Burned Material in Carbon-Oxygen White Dwarfs

    Science.gov (United States)

    Gamezo, V. N.; Oran, E. S.

    2006-06-01

    In three-dimensional delayed-detonation models of type Ia supernovae, detonations propagate through funnels of degenerate carbon-oxygen matter that are left unburned by turbulent deflagrations in central parts of a white dwarf. Some of these funnels can be disconnected from the rest of the unburned material, thus creating unburned pockets that cannot be directly reached by a detonation wave. These pockets may or may not ignite when strong shocks generated by detonations reach them through layers of burned material. In this work, we study the detonation transmission phenomena in exploding white dwarfs using one-dimensional time-dependent numerical simulations based on reactive Euler equations. The thermonuclear burning of carbon-oxygen mixture is modeled by a 13-nuclei alpha network. We use a steady-state solution for the reaction-zone structure of a one-dimensional detonation wave as an initial condition. Time-dependent computations performed for a fully resolved carbon reaction scale show that a detonation shock passing through a layer of burned material can initiate a new detonation or decay. The critical thickness of burned material that allows the detonation reignition is a function of density. This work was supported in part by the NASA ATP program (NRA-02-OSS-01-ATP) and by the Naval Research Laboratory (NRL) through the Office of Naval Research.

  9. Constraining the neutrino magnetic dipole moment from white dwarf pulsations

    Energy Technology Data Exchange (ETDEWEB)

    Córsico, A.H.; Althaus, L.G. [Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, (1900) La Plata (Argentina); Bertolami, M.M. Miller [Instituto de Astrofísica La Plata, CONICET-UNLP, Paseo del Bosque s/n, (1900) La Plata (Argentina); Kepler, S.O. [Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, Porto Alegre 91501-970, RS (Brazil); García-Berro, E., E-mail: acorsico@fcaglp.unlp.edu.ar, E-mail: althaus@fcaglp.unlp.edu.ar, E-mail: marcelo@MPA-Garching.MPG.DE, E-mail: kepler@if.ufrgs.br, E-mail: enrique.garcia-berro@upc.edu [Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860, Castelldefels (Spain)

    2014-08-01

    Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment (μ{sub ν}) using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. We employ state-of-the-art evolutionary and pulsational codes which allow us to perform a detailed asteroseismological period fit based on fully DB white dwarf evolutionary sequences. Plasmon neutrino emission is the dominant cooling mechanism for this class of hot pulsating white dwarfs, and so it is the main contributor to the rate of change of period with time (Pidot) for the DBV class. Thus, the inclusion of an anomalous neutrino emission through a non-vanishing magnetic dipole moment in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DBV stars. By comparing the theoretical Pidot value with the rate of change of period with time of PG 1351+489, we assess the possible existence of additional cooling by neutrinos with magnetic dipole moment. Our models suggest the existence of some additional cooling in this pulsating DB white dwarf, consistent with a non-zero magnetic dipole moment with an upper limit of μ{sub ν} ∼< 10{sup -11} μ{sub B}. This bound is somewhat less restrictive than, but still compatible with, other limits inferred from the white dwarf luminosity function or from the color-magnitude diagram of the Globular cluster M5. Further improvements of the measurement of the rate of period change of the dominant pulsation mode of PG 1351+489 will be necessary to confirm our bound.

  10. Constraining the neutrino magnetic dipole moment from white dwarf pulsations

    Science.gov (United States)

    Córsico, A. H.; Althaus, L. G.; Miller Bertolami, M. M.; Kepler, S. O.; García-Berro, E.

    2014-08-01

    Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment (μν) using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. We employ state-of-the-art evolutionary and pulsational codes which allow us to perform a detailed asteroseismological period fit based on fully DB white dwarf evolutionary sequences. Plasmon neutrino emission is the dominant cooling mechanism for this class of hot pulsating white dwarfs, and so it is the main contributor to the rate of change of period with time (Pi dot) for the DBV class. Thus, the inclusion of an anomalous neutrino emission through a non-vanishing magnetic dipole moment in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DBV stars. By comparing the theoretical Pi dot value with the rate of change of period with time of PG 1351+489, we assess the possible existence of additional cooling by neutrinos with magnetic dipole moment. Our models suggest the existence of some additional cooling in this pulsating DB white dwarf, consistent with a non-zero magnetic dipole moment with an upper limit of μν lesssim 10-11 μB. This bound is somewhat less restrictive than, but still compatible with, other limits inferred from the white dwarf luminosity function or from the color-magnitude diagram of the Globular cluster M5. Further improvements of the measurement of the rate of period change of the dominant pulsation mode of PG 1351+489 will be necessary to confirm our bound.

  11. VW Hyi - The white dwarf revealed

    Science.gov (United States)

    Mateo, M.; Szkody, P.

    1984-01-01

    Nonsimultaneous IUE, optical, and near-IR observations of VW Hyi at quiescence are presented. Using these and UV data from other investigations, a broad feature in the ultraviolet is identified with L-alpha absorption. The presence and width of the line imply that (1) the white dwarf in VW Hyi is directly visible in the UV and (2) the effective temperature of this star is approximately 18,000 + or - 2000 K for log g = 8. The continuum observations, combined with the J and K photometry of Sherrington et al., (1980), can be fit with a combination of this relatively cool white dwarf and a steady-state disk model with an accretion rate of 10 to the -11th solar masses/yr. Additional observations of the hump in the optical light curve can be reasonably fit by a 12,000-K blackbody. Such a source is consistent with the hump being a minor contribution to the system's overall continuum distribution shortward of 2000 A and longward of about 1 micron.

  12. Pulsations powered by hydrogen shell burning in white dwarfs

    Science.gov (United States)

    Camisassa, M. E.; Córsico, A. H.; Althaus, L. G.; Shibahashi, H.

    2016-10-01

    Context. In the absence of a third dredge-up episode during the asymptotic giant-branch phase, white dwarf models evolved from low-metallicity progenitors have a thick hydrogen envelope, which makes hydrogen shell burning be the most important energy source. Aims: We investigate the pulsational stability of white dwarf models with thick envelopes to see whether nonradial g-mode pulsations are triggered by hydrogen burning, with the aim of placing constraints on hydrogen shell burning in cool white dwarfs and on a third dredge-up during the asymptotic giant-branch evolution of their progenitor stars. Methods: We construct white-dwarf sequences from low-metallicity progenitors by means of full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally pulsing and the post-asymptotic giant-branch phases, and analyze their pulsation stability by solving the linear, nonadiabatic, nonradial pulsation equations for the models in the range of effective temperatures Teff 15 000-8000 K. Results: We demonstrate that, for white dwarf models with masses M⋆ ≲ 0.71 M⊙ and effective temperatures 8500 ≲ Teff ≲ 11 600 K that evolved from low-metallicity progenitors (Z = 0.0001, 0.0005, and 0.001), the dipole (ℓ = 1) and quadrupole (ℓ = 2) g1-modes are excited mostly as a result of the hydrogen-burning shell through the ɛ-mechanism, in addition to other g-modes driven by either the κ - γ or the convective driving mechanism. However, the ɛ mechanism is insufficient to drive these modes in white dwarfs evolved from solar-metallicity progenitors. Conclusions: We suggest that efforts should be made to observe the dipole g1-mode in white dwarfs associated with low-metallicity environments, such as globular clusters and/or the galactic halo, to place constraints on hydrogen shell burning in cool white dwarfs and the third dredge-up episode during the preceding asymptotic giant-branch phase.

  13. The structure of white dwarf stars | Babaji | Journal of the Nigerian ...

    African Journals Online (AJOL)

    A FORTRAN code to compute the structure of white dwarf Stars has been written. It is assumed that a good model for the matter in white dwarf stars is the free Fermi gas of electrons at zero temperature, treated with relativistic kinematics. The code written essentially solves numerically the two coupled first-order differential ...

  14. A Pulsar and White Dwarf in an Unexpected Orbit

    Science.gov (United States)

    Kohler, Susanna

    2016-11-01

    Astronomers have discovered a binary system consisting of a low-mass white dwarf and a millisecond pulsar but its eccentric orbit defies all expectations of how such binaries form.Observed orbital periods and binary eccentricities for binary millisecond pulsars. PSR J2234+0511 is the furthest right of the green stars that mark the five known eccentric systems. [Antoniadis et al. 2016]Unusual EccentricityIt would take a low-mass (0.4 solar masses) white dwarf over 100 billion years to form from the evolution of a single star. Since this is longer than the age of the universe, we believe that these lightweights are instead products of binary-star evolution and indeed, we observe many of these stars to still be in binary systems.But the binary evolution that can create a low-mass white dwarf includes a period of mass transfer, in which efficient tidal dissipation damps the systems orbital eccentricity. Because of this, we would expect all systems containing low-mass white dwarfs to have circular orbits.In the past, our observations of low-mass white dwarfmillisecond pulsar binaries have all been consistent with this expectation. But a new detection has thrown a wrench in the works: the unambiguous identification of a low-mass white dwarf thats in an eccentric (e=0.13) orbit with the millisecond pulsar PSR J2234+0511. How could this system have formed?Eliminating Formation ModelsLed by John Antoniadis (Dunlap Institute at University of Toronto), a team of scientists has used newly obtained optical photometry (from the Sloan Digital Sky Survey) and spectroscopy (from the Very Large Telescope in Chile) of the white dwarf to confirm the identification of this system.Antoniadis and collaborators then use measurements of the bodies masses (0.28 and 1.4 solar masses for the white dwarf and pulsar, respectively) and velocities, and constraints on the white dwarfs temperature, radius and surface gravity, to address three proposed models for the formation of this system.The 3D

  15. The onset of convective coupling and freezing in the white dwarfs of 47 Tucanae

    Science.gov (United States)

    Obertas, Alysa; Caiazzo, Ilaria; Heyl, Jeremy; Richer, Harvey; Kalirai, Jason; Tremblay, Pier-Emmanuel

    2018-02-01

    Using images from the Hubble Space Telescope Advanced Camera for Surveys, we measure the rate of cooling of white dwarfs in the globular cluster 47 Tucanae and compare it to modelled cooling curves. We examine the effects of the outer convective envelope reaching the nearly isothermal degenerate core and the release of latent heat during core crystallization on the white dwarf cooling rates. For white dwarfs typical of 47 Tuc, the onset of these effects occur at similar times. The latent heat released during crystallization is a small heat source. In contrast, the heat reservoir of the degenerate core is substantially larger. When the convective envelope reaches the nearly isothermal interior of the white dwarf, the star becomes brighter than it would be in the absence of this effect. Our modelled cooling curves that include this convective coupling closely match the observed luminosity function of the white dwarfs in 47 Tuc.

  16. White Dwarfs Cosmological and Galactic Probes

    CERN Document Server

    Sion, Edward M; Vennes, Stéphane

    2005-01-01

    The emphasis on white dwarf stars and cosmology arises from the most recent advances in cosmological and galactic structure research in which white dwarf stars are playing a very prominent role. Examples are Type Ia supernovae (i.e. white dwarf supernovae), the origin and evolution of the universe, the age of the galactic disk, cosmochronology using white dwarfs in globular clusters and galactic clusters, and the physics of accretion onto compact (very dense) stars. As an assisting guide to the reader, we have included, by invitation, comprehensive review articles in each of the four major areas of the book, white dwarf supernovae, cosmology, accretion physics and galactic structure. The reviews include introductory material that they build upon. The book is suitable and most useful to advanced undergraduates, graduate students and scientific professionals (e.g. astronomers, astrophysicists, cosmologists, physicists).

  17. PREFACE: 16th European White Dwarfs Workshop

    Science.gov (United States)

    Garcia-Berro, Enrique; Hernanz, Margarita; Isern, Jordi; Torres, Santiago

    2009-07-01

    The 16th European Workshop on White Dwarfs was held in Barcelona, Spain, from 30 June to 4 July 2008 at the premises of the UPC. Almost 120 participants from Europe (France, Germany, United Kingdom, Italy, and several others), America (USA, Canada, Argentina, Brazil, and Chile), and other continents (Australia, South Africa, . . . ) attended the workshop. Among these participants were the most relevant specialists in the field. The topics covered by the conference were: White dwarf structure and evolution Progenitors and Planetary Nebulae White dwarfs in binaries: cataclysmic variables, double degenerates and other binaries White dwarfs, dust disks and planetary systems Atmospheres, chemical composition, magnetic fields Variable white dwarfs White dwarfs in stellar clusters and the halo White Dwarfs as SNIa progenitors The programme included 54 talks, and 45 posters. The oral presentations were distributed into the following sessions: Luminosity function, mass function and populations White dwarf structure and evolution White dwarf ages White dwarf catalogs and surveys Central stars of planetary nebulae Supernovae progenitors White dwarfs in novae and CVs Physical processes in white dwarfs and magnetic white dwarfs Disks, dust and planets around white dwarfs Pulsating white dwarfs Additionally we had a special open session about Spitzer and white dwarfs. The Proceedings of the 16th European Workshop on White Dwarfs are representative of the current state-of-the-art of the research field and include new and exciting results. We acknowledge the very positive attitude of the attendants to the workshop, which stimulated very fruitful discussions that took place in all the sessions and after the official schedule. Also, the meeting allowed new collaborations tp start that will undoubtedly result in significant advances in the research field. We also acknowledge the willingness of the participants to deliver their contributions before the final deadline. We sincerely

  18. Chandra Grating Spectroscopy of Three Hot White Dwarfs

    Science.gov (United States)

    Adamczak, J.; Werner, K.; Rauch, T.; Schuh, S.; Drake, J. J.; Kruk, J. W.

    2013-01-01

    High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB1919) and the other is a non-DA of spectral type PG1159 (PG1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD246). Aims. The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. Methods. The Chandra spectra are analyzed with chemically homogeneous as well as stratified NLTE model atmospheres that assume equilibrium between gravitational settling and radiative acceleration of chemical elements. Archival EUV and UV spectra obtained with EUVE, FUSE, and HST are utilized to support the analysis. Results. No metals could be identified in LB1919. All observations are compatible with a pure hydrogen atmosphere. This is in stark contrast to the vast majority of hot DA white dwarfs that exhibit light and heavy metals and to the stratified models that predict significant metal abundances in the atmosphere. For GD246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG1520+525 constrains the effective temperature to T(sub eff) = 150 000 +/- 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GWVir class (PG1159-035) defines the location of the blue edge of the GWVir instability region

  19. What fraction of white dwarfs are members of binary systems?

    Science.gov (United States)

    Holberg, J. B.

    2009-06-01

    White dwarfs were originally discovered as the subordinate faint companions of bright nearby stars (i.e. Sirius B and 40 Eri B). Several general categories of binary systems involving white dwarfs are recognized: Sirius-like systems, where the white dwarf may be difficult to detect, binary systems containing white dwarfs and low mass stars, where the white dwarf is often readily discerned; and double degenerate systems. Different modes of white dwarf discovery influence our perception of both the overall binary fraction and the nature of these systems; proper motion surveys emphasize resolved systems, while photometric surveys emphasize unresolved systems containing relatively hot white dwarfs. Recent studies of the local white dwarf population offer some hope of achieving realistic estimates of the relative number of binary systems containing white dwarfs. A sample of 132 white dwarfs within 20 pc indicates that an individual white dwarf has a probability of 32 ± 8% of occurring within a binary or multiple star system.

  20. SIM's Search for Planets Orbiting White Dwarfs

    Science.gov (United States)

    Subasavage, John P., Jr.

    2009-01-01

    Once launched, The Space Interferometry Mission (SIM) will be the most precise astrometric instrument ever developed. These capabilities are vital to exoplanetary studies, in particular, for low-mass, Earthlike planets. I propose to use SIM to observe a sample ( 25-50) of nearby white dwarfs in hopes of detecting planetary companions with masses in the 10 Earth mass range on average. Because of the nature of white dwarfs' spectral signatures (a few broad, if any, absorption lines), current radial velocity planet hunting techniques are not viable. Astrometry is currently the only technique capable of detecting low mass planets around white dwarfs and SIM would be the best suited astrometric instrument to do so. Planetary detections around white dwarfs would better enable us to probe planetary formation theory as well as planetary evolution theory in conjunction with stellar evolution. Because astrometric signatures are inversely related to distance, the closer the system, the larger the signature (all else being equal). Because most stars will eventually end their lives as white dwarfs, these objects are plentiful and on average, closer to the Sun than more rare objects. Thus, a number of white dwarfs are close enough to the Sun to permit low mass planetary signature detections. Given that white dwarfs are the remnants of main-sequence dwarfs with spectral classes from B to K (thus far), we could better understand planetary formation over a broader range of objects than those currently investigated using radial velocity techniques (F, G, and K stars primarily).

  1. High-Resolution EUV Spectroscopy of White Dwarfs

    Science.gov (United States)

    Kowalski, Michael P.; Wood, K. S.; Barstow, M. A.

    2014-01-01

    We compare results of high-resolution EUV spectroscopic measurements of the isolated white dwarf G191-B2B and the binary system Feige 24 obtained with the J-PEX (Joint Plasmadynamic Experiment), which was sponsored jointly by the U.S. Naval Research Laboratory and NASA. J-PEX delivers the world's highest resolution in EUV and does so at high effective area (e.g., more effective area in a sounding rocket than is available with Chandra at adjacent energies, but in a waveband Chandra cannot reach). The capability J-PEX represents is applicable to the astrophysics of hot plasmas in stellar coronae, white dwarfs and the ISM. G191-B2B and Feige 24 are quite distinct hot white dwarf systems having in common that they are bright in the portion of the EUV where He emission features and edges occur, hence they can be exploited to probe both the stellar atmosphere and the ISM, separating those components by model-fitting that sums over all relevant (He) spectral features in the band. There is evidence from these fits that atmospheric He is being detected but the result is more conservatively cast as a pair of upper limits. We discuss how longer duration satellite observations with the same instrumentation could increase exposure to detect atmospheric He in these and other nearby hot white dwarfs.

  2. An Ultraviolet Spectral Legacy of Polluted White Dwarfs

    Science.gov (United States)

    Farihi, Jay

    2016-10-01

    Polluted white dwarfs represent the best path to empirically measure the bulk chemical compositions and limiting masses of extrasolar planetesimals. A wealth of data now firmly link metal-enriched white dwarfs with circumstellar debris that derives from rocky planetary building blocks or fragments of larger parent bodies. Ultraviolet spectroscopy is the only way to detect critical elements such as carbon and oxygen, and the small handful of stars studied in this way have yielded spectacular results. Our recent HST successes demonstrate the power of this technique, and we wish to take the next logical step with a large survey. In the same way that planetary scientists utilize diverse meteorite classes to understand the formation and composition of the terrestrial planets, we need a statistical census of extrasolar planetesimal compositions to fully understand their physical and chemical role as exoplanet precursors. A large (N 40) dataset of detailed metal abundances in polluted white dwarfs is essential to our understanding of the assembly and chemistry of Earth-like exoplanets.We propose to observe a brightness-limited sample of metal-rich white dwarfs, by selecting 31 targets that can be observed in a single COS / STIS visit, and thereby making the most efficient use of HST time for any sample of this size. This will increase the number of polluted white dwarfs observed in the far ultraviolet by a factor of five, and thereby perform the most robust and statistically meaningful study that is currently possible. This is a legacy that HST should leave to future planet formation modelers and cosmochemists, and it requires the ultraviolet capacity of HST.

  3. White dwarf-red dwarf binaries in the Galaxy

    NARCIS (Netherlands)

    Besselaar, E.J.M. van den

    2007-01-01

    This PhD thesis shows several studies on white dwarf - red dwarf binaries. White dwarfs are the end products of most stars and red dwarfs are normal hydrogen burning low-mass stars. White dwarf - red dwarf binaries are both blue (white dwarf) and red (red dwarf). Together with the fact that they are

  4. Transit probabilities for debris around white dwarfs

    Science.gov (United States)

    Lewis, John Arban; Johnson, John A.

    2017-01-01

    The discovery of WD 1145+017 (Vanderburg et al. 2015), a metal-polluted white dwarf with an infrared-excess and transits confirmed the long held theory that at least some metal-polluted white dwarfs are actively accreting material from crushed up planetesimals. A statistical understanding of WD 1145-like systems would inform us on the various pathways for metal-pollution and the end states of planetary systems around medium- to high-mass stars. However, we only have one example and there are presently no published studies of transit detection/discovery probabilities for white dwarfs within this interesting regime. We present a preliminary look at the transit probabilities for metal-polluted white dwarfs and their projected space density in the Solar Neighborhood, which will inform future searches for analogs to WD 1145+017.

  5. Characterization of the galactic white dwarf population in the next generation Virgo Cluster survey

    Science.gov (United States)

    Fantin, Nicholas

    Halo white dwarfs remain one of the least studied stellar populations in the Milky Way because of their faint luminosities. Recent work has uncovered a population of hot white dwarfs which are thought to be remnants of low-mass Population II stars. This thesis uses optical data from the Next Generation Virgo Cluster Survey (NGVS) and ultravoilet data from the GALEX Ultraviolet Virgo Cluster Survey (GUViCS) to select candidates which may belong to this population of recently formed halo white dwarfs. A colour selection was used to separate white dwarfs from QSOs and main-sequence stars. Photometric distances are calculated using model colour-absolute magnitude relations. Proper motions are calculated by using the difference in positions between objects from the Sloan Digital Sky Survey and the NGVS. The proper motions are combined with the calculated photometric distances to calculate tangential velocities, as well as approximate Galactic space velocities. White dwarf candidates are characterized as belonging to either the disk or the halo using a variety of methods, including calculated scale heights (z> 1 kpc), tangential velocities (vt >200 km/s), and their location in (V,U) space. The 20 halo white dwarf candidates which were selected using Galactic space velocities are analyzed, and their colours and temperatures suggest that these objects represent some of the youngest white dwarfs in the Galactic halo.

  6. A Study of the SDSS White Dwarf Component in the LSPM Proper Motion Survey

    Science.gov (United States)

    Darveau-Bernier, A.; Bergeron, P.; Lépine, S.

    2017-03-01

    We present a model atmosphere analysis of the white dwarfs identified in the Sloan Digital Sky Survey with proper motions measured in the LSPM proper motion survey of Lépine et al. We rely on reduced proper motion diagrams to build a sample of white dwarfs in the SDSS footprint, and cross correlate this sample with the SDSS spectroscopic database to understand the systematics related to completeness, contamination, WD+M dwarf binaries, reddening, etc. We then determine a white dwarf luminosity function for this sample using various methods.

  7. mocca-SURVEY database I. Accreting white dwarf binary systems in globular clusters - III. Cataclysmic variables - implications of model assumptions

    Science.gov (United States)

    Belloni, Diogo; Zorotovic, Mónica; Schreiber, Matthias R.; Leigh, Nathan W. C.; Giersz, Mirek; Askar, Abbas

    2017-06-01

    In this third of a series of papers related to cataclysmic variables (CVs) and related objects, we analyse the population of CVs in a set of 12 globular cluster models evolved with the MOCCA Monte Carlo code, for two initial binary populations (IBPs), two choices of common-envelope phase (CEP) parameters, and three different models for the evolution of CVs and the treatment of angular momentum loss. When more realistic models and parameters are considered, we find that present-day cluster CV duty cycles are extremely low (≲0.1 per cent) that makes their detection during outbursts rather difficult. Additionally, the IBP plays a significant role in shaping the CV population properties, and models that follow the Kroupa IBP are less affected by enhanced angular momentum loss. We also predict from our simulations that CVs formed dynamically in the past few Gyr (massive CVs) correspond to bright CVs (as expected) and that faint CVs formed several Gyr ago (dynamically or not) represent the overwhelming majority. Regarding the CV formation rate, we rule out the notion that it is similar irrespective of the cluster properties. Finally, we discuss the differences in the present-day CV properties related to the IBPs, the initial cluster conditions, the CEP parameters, formation channels, the CV evolution models and the angular momentum loss treatments.

  8. 2D simulations of the double-detonation model for thermonuclear transients from low-mass carbon-oxygen white dwarfs

    Science.gov (United States)

    Sim, S. A.; Fink, M.; Kromer, M.; Röpke, F. K.; Ruiter, A. J.; Hillebrandt, W.

    2012-03-01

    Thermonuclear explosions may arise in binary star systems in which a carbon-oxygen (CO) white dwarf (WD) accretes helium-rich material from a companion star. If the accretion rate allows a sufficiently large mass of helium to accumulate prior to ignition of nuclear burning, the helium surface layer may detonate, giving rise to an astrophysical transient. Detonation of the accreted helium layer generates shock waves that propagate into the underlying CO WD. This might directly ignite a detonation of the CO WD at its surface (an edge-lit secondary detonation) or compress the core of the WD sufficiently to trigger a CO detonation near the centre. If either of these ignition mechanisms works, the two detonations (helium and CO) can then release sufficient energy to completely unbind the WD. These 'double-detonation' scenarios for thermonuclear explosion of WDs have previously been investigated as a potential channel for the production of Type Ia supernovae from WDs of ˜ 1 M⊙. Here we extend our 2D studies of the double-detonation model to significantly less massive CO WDs, the explosion of which could produce fainter, more rapidly evolving transients. We investigate the feasibility of triggering a secondary core detonation by shock convergence in low-mass CO WDs and the observable consequences of such a detonation. Our results suggest that core detonation is probable, even for the lowest CO core masses that are likely to be realized in nature. To quantify the observable signatures of core detonation, we compute spectra and light curves for models in which either an edge-lit or compression-triggered CO detonation is assumed to occur. We compare these to synthetic observables for models in which no CO detonation was allowed to occur. If significant shock compression of the CO WD occurs prior to detonation, explosion of the CO WD can produce a sufficiently large mass of radioactive iron-group nuclei to significantly affect the light curves. In particular, this can lead

  9. Resonant excitation of white dwarf oscillations in compact object binaries

    Science.gov (United States)

    Rathore, Yasser

    2005-12-01

    ]-modes is found to be unimportant, however, but g -mode are shown to potentially introduce significant errors in parameter estimation for such systems. The exact magnitude of the errors depends upon the density of resonances during the period of observation, and therefore upon details of the white dwarf model.

  10. Rapid Rotation of a Heavy White Dwarf

    Science.gov (United States)

    Kohler, Susanna

    2017-05-01

    New Kepler observations of a pulsating white dwarf have revealed clues about the rotation of intermediate-mass stars.Learning About ProgenitorsStars weighing in at under 8 solar masses generally end their lives as slowly cooling white dwarfs. By studying the rotation of white dwarfs, therefore, we are able to learn about the final stages of angular momentum evolution in these progenitor stars.Most isolated field white dwarfs cluster in mass around 0.62 solar masses, which corresponds to a progenitor mass of around 2.2 solar masses. This abundance means that weve already learned a good deal about the final rotation of low-mass (13 solar-mass) stars. Our knowledge about the angular momentum of intermediate-mass (38 solar-mass) stars, on the other hand, remains fairly limited.Fourier transform of the pulsations from SDSSJ0837+1856. The six frequencies of stellar variability, marked with red dots, reveal a rotation period of 1.13 hours. [Hermes et al. 2017]Record-Breaking FindA newly discovered white dwarf, SDSSJ0837+1856, is now helping to shed light on this mass range. SDSSJ0837+1856 appears to be unusually massive: its measured at 0.87 solar masses, which corresponds to a progenitor mass of roughly 4.0 solar masses. Determining the rotation of this white dwarf would therefore tell us about the final stages of angular momentum in an intermediate-mass star.In a new study led by J.J. Hermes (Hubble Fellow at University of North Carolina, Chapel Hill), a team of scientists presents a series of measurements of SDSSJ0837+1856 that suggest its the highest-mass and fastest-rotating isolated pulsating white dwarf known.Histogram of rotation rates determined from the asteroseismology of pulsating white dwarfs (marked in red). SDSSJ0837+1856 (indicated in black) is more massive and rotates faster than any other known pulsating white dwarf. [Hermes et al. 2017]Rotation from PulsationsWhy pulsating? In the absence of measurable spots and other surface features, the way we

  11. Polluting white dwarfs with perturbed exo-comets

    Science.gov (United States)

    Caiazzo, Ilaria; Heyl, Jeremy S.

    2017-08-01

    We present a model to account for the observed debris discs around young white dwarfs and the presence of metal lines in their spectra. Stellar evolution models predict that the mass-loss on the AGB will be pulsed; furthermore, observations indicate that the bulk of the mass-loss occurs on the AGB. In this case, if the progenitors of the white dwarfs had remnants of planetary formation like the Sun's Oort cloud or the Kuiper Belt and a planet lying within that cloud or nearby, we find that up to 2 per cent of the planetesimals will fall either into planet-crossing orbits or into chaotic regions after the mass-loss, depending on the location and mass of the planet (from Mars to Neptune). This yields a sufficient mass of comets that can be scattered towards the star, form a debris disc and pollute the atmosphere.

  12. White dwarf pollution by planets in stellar binaries

    OpenAIRE

    Hamers, S.; Portegies, F, Zwart S.

    2016-01-01

    Approximately $0.2 \\pm 0.2$ of white dwarfs (WDs) show signs of pollution by metals, which is likely due to the accretion of tidally disrupted planetary material. Models invoking planet-planet interactions after WD formation generally cannot explain pollution at cooling times of several Gyr. We consider a scenario in which a planet is perturbed by Lidov-Kozai oscillations induced by a binary companion and exacerbated by stellar mass loss, explaining pollution at long cooling times. Our comput...

  13. SPIRAL INSTABILITY CAN DRIVE THERMONUCLEAR EXPLOSIONS IN BINARY WHITE DWARF MERGERS

    Energy Technology Data Exchange (ETDEWEB)

    Kashyap, Rahul; Fisher, Robert [Department of Physics, University of Massachusetts Dartmouth, 285 Old Westport Road, North Dartmouth, MA 02740 (United States); García-Berro, Enrique; Aznar-Siguán, Gabriela [Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades, 5, E-08860 Castelldefels (Spain); Ji, Suoqing [Department of Physics, Broida Hall, University of California Santa Barbara, Santa Barbara, CA 93106–9530 (United States); Lorén-Aguilar, Pablo [School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL (United Kingdom)

    2015-02-10

    Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion of carbon–oxygen white dwarfs, and serve as standardizable cosmological candles. However, despite their importance, the nature of the progenitor systems that give rise to SNe Ia has not been hitherto elucidated. Observational evidence favors the double-degenerate channel in which merging white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies exist between observations and theory, and to date, there has been no self-consistent merger model that yields a SNe Ia. Here we show that a spiral mode instability in the accretion disk formed during a binary white dwarf merger leads to a detonation on a dynamical timescale. This mechanism sheds light on how white dwarf mergers may frequently yield SNe Ia.

  14. The connection between period spectra and constraints in white dwarf asteroseismology

    Directory of Open Access Journals (Sweden)

    Bischoff-Kim Agnès

    2017-01-01

    Full Text Available White dwarfs are the end product of evolution for around 98% of the stars in our Galaxy. Buried in their interiors are the records of physical processes that take place during earlier stages in the life of the star. In recent years, a well-established theory of non-radial oscillations, improved white dwarf models, year of expertise built up in the field of white dwarf asteroseismic fitting, and computing power have culminated in the asteroseismology finally delivering what it promised: a detailed map of the interior structure of white dwarfs. As always in science, new results raise new questions. We perform a number of numerical experiments to better understand the connection between a given set of periods varying in the number of periods and in the set of radial overtones and the quality of the constraints on interior structure one obtains from fitting these periods.

  15. White Dwarfs in Gaia Data Release 1

    Science.gov (United States)

    Jordan, S.

    2017-03-01

    On September 14, the Gaia archives opened for access to the Gaia DR1. The catalogue with more than one billion star positions and more than two million parallaxes and proper motions will have enormous influence on many topics in astronomy. However, due to their extremely blue colour, parallaxes and proper motions of only six white dwarfs were directly measured. Tremblay et al. used these data and those for 46 white dwarfs in binaries in order to construct an empirical mass-radius relation. As it was the case for Hipparcos, the precision of the data does not allow for the characterisation of hydrogen envelope masses. With Gaia DR2 coming in late 2017 the prospects for white dwarf research are much better.

  16. Actively Disintegrating Astroids around a White Dwarf

    Science.gov (United States)

    Xu, Siyi

    2017-08-01

    Recent studies show that planetary systems can be widespread around white dwarfs. It has been proposed that planetary systems are responsible for the pollution observed in a white dwarf's atmosphere and the excess infrared radiation. This scenario is greatly strengthened by the recent discovery of actively disintegrating bodies orbiting around the white dwarf WD 1145+017. In addition, this system has a heavily polluted atmosphere, a dust disk, and circumstellar gas. Our team has been monitoring this system since its discovery and our recent COS data have revealed many new surprises. We propose to continue studying this system for the next two cycles and further constrain the evolution of the disintegrating bodies: what are the main mechanisms responsible for its destruction? How is circumstellar gas produced and maintained?

  17. Testing the white dwarf mass-radius relationship with eclipsing binaries

    Science.gov (United States)

    Parsons, S. G.; Gänsicke, B. T.; Marsh, T. R.; Ashley, R. P.; Bours, M. C. P.; Breedt, E.; Burleigh, M. R.; Copperwheat, C. M.; Dhillon, V. S.; Green, M.; Hardy, L. K.; Hermes, J. J.; Irawati, P.; Kerry, P.; Littlefair, S. P.; McAllister, M. J.; Rattanasoon, S.; Rebassa-Mansergas, A.; Sahman, D. I.; Schreiber, M. R.

    2017-10-01

    We present high-precision, model-independent, mass and radius measurements for 16 white dwarfs in detached eclipsing binaries and combine these with previously published data to test the theoretical white dwarf mass-radius relationship. We reach a mean precision of 2.4 per cent in mass and 2.7 per cent in radius, with our best measurements reaching a precision of 0.3 per cent in mass and 0.5 per cent in radius. We find excellent agreement between the measured and predicted radii across a wide range of masses and temperatures. We also find the radii of all white dwarfs with masses less than 0.48 M⊙ to be fully consistent with helium core models, but they are on average 9 per cent larger than those of carbon-oxygen core models. In contrast, white dwarfs with masses larger than 0.52 M⊙ all have radii consistent with carbon-oxygen core models. Moreover, we find that all but one of the white dwarfs in our sample have radii consistent with possessing thick surface hydrogen envelopes (10-5 ≥ MH/MWD ≥ 10-4), implying that the surface hydrogen layers of these white dwarfs are not obviously affected by common envelope evolution.

  18. A large oxygen-dominated core from the seismic cartography of a pulsating white dwarf

    Science.gov (United States)

    Giammichele, N.; Charpinet, S.; Fontaine, G.; Brassard, P.; Green, E. M.; Van Grootel, V.; Bergeron, P.; Zong, W.; Dupret, M.-A.

    2018-02-01

    White-dwarf stars are the end product of stellar evolution for most stars in the Universe. Their interiors bear the imprint of fundamental mechanisms that occur during stellar evolution. Moreover, they are important chronometers for dating galactic stellar populations, and their mergers with other white dwarfs now appear to be responsible for producing the type Ia supernovae that are used as standard cosmological candles. However, the internal structure of white-dwarf stars—in particular their oxygen content and the stratification of their cores—is still poorly known, because of remaining uncertainties in the physics involved in stellar modelling codes. Here we report a measurement of the radial chemical stratification (of oxygen, carbon and helium) in the hydrogen-deficient white-dwarf star KIC08626021 (J192904.6+444708), independently of stellar-evolution calculations. We use archival data coupled with asteroseismic sounding techniques to determine the internal constitution of this star. We find that the oxygen content and extent of its core exceed the predictions of existing models of stellar evolution. The central homogeneous core has a mass of 0.45 solar masses, and is composed of about 86 per cent oxygen by mass. These values are respectively 40 per cent and 15 per cent greater than those expected from typical white-dwarf models. These findings challenge present theories of stellar evolution and their constitutive physics, and open up an avenue for calibrating white-dwarf cosmochronology.

  19. White dwarf atmospheres and circumstellar environments

    CERN Document Server

    Hoard, Donald W

    2012-01-01

    Written by selected astronomers at the forefront of their fields, this timely and novel book compiles the latest results from research on white dwarf stars, complementing existing literature by focusing on fascinating new developments in our understanding of the atmospheric and circumstellar environments of these stellar remnants. Complete with a thorough refresher on the observational characteristics and physical basis for white dwarf classification, this is a must-have resource for researchers interested in the late stages of stellar evolution, circumstellar dust and nebulae, and the future

  20. The angular momentum of isolated white dwarfs

    Directory of Open Access Journals (Sweden)

    Brassard P.

    2013-03-01

    Full Text Available This is a very brief report on an ongoing program aimed at mapping the internal rotation profiles of stars through asteroseismology. Three years ago, we developed and applied successfully a new technique to the pulsating GW Vir white dwarf PG 1159−035, and were able to infer that it rotates very slowly and rigidly over some 99% of its mass. We applied the same approach to the three other GW Vir pulsators with available rotational splitting data, and found similar results. We discuss the implications of these findings on the question of the angular momentum of white dwarfs resulting from single star evolution.

  1. Accreting White Dwarfs as Universal Accretion Laboratories

    Science.gov (United States)

    Knigge, Christian

    Accreting white dwarfs (AWDs) are numerous, bright and nearby, making them excellent laboratories for the study of accretion physics. Since their accretion flows are unaffected by relativistic effects or ultra-strong magnetic fields, they provide a crucial "control" group for efforts to understand more complex/compact systems, such as accreting neutron stars (NSs) and black holes (BHs). Here, I will review recent work on AWDs, which has revealed that these superficially simple systems actually exhibit the full range of accretion-related phenomenology seen in accreting NSs and BHs. For example, (i) AWDs undergo mass loss in the form of both disk winds and radio jets; (ii) their disk winds are only seen in high-Mdot states, similar to what is observed in accreting BHs; (iii) they exhibit (possibly hysteretic) outbursts produced by disk instabilities, as also seen in NS and BH transients; and (iv) they produce accretion-induced stochastic variability ("flickering") that exhibits the same rms-flux relation as observed in low-mass X-ray binaries and AGN. Based on this rich and shared phenomenology, it is reasonable to hope that much of accretion physics is universal. In this context, AWDs hold great promise as observational testing grounds for attempts to model and understand these physics.

  2. COS Spectroscopy of White Dwarf Companions to Blue Stragglers

    Science.gov (United States)

    Gosnell, Natalie M.; Geller, Aaron M.; Knigge, Christian; Mathieu, Robert D.; Sills, Alison; Leiner, Emily; Leigh, Nathan

    2017-01-01

    Complete membership studies of open stellar clusters reveal that 25% of the evolved stars follow alternative pathways in stellar evolution, meaning something in the history of these stars changed their composition or mass (or both). In order to draw a complete picture of stellar evolution we must include these canonically "strange" stars in our definition of standard stellar populations. The formation mechanism of blue straggler stars, traditionally defined to be brighter and bluer than the main sequence turnoff in a star cluster, has been an outstanding question for almost six decades. Recent Hubble Space Telescope (HST) far-ultraviolet (far-UV) observations directly reveal that the blue straggler stars in the old (7 Gyr) open cluster NGC 188 are predominantly formed through mass transfer. We will present HST far-UV COS spectroscopy of white dwarf companions to blue stragglers. These white dwarfs are the remnants of the mass transfer formation process. The effective temperatures and surface gravities of the white dwarfs delineate the timeline of blue straggler formation in this cluster. The existence of these binaries in a well-studied cluster environment provides an unprecedented opportunity to observationally constrain mass transfer models and inform our understanding of many other alternative pathway stellar products.

  3. Fundamental Physics from Observations of White Dwarf Stars

    Science.gov (United States)

    Bainbridge, M. B.; Barstow, M. A.; Reindl, N.; Barrow, J. D.; Webb, J. K.; Hu, J.; Preval, S. P.; Holberg, J. B.; Nave, G.; Tchang-Brillet, L.; Ayres, T. R.

    2017-03-01

    Variation in fundamental constants provide an important test of theories of grand unification. Potentially, white dwarf spectra allow us to directly observe variation in fundamental constants at locations of high gravitational potential. We study hot, metal polluted white dwarf stars, combining far-UV spectroscopic observations, atomic physics, atmospheric modelling and fundamental physics, in the search for variation in the fine structure constant. This registers as small but measurable shifts in the observed wavelengths of highly ionized Fe and Ni lines when compared to laboratory wavelengths. Measurements of these shifts were performed by Berengut et al (2013) using high-resolution STIS spectra of G191-B2B, demonstrating the validity of the method. We have extended this work by; (a) using new (high precision) laboratory wavelengths, (b) refining the analysis methodology (incorporating robust techniques from previous studies towards quasars), and (c) enlarging the sample of white dwarf spectra. A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature. We describe our approach and present preliminary results.

  4. Mystery of a Dimming White Dwarf

    Science.gov (United States)

    Kohler, Susanna

    2015-12-01

    In the wake of the recent media attention over an enigmatic, dimming star, another intriguing object has been discovered: J1529+2928, a white dwarf that periodically dims. This mystery, however, may have a simple solution with interesting consequences for future surveys of white dwarfs.Unexpected VariabilityJ1529+2928 is an isolated white dwarf that appears to have a mass of slightly more than the Sun. But rather than radiating steadily, J1529+2928 dims once every 38 minutes almost as though it were being eclipsed.The team that discovered these variations, led by Mukremin Kilic (University of Oklahoma), used telescopes at the Apache Point Observatory and the McDonald Observatory to obtain follow-up photometric data of J1529+2928 spread across 66 days. The team also took spectra of the white dwarf with the Gemini North telescope.Kilic and collaborators then began, one by one, to rule out possible causes of this objects variability.Eliminating OptionsThe period of the variability is too long for J1529+2928 to be a pulsating white dwarf with luminosity variation caused by gravity-wave pulsations.The variability cant be due to an eclipse by a stellar or brown-dwarf companion, because there isnt any variation in J1529+2928s radial velocity.Its not due to the orbit of a solid-body planetary object; such a transit would be too short to explain observations.It cant be due to the orbit of a disintegrated planet; this wouldnt explain the light curves observed in different filters plus the light curve doesnt change over the 66-day span.Spotty SurfaceTop and middle two panels: light curves from three different nights observing J1529+2928s periodic dimming. Bottom panel: The Fourier transform shows a peak at 37.7 cycles/day (and another, smaller peak at its first harmonic). [Kilic et al. 2015]So what explanation is left? The authors suggest that J1529+2928s variability is likely caused by a starspot on the white dwarfs surface that rotates into and out of our view. Estimates

  5. Infrared observations of white dwarfs and the implications for the accretion of dusty planetary material

    Science.gov (United States)

    Bonsor, Amy; Farihi, Jay; Wyatt, Mark C.; van Lieshout, Rik

    2017-06-01

    Infrared excesses around metal-polluted white dwarfs have been associated with the accretion of dusty planetary material. This work analyses the available infrared data for an unbiased sample of white dwarfs and demonstrates that no more than 3.3 per cent can have a wide, flat, opaque dust disc, extending to the Roche radius, with a temperature at the disc inner edge of Tin = 1400 K, the standard model for the observed excesses. This is in stark contrast to the incidence of pollution of about 30 per cent. We present four potential reasons for the absence of an infrared excess in polluted white dwarfs, depending on their stellar properties and inferred accretion rates: (I) their dust discs are opaque, but narrow, thus evading detection if more than 85 per cent of polluted white dwarfs have dust discs narrower than δr white dwarfs with sinking time-scales longer than hundreds of years, (III) their dust is optically thin, which can supply low accretion rates of 20 000 K. Future observations sensitive to faint infrared excesses or the presence of gas can test the scenarios presented here, thereby better constraining the nature of the material fuelling accretion in polluted white dwarfs.

  6. New halo white dwarf candidates in the Sloan Digital Sky Survey

    Science.gov (United States)

    Dame, Kyra; Gianninas, A.; Kilic, Mukremin; Munn, Jeffrey A.; Brown, Warren R.; Williams, Kurtis A.; von Hippel, Ted; Harris, Hugh C.

    2016-12-01

    We present optical spectroscopy and near-infrared photometry of 57 faint (g = 19-22) high proper motion white dwarfs identified through repeat imaging of ≈3100 deg2 of the Sloan Digital Sky Survey footprint by Munn et al. We use ugriz and JH photometry to perform a model atmosphere analysis, and identify 10 ultracool white dwarfs with Teff 120 km s-1) and UVW velocities that are more consistent with the halo than the Galactic disc. For typical 0.6 M⊙ white dwarfs, the cooling ages for these halo candidates range from 2.3 to 8.5 Gyr. However, the total main-sequence+white dwarf cooling ages of these stars would be consistent with the Galactic halo if they are slightly undermassive. Given the magnitude limits of the current large-scale surveys, many of the coolest and oldest white dwarfs remain undiscovered in the solar neighbourhood, but upcoming surveys such as Gaia and the Large Synoptic Survey Telescope should find many of these elusive thick disc and halo white dwarfs.

  7. Population Synthesis Studies of the White Dwarfs of the Galactic Disk and Halo

    Science.gov (United States)

    Cojocaru, Elena-Ruxandra

    2016-09-01

    White dwarfs are fossil stars that can encode valuable information about the formation, evolution and other properties of the different Galactic stellar populations. They are the direct descendants of main-sequence stars with masses ranging from ∼0.8 M⊙ to ∼10 M⊙, which means that over 95% of the stars in our Galaxy will eventually become white dwarfs. This fact, correlated with the excellent quality of modern white dwarf cooling models, clearly marks their potential as cosmic clocks for estimating the ages of Galactic stellar populations, as well as place white dwarfs as privileged objects in understanding several actual astrophysical problems. Stellar population synthesis methods (Tinsley, 1968) use theoretical evolutionary sequences to reproduce luminosities, temperatures and other parameters building up to a synthetic population that can be readily compared to an observed sample of stars. Such techniques are perfect for the study of the different white dwarf populations in our Galaxy and their strength has only grown in recent years, fueled both by improved evolutionary sequences and detailed cooling tracks and also by the ever growing samples of white dwarfs identified through modern survey missions. In particular, the work presented in this thesis uses an updated population synthesis code based on previous versions of the code from our group (García-Berro et al., 1999; Torres et al., 2002; García-Berro et al., 2004; Torres et al., 2005; Camacho et al., 2014). Our synthetic population code, based on Monte Carlo statistical techniques, has been extensively used in the study of the disk (García-Berro et al., 1! 999; Torres et al., 2001; Torres & García-Berro, 2016) and halo (Torres et al., 2002; García-Berro et al., 2004) single white-dwarf population, white dwarf plus main sequence stars (Camacho et al., 2014), as well as open clusters such as NGC 6791 (García-Berro et al., 2010; García-Berro et al., 2011) or globular clusters, as 47 Tuc (Garc

  8. A wide binary trigger for white dwarf pollution

    OpenAIRE

    Bonsor, Amy; Veras, Dimitri

    2015-01-01

    Metal pollution in white dwarf atmospheres is likely to be a signature of remnant planetary systems. Most explanations for this pollution predict a sharp decrease in the number of polluted systems with white dwarf cooling age. Observations do not confirm this trend, and metal pollution in old (1-5 Gyr) white dwarfs is difficult to explain. We propose an alternative, time-independent mechanism to produce the white dwarf pollution. The orbit of a wide binary companion can be perturbed by Galact...

  9. Period changes in ultracompact double white dwarfs

    NARCIS (Netherlands)

    Marsh, T.R.; Nelemans, G.A.

    2005-01-01

    In recent years there has been much interest in the nature of two stars, V407 Vul and RX J0806+1527, which are widely thought to be binary white dwarfs of very short orbital period, 570 and 321s, respectively. As such they should be strong sources of gravitational waves and possible ancestors of the

  10. The White Dwarf Companions of Recycled Pulsars

    OpenAIRE

    van Kerkwijk, M. H.

    1996-01-01

    I review what properties of the white-dwarf companions of recycled pulsars can be inferred from optical observations, and discuss how these can help us understand the characteristics and evolution of these binaries. I focus on spectroscopic observations, describing results obtained recently, and looking forward to what may come.

  11. Pulsating low-mass white dwarfs in the frame of new evolutionary sequences. V. Asteroseismology of ELMV white dwarf stars

    Science.gov (United States)

    Calcaferro, Leila M.; Córsico, Alejandro H.; Althaus, Leandro G.

    2017-11-01

    Context. Many pulsating low-mass white dwarf stars have been detected in the past years in the field of our Galaxy. Some of them exhibit multiperiodic brightness variation, therefore it is possible to probe their interiors through asteroseismology. Aims: We present a detailed asteroseismological study of all the known low-mass variable white dwarf stars based on a complete set of fully evolutionary models that are representative of low-mass He-core white dwarf stars. Methods: We employed adiabatic radial and nonradial pulsation periods for low-mass white dwarf models with stellar masses ranging from 0.1554 to 0.4352 M⊙ that were derived by simulating the nonconservative evolution of a binary system consisting of an initially 1 M⊙ zero-age main-sequence (ZAMS) star and a 1.4 M⊙ neutron star companion. We estimated the mean period spacing for the stars under study (where this was possible), and then we constrained the stellar mass by comparing the observed period spacing with the average of the computed period spacings for our grid of models. We also employed the individual observed periods of every known pulsating low-mass white dwarf star to search for a representative seismological model. Results: We found that even though the stars under analysis exhibit few periods and the period fits show multiplicity of solutions, it is possible to find seismological models whose mass and effective temperature are in agreement with the values given by spectroscopy for most of the cases. Unfortunately, we were not able to constrain the stellar masses by employing the observed period spacing because, in general, only few periods are exhibited by these stars. In the two cases where we were able to extract the period spacing from the set of observed periods, this method led to stellar mass values that were substantially higher than expected for this type of stars. Conclusions: The results presented in this work show the need for further photometric searches, on the one hand

  12. Prestack exploding reflector modeling and migration in TI media

    KAUST Repository

    Wang, H.

    2014-01-01

    Prestack depth migration in anisotropic media, especially those that exhibit tilt, can be costly using reverse time migration (RTM). We present two-way spectral extrapolation of prestack exploding reflector modeling and migration (PERM) in acoustic transversely isotropic (TI) media. We construct systematic ways to evaluate phase angles and phase velocities in dip oriented TI (DTI), vertical TI (VTI) and tilted TI (TTI) media. Migration results from the Marmousi VTI model and the BP2007 TTI model show the feasibility of our approach.

  13. Stellar explosions from accreting white dwarfs

    Science.gov (United States)

    Moore, Kevin L.

    Unstable thermonuclear burning on accreting white dwarfs (WDs) can lead to a wide variety of outcomes, and induce shock waves in several contexts. In classical and recurrent novae, a WD accreting hydrogen-rich material from a binary companion can experience thermonuclear runaways, ejecting mass into the interstellar/circumbinary environment at ~1000 km/s. This highly supersonic ejecta drives shock waves into the interstellar gas which may be relevant for sweeping out gas from globular clusters or forming circumstellar absorption regions in interacting supernovae. While runaway nuclear burning in novae releases enough energy for these objects to brighten by a factor of ~10 4 over roughly a weeklong outburst, it does not become dynamically unstable. In contrast, certain helium accretion scenarios may allow for dynamical burning modes, in part due to the higher temperature sensitivity of helium burning reactions and larger accreted envelopes. The majority of this thesis involves such dynamical burning modes, specifically detonations - shock waves sustained by nuclear energy release behind the shock front. We investigate when steady-state detonations are realizable in accreted helium layers on WDs, and model their strength and burning products using both semi-analytic and numerical models. We find the minimum helium layer thickness that will sustain a steady laterally propagating detonation and show that it depends on the density and composition of the helium layer, specifically 12 C and 16O. Though gravitationally unbound, the ashes still have unburned helium (~80% in the thinnest cases) and only reach up to heavy elements such as 40Ca, 44Ti, 48Cr, and 52Fe. It is rare for these thin shells to generate large amounts of radioactive isotopes necessary to power light curves, such as 56Ni. This has important implications on whether the unbound helium burning ashes may create faint and fast peculiar supernovae or events with virtually no radioactivity, as well as on off

  14. Simple scaling model for exploding pusher targets

    Energy Technology Data Exchange (ETDEWEB)

    Storm, E.K.; Larsen, J.T.; Nuckolls, J.H.; Ahlstrom, H.G.; Manes, K.R.

    1977-11-04

    A simple model has been developed which when normalized by experiment or Lasnex calculations can be used to scale neutron yields for variations in laser input power and pulse length and target radius and wall thickness. The model also illucidates some of the physical processes occurring in this regime of laser fusion experiments. Within certain limitations on incident intensity and target geometry, the model scales with experiments and calculations to within a factor of two over six decades in neutron yield.

  15. White dwarf cosmochronology in the solar neighborhood

    Energy Technology Data Exchange (ETDEWEB)

    Tremblay, P.-E.; Kalirai, J. S.; Soderblom, D. R.; Cignoni, M. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Cummings, J., E-mail: tremblay@stsci.edu [Center for Astrophysical Sciences, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States)

    2014-08-20

    The study of the stellar formation history in the solar neighborhood is a powerful technique to recover information about the early stages and evolution of the Milky Way. We present a new method that consists of directly probing the formation history from the nearby stellar remnants. We rely on the volume complete sample of white dwarfs within 20 pc, where accurate cooling ages and masses have been determined. The well characterized initial-final mass relation is employed in order to recover the initial masses (1 ≲ M {sub initial}/M {sub ☉} ≲ 8) and total ages for the local degenerate sample. We correct for moderate biases that are necessary to transform our results to a global stellar formation rate, which can be compared to similar studies based on the properties of main-sequence stars in the solar neighborhood. Our method provides precise formation rates for all ages except in very recent times, and the results suggest an enhanced formation rate for the solar neighborhood in the last 5 Gyr compared to the range 5 < Age (Gyr) < 10. Furthermore, the observed total age of ∼10 Gyr for the oldest white dwarfs in the local sample is consistent with the early seminal studies that have determined the age of the Galactic disk from stellar remnants. The main shortcoming of our study is the small size of the local white dwarf sample. However, the presented technique can be applied to larger samples in the future.

  16. White Dwarf Cosmochronology in the Solar Neighborhood

    Science.gov (United States)

    Tremblay, P.-E.; Kalirai, J. S.; Soderblom, D. R.; Cignoni, M.; Cummings, J.

    2014-08-01

    The study of the stellar formation history in the solar neighborhood is a powerful technique to recover information about the early stages and evolution of the Milky Way. We present a new method that consists of directly probing the formation history from the nearby stellar remnants. We rely on the volume complete sample of white dwarfs within 20 pc, where accurate cooling ages and masses have been determined. The well characterized initial-final mass relation is employed in order to recover the initial masses (1 ages for the local degenerate sample. We correct for moderate biases that are necessary to transform our results to a global stellar formation rate, which can be compared to similar studies based on the properties of main-sequence stars in the solar neighborhood. Our method provides precise formation rates for all ages except in very recent times, and the results suggest an enhanced formation rate for the solar neighborhood in the last 5 Gyr compared to the range 5 Age (Gyr) age of ~10 Gyr for the oldest white dwarfs in the local sample is consistent with the early seminal studies that have determined the age of the Galactic disk from stellar remnants. The main shortcoming of our study is the small size of the local white dwarf sample. However, the presented technique can be applied to larger samples in the future.

  17. Unlocking the secrets of white dwarf stars

    CERN Document Server

    Van Horn, Hugh M

    2015-01-01

    White dwarfs, each containing about as much mass as our Sun but packed into a volume about the size of Earth, are the endpoints of evolution for most stars. Thousands of these faint objects have now been discovered, though only a century ago only three were known. They are among the most common stars in the Milky Way Galaxy, and they have become important tools in understanding the universe. Yet a century ago only three white dwarfs were known.   The existence of these stars completely baffled the scientists of the day, and solving the mysteries of these strange objects required revolutionary advances in science and technology, including the development of quantum physics, the construction and utilization of large telescopes, the invention of the digital computer, and the ability to make astronomical observations from space.   This book tells the story of the growth in our understanding of white dwarf stars, set within the context of the relevant scientific and technological advances. Part popular science, ...

  18. Exoplanet recycling in massive white-dwarf debris discs

    Science.gov (United States)

    Van Lieshout, Rik

    2017-06-01

    When a star evolves into a white dwarf, the planetary system it hosts can become unstable. Planets in such systems may then be scattered onto star-grazing orbits, leading to their tidal disruption as they pass within the white dwarf’s Roche limit. We study the massive, compact debris discs that may arrise from this process using a combination of analytical estimates and numerical modelling. The discs are gravitationally unstable, resulting in an enhanced effective viscosity due to angular momentum transport associated with self-gravity wakes. For disc masses greater than ~1026 g (corresponding to progenitor objects comparable to the Galilean moons), viscous spreading dominates over Poynting-Robertson drag in the outer parts of the disc. In such massive discs, mass is transported both in- and outwards. When the outward-flowing material spreads beyond the Roche limit, it coagulates into new (minor) planets in a process analogous to the ongoing formation of Saturn’s innermost moonlets. This process recycles a substantial fraction of the original disc mass (tens of percents), with the bulk of the mass locked in a single large body orbitting in a 2:1 mean-motion resonance with the Roche limit. As such, the recycling of a tidally disrupted super-Earth could yield an Earth-mass planet on a 10--20 hr orbit. For white dwarfs with a temperature below 6000-7000 K (corresponding to a cooling age of >1--2 Gyr), this orbit is located in the white dwarf’s habitable zone. The recycling process also creates a string of smaller bodies just outside the Roche limit. These may account for the collection of minor planets postulated to orbit white dwarf WD 1145+017.

  19. New insights on pulsating white dwarfs from 3D radiation-hydrodynamical simulations

    Science.gov (United States)

    Tremblay, Pier-Emmanuel; Fontaine, Gilles; Ludwig, Hans-Günter; Gianninas, Alexandros; Kilic, Mukremin

    We have recently computed a grid of 3D radiation-hydrodynamical simulations for the atmosphere of pure-hydrogen DA white dwarfs in the range 5.0 white dwarfs are located. We have significantly improved the theoretical framework to study these objects by removing the free parameters of 1D convection, which were previously a major modeling hurdle. We present improved atmospheric parameter determinations based on spectroscopic fits with 3D model spectra, allowing for an updated definition of the empirical edges of the ZZ Ceti instability strip. Our 3D simulations also precisely predict the depth of the convection zones, narrowing down the internal layers where pulsation are being driven. We hope that these 3D effects will be included in asteroseismic models in the future to predict the region of the HR diagram where white dwarfs are expected to pulsate.

  20. SIM's Search for Planets Orbiting Nearby White Dwarfs - Update

    Science.gov (United States)

    Subasavage, John P., Jr.

    2009-05-01

    I propose to use the Space Interferometry Mission (SIM) to observe a sample ( 25-50) of nearby white dwarfs in hopes of detecting planetary companions with masses in the 10 Earth mass range on average. Because of the nature of white dwarfs' spectral signatures (a few broad, if any, absorption lines), current radial velocity planet hunting techniques are not viable. Astrometry is currently the only technique capable of detecting low mass planets around white dwarfs and SIM would be the best suited astrometric instrument to do so once launched. As part of a SIM Science Study, I present a detailed evaluation of the star fields in the vicinity of nearby white dwarfs within 20 pc and with V white dwarfs with accuate trigonometric parallaxes and photometry. This effort will aid in the selection of white dwarfs to be targeted for planet searches using SIM by maximizing planetary sensitivities while minimizing total mission time spent on these observations.

  1. Anderson and Stoner Published White Dwarf Mass Limits Before Chandrasekhar

    CERN Document Server

    Blackman, Eric G

    2011-01-01

    In their engaging recountals of Chandrasekhar's extraordinary career (Physics Today, vol 63, Issue 12, Dec 2010), neither Dyson nor Wali mention that Chandrasekhar was the third person not the first, to publish a white dwarf mass limit incorporating a relativistic treatment of degenerate electrons. As it has become a common misconception that Chandrasekhar was the first, a clarifying reminder on this historical point is warranted. In short, the white dwarf mass limit widely attributed to Chandrasekhar (1931) should be the specific white dwarf mass limit calculated for a polytrope. The insight that a relativistic treatment of degeneracy leads to the existence of a white dwarf mass limit first appeared in papers of W. Anderson (1929) and E.C. Stoner (1930) for a uniform density star. Accordingly, Chandrasekhar (1931) cites Stoner (1930) and points out that the polytrope white dwarf mass limit is less than Stoner's uniform density white dwarf mass limit by about 20%.

  2. Current and future white dwarf mass-radius constraints on varying fundamental couplings and unification scenarios

    Science.gov (United States)

    Magano, D. M. N.; Vilas Boas, J. M. A.; Martins, C. J. A. P.

    2017-10-01

    We discuss the feasibility of using astrophysical observations of white dwarfs as probes of fundamental physics. We quantify the effects of varying fundamental couplings on the white dwarf mass-radius relation in a broad class of unification scenarios, both for the simple case of a polytropic stellar structure model and for more general models. Independent measurements of the mass and radius, together with direct spectroscopic measurements of the fine-structure constant in white dwarf atmospheres lead to constraints on combinations of the two phenomenological parameters describing the underlying unification scenario (one of which is related to the strong sector of the theory while the other is related to the electroweak sector). While currently available measurements do not yet provide stringent constraints, we show that forthcoming improvements, expected for example from the Gaia satellite, can break parameter degeneracies and lead to constraints that ideally complement those obtained from local laboratory tests using atomic clocks.

  3. The fate of exomoons in white dwarf planetary systems

    Science.gov (United States)

    Payne, Matthew J.; Veras, Dimitri; Gänsicke, Boris T.; Holman, Matthew J.

    2017-01-01

    Roughly 1000 white dwarfs are known to be polluted with planetary material, and the progenitors of this material are typically assumed to be asteroids. The dynamical architectures which perturb asteroids into white dwarfs are still unknown, but may be crucially dependent on moons liberated from parent planets during post-main-sequence gravitational scattering. Here, we trace the fate of these exomoons, and show that they more easily achieve deep radial incursions towards the white dwarf than do scattered planets. Consequently, moons are likely to play a significant role in white dwarf pollution, and in some cases may be the progenitors of the pollution itself.

  4. Iron abundance in the hot DA white dwarfs Feige 24 and G191 B2B

    Science.gov (United States)

    Vennes, Stephane; Chayer, Pierre; Thorstensen, John R.; Bowyer, Stuart; Shipman, Harry L.

    1992-01-01

    Attention is given to model calculations of the far- and extreme-UV line spectra of highly ionized Fe species (Fe IV, Fe V, and Fe VI) for hot high-gravity H-rich stars. A spectral analysis of 31 hr of exposure of the DA white dwarf Feige 24 with IUE in the echelle mode reveals the presence of Fe with an abundance relative to H by number of (5-10) x 10 exp -6 with an uncertainty dominated by the determination of stellar parameters. An analysis of IUE data from the white dwarf G191 B2B results in a similar Fe abundance if this star shares similar atmospheric parameters (Teff, g) with Feige 24. Fe is thus the second most abundant photospheric element in hot DA white dwarfs.

  5. Cool White Dwarfs from the SuperCOSMOS and Sloan Digital Sky Surveys

    Science.gov (United States)

    Hambly, N. C.; Digby, A. P.; Oppenheimer, B. R.

    2005-07-01

    We have used datamining techniques in the SuperCOSMOS Science Archive (http://surveys.roe.ac.uk/ssa) to obtain a large, well defined proper motion and magnitude selected sample of cool white dwarfs. Using accurate 5-colour photometry from the Sloan Digital Sky Survey DR1 and SuperCOSMOS Sky Survey photometry and astrometry, we demonstrate the power of reduced proper motion in obtaining a sample of >700 white dwarfs. We examine the characteristics of these objects in various two-colour diagrams in conjunction with new model atmosphere predictions recently computed in the SDSS photometric system. Ultimately, we intend to analyse these data with techniques similar to those already used to examine the subdwarf luminosity function (Digby et al. 2003). In this way, we aim to decompose the contribution of thin disk, thick disk and spheroid white dwarfs in the sample to enable computation of accurate luminosity functions for those respective populations.

  6. The stellar seismology of hot white dwarfs and planetary nebula nuclei

    Science.gov (United States)

    Kawaler, Steven D.

    1987-01-01

    The pulsation properties of hot white dwarfs make it possible to determine their mass, surface composition, rotation, and rate of evolution, and provide constraints on their internal structure. Period spacings are sensitive measures of stellar mass and indicate surface layer structure. Measurement of the rate of period change for these stars provide a way to determine their cooling rates. Attention is also given to how well (or poorly) models of excitation of the pulsations fit within current models of planetary nebula nuclei and hot white dwarfs.

  7. Mass transfer in white dwarf-neutron star binaries

    Science.gov (United States)

    Bobrick, Alexey; Davies, Melvyn B.; Church, Ross P.

    2017-05-01

    We perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (WD-NS binaries), and measure the specific angular momentum of material lost from the binary in disc winds. By incorporating our results within a long-term evolution model, we measure the long-term stability of mass transfer in these binaries. We find that only binaries containing helium white dwarfs (WDs) with masses less than a critical mass of MWD, crit = 0.2 M⊙ undergo stable mass transfer and evolve into ultracompact X-ray binaries. Systems with higher mass WDs experience unstable mass transfer, which leads to tidal disruption of the WD. Our low critical mass compared to the standard jet-only model of mass-loss arises from the efficient removal of angular momentum in the mechanical disc winds, which develop at highly super-Eddington mass-transfer rates. We find that the eccentricities expected for WD-NS binaries when they come into contact do not affect the loss of angular momentum, and can only affect the long-term evolution if they change on shorter time-scales than the mass-transfer rate. Our results are broadly consistent with the observed numbers of both ultracompact X-ray binaries and radio pulsars with WD companions. The observed calcium-rich gap transients are consistent with the merger rate of unstable systems with higher mass WDs.

  8. Search for Higgs shifts in white dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Onofrio, Roberto [Dipartimento di Fisica e Astronomia " Galileo Galilei," Università di Padova, Via Marzolo 8, I-35131 Padova (Italy); Wegner, Gary A., E-mail: onofrior@gmail.com, E-mail: gary.a.wegner@dartmouth.edu [Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755 (United States)

    2014-08-20

    We report on a search for differential shifts between electronic and vibronic transitions in carbon-rich white dwarfs BPM 27606 and Procyon B. The absence of differential shifts within the spectral resolution and taking into account systematic effects such as space motion and pressure shifts allows us to set the first upper bound of astrophysical origin on the coupling between the Higgs field and the Kreschmann curvature invariant. Our analysis provides the basis for a more general methodology to derive bounds to the coupling of long-range scalar fields to curvature invariants in an astrophysical setting complementary to the ones available from high-energy physics or table-top experiments.

  9. Crystallization of the Pulsating White Dwarf Star, BPM 37093

    Science.gov (United States)

    Salois, Amee; Winget, D.

    2010-01-01

    BPM 37093 is unique among pulsating white dwarf stars because it is expected to have a highly crystallized interior. By understanding how this star is crystallizing, we gain a better understanding of extreme physics. Theoretical models of the evolution of white dwarf stars suggest that they crystallize from the inside out. The pulsations of the star, which we see as intensity variations, cannot penetrate this crystallized interior. Therefore, as the star crystallizes there is a smaller volume for the propagation of the pulsations and the pulsation periods are changed accordingly. We studied these changes in the periods of the pulsations of the star over ten weeks during the McDonald Observatory Research Experience for Undergraduates Program. By studying the changes in the pulsations periods of the star we can determine the mass fraction of the star that is crystallized. Comparing Fourier transforms of our observed light curves taken in 2004 and 2005 at CTIO with data taken in 1998 and 1999 by Kanaan et al. we hope to see the changes that have occurred in the star as well as determining a better approximation of the star's crystallized mass fraction.

  10. Prevention of accretion onto white dwarfs by stellar winds

    Science.gov (United States)

    Macdonald, James

    1992-01-01

    There is indirect observational evidence that hot white dwarfs may have weak stellar winds. In this paper, the interaction between such a wind and the flow of ISM material in the gravitational field of the white dwarf is investigated with the aim of finding limits on the mass-loss rate and terminal velocity of winds capable of preventing accretion from the ISM. The limiting cases of no relative motion of the star and the ISM and supersonic relative motion of the star through ISM are separately investigated. Each case is treated by generalizing models for the interaction between the solar wind and the local ISM to include the effects of gravity. It is found that, for wind velocities expected for radiatively driven winds, mass-loss rates as low as 10 exp -21 solar mass/yr are sufficient to prevent accretion from the hot phase of the ISM. To prevent accretion during passages through cold clouds, wind mass-loss rates of order 10 exp -18 to 10 exp -17 are required.

  11. On the Detection and Characterization of Polluted White Dwarfs

    Science.gov (United States)

    Steele, Amy; Debes, John H.; Deming, Drake

    2017-06-01

    There is evidence of circumstellar material around main sequence, giant, and white dwarf stars. What happens to this material after the main sequence? With this work, we focus on the characterization of the material around WD 1145+017. The goals are to monitor the white dwarf—which has a transiting, disintegrating planetesimal and determine the composition of the evaporated material for that same white dwarf by looking at high-resolution spectra. We also present preliminary results of follow-up photometric observations of known polluted WDs. If rocky bodies survive red giant branch evolution, then the material raining down on a WD atmosphere is a direct probe of main sequence cosmochemistry. If rocky bodies do not survive the evolution, then this informs the degree of post-main-sequence processing. These case studies will provide the community with further insight about debris disk modeling, the degree of post-main-sequence processing of circumstellar material, and the composition of a disintegrating planetesimal.

  12. GD1212: Probing deep into the interior of a pulsating white dwarf star

    Directory of Open Access Journals (Sweden)

    Giammichele N.

    2015-01-01

    Full Text Available We present the first self-consistent seismic analysis of a white dwarf star, GD 1212, in the Kepler2 field. We precisely establish the fundamental parameters of the star using the forward method based on physically sound models. We unravel the internal structure as well as the rotation profile of GD1212 deeper than in any other ZZCeti stars studied so far. This opens up interesting prospects for future analyses of the white dwarf pulsators monitored in the Kepler and Kepler2 fields.

  13. Accretion on to Magnetic White Dwarfs

    Directory of Open Access Journals (Sweden)

    Wickramasinghe Dayal

    2014-01-01

    The polars have no counterparts in neutron star systems and their study provides unique insights into the complex nature of the magnetospheric boundary. The observed properties of accretion shocks at the white dwarf surface such as the anomalous soft-X-ray excess and its time variability provide strong support for the hypothesis that under certain circumstances the field channelled funnel flow is “blobby”. This has been attributed to interchange instabilities such as the Magnetic Rayleigh-Taylor instability in the shocked gas at the stream-magnetosphere boundary where the stream fragments into discrete clumps of gas. As the clumps penetrate into the magnetosphere, they are shredded into smaller mass blobs via the Kelvin-Helmholtz instability that then couple on to field lines over an extended inner transition region in the orbital plane. The more massive blobs penetrate deep into the photosphere of the white dwarf releasing their energy as a reprocessed soft-X-ray black body component. Although similar instabilities are expected in the inner transition region in disced accretion albeit on a different scale there has been no direct observational evidence for blobby accretion in the generally lower field and disced IPs.

  14. Sirius B: Confronting the Limits of our Understanding of White Dwarfs

    Science.gov (United States)

    Barstow, M. A.; Joyce, S.; Casewell, S. L.; Holberg, J. B.; Bond, H. E.; Burleigh, M. R.

    2017-03-01

    Sirius B is the visually brightest and closest of all white dwarfs and we should understand it better than any other. However, as part of a binary system, its proximity to the main sequence companion Sirius A makes it very difficult to observe from the ground. Consequently, detailed study of this white dwarf has relied on a range of space-based observatories, including ROSAT, EUVE, FUSE and HST. Photometry and spectroscopy of exquisite quality and the highest signal-to- noise have been obtained from these missions from which we have been able to study the star in great detail. In principle, the measurements made are the most precise of any white dwarf. Nevertheless, Sirius B remains a challenging object to understand. So far it has proved impossible to compute a self-consistent model atmosphere that can match observations across its full energy distribution. Furthermore, separate determinations of its mass and radius from Balmer line fitting, measurement of the gravitational redshift and astrometry of the binary remain stubbornly in significant disagreement. We examine all the systematic effects that come into play with the various models and measurements and consider what improvements need to be made to finally understand Sirius B and, by implication, many other white dwarfs.

  15. Measurements of Physical Parameters of White Dwarfs: A Test of the Mass-Radius Relation

    Science.gov (United States)

    Bédard, A.; Bergeron, P.; Fontaine, G.

    2017-10-01

    We present a detailed spectroscopic and photometric analysis of 219 DA and DB white dwarfs for which trigonometric parallax measurements are available. Our aim is to compare the physical parameters derived from the spectroscopic and photometric techniques, and then to test the theoretical mass-radius relation for white dwarfs using these results. The agreement between spectroscopic and photometric parameters is found to be excellent, especially for effective temperatures, showing that our model atmospheres and fitting procedures provide an accurate, internally consistent analysis. The values of surface gravity and solid angle obtained, respectively, from spectroscopy and photometry, are combined with parallax measurements in various ways to study the validity of the mass-radius relation from an empirical point of view. After a thorough examination of our results, we find that 73% and 92% of the white dwarfs are consistent within 1σ and 2σ confidence levels, respectively, with the predictions of the mass-radius relation, thus providing strong support to the theory of stellar degeneracy. Our analysis also allows us to identify 15 stars that are better interpreted in terms of unresolved double degenerate binaries. Atmospheric parameters for both components in these binary systems are obtained using a novel approach. We further identify a few white dwarfs that are possibly composed of an iron core rather than a carbon/oxygen core, since they are consistent with Fe-core evolutionary models.

  16. SIM's Search for Planets Orbiting White Dwarfs - Final Report

    Science.gov (United States)

    Subasavage, John P., Jr.

    2010-01-01

    I propose to use Space Interferometry Mission (SIM) Lite to observe a sample ( 25-50) of nearby white dwarfs in hopes of detecting planetary companions with masses in the 1-10 Earth mass range on average. Because of the nature of white dwarfs' spectral signatures (a few broad, if any, absorption lines), current radial velocity planet hunting techniques are not viable. Astrometry is currently the only technique capable of detecting low mass planets around white dwarfs and SIM would be the best suited astrometric instrument to do so once launched. As part of a SIM Science Study, I present a detailed evaluation of the star fields in the vicinities of nearby white dwarfs within 20 pc and with V white dwarfs with accuate trigonometric parallaxes and photometry. Simulations indicate that planetary mass detections in the range of 1-10 Earth masses are possible for the majority of the 54 white dwarfs evaluated, provided the planets exist. This effort will aid in the selection of white dwarfs to be targeted for planet searches using SIM by maximizing planetary sensitivity while minimizing total mission time spent on these observations.

  17. The Local White Dwarf Population: The 25 pc Sample

    Science.gov (United States)

    Holberg, Jay B.; Sion, E. M.; Oswalt, T. D.

    2011-01-01

    Currently the most complete sample of white dwarf stars comes from the local white dwarf population within 20 pc, which is believed to be 80 percent complete. This sample contains some 132 degenerate stars. We hope to effectively double the number of known local white dwarfs by extending the sample boundary to 25 pc, while still maintaining a high level of completeness. We discuss plans for extending the local sample and some of the studies that will be possible with this enlarged population. This work was funded in part by NSF Grant AST-1008845.

  18. General relativistic white dwarfs and their astrophysical implications

    Energy Technology Data Exchange (ETDEWEB)

    Boshkayev, Kuantay [Al-Farabi Kazakh National University, Almaty (Kazakhstan); Rueda, Jorge A.; Ruffini, Remo [Sapienza University of Rome, Rome (Italy); Siutsou, Ivan [ICRANet, Square of Republic, Pescara (Italy)

    2014-09-15

    We consider applications of general relativistic uniformly-rotating white dwarfs to several astrophysical phenomena related to the spin-up and the spin-down epochs and to delayed type Ia supernova explosions of super-Chandrasekhar white dwarfs, where we estimate the 'spinning down' lifetime due to magnetic-dipole braking. In addition, we describe the physical properties of Soft Gamma Repeaters and Anomalous X-Ray Pulsars as massive rapidly-rotating highly-magnetized white dwarfs. Particularly we consider one of the so-called low-magnetic-field magnetars SGR 0418+5729 as a massive rapidly-rotating highly- magnetized white dwarf and give bounds for the mass, radius, moment of inertia, and magnetic field by requiring the general relativistic uniformly rotating configurations to be stable.

  19. The Diversity of Rocky Bodies from White Dwarf Pollution

    Science.gov (United States)

    Feng, W.; Desch, S.

    2017-11-01

    Insight to the chemical diversity of extrasolar rocky bodies may be gained by observing heavy elements in white dwarf atmospheres. The disk accretion scenario is key to understanding the accreted rocky bodies.

  20. Circumstellar debris and pollution at white dwarf stars

    Science.gov (United States)

    Farihi, J.

    2016-04-01

    Circumstellar disks of planetary debris are now known or suspected to closely orbit hundreds of white dwarf stars. To date, both data and theory support disks that are entirely contained within the preceding giant stellar radii, and hence must have been produced during the white dwarf phase. This picture is strengthened by the signature of material falling onto the pristine stellar surfaces; disks are always detected together with atmospheric heavy elements. The physical link between this debris and the white dwarf host abundances enables unique insight into the bulk chemistry of extrasolar planetary systems via their remnants. This review summarizes the body of evidence supporting dynamically active planetary systems at a large fraction of all white dwarfs, the remnants of first generation, main-sequence planetary systems, and hence provide insight into initial conditions as well as long-term dynamics and evolution.

  1. Calibrating Cosmological Chronometers: White Dwarf Masses via Astrometry

    Science.gov (United States)

    2009-01-01

    Calibrating cosmological chronometers: white dwarf masses via astrometry John P. Subasavage1, Todd J. Henry1, Wei-Chun Jao1, Edmund P. Nelan2, Hugh C...control number. 1. REPORT DATE 2009 2. REPORT TYPE 3. DATES COVERED 00-00-2009 to 00-00-2009 4. TITLE AND SUBTITLE Calibrating cosmological ...Workshop on White Dwarfs Held at Kiel, Germany, 29 August - 1 September 1994. Lecture Notes in Physics vol. 443 eds D. Koester and K. Werner (Heidelberg

  2. White dwarfs as physics laboratories: the case of axions

    OpenAIRE

    Isern, J.; Althaus, L.; Catalan, S.; Corsico, A.; Garcia-Berro, E.; Salaris, M.; Torres, S.

    2012-01-01

    White dwarfs are almost completely degenerate objects that cannot obtain energy from thermonuclear sources, so their evolution is just a gravothermal cooling process. Recent improvements in the accuracy and precision of the luminosity function and in pulsational data of variable white dwarfs suggest that they are cooling faster than expected from conventional theory. In this contribution we show that the inclusion of an additional cooling term due to axions able to interact with electrons wit...

  3. SHORT-PERIOD g-MODE PULSATIONS IN LOW-MASS WHITE DWARFS TRIGGERED BY H-SHELL BURNING

    Energy Technology Data Exchange (ETDEWEB)

    Córsico, A. H.; Althaus, L. G., E-mail: acorsico@fcaglp.unlp.edu.ar [Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata (Argentina)

    2014-09-20

    The detection of pulsations in white dwarfs with low mass offers the possibility of probing their internal structures through asteroseismology and placing constraints on the binary evolutionary processes involved in their formation. In this Letter, we assess the impact of stable H burning on the pulsational stability properties of low-mass He-core white dwarf models resulting from binary star evolutionary calculations. We found that besides a dense spectrum of unstable radial modes and nonradial g and p modes driven by the κ mechanism due to the partial ionization of H in the stellar envelope, some unstable g modes with short pulsation periods are also powered by H burning via the ε mechanism of mode driving. This is the first time that ε destabilized modes are found in models representative of cool white dwarf stars. The short periods recently detected in the pulsating low-mass white dwarf SDSS J111215.82+111745.0 could constitute the first evidence of the existence of stable H burning in these stars, in particular in the so-called extremely low-mass white dwarfs.

  4. Double-detonation supernovae of sub-Chandrasekhar mass white dwarfs

    Science.gov (United States)

    Fink, M.; Hillebrandt, W.; Röpke, F. K.

    2007-12-01

    Type Ia supernovae are believed to be white dwarfs disrupted by a thermonuclear explosion. Here we investigate the scenario in which a rather low-mass, carbon-oxygen (C + O) white dwarf accumulates helium on its surface in a sufficient amount for igniting a detonation in the helium shell before the Chandrasekhar mass is reached. In principle, this can happen on white dwarfs accreting from a non-degenerate companion or by merging a C + O white dwarf with a low-mass helium one. In this scenario, the helium detonation is thought to trigger a secondary detonation in the C + O core. It is therefore called the “double-detonation sub-Chandrasekhar” supernova model. By means of a set of numerical simulations, we investigate the robustness of this explosion mechanism for generic 1-{M_⊙} models and analyze its observable predictions. Also a resolution dependence in numerical simulations is analyzed. Hydrodynamic simulations of the double-detonation sub-Chandrasekhar scenario are conducted in two and three spatial dimensions. The propagation of thermonuclear detonation fronts, both in helium and in the carbon-oxygen mixture, is computed by means of both a level-set function and a simplified description for nuclear reactions. The decision whether a secondary detonation is triggered in the white dwarf's core or not is made based on criteria given in the literature. In a parameter study involving different initial flame geometries for He-shell masses of 0.2 and 0.1 {M_⊙} (and thus 0.8 and 0.9 {M_⊙} of C + O), we find that a secondary detonation ignition is a very robust process. Converging shock waves originating from the detonation in the He shell generate the conditions for a detonation near the center of the white dwarf in most of the cases considered. Finally, we follow the complete evolution of three selected models with 0.2 {M_⊙} of He through the C/O-detonation phase and obtain 56Ni-masses of about 0.40 to 0.45 {M_⊙}. Although we have not done a complete

  5. An unusual white dwarf star may be a surviving remnant of a subluminous Type Ia supernova

    Science.gov (United States)

    Vennes, S.; Nemeth, P.; Kawka, A.; Thorstensen, J. R.; Khalack, V.; Ferrario, L.; Alper, E. H.

    2017-08-01

    Subluminous Type Ia supernovae, such as the Type Iax-class prototype SN 2002cx, are described by a variety of models such as the failed detonation and partial deflagration of an accreting carbon-oxygen white dwarf star or the explosion of an accreting, hybrid carbon-oxygen-neon core. These models predict that bound remnants survive such events with, according to some simulations, a high kick velocity. We report the discovery of a high proper motion, low-mass white dwarf (LP 40-365) that travels at a velocity greater than the Galactic escape velocity and whose peculiar atmosphere is dominated by intermediate-mass elements. Strong evidence indicates that this partially burnt remnant was ejected following a subluminous Type Ia supernova event. This supports the viability of single-degenerate supernova progenitors.

  6. White dwarf stars exceeding the Chandrasekhar mass limit

    Science.gov (United States)

    Tomaschitz, Roman

    2018-01-01

    The effect of nonlinear ultra-relativistic electron dispersion on the mass-radius relation of high-mass white dwarfs is studied. The dispersion is described by a permeability tensor in the Dirac equation, generated by the ionized high-density stellar matter, which constitutes the neutralizing background of the nearly degenerate electron plasma. The electron dispersion results in a stable mass-radius relation for high-mass white dwarfs, in contrast to a mass limit in the case of vacuum permeabilities. In the ultra-relativistic regime, the dispersion relation is a power law whose amplitude and scaling exponent is inferred from mass and radius estimates of two high-mass white dwarfs, Sirius B and LHS 4033. Evidence for the existence of super-Chandrasekhar mass white dwarfs is provided by several Type Ia supernovae (e.g., SN 2013cv, SN 2003fg, SN 2007if and SN 2009dc), whose mass ejecta exceed the Chandrasekhar limit by up to a factor of two. The dispersive mass-radius relation is used to estimate the radii, central densities, Fermi temperatures, bulk and compression moduli and sound velocities of their white dwarf progenitors.

  7. Do all barium stars have a white dwarf companion?

    Science.gov (United States)

    Dominy, J. F.; Lambert, D. L.

    1983-01-01

    International Ultraviolet Explorer short-wavelength, low-dispersion spectra were analyzed for four barium, two mild barium, and one R-type carbon star in order to test the hypothesis that the barium and related giants are produced by mass transfer from a companion now present as a white dwarf. An earlier tentative identification of a white dwarf companion to the mild barium star Zeta Cyg is confirmed. For the other stars, no ultraviolet excess attributable to a white dwarf is seen. Limits are set on the bolometric magnitude and age of a possible white dwarf companion. Since the barium stars do not have obvious progenitors among main-sequence and subgiant stars, mass transfer must be presumed to occur when the mass-gaining star is already on the giant branch. This restriction, and the white dwarf's minimum age, which is greater than 8 x 10 to the 8th yr, determined for several stars, effectively eliminates the hypothesis that mass transfer from an asymptotic giant branch star creates a barium star. Speculations are presented on alternative methods of producing a barium star in a binary system.

  8. A catalogue of white dwarf candidates in VST ATLAS

    Science.gov (United States)

    Gentile Fusillo, Nicola Pietro; Raddi, Roberto; Gänsicke, Boris T.; Hermes, J. J.; Pala, Anna F.; Fuchs, Joshua T.; Chehade, Ben; Metcalfe, Nigel; Shanks, Tom

    2017-07-01

    The Sloan Digital Sky Survey (SDSS) has created a knowledge gap between the Northern and the Southern hemispheres, which is very marked for white dwarfs: Only ≃15 per cent of the known white dwarfs are south of the equator. Here, we make use of the VLT Survey Telescope (VST) ATLAS survey, one of the first surveys obtaining deep, optical, multiband photometry over a large area of the southern skies, to remedy this situation. Applying the colour and proper-motion selection developed in our previous work on SDSS to the most recent internal data release (2016 April 25) of VST ATLAS, we created a catalogue of ≃4200 moderately bright (g ≤ 19), high-confidence southern white dwarf candidates, which can be followed up individually with both the large array of southern telescopes or in bulk with ESO's forthcoming multi-object spectrograph 4MOST.

  9. Metal Accretion onto White Dwarfs. I. The Approximate Approach Based on Estimates of Diffusion Timescales

    Science.gov (United States)

    Fontaine, G.; Brassard, P.; Dufour, P.; Tremblay, P.-E.

    2015-06-01

    The accretion-diffusion picture is the model par excellence for describing the presence of planetary debris polluting the atmospheres of relatively cool white dwarfs. Some important insights into the process may be derived using an approximate approach which combines static stellar models with estimates of diffusion timescales at the base of the outer convection zone or, in its absence, at the photosphere. Until recently, and to our knowledge, values of diffusion timescales in white dwarfs have all been obtained on the basis of the same physics as that developed initially by Paquette et al., including their diffusion coefficients and thermal diffusion coefficients. In view of the recent exciting discoveries of a plethora of metals (including some never seen before) polluting the atmospheres of an increasing number of cool white dwarfs, we felt that a new look at the estimates of settling timescales would be worthwhile. We thus provide improved estimates of diffusion timescales for all 27 elements from Li to Cu in the periodic table in a wide range of the surface gravity-effective temperature domain and for both DA and non-DA stars.

  10. Astro-archaeology - The white dwarfs and hot subwarfs

    Science.gov (United States)

    Van Horn, Hugh M.

    1991-01-01

    By 'astroarcheology' is presently meant the effort to ascertain the Galaxy's past in light of what is found in its most ancient, white dwarf constituents. Attention is given to the controversial role of the hot subdwarfs and the theory of white dwarf spectral evolution, as well as to the concept of the 'Whole Earth Telescope', involving continuous photometric coverage of rapidly varying astronomical sources and thereby eliminating the otherwise troublesome diurnal gaps in data. Much higher resolution of the power spectra of these objects is attainable by these means than any current alternative.

  11. White dwarfs in the UKIRT infrared deep sky survey data release

    Energy Technology Data Exchange (ETDEWEB)

    Tremblay, P.-E.; Kalirai, J. S. [Space Telescope Science Institute, 700 San Martin Drive, Baltimore, MD 21218 (United States); Leggett, S. K. [Gemini Observatory, Northern Operations Center, 670 North A' ohoku Place, Hilo, HI 96720 (United States); Lodieu, N. [Instituto de Astrofísica de Canarias (IAC), C/Vía Láctea s/n, E-38200 La Laguna, Tenerife (Spain); Freytag, B. [Astronomical Observatory, Uppsala University, Regementsvägen 1, Box 515, SE-75120 Uppsala (Sweden); Bergeron, P. [Département de Physique, Université de Montréal, C. P. 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7 (Canada); Ludwig, H.-G., E-mail: tremblay@stsci.edu [Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, D-69117 Heidelberg (Germany)

    2014-06-20

    We have identified 8 to 10 new cool white dwarfs from the Large Area Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data set was paired with the Sloan Digital Sky Survey to obtain proper motions and a broad ugrizYJHK wavelength coverage. Optical spectroscopic observations were secured at Gemini Observatory and confirm the degenerate status for eight of our targets. The final sample includes two additional white dwarf candidates with no spectroscopic observations. We rely on improved one-dimensional model atmospheres and new multi-dimensional simulations with CO5BOLD to review the stellar parameters of the published LAS white dwarf sample along with our additional discoveries. Most of the new objects possess very cool atmospheres with effective temperatures below 5000 K, including two pure-hydrogen remnants with a cooling age between 8.5 and 9.0 Gyr, and tangential velocities in the range 40 km s{sup –1} ≤v {sub tan} ≤ 60 km s{sup –1}. They are likely thick disk 10-11 Gyr old objects. In addition, we find a resolved double degenerate system with v {sub tan} ∼ 155 km s{sup –1} and a cooling age between 3.0 and 5.0 Gyr. These white dwarfs could be disk remnants with a very high velocity or former halo G stars. We also compare the LAS sample with earlier studies of very cool degenerates and observe a similar deficit of helium-dominated atmospheres in the range 5000 < T {sub eff} (K) < 6000. We review the possible explanations for the spectral evolution from helium-dominated toward hydrogen-rich atmospheres at low temperatures.

  12. Turbulent Mixing on Helium-accreting White Dwarfs

    Science.gov (United States)

    Piro, Anthony L.

    2015-03-01

    An attractive scenario for producing Type Ia supernovae (SNe Ia) is a double detonation, where detonation of an accreted helium layer triggers ignition of a C/O core. Whether or not such a mechanism can explain some or most SNe Ia depends on the properties of the helium burning, which in turn is set by the composition of the surface material. Using a combination of semi-analytic and simple numerical models, I explore when turbulent mixing due to hydrodynamic instabilities during the accretion process can mix C/O core material up into the accreted helium. Mixing is strongest at high accretion rates, large white dwarf (WD) masses, and slow spin rates. The mixing would result in subsequent helium burning that better matches the observed properties of SNe Ia. In some cases, there is considerable mixing that can lead to more than 50% C/O in the accreted layer at the time of ignition. These results will hopefully motivate future theoretical studies of such strongly mixed conditions. Mixing also has implications for other types of WD surface explosions, including the so-called .Ia supernovae, the calcium-rich transients (if they arise from accreting WDs), and metal-enriched classical novae.

  13. White dwarf pollution by planets in stellar binaries

    Science.gov (United States)

    Hamers, Adrian S.; Portegies Zwart, Simon F.

    2016-10-01

    Approximately 0.2 ± 0.2 of white dwarfs (WDs) show signs of pollution by metals, which is likely due to the accretion of tidally disrupted planetary material. Models invoking planet-planet interactions after WD formation generally cannot explain pollution at cooling times of several Gyr. We consider a scenario in which a planet is perturbed by Lidov-Kozai oscillations induced by a binary companion and exacerbated by stellar mass-loss, explaining pollution at long cooling times. Our computed accretion rates are consistent with observations assuming planetary masses between ˜0.01 and 1 MMars, although non-gravitational effects may already be important for masses ≲0.3 MMars. The fraction of polluted WDs in our simulations, ˜0.05, is consistent with observations of WDs with intermediate cooling times between ˜0.1 and 1 Gyr. For cooling times ≲0.1 Gyr and ≳1 Gyr, our scenario cannot explain the high observed pollution fractions of up to 0.7. Nevertheless, our results motivate searches for companions around polluted WDs.

  14. The Physical Nature of Subdwarf A Stars: White Dwarf Impostors

    Science.gov (United States)

    Brown, Warren R.; Kilic, Mukremin; Gianninas, A.

    2017-04-01

    We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ˜1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ≃1.2 M ⊙ main sequence stars with ≃0.8 M ⊙ companions. While WDs must exist at sdA temperatures, only ˜1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A-F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.

  15. The Physical Nature of Subdwarf A Stars: White Dwarf Impostors

    Energy Technology Data Exchange (ETDEWEB)

    Brown, Warren R. [Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 (United States); Kilic, Mukremin; Gianninas, A., E-mail: wbrown@cfa.harvard.edu, E-mail: kilic@ou.edu, E-mail: alexg@nhn.ou.edu [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, OK, 73019 (United States)

    2017-04-10

    We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ∼1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ≃1.2 M {sub ⊙} main sequence stars with ≃0.8 M {sub ⊙} companions. While WDs must exist at sdA temperatures, only ∼1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A–F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.

  16. Determining the core stratification in white dwarfs with asteroseismology

    Directory of Open Access Journals (Sweden)

    Charpinet S.

    2017-01-01

    Full Text Available Using the forward modeling approach and a new parameterization for the core chemical stratification in ZZ Ceti stars, we test several situations typical of the usually limited constraints available, such as small numbers of observed independent modes, to carry out asteroseismology of these stars. We find that, even with a limited number of modes, the core chemical stratification (in particular, the location of the steep chemical transitions expected in the oxygen profile can be determined quite precisely due to the significant sensitivity of some confined modes to partial reflexion (trapping effects. These effects are similar to the well known trapping induced by the shallower chemical transitions at the edge of the core and at the bottom of the H-rich envelope. We also find that success to unravel the core structure depends on the information content of the available seismic data. In some cases, it may not be possible to isolate a unique, well-defined seismic solution and the problem remains degenerate. Our results establish that constraining the core chemical stratification in white dwarf stars based solely on asteroseismology is possible, an opportunity that we have started to exploit.

  17. Mass Transfer and Tidal Dynamics in White Dwarf Binary Systems

    Science.gov (United States)

    Gerber, Jeffrey; Fuller, J.

    2014-01-01

    Compact white dwarf (WD) binary systems (with orbital periods ranging from minutes to hours) can produce a variety of interesting astrophysical objects (e.g., type Ia supernovae, AM CVn systems, R Cor Bor stars, sdB stars) upon the onset of mass transfer. These systems are driven toward Roche lobe overflow by the emission of gravitational radiation, but it is not known whether the mass transfer will be stable (forming an Am CVn system) or become unstable (resulting in a merger). We analyze how the combined effects of mass transfer and tidal torques affect the evolution of these systems by creating numerical models with the MESA stellar evolution program. Using new calculations of the tidal torque in rotating WDs, we predict the outcome of mass transfer in these systems as a function of the masses of the WD components. We find that the stability of mass transfer depends primarily on the peak mass transfer rate near the period minimum, which is highly dependent on the WD masses and on the strength of the tidal torques. Except for low WD accretor masses, the tidal torques are insufficient to significantly increase the stability of mass transfer. We find that mass transfer is generally unstable for WD donor masses greater than about 0.25 solar masses, and that the 12 minute system SDSS J0615 will end its inspiral in a WD merger, likely producing an R Cor Bor star.

  18. Binary white dwarfs in the halo of the Milky Way

    NARCIS (Netherlands)

    van Oirschot, Pim; Nelemans, Gijs; Toonen, Silvia; Pols, Onno; Brown, Anthony G. A.; Helmi, Amina; Portegies Zwart, Simon

    Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the

  19. A Common Origin of Magnetism from Planets to White Dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Isern, Jordi; Külebi, Baybars [Institut de Ciències de l’Espai (CSIC), Campus UAB, 08193 Cerdanyola (Spain); García-Berro, Enrique [Institut d’Estudis Espacials de Catalunya, Ed. Nexus-201, c/Gran Capità 2-4, E-08034 Barcelona (Spain); Lorén-Aguilar, Pablo [School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL (United Kingdom)

    2017-02-20

    Isolated magnetic white dwarfs have field strengths ranging from kilogauss to gigagauss. However, the origin of the magnetic field has not been hitherto elucidated. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of their progenitor stars, or are the result of binary interactions, or, finally, they are produced by other internal physical mechanisms during the cooling of the white dwarf itself, remains a mystery. At sufficiently low temperatures, white dwarfs crystallize. Upon solidification, phase separation of its main constituents, {sup 12}C and {sup 16}O, and of the impurities left by previous evolution occurs. This process leads to the formation of a Rayleigh–Taylor unstable liquid mantle on top of a solid core. This convective region, as it occurs in solar system planets like the Earth and Jupiter, can produce a dynamo able to yield magnetic fields of strengths of up to 0.1 MG, thus providing a mechanism that could explain magnetism in single white dwarfs.

  20. Planetary Engulfment as a Trigger for White Dwarf Pollution

    Science.gov (United States)

    Petrovich, Cristobal; Muñoz, Diego J.

    2017-01-01

    The presence of a planetary system can shield a planetesimal disk from the secular gravitational perturbations due to distant outer massive objects (planets or stellar companions). As the host star evolves off the main sequence to become a white dwarf, these planets can be engulfed during the giant phase, triggering secular instabilities and leading to the tidal disruptions of small rocky bodies. These disrupted bodies can feed the white dwarfs with rocky material and possibly explain the high-metallicity material in their atmospheres. We illustrate how this mechanism can operate when the gravitational perturbations are due to the KL mechanism from a stellar binary companion, a process that is activated only after the planet has been removed/engulfed. We show that this mechanism can explain the observed accretion rates if: (1) the planetary engulfment happens rapidly compared to the secular timescale, which is generally the case for wide binaries (> 100 au) and planetary engulfment during the asymptotic giant branch; (2) the planetesimal disk has a total mass of ˜ {10}-4-{10}-2{M}\\oplus . We show that this new mechanism can provide a steady supply of material throughout the entire life of the white dwarfs for all cooling ages and can account for a large fraction (up to nearly half) of the observed polluted white dwarfs.

  1. A Dark Spot on a Massive White Dwarf

    Science.gov (United States)

    Kilic, Mukremin; Gianninas, Alexandros; Bell, Keaton J.; Curd, Brandon; Brown, Warren R.; Hermes, J. J.; Dufour, Patrick; Wisniewski, John P.; Winget, D. E.; Winget, K. I.

    2015-12-01

    We present the serendipitous discovery of eclipse-like events around the massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We selected J1529+2928 for time-series photometry based on its spectroscopic temperature and surface gravity, which place it near the ZZ Ceti instability strip. Instead of pulsations, we detect photometric dips from this white dwarf every 38 minutes. Follow-up optical spectroscopy observations with Gemini reveal no significant radial velocity variations, ruling out stellar and brown dwarf companions. A disintegrating planet around this white dwarf cannot explain the observed light curves in different filters. Given the short period, the source of the photometric dips must be a dark spot that comes into view every 38 minutes due to the rotation of the white dwarf. Our optical spectroscopy does not show any evidence of Zeeman splitting of the Balmer lines, limiting the magnetic field strength to B Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).

  2. First Detection of Krypton and Xenon in a White Dwarf

    Science.gov (United States)

    Werner, Klaus; Rauch, Thomas; Ringat, Ellen; Kruk, Jeffrey W.

    2012-01-01

    We report on the first detection of the noble gases krypton (Z = 36) and xenon (54) in a white dwarf. About 20 KrVI-VII and Xe VI-VII lines were discovered in the ultraviolet spectrum of the hot DO-type white dwarf RE 0503-289. The observations, performed with the Far Ultraviolet Spectroscopic Explorer, also reveal highly ionized photospheric lines from other trans-iron group elements, namely Ga (31), Ge (32), As (33), Se (34), Mo (42), Sn (50), Te (52), and I (53), from which gallium and molybdenum are new discoveries in white dwarfs, too. For Kr and Xe, we performed an NLTE analysis and derived mass fractions of log Kr = -4.3 plus or minus 0.5 and log Xe = -4.2 plus or minus 0.6, corresponding to an enrichment by factors of 450 and 3800, respectively, relative to the Sun. The origin of the large overabundances is unclear. We discuss the roles of neutron-capture nucleosynthesis in the-precursor star and radiation-driven diffusion. It is possible that diffusion is insignificant and thaI the observed metal abundances constrain the evolutionary history of the star. Its hydrogen deficiency may be the consequence of a late helium-shell nash or a binary white dwarf merger.

  3. The Dynamics of White Dwarfs, Black Holes and Stellar Cusps

    Science.gov (United States)

    Wegg, Christopher

    2013-03-01

    This thesis contains topics related mostly to the dynamics of white dwarfs (chapter 2), the dynamics of stars around binary super massive black holes (chapters 4, 5 and 6) and dynamics in the singular isothermal sphere (chapter 7). In chapter 2 the kinematics of young (white dwarfs are investigated. A relationship between the mass and kinematics of white dwarfs is demonstrated, whereby high-mass white dwarfs have low velocity dispersion. This is the result of less scattering during the shorter lifetime of their more massive precursors. The kinematics of the highest-mass white dwarfs (> 0.95 M⊙ ) are also investigated, and it is shown that they are consistent with the majority being formed via single-star evolution from massive progenitor stars. In chapter 3 it is shown that the coolest, oldest white dwarfs can be identified photometrically from their unique colors, and five new ultracool white dwarfs are spectroscopically confirmed. In chapter 4 it is shown that close binary supermassive black holes (SMBHs) should produce a burst of tidal disruptions of up to 0.1 yr-1 as they form. The quiescent rate is ˜ 10-5 yr-1 per galaxy, and it is therefore shown that binary SMBHs can potentially be identified via multiple tidal disruptions from the same system. In chapter 5 we perform more extensive simulations of the dynamics of stars around binary SMBHs to better quantify and understand the stellar dynamics. By incorporating general relativistic corrections, we also investigate the processes undergone by compact remnants orbiting the binary SMBHs, analyzing both objects that plunge directly into the SMBHs, and those that undergo extreme mass ratio inspirals (EMRIs). The potential used to mimic general relativistic precession in these simulations is novel, and more accurate for the type of nearly parabolic orbits considered in this work: It is described in chapter 6. In chapter 7 an analytic solution to the manner in which stars diffuse in the background of a singular

  4. Can magnetic fields suppress convection in the atmosphere of cool white dwarfs? A case study on WD2105-820

    Science.gov (United States)

    Gentile Fusillo, N. P.; Tremblay, P.-E.; Jordan, S.; Gänsicke, B. T.; Kalirai, J. S.; Cummings, J.

    2018-01-01

    Around 10 per cent of white dwarfs exhibit global magnetic structures with fields ranging from 1 kG to hundreds of MG. Recently, the first radiation magnetohydrodynamics simulations of the atmosphere of white dwarfs showed that convection should be suppressed in their photospheres for magnetic fields with strengths B ≳ 50 kG. These predictions are in agreement with our knowledge of stellar physics (e.g. energy transfer in strong magnetic field regions of the solar photosphere), but have yet to be directly confirmed from white dwarf observations. We obtained Cosmic Origins Spectrograph (COS) far-ultraviolet (FUV) spectroscopy of the weakly magnetic, hydrogen-atmosphere, white dwarf WD2105-820 and of three additional non-magnetic, convective remnants (all in the Teff range 9000-11 000 K). We fitted both the COS and the already available optical spectra with convective and radiative atmospheric models. As expected, we find that for two of the non-magnetic comparison stars only convective model fits predicted consistent Teff values from both the optical and the FUV spectra. In contrast, for WD2105-820 only the best-fitting radiative model produced consistent results.

  5. TOWARD HIGH-PRECISION SEISMIC STUDIES OF WHITE DWARF STARS: PARAMETRIZATION OF THE CORE AND TESTS OF ACCURACY

    Energy Technology Data Exchange (ETDEWEB)

    Giammichele, N.; Fontaine, G.; Brassard, P. [Département de Physique, Université de Montréal, Montréal, QC H3C 3J7 (Canada); Charpinet, S. [Université de Toulouse, UPS-OMP, IRAP, Toulouse F-31400 (France)

    2017-01-10

    We present a prescription for parametrizing the chemical profile in the core of white dwarfs in light of the recent discovery that pulsation modes may sometimes be deeply confined in some cool pulsating white dwarfs. Such modes may be used as unique probes of the complicated chemical stratification that results from several processes that occurred in previous evolutionary phases of intermediate-mass stars. This effort is part of our ongoing quest for more credible and realistic seismic models of white dwarfs using static, parametrized equilibrium structures. Inspired by successful techniques developed in design optimization fields (such as aerodynamics), we exploit Akima splines for the tracing of the chemical profile of oxygen (carbon) in the core of a white dwarf model. A series of tests are then presented to better seize the precision and significance of the results that can be obtained in an asteroseismological context. We also show that the new parametrization passes an essential basic test, as it successfully reproduces the chemical stratification of a full evolutionary model.

  6. Numerical Simulations of Gaseous Disks Generated from Collisional Cascades at the Roche Limits of White Dwarf Stars

    Science.gov (United States)

    Kenyon, Scott J.; Bromley, Benjamin C.

    2017-11-01

    We consider the long-term evolution of gaseous disks fed by the vaporization of small particles produced in a collisional cascade inside the Roche limit of a 0.6 {M}⊙ white dwarf. Adding solids with radius {r}0 at a constant rate {\\dot{M}}0 into a narrow annulus leads to two distinct types of evolution. When {\\dot{M}}0≳ {\\dot{M}}0,{crit}≈ 3× {10}4 {({r}0/1{km})}3.92 {{g}} {{{s}}}-1, the cascade generates a fairly steady accretion disk where the mass transfer rate of gas onto the white dwarf is roughly {\\dot{M}}0 and the mass in gas is {M}g≈ 2.3× {10}22 ({\\dot{M}}0/{10}10 {{g}} {{{s}}}-1) (1500 {{K}}/{T}0) ({10}-3/α ) g, where T 0 is the temperature of the gas near the Roche limit and α is the dimensionless viscosity parameter. If {\\dot{M}}0≲ {\\dot{M}}0,{crit}, the system alternates between high states with large mass transfer rates and low states with negligible accretion. Although either mode of evolution adds significant amounts of metals to the white dwarf photosphere, none of our calculations yield a vertically thin ensemble of solids inside the Roche limit. X-ray observations can place limits on the mass transfer rate and test this model for metallic line white dwarfs.

  7. The nature of millisecond pulsars with helium white dwarf companions

    Science.gov (United States)

    Smedley, Sarah L.; Tout, Christopher A.; Ferrario, Lilia; Wickramasinghe, Dayal T.

    2014-01-01

    We examine the growing data set of binary millisecond pulsars that are thought to have a helium white dwarf companion. These systems are believed to form when a low- to intermediate-mass companion to a neutron star fills its Roche lobe between central hydrogen exhaustion and core helium ignition. We confirm that our own stellar models reproduce a well-defined period-companion mass relation irrespective of the details of the mass transfer process. With magnetic braking, this relation extends to periods of less than 1 d for a 1 M⊙ giant donor. With this and the measured binary mass functions, we calculate the orbital inclination of each system for a given pulsar mass. We expect these inclinations to be randomly oriented in space. If the masses of the pulsars were typically 1.35 M⊙, then there would appear to be a distinct dearth of high-inclination systems. However, if the pulsar masses are more typically from 1.55 to 1.65 M⊙, then the distribution of inclinations is indeed indistinguishable from random. If it were as much as 1.75 M⊙, then there would appear to be an excess of high-inclination systems. Thus, with the available data, we can argue that the neutron star masses in binary millisecond pulsars recycled by mass transfer from a red giant typically lie around 1.6 M⊙ and that there is no preferred inclination at which these systems are observed. Hence, there is reason to believe that pulsar beams are either sufficiently broad or show no preferred direction relative to the pulsar's spin axis which is aligned with the binary orbit. This is contrary to some previous claims, based on a subset of the data available today, that there might be a tendency for the pulsar beams to be perpendicular to their spin.

  8. Deposition of steeply infalling debris around white dwarf stars

    Science.gov (United States)

    Brown, John C.; Veras, Dimitri; Gänsicke, Boris T.

    2017-06-01

    High-metallicity pollution is common in white dwarf (WD) stars hosting remnant planetary systems. However, they rarely have detectable debris accretion discs, possibly because much of the influx is fast steeply infalling debris in star-grazing orbits, producing a more tenuous signature than a slowly accreting disc. Processes governing such deposition between the Roche radius and photosphere have so far received little attention and we model them here analytically by extending recent work on sun-grazing comets to WD systems. We find that the evolution of cm-to-km size (a0) infallers most strongly depends on two combinations of parameters, which effectively measure sublimation rate and binding strength. We then provide an algorithm to determine the fate of infallers for any WD, and apply the algorithm to four limiting combinations of hot versus cool (young/old) WDs with snowy (weak, volatile) versus rocky (strong, refractory) infallers. We find: (I) Total sublimation above the photosphere befalls all small infallers across the entire WD temperature (TWD) range, the threshold size rising with TWD and 100× larger for rock than snow. (II) All very large objects fragment tidally regardless of TWD: for rock, a0 ≽ 105 cm; for snow, a0 ≽ 103-3 × 104 cm across all WD cooling ages. (III) A considerable range of a0 avoids fragmentation and total sublimation, yielding impacts or grazes with cold WDs. This range rapidly narrows with increasing TWD, especially for snowy bodies. Finally, we briefly discuss how the various forms of deposited debris may finally reach the photosphere surface itself.

  9. Two new color-selected magnetic DA white dwarfs

    Science.gov (United States)

    Liebert, J.; Schmidt, G. D.; Sion, E. M.; Starrfield, S. G.; Green, R. F.; Boroson, T. A.

    1985-01-01

    The discovery of two magnetic white dwarfs culled from blue star surveys is reported. The surveys were carried out with the Mount Lemnon 1.5-meter reflecting telescope attached to a two-holer polarimeter/photometer. Spectral observations of the objects, (PG 1533 - 057, and K813 - 14), indicate the presence of hydrogen and Zeeman components. On the basis of dipolar field simulations, it is shown that PG 1533 - 057 has a polar field strength of 31 megagauss (MG) while K813 - 14 has a polar field strength of 29 MG. A third known white dwarf has a polar field strength of 18 MG. All the dwarfs had temperatures in the 11,000-20,000 K range. The possibility that a significant fraction of isolated magnetic degenerate stars could be the progeny of magnetic accreting binary systems is considered.

  10. A circumbinary debris disk in a polluted white dwarf system

    Science.gov (United States)

    Farihi, J.; Parsons, S. G.; Gänsicke, B. T.

    2017-03-01

    Planetary systems commonly survive the evolution of single stars, as evidenced by terrestrial-like planetesimal debris observed orbiting and polluting the surfaces of white dwarfs 1,2 . Here, we report the identification of a circumbinary dust disk surrounding a white dwarf with a substellar companion in a 2.27 h orbit. The system bears the dual hallmarks of atmospheric metal pollution and infrared excess 3,4 ; however, the standard (flat and opaque) disk configuration is dynamically precluded by the binary. Instead, the detected reservoir of debris must lie well beyond the Roche limit in an optically thin configuration, where erosion by stellar irradiation is relatively rapid. This finding shows that rocky planetesimal formation is robust around close binaries, even those with low mass ratios.

  11. Planet-Planet Scattering and White Dwarf Pollution

    Science.gov (United States)

    Joasil, Arielle; Payne, Matthew John; Veras, Dimitri

    2017-01-01

    About one-quarter to one-half of white dwarfs are observed to have polluted atmospheres. White dwarfs (WD) are expected to be chemically stratified, with heavy elements rapidly sinking. The frequent observation of heavy element pollution in WD atmospheres indicates that there must be a copious and frequent supply of rocky material from remnant planetary systems acting as a pollutant. Recently, the white dwarf WD 1145+017 has been observed to have been transited by a rocky body apparently in the process of disintegrating (Vanderburg et al. 2015).Post-main sequence expansion may render the planetary system unstable (Veras 2016). Planets orbiting the white dwarf may perturb and scatter one another. If this scattering happens, any moons can be scattered about the system. As such, one possible source of the material polluting WDs is destabilized exomoons (Payne et al. 2016a, 2016b). Moons offer a plausible source of pollution due to their large total mass (in the Solar system), and their generally rocky composition that matches that found in the atmospheric pollution of WDs. During a planet-planet scattering event, the probability that a moon will be ejected from its parent planet is a function of the velocity of the perturbing planet and the distance between the perturbed moon and the perturbing planet (as well as the initial orbit of the moon). We review the results of Payne et al. (2016a, 2016b) and present new results illustrating the probability of moon ejection as a function of these key parameters. We demonstrate the utility of these results for (a) the pollution and WDs, and for (b) general planet-planet scattering scenarios around main-sequence stars.

  12. The white dwarf cooling sequence of 47 Tucanae

    OpenAIRE

    García Berro, Enrique; Torres, Santiago; Althaus, Leandro Gabriel; Miller Bertolami, Marcelo Miguel

    2017-01-01

    Context. 47 Tucanae is one of the most interesting, well-observed, and theoretically studied globular clusters. This allows us to determine the reliability of our understanding of white dwarf cooling sequences, to compare different methods of determining its age, and to assess other important characteristics, such as its star formation history. Aims. Here we present a population synthesis study of the cooling sequence of the globular cluster 47 Tucanae. In particular, we study the d...

  13. Probing the accretion induced collapse of white dwarfs in millisecond pulsars

    Science.gov (United States)

    Taani, A.; Khasawneh, A.

    2017-06-01

    This paper investigates the progenitors of Millisecond Pulsars (MSPs) with a distribution of long orbital periods (Porb > 2 d), to show the link between white dwarf (WD) binaries and long orbits for some binary MSPs through the Accretion Induced Collapse (AIC) of a WD. For this purpose, a model is presented to turn binary MSPs into wide binaries and highly circular orbits (e pulsar during the AIC process, which may indicate a sizeable kick velocity along the rotation of the proto-neutron star. The results show the effects of shock wave, binding energy, and mass loss (0.2M⊙). The model shows the pulsar systems are relevant to AIC-candidates.

  14. Future Far-UV Studies of Hot White Dwarfs

    Science.gov (United States)

    Barstow, M. A.

    We are beginning to understand the evolution of the hot white dwarfs, but even with telescopes such as IUE and HST, we have still only observed a modest number of the most interesting objects with the spectral resolution and signal-to-noise required. This is in part arises from the intense competition for HST time, against both optical and IR observations as well as other far-UV bids. A key requirement of any new far-UV telescope is sufficient sensitivity to observe most of the ~100 or so brightest hot white dwarfs at high spectral resolution. A spectral resolving power of at least 30,000 is required for studies of white dwarfs. First it is necessary to detect and resolve photospheric, circumstellar and interstellar absorption features. Interstellar/circumstellar features may have several components that can only be separated in velocity space. For example, it is interesting to note that IUE was unable to resolve the photospheric and circumstellar CIV components of G191-B2B, discovered by the HST STIS instrument, leading to a serious overestimate of the carbon abundance in this star. There may be similar components in other stars observed only by IUE. In those stars that have highly stratified atmospheres, the detailed shape of the absorption lines is sensitive to the atmospheric structure. High-resolution observations of the line shapes can provide us with a direct probe of atmospheric structure.

  15. White dwarfs in the building blocks of the Galactic spheroid

    Science.gov (United States)

    van Oirschot, Pim; Nelemans, Gijs; Starkenburg, Else; Toonen, Silvia; Helmi, Amina; Zwart, Simon Portegies

    2017-11-01

    Aims: The Galactic halo likely grew over time in part by assembling smaller galaxies, the so-called building blocks (BBs). We investigate if the properties of these BBs are reflected in the halo white dwarf (WD) population in the solar neighbourhood. Furthermore, we compute the halo WD luminosity functions (WDLFs for four major BBs of five cosmologically motivated stellar haloes). We compare the sum of these to the observed WDLF of the Galactic halo, derived from selected halo WDs in the SuperCOSMOS Sky Survey, aiming to investigate if they match better than the WDLFs predicted by simpler models. Methods: We couple the SeBa binary population synthesis model to the Munich-Groningen semi-analytic galaxy formation model applied to the high-resolution Aquarius dark matter simulations. Although the semi-analytic model assumes an instantaneous recycling approximation, we model the evolution of zero-age main sequence stars to WDs, taking age and metallicity variations of the population into account. To be consistent with the observed stellar halo mass density in the solar neighbourhood (ρ0), we simulate the mass in WDs corresponding to this density, assuming a Chabrier initial mass function (IMF) and a binary fraction of 50%. We also normalize our WDLFs to ρ0. Results: Although the majority of halo stars are old and metal-poor and therefore the WDs in the different BBs have similar properties (including present-day luminosity), we find in our models that the WDs originating from BBs that have young and/or metal-rich stars can be distinguished from WDs that were born in other BBs. In practice, however, it will be hard to prove that these WDs really originate from different BBs, as the variations in the halo WD population due to binary WD mergers result in similar effects. The five joined stellar halo WD populations that we modelled result in WDLFs that are very similar to each other. We find that simple models with a Kroupa or Salpeter IMF fit the observed luminosity

  16. THE SPECTRUM OF THE RECYCLED PSR J0437-4715 AND ITS WHITE DWARF COMPANION

    Energy Technology Data Exchange (ETDEWEB)

    Durant, Martin; Kargaltsev, Oleg [Department of Astronomy, University of Florida, FL 32611-2055 (United States); Pavlov, George G.; Posselt, Bettina [Department of Astronomy and Astrophysics, Pennsylvania State University, PA 16802 (United States); Kowalski, Piotr M. [Helmholtz Centre Potsdam-GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam (Germany); Van Kerkwijk, Marten H. [Department of Astronomy, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada); Kaplan, David L., E-mail: martin.durant@astro.ufl.edu [Physics Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53211 (United States)

    2012-02-10

    We present extensive spectral and photometric observations of the recycled pulsar/white dwarf binary containing PSR J0437-4715, which we analyzed together with archival X-ray and gamma-ray data, to obtain the complete mid-infrared to gamma-ray spectrum. We first fit each part of the spectrum separately, and then the whole multi-wavelength spectrum. We find that the optical-infrared part of the spectrum is well fit by a cool white dwarf atmosphere model with pure hydrogen composition. The model atmosphere (T{sub eff} = 3950 {+-} 150 K, log g = 6.98 {+-} 0.15, R{sub WD} = (1.9 {+-} 0.2) Multiplication-Sign 10{sup 9} cm) fits our spectral data remarkably well for the known mass and distance (M = 0.25 {+-} 0.02 M{sub Sun }, d = 156.3 {+-} 1.3 pc), yielding the white dwarf age ({tau}{sub WD} = 6.0 {+-} 0.5 Gyr). In the UV, we find a spectral shape consistent with thermal emission from the bulk of the neutron star surface, with surface temperature between 1.25 Multiplication-Sign 10{sup 5} and 3.5 Multiplication-Sign 10{sup 5} K. The temperature of the thermal spectrum suggests that some heating mechanism operates throughout the life of the neutron star. The temperature distribution on the neutron star surface is non-uniform. In the X-rays, we confirm the presence of a high-energy tail which is consistent with a continuation of the cutoff power-law component ({Gamma} = 1.56 {+-} 0.01, E{sub cut} = 1.1 {+-} 0.2 GeV) that is seen in gamma rays and perhaps even extends to the near-UV.

  17. Topics in solid-state astrophysics: Magnetized neutron star crusts and multicomponent crusts/white dwarfs

    Science.gov (United States)

    Engstrom, Tyler A.

    Two research endeavors are described in this dissertation; both undertake problems in solid-state astrophysics, which is a branch of solid-state physics concerning the extreme conditions found within white dwarfs and the solid crusts of neutron stars. As much of our knowledge about these compact objects comes from observation of astrophysical phenomena, Chapter 1 is devoted to the phenomena, and how they can be exploited as material property probes. Several of the most interesting phenomena involve the enormous magnetic fields (B ≥ 1012 gauss) harbored by many neutron stars, and the interaction between these fields and the charged particles within the solid crust. Accordingly, Chapter 2 reviews some theory of strongly-magnetized electrons, which both sets the stage for Chapter 3, and (hopefully) serves as a useful reference for future research. Let it now be made clear that this dissertation focuses exclusively on the "outer crusts," of neutron stars, where no free neutrons are present (rho white dwarfs, which have central densities ˜ 107 g/cc. For the most part we specialize to even lower densities. In Chapter 3, static and dynamic properties of low density (rho ≥ 106 g/cc) outer envelopes of neutron stars are calculated within the nonlinear magnetic Thomas-Fermi model, assuming degenerate electrons. A novel domain decomposition enables proper description of lattice symmetry and may be seen as a prototype for the general class of problems involving nonlinear charge screening of periodic, quasi-low-dimensionality structures, e.g. liquid crystals. We describe a scalable implementation of the method using Hypre. Over the density range considered, the effective shear modulus appears to be a factor of ≈ 20 larger than in the linearlyscreened Coulomb crystal model, which could have implications for observables related to astroseismology as well as low temperature phonon-mediated thermal conductivity. Other findings include incipient c' white dwarfs. Candidate

  18. Radio pulsar death lines to SGRs/AXPs and white dwarfs pulsars

    Energy Technology Data Exchange (ETDEWEB)

    Lobato, Ronaldo V.; Malheiro, M. [Departamento de Física, Instituto Tecnológico de Aeronáutica, ITA - DCTA, Vila das Acácias, São José dos Campos, 12228-900 SP (Brazil); Coelho, J. G. [INPE - Instituto Nacional de Pesquisas Espaciais, Divisão de Astrofísica, Av. dos Astronautas 1758, São José dos Campos, 12227-010 SP (Brazil)

    2015-12-17

    Recently, an alternative model based on white dwarfs pulsars has been proposed to explain a class of pulsars known as Soft Gamma Repeaters (SGR) and Anomalus X-Ray Pulsars (AXP) [1], usually named as magnetars. In this model, the magnetized white dwarfs can have surface magnetic field B ∼ 10{sup 7} − 10{sup 10} G and rotate very fast with angular frequencies Ω ∼ 1 rad/s, allowing them to produce large electromagnetic (EM) potentials and generate electron-positron pairs. These EM potentials are comparable with the ones of neutron star pulsars with strong magnetic fields and even larger. In this study we consider two possible processes associated with the particle acceleration, both of them are common used to explain radio emission in neutron star pulsars: in the first process the pair production happens near to the star polar caps, i.e. inside of the light cylinder where magnetic field lines are closed; in the second one the creation of pair happens in the outer magnetosphere, i.e. far away of the star surface where magnetic field lines are open [2]. The analysis of the possibility of radio emission were done for 23 SGRs/AXPs of the McGill Online Magnetar Catalog [3] that contains the current information available on these sources. The results of this work show that the model where the particles production occur in the outer magnetosphere emission “o2” is the process compatible with the astronomical observations of absence of radio emission for almost all SGRs/AXPs when these sources are understood as white dwarf pulsars. Our work is a first attempted to find an explanation for the puzzle why for almost all the SGRs/AXPs was expected radio emission, but it was observed in only four of them. These four sources, as it was suggested recently [4], seem to belong to an high magnetic field neutron star pulsar category, different from all the others SGRs/AXPs that our work indicate to belong to a new class of white dwarf pulsars, very fast and magnetized.

  19. Revisiting the axion bounds from the Galactic white dwarf luminosity function

    Science.gov (United States)

    Miller Bertolami, M. M.; Melendez, B. E.; Althaus, L. G.; Isern, J.

    2014-10-01

    It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to check for the possible existence of DFSZ-axions, a proposed but not yet detected type of weakly interacting particles. With the aim of deriving new constraints on the axion mass, we compute in this paper new theoretical WDLFs on the basis of WD evolving models that incorporate the feedback of axions on the thermal structure of the white dwarf. We find that the impact of the axion emission into the neutrino emission can not be neglected at high luminosities M Bollesssim 8) and that the axion emission needs to be incorporated self-consistently into the evolution of the white dwarfs when dealing with axion masses larger than macos2β≳ 5 meV (i.e. axion-electron coupling constant gae≳ 1.4× 10-13). We went beyond previous works by including 5 different derivations of the WDLF in our analysis. Then we have performed χ2-tests to have a quantitative measure of the agreement between the theoretical WDLFs — computed under the assumptions of different axion masses and normalization methods --- and the observed WDLFs of the Galactic disk. While all the WDLF studied in this work disfavour axion masses in the range suggested by asteroseismology macos2β≳ 10 meV; gae≳ 2.8× 10-13) lower axion masses can not be discarded from our current knowledge of the WDLF of the Galactic Disk. A larger set of completely independent derivations of the WDLF of the galactic disk as well as a detailed study of the uncertainties of the theoretical WDLFs is needed before quantitative constraints on the axion-electron coupling constant can be made.

  20. Activity and Kinematics of White Dwarf-M Dwarf Binaries from the SUPERBLINK Proper Motion Survey

    Energy Technology Data Exchange (ETDEWEB)

    Skinner, Julie N. [Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States); Morgan, Dylan P.; West, Andrew A. [Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States); Lépine, Sébastien [Department of Physics and Astronomy, Georgia State University, 25 Park Place NE, Atlanta, GA, 30303 (United States); Thorstensen, John R., E-mail: jskinner@bu.edu [Department of Physics and Astronomy, 6127 Wilder Laboratory, Dartmouth College, Hanover, NH 03755 (United States)

    2017-09-01

    We present an activity and kinematic analysis of high proper motion white dwarf-M dwarf binaries (WD+dMs) found in the SUPERBLINK survey, 178 of which are new identifications. To identify WD+dMs, we developed a UV–optical–IR color criterion and conducted a spectroscopic survey to confirm each candidate binary. For the newly identified systems, we fit the two components using model white dwarf spectra and M dwarf template spectra to determine physical parameters. We use H α chromospheric emission to examine the magnetic activity of the M dwarf in each system, and investigate how its activity is affected by the presence of a white dwarf companion. We find that the fraction of WD+dM binaries with active M dwarfs is significantly higher than their single M dwarf counterparts at early and mid-spectral types. We corroborate previous studies that find high activity fractions at both close and intermediate separations. At more distant separations, the binary fraction appears to approach the activity fraction for single M dwarfs. Using derived radial velocities and the proper motions, we calculate 3D space velocities for the WD+dMs in SUPERBLINK. For the entire SUPERBLINK WD+dMs, we find a large vertical velocity dispersion, indicating a dynamically hotter population compared to high proper motion samples of single M dwarfs. We compare the kinematics for systems with active M dwarfs and those with inactive M dwarfs, and find signatures of asymmetric drift in the inactive sample, indicating that they are drawn from an older population.

  1. Activity and Kinematics of White Dwarf-M Dwarf Binaries from the SUPERBLINK Proper Motion Survey

    Science.gov (United States)

    Skinner, Julie N.; Morgan, Dylan P.; West, Andrew A.; Lépine, Sébastien; Thorstensen, John R.

    2017-09-01

    We present an activity and kinematic analysis of high proper motion white dwarf-M dwarf binaries (WD+dMs) found in the SUPERBLINK survey, 178 of which are new identifications. To identify WD+dMs, we developed a UV-optical-IR color criterion and conducted a spectroscopic survey to confirm each candidate binary. For the newly identified systems, we fit the two components using model white dwarf spectra and M dwarf template spectra to determine physical parameters. We use Hα chromospheric emission to examine the magnetic activity of the M dwarf in each system, and investigate how its activity is affected by the presence of a white dwarf companion. We find that the fraction of WD+dM binaries with active M dwarfs is significantly higher than their single M dwarf counterparts at early and mid-spectral types. We corroborate previous studies that find high activity fractions at both close and intermediate separations. At more distant separations, the binary fraction appears to approach the activity fraction for single M dwarfs. Using derived radial velocities and the proper motions, we calculate 3D space velocities for the WD+dMs in SUPERBLINK. For the entire SUPERBLINK WD+dMs, we find a large vertical velocity dispersion, indicating a dynamically hotter population compared to high proper motion samples of single M dwarfs. We compare the kinematics for systems with active M dwarfs and those with inactive M dwarfs, and find signatures of asymmetric drift in the inactive sample, indicating that they are drawn from an older population. Based on observations obtained at the MDM Observatory operated by Dartmouth College, Columbia University, The Ohio State University, and the University of Michigan.

  2. The binarity of the local white dwarf population

    Science.gov (United States)

    Toonen, S.; Hollands, M.; Gänsicke, B. T.; Boekholt, T.

    2017-06-01

    Context. As endpoints of stellar evolution, white dwarfs (WDs) are powerful tools to study the evolutionary history of the Galaxy. In particular, the multiplicity of WDs contains information regarding the formation and evolution of binary systems. Aims: Can we understand the multiplicity of the local WD sample from a theoretical point of view? Population synthesis methods are often applied to estimate stellar space densities and event rates, but how well are these estimates calibrated? This can be tested by a comparison with the 20 pc sample, which contains ≃100 stars and is minimally affected by selection biases. Methods: We model the formation and evolution of single stars and binaries within 20 pc with a population synthesis approach. We construct a model of the current sample of WDs and differentiate between WDs in different configurations, that is single WDs, and resolved and unresolved binaries containing a WD with either a main-sequence (MS) component or with a second WD. We also study the effect of different assumptions concerning the star formation history, binary evolution, and the initial distributions of binary parameters. We compile from the literature the available information on the sample of WDs within 20 pc, with a particular emphasis on their multiplicity, and compare this to the synthetic models. Results: The observed space densities of single and binary WDs are well reproduced by the models. The space densities of the most common WD systems (single WDs and unresolved WD-MS binaries) are consistent within a factor two with the observed value. We find a discrepancy only for the space density of resolved double WDs. We exclude that observational selection effects, fast stellar winds, or dynamical interactions with other objects in the Milky Way explain this discrepancy. We find that either the initial mass ratio distribution in the solar neighbourhood is biased towards low mass-ratios, or more than ten resolved DWDs have been missed

  3. Halo white dwarfs in the Gaia era

    NARCIS (Netherlands)

    van Oirschot, P.; Nelemans, G.; Pols, O.; Helmi, A.; Tolstoy, E.; Brown, A. G. A.; Pugliese, G.; de Koter, A.; Wijburg, M.

    The Galactic Halo is the oldest and most metal-poor component of the Galaxy. It is studied in detail both to understand the formation and evolution of galaxies, as well as the formation and evolution of the earliest stars. With this aim in mind, we plan to couple a population synthesis model to a

  4. VizieR Online Data Catalog: NLTE spectral analysis of white dwarf G191-B2B (Rauch+, 2013)

    Science.gov (United States)

    Rauch, T.; Werner, K.; Bohlin, R.; Kruk, J. W.

    2013-08-01

    In the framework of the Virtual Observatory, the German Astrophysical Virtual Observatory developed the registered service TheoSSA. It provides easy access to stellar spectral energy distributions (SEDs) and is intended to ingest SEDs calculated by any model-atmosphere code. In case of the DA white dwarf G191-B2B, we demonstrate that the model reproduces not only its overall continuum shape but also the numerous metal lines exhibited in its ultraviolet spectrum. (3 data files).

  5. The critical binary star separation for a planetary system origin of white dwarf pollution

    OpenAIRE

    Veras, Dimitri; Xu, Siyi; Rebassa-Mansergas, Alberto

    2017-01-01

    The atmospheres of between one quarter and one half of observed single white dwarfs in the Milky Way contain heavy element pollution from planetary debris. The pollution observed in white dwarfs in binary star systems is, however, less clear, because companion star winds can generate a stream of matter which is accreted by the white dwarf. Here we (i) discuss the necessity or lack thereof of a major planet in order to pollute a white dwarf with orbiting minor planets in both single and binary...

  6. The WIRED Survey. IV. New Dust Disks from the McCook & Sion White Dwarf Catalog

    Science.gov (United States)

    Hoard, D. W.; Debes, John H.; Wachter, Stefanie; Leisawitz, David T.; Cohen, Martin

    2013-06-01

    We have compiled photometric data from the Wide-field Infrared Survey Explorer All Sky Survey and other archival sources for the more than 2200 objects in the original McCook & Sion Catalog of Spectroscopically Identified White Dwarfs. We applied color-selection criteria to identify 28 targets whose infrared spectral energy distributions depart from the expectation for the white dwarf (WD) photosphere alone. Seven of these are previously known WDs with circumstellar dust disks, five are known central stars of planetary nebulae, and six were excluded for being known binaries or having possible contamination of their infrared photometry. We fit WD models to the spectral energy distributions of the remaining ten targets, and find seven new candidates with infrared excess suggesting the presence of a circumstellar dust disk. We compare the model dust disk properties for these new candidates with a comprehensive compilation of previously published parameters for known WDs with dust disks. It is possible that the current census of WDs with dust disks that produce an excess detectable at K-band and shorter wavelengths is close to complete for the entire sample of known WDs to the detection limits of existing near-IR all-sky surveys. The WD dust disk candidates now being found using longer wavelength infrared data are drawn from a previously underrepresented region of parameter space, in which the dust disks are overall cooler, narrower in radial extent, and/or contain fewer emitting grains.

  7. Prestack exploding reflector modelling and migration for anisotropic media

    KAUST Repository

    Alkhalifah, Tariq Ali

    2014-10-09

    The double-square-root equation is commonly used to image data by downward continuation using one-way depth extrapolation methods. A two-way time extrapolation of the double-square-root-derived phase operator allows for up and downgoing wavefields but suffers from an essential singularity for horizontally travelling waves. This singularity is also associated with an anisotropic version of the double-square-root extrapolator. Perturbation theory allows us to separate the isotropic contribution, as well as the singularity, from the anisotropic contribution to the operator. As a result, the anisotropic residual operator is free from such singularities and can be applied as a stand alone operator to correct for anisotropy. We can apply the residual anisotropy operator even if the original prestack wavefield was obtained using, for example, reverse-time migration. The residual correction is also useful for anisotropic parameter estimation. Applications to synthetic data demonstrate the accuracy of the new prestack modelling and migration approach. It also proves useful in approximately imaging the Vertical Transverse Isotropic Marmousi model.

  8. Dynamo generation of magnetic field in the white dwarf GD 358

    Science.gov (United States)

    Markiel, J. Andrew; Thomas, John H.; Van Horn, H. M.

    1994-01-01

    On the basis of Whole Earth Telescope observations of the g-mode oscillation spectrum of the white dwarf GD 358, Winget et al. find evidence for significant differential rotation and for a time-varying magnetic field concentrated in the surface layers of this star. Here we argue on theoretical grounds that this magnetic field is produced by an alpha omega dynamo operating in the lower part of a surface convection zone in GD 358. Our argument is based on numerical solutions of the nonlinear, local dynamo equations of Robinson & Durney, with specific parameters based on our detailed models of white-dwarf convective envelopes, and universal constants determined by a calibration with the the Sun's dynamo. The calculations suggest a dynamo cycle period of about 6 years for the fundamental mode, and periods as short as 1 year for the higher-order modes that are expected to dominate in view of the large dynamo number we estimate for GD 358. These dynamo periods are consistent with the changes in the magnetic field of GD 358 over the span of 1 month inferred by Winget et. al. from their observations. Our calculations also suggest a peak dynamo magnetic field strength at the base of the surface convection zone of about 1800 G, which is consistent with the field strength inferred from the observations.

  9. Formation of Millisecond Pulsars with Heavy White Dwarf Companions: Extreme Mass Transfer on Subthermal Timescales.

    Science.gov (United States)

    Tauris; van Den Heuvel EP; Savonije

    2000-02-20

    We have performed detailed numerical calculations of the nonconservative evolution of close X-ray binary systems with intermediate-mass (2.0-6.0 M middle dot in circle) donor stars and a 1.3 M middle dot in circle accreting neutron star. We calculated the thermal response of the donor star to mass loss in order to determine its stability and follow the evolution of the mass transfer. Under the assumption of the "isotropic reemission model," we demonstrate that in many cases it is possible for the binary to prevent a spiral-in and survive a highly super-Eddington mass transfer phase (1millisecond pulsars with heavy CO white dwarfs and relatively short orbital periods (3-50 days). However, we conclude that to produce a binary pulsar with a O-Ne-Mg white dwarf or Porb approximately 1 day (e.g., PSR B0655+64) the above scenario does not work, and a spiral-in phase is still considered the most plausible scenario for the formation of such a system.

  10. The double helium-white dwarf channel for the formation of AM CVn binaries

    Science.gov (United States)

    Zhang, Xian-Fei; Liu, Jin-Zhong; Jeffery, C. Simon; Hall, Philip D.; Bi, Shao-Lan

    2018-01-01

    Most close double helium white dwarfs will merge within a Hubble time due to orbital decay by gravitational wave radiation. However, a significant fraction with low mass ratios will survive for a long time as a consequence of stable mass transfer. Such stable mass transfer between two helium white dwarfs (HeWDs) provides one channel for the production of AM CVn binary stars. In previous calculations of double HeWD progenitors, the accreting HeWD was treated as a point mass. We have computed the evolution of 16 double HeWD models in order to investigate the consequences of treating the evolution of both components in detail. We find that the boundary between binaries having stable and unstable mass transfer is slightly modified by this approach. By comparing with observed periods and mass ratios, we redetermine masses of eight known AM CVn stars by our double HeWDs channel, i.e. HM Cnc, AM CVn, V406 Hya, J0926, J1240, GP Com, Gaia14aae and V396 Hya.We propose that central spikes in the triple-peaked emission spectra of J1240, GP Com and V396 Hya and the surface abundance ratios of N/C/O in GP Com can be explained by the stable double HeWD channel. The mass estimates derived from our calculations are used to discuss the predicted gravitational wave signal in the context of the Laser Interferometer Space Antenna (LISA) project.

  11. White dwarfs as a source of constraints on exotic physics

    CERN Document Server

    Malec, B

    2001-01-01

    In this paper we briefly review main ideas underlying the constraints on exotic physics coming from Astrophysics already used by the others. Next we present a new bound coming from the White Dwarf cooling. Such stringent bound is possible due to accurate measurements offered by astro-seismology. Specifically we consider the G117-B15A pulsating white dwarf (ZZ Ceti star) for which the speed of the period increase has been accurately measured for its fundamental oscillation mode. It has been claimed that this mode detected in G117-B15A is perhaps the most stable oscillation ever recorded in the optical band. Then we review our result concerning the bounds on compactification scale in the theory with large extra dimensions according to Arkani-Hamed, Dimopoulos and Dvali (1998). Because an additional channel of energy loss (Kaluza-Klein gravitons) would speed up the cooling rate, one is able to use the aforementioned stability to derive a bound on compactification scale. We find the lower bound on compactificatio...

  12. Does a Differentiated, Carbonate-rich, Rocky Object Pollute the White Dwarf SDSS J104341.53+085558.2?

    Science.gov (United States)

    Melis, Carl; Dufour, P.

    2017-01-01

    We present spectroscopic observations of the dust- and gas-enshrouded, polluted, single white dwarf star SDSS J104341.53+085558.2 (hereafter SDSS J1043+0855). Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet spectra combined with deep Keck HIRES optical spectroscopy reveal the elements C, O, Mg, Al, Si, P, S, Ca, Fe, and Ni and enable useful limits for Sc, Ti, V, Cr, and Mn in the photosphere of SDSS J1043+0855. From this suite of elements we determine that the parent body being accreted by SDSS J1043+0855 is similar to the silicate Moon or the outer layers of Earth in that it is rocky and iron-poor. Combining this with comparison to other heavily polluted white dwarf stars, we are able to identify the material being accreted by SDSS J1043+0855 as likely to have come from the outermost layers of a differentiated object. Furthermore, we present evidence that some polluted white dwarfs (including SDSS J1043+0855) allow us to examine the structure of differentiated extrasolar rocky bodies. Enhanced levels of carbon in the body polluting SDSS J1043+0855 relative to the Earth-Moon system can be explained with a model where a significant amount of the accreted rocky minerals took the form of carbonates; specifically, through this model the accreted material could be up to 9% calcium-carbonate by mass.

  13. Ultraviolet carbon lines in the spectrum of the white dwarf BPM 11668

    Science.gov (United States)

    Wegner, G.

    1983-01-01

    The southern hemisphere DC white dwarf BPM 11668 has been found to show strong ultraviolet lines of neutral carbon using observations from the IUE satellite. This star seems typical of the growing number of DC white dwarfs found to be of this type and appears to have a carbon abundance near C:He = 0.0001, with an effective temperature of 8500 K.

  14. A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes.

    Science.gov (United States)

    García-Berro, Enrique; Torres, Santiago; Althaus, Leandro G; Renedo, Isabel; Lorén-Aguilar, Pablo; Córsico, Alejandro H; Rohrmann, René D; Salaris, Maurizio; Isern, Jordi

    2010-05-13

    NGC 6791 is a well studied open cluster that it is so close to us that can be imaged down to very faint luminosities. The main-sequence turn-off age ( approximately 8 Gyr) and the age derived from the termination of the white dwarf cooling sequence ( approximately 6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, (22)Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallize and phase separation of the main constituents of the core of a typical white dwarf ((12)C and (16)O) is expected to occur. This sequence of events is expected to introduce long delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, resolving the age discrepancy for NGC 6791.

  15. AR Sco as a possible seed of highly magnetized white dwarf

    Science.gov (United States)

    Mukhopadhyay, Banibrata; Rao, A. R.; Bhatia, Tanayveer Singh

    2017-12-01

    We explore the possibility that the recently discovered white dwarf pulsar AR Sco acquired its high spin and magnetic field due to repeated episodes of accretion and spin-down. An accreting white dwarf can lead to a larger mass and consequently a smaller radius thus causing an enhanced rotation period and a magnetic field. This spinning magnetic white dwarf temporarily can inhibit accretion, spin down and eventually, the accretion can start again due to the shrinking of the binary period by gravitational radiation. A repetition of the above cycle can eventually lead to a high magnetic field white dwarf, recently postulated to be the reason for overluminous type Ia supernovae. We also point out that these high magnetic field spinning white dwarfs are attractive sites for gravitational radiation.

  16. White Dwarfs in Cataclysmic Variable Stars: Surface Temperatures and Evolution

    Directory of Open Access Journals (Sweden)

    Edward M. Sion

    2012-06-01

    Full Text Available A summary is presented of what is currently known about the surface temperatures of accreting white dwarfs (WDs detected in non-magnetic and magnetic cataclysmic variables (CVs based upon synthetic spectral analyses of far ultraviolet data. A special focus is placed on WD temperatures above and below the CV period gap as a function of the orbital period, Porb. The principal uncertainty of the temperatures for the CV WDs in the Teff - Porb distribution, besides the distance to the CV, is the mass of the WD. Only in eclipsing CV systems, an area of eclipsing binary studies, which was so central to Robert H. Koch’s career, is it possible to know CV WD masses with high precision.

  17. Pre-nova Mixing at the Surface of White Dwarfs

    Science.gov (United States)

    Rosner, R.; Young, Y. N.; Alexakis, A.; Dursi, L. J.; Truran, J.; Calder, A. C.; Fryxell, B.; Olson, K.; Ricker, P. M.; Timmes, F. X.; Zingale, M.; Tufo, H. M.; MacNeice, P.

    2000-12-01

    We report on studies of shear-driven mixing at the interface between a hydrogen-helium envelope and the carbon-oxygen surface of a pre-nova outburst white dwarf. Linear theory shows distinct regimes of unstable gravity surface waves and Kelvin-Helmholtz modes, which we follow into the weakly nonlinear regime by analytical techniques; we then follow the fully nonlinear evolution to the point of wave breaking, using the Chicago Flash Code. We also report on estimates of the mixing efficiency resulting from the combination of weakly driven convection and wave breaking; and discuss their implications for nova outbursts. This work was supported by the DOE ASCI/Alliances program at the University of Chicago, under grant No. B341495.

  18. Compact objects for everyone: I. White dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Jackson, C B; Taruna, J; Pouliot, S L; Ellison, B W; Lee, D D; Piekarewicz, J [Department of Physics, Florida State University, Tallahassee, FL 32306 (United States)

    2005-09-01

    Based upon previous discussions on the structure of compact stars geared towards undergraduate physics students, a real experiment involving two upper-level undergraduate physics students, a beginning physics graduate and two advanced graduate students was conducted. A recent addition to the physics curriculum at Florida State University, The Physics of Stars, sparked quite a few students' interests in the subject matter involving stellar structure. This, coupled with Stars and statistical physics by Balian and Blaizot (1999 Am. J. Phys. 67 1189) and Neutron stars for undergraduates by Silbar and Reddy (2004 Am. J. Phys. 72 892), is the cornerstone of this small research group who tackled solving the structure equations for compact objects in the summer of 2004. Through the use of a simple finite-difference algorithm coupled to Microsoft Excel and Maple, solutions to the equations for stellar structure are presented in the Newtonian regime appropriate to the physics of white dwarf stars.

  19. On white dwarfs and neutron stars in Palatini f(R) gravity

    CERN Document Server

    Reijonen, Vappu

    2009-01-01

    In Palatini $f(R)$ gravity, the parameters of the Schwarzschild - de Sitter solution as well as the whole interior solutions of compact objects are expected to change when compared to general relativity. We solve the Palatini field equations numerically in the case of the models $f(R) = R + \\alpha R^2$ and $f(R) = R - \\mu^4/R$, and using the equation of state of Fermi gas. We show how the density profiles and the prediction for the maximum masses of white dwarfs (the Chandrasekhar limit) and neutron stars (the Tolman-Oppenheimer-Volkoff limit) are altered, and thereby conclude that observations on compact stars may be used to exclude alternative gravity models.

  20. White Dwarf Pollution by Disk Accretion of Tidally Disrupted Rocky Bodies

    Science.gov (United States)

    Feng, Wanda; Desch, Steven

    2017-01-01

    Approximately 30% of cool white dwarfs (WDs) show heavy elements which should otherwise sediment out of their atmospheres (Koester et al. 2014; Zuckerman et al. 2010). The prevailing model for the pollution of white dwarf photospheres invokes the formation of a solid disk upon a rocky body falling within the WD Roche radius, which is then transported inward by Poynting-Robertson drag (e.g., Metzger et al. 2012, Rafikov 2011). At high temperatures close to the WD, solid particles sublimate to gas that accretes onto the WD and viscously spreads outward. This concept is supported by observations of Ca II emission from WD disks (e.g., Manser et al. 2016). The model by Metzger et al. (2012) successfully explains the range in inferred mass accretion rates (10^10 g/s, Farihi et al. 2010), provided the gaseous disks viscously spread at rates consistent with a partially suppressed magnetorotational instability (MRI). However, Metzger et al. (2012) do not consider disk chemistry or dust-to-gas mixing in their model, and do not calculate the degree of ionization to explore the extent of MRI in WD disks.We present a 1-D model of a gaseous WD disk accretion, to assess the extent of the magnetorotational instability in WD disks. The disk composition is considered with changes in sublimation rate by pressure. The degree of ionization is determined by considering UV, X-ray, and high-temperature ionization. We calculate the rate of viscous spreading and accretion rates of metals onto WDs.

  1. Multiband photometry and spectroscopy of an all-sky sample of bright white dwarfs

    Science.gov (United States)

    Raddi, R.; Gentile Fusillo, N. P.; Pala, A. F.; Hermes, J. J.; Gänsicke, B. T.; Chote, P.; Hollands, M. A.; Henden, A.; Catalán, S.; Geier, S.; Koester, D.; Munari, U.; Napiwotzki, R.; Tremblay, P.-E.

    2017-12-01

    The upcoming NASA Transiting Exoplanet Survey Satellite (TESS) will obtain space-based uninterrupted light curves for a large sample of bright white dwarfs distributed across the entire sky, providing a very rich resource for asteroseismological studies and the search for transits from planetary debris. We have compiled an all-sky catalogue of ultraviolet, optical and infrared photometry as well as proper motions, which we propose as an essential tool for the preliminary identification and characterization of potential targets. We present data for 1864 known white dwarfs and 305 high-probability white dwarf candidates brighter than 17 mag. We describe the spectroscopic follow-up of 135 stars, of which 82 are white dwarfs and 25 are hot subdwarfs. The new confirmed stars include six pulsating white dwarf candidates (ZZ Cetis), and nine white dwarf binaries with a cool main-sequence companion. We identify one star with a spectroscopic distance of only 25 pc from the Sun. Around the time TESS is launched, we foresee that all white dwarfs in this sample will have trigonometric parallaxes measured by the ESA Gaia mission next year.

  2. Can Oort clouds pollute their parent stars after they become white dwarfs?

    Science.gov (United States)

    Veras, D.; Shannon, A.; Gänsicke, B. T.

    2017-09-01

    Comets impact the Sun frequently. In fact, coronographs like those which are part of Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph Experiment (LASCO) reveal that a comet grazes the Sun every few days, with a total of about 2400 grazers from 1996 to 2008. This frequency underscores an outstanding question in the quest to understand planetary systems: what types of small bodies - pebbles, asteroids, comets or moons - are the primary polluter of white dwarfs? We determine how often remnant exo-Oort clouds, freshly excited from post-main-sequence stellar mass loss, dynamically inject comets inside the white dwarf's Roche radius. We improve upon previous studies by considering a representative range of single white dwarf masses (0.52-1.00 M⊙) and incorporating different cloud architectures, giant branch stellar mass loss, stellar flybys, Galactic tides and a realistic escape ellipsoid in self-consistent numerical simulations that integrate beyond 8 Gyr ages of white dwarf cooling. We find that ˜10^(-5) of the material in an exo-Oort cloud is typically amassed onto the white dwarf, and that hydrogen deposits accumulate even as the cloud dissipates. This accumulation may account for the relatively large amount of trace hydrogen, 10^(22) -10^(25) g, that is determined frequently among white dwarfs with cooling ages ≥1 Gyr. Our results also reaffirm the notion that exo-Oort cloud comets are not the primary agents of the metal budgets observed in polluted white dwarf atmospheres.

  3. Trace hydrogen in helium atmosphere white dwarfs as a possible signature of water accretion

    Science.gov (United States)

    Gentile Fusillo, Nicola Pietro; Gänsicke, Boris T.; Farihi, Jay; Koester, Detlev; Schreiber, Matthias R.; Pala, Anna F.

    2017-06-01

    A handful of white dwarfs with helium-dominated atmospheres contain exceptionally large masses of hydrogen in their convection zones, with the metal-polluted white dwarf GD 16 being one of the earliest recognized examples. We report the discovery of a similar star: the white dwarf coincidentally named GD 17. We obtained medium-resolution spectroscopy of both GD 16 and GD 17 and calculated abundances and accretion rates of photospheric H, Mg, Ca, Ti, Fe and Ni. The metal abundance ratios indicate that the two stars recently accreted debris, which is Mg-poor compared to the composition of bulk Earth. However, unlike the metal pollutants, H never diffuses out of the atmosphere of white dwarfs and we propose that the exceptionally high atmospheric H content of GD 16 and GD 17 (2.2 × 1024 and 2.9 × 1024 g, respectively) could result from previous accretion of water bearing planetesimals. Comparing the detection of trace H and metal pollution among 729 helium atmosphere white dwarfs, we find that the presence of H is nearly twice as common in metal-polluted white dwarfs compared to their metal-free counterparts. This highly significant correlation indicates that, over the cooling age of the white dwarfs, at least some fraction of the H detected in many He atmospheres (including GD 16 and GD 17) is accreted alongside metal pollutants, where the most plausible source is water. In this scenario, water must be common in systems with rocky planetesimals.

  4. NuSTAR and swift observations of the fast rotating magnetized white dwarf AE Aquarii

    DEFF Research Database (Denmark)

    Kitaguchi, Takao; An, Hongjun; Beloborodov, Andrei M.

    2014-01-01

    AE Aquarii is a cataclysmic variable with the fastest known rotating magnetized white dwarf (P-spin = 33.08 s). Compared to many intermediate polars, AE Aquarii shows a soft X-ray spectrum with a very low luminosity (L-X similar to 10(31) erg s(-1)). We have analyzed overlapping observations...... model with two temperatures of 1.00(-0.23)(+0.34) and 4.64(-0.84)(+1.58) keV plus a power-law component with photon index of 2.50(-0.23)(+0.17). The pulse profile in the 3-20 keV band is broad and approximately sinusoidal, with a pulsed fraction of 16.6% +/- 2.3%. We do not find any evidence...

  5. Explosive nucleosynthesis in tidal disruption events of massive white dwarfs, and their debris

    Science.gov (United States)

    Tanikawa, Ataru; Sato, Yushi; Nomoto, Ken'Ichi; Maeda, Keiichi; Nakasato, Naohito; Hachisu, Izumi

    We perform SPH simulations coupled with nuclear reactions to follow tidal disruption events (TDEs) of white dwarfs (WDs) by intermediate mass black holes (IMBHs). We consider an oxygen-neon-magnesium (ONeMg) WD with 1.2M ⊙ as well as a helium (He) WD with 0.3M ⊙, and a carbon-oxygen (CO) WD with 0.6M ⊙. Our WD models have different numbers of SPH particles, N, up to a few 10 million. We find that nucleosynthesis does not converge against N even for N > 107. For all the WDs, the amount of radioactive nuclei, such as 56Ni, decreases with increasing N. Nuclear reactions might be extinguished for infinitely large N. Our results show that these kinds of TDEs, if solely powered by radioactive decays, are much dimmer optical transients similar to Type Ia supernovae as previously suggested.

  6. Efficient Modeling and Migration in Anisotropic Media Based on Prestack Exploding Reflector Model and Effective Anisotropy

    KAUST Repository

    Wang, Hui

    2014-05-01

    This thesis addresses the efficiency improvement of seismic wave modeling and migration in anisotropic media. This improvement becomes crucial in practice as the process of imaging complex geological structures of the Earth\\'s subsurface requires modeling and migration as building blocks. The challenge comes from two aspects. First, the underlying governing equations for seismic wave propagation in anisotropic media are far more complicated than that in isotropic media which demand higher computational costs to solve. Second, the usage of whole prestack seismic data still remains a burden considering its storage volume and the existing wave equation solvers. In this thesis, I develop two approaches to tackle the challenges. In the first part, I adopt the concept of prestack exploding reflector model to handle the whole prestack data and bridge the data space directly to image space in a single kernel. I formulate the extrapolation operator in a two-way fashion to remove he restriction on directions that waves propagate. I also develop a generic method for phase velocity evaluation within anisotropic media used in this extrapolation kernel. The proposed method provides a tool for generating prestack images without wavefield cross correlations. In the second part of this thesis, I approximate the anisotropic models using effective isotropic models. The wave phenomena in these effective models match that in anisotropic models both kinematically and dynamically. I obtain the effective models through equating eikonal equations and transport equations of anisotropic and isotropic models, thereby in the high frequency asymptotic approximation sense. The wavefields extrapolation costs are thus reduced using isotropic wave equation solvers while the anisotropic effects are maintained through this approach. I benchmark the two proposed methods using synthetic datasets. Tests on anisotropic Marmousi model and anisotropic BP2007 model demonstrate the applicability of my

  7. A consistency test of white dwarf and main sequence ages: NGC 6791

    Directory of Open Access Journals (Sweden)

    Córsico A.H.

    2013-03-01

    Full Text Available NGC 6791 is an open cluster that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (∼8 Gyr and the age derived from the cut-off of the white dwarf luminosity function (∼6 Gyr were found to be significantly different. Here we demonstrate that the origin of this age discrepancy lies in an incorrect evaluation of the white dwarf cooling ages, and we show that when the relevant physical separation processes are included in the calculation of white dwarf sequences both ages are coincident.

  8. Super-Nyquist White Dwarf Pulsations in K2 Long-Cadence Data

    Science.gov (United States)

    Bell, Keaton J.; Hermes, JJ; Montgomery, Michael H.; Vanderbosch, Zach

    2017-06-01

    The Kepler and K2 missions have recently revolutionized the field of white dwarf asteroseismology. Since white dwarfs pulsate on timescales of order 10 minutes, we aim to observe these objects at K2’s short cadence (1 minute). Occasionally we find signatures of pulsations in white dwarf targets that were only observed by K2 at long cadence (30 minute). These signals suffer extreme aliasing since the intrinsic frequencies exceed the Nyquist sampling limit. We present our work to recover accurate frequency determinations for these targets, guided by a limited amount of supplementary, ground-based photometry from McDonald Observatory.

  9. Fast and Luminous Transients from the Explosions of Long-lived Massive White Dwarf Merger Remnants

    Science.gov (United States)

    Brooks, Jared; Schwab, Josiah; Bildsten, Lars; Quataert, Eliot; Paxton, Bill; Blinnikov, Sergei; Sorokina, Elena

    2017-12-01

    We study the evolution and final outcome of long-lived (≈ {10}5 years) remnants from the merger of an He white dwarf (WD) with a more massive C/O or O/Ne WD. Using Modules for Experiments in Stellar Astrophysics ({\\mathtt{MESA}}), we show that these remnants have a red giant configuration supported by steady helium burning, adding mass to the WD core until it reaches {M}{core}≈ 1.12{--}1.20 {M}⊙ . At that point, the base of the surface convection zone extends into the burning layer, mixing the helium-burning products (primarily carbon and magnesium) throughout the convective envelope. Further evolution depletes the convective envelope of helium and dramatically slows the mass increase of the underlying WD core. The WD core mass growth re-initiates after helium depletion, as then an uncoupled carbon-burning shell is ignited and proceeds to burn the fuel from the remaining metal-rich extended envelope. For large enough initial total merger masses, O/Ne WD cores would experience electron-capture triggered collapse to neutron stars (NSs) after growing to near Chandrasekhar mass ({M}{Ch}). Massive C/O WD cores could suffer the same fate after a carbon-burning flame converts them to ONe. The NS formation would release ≈ {10}50 erg into the remaining extended low mass envelope. Using the STELLA radiative transfer code, we predict the resulting optical light curves from these exploded envelopes. Reaching absolute magnitudes of {M}V≈ -17, these transients are bright for about one week and have many features of the class of luminous, rapidly evolving transients studied by Drout and collaborators.

  10. Exploding Stars and Stripes

    Science.gov (United States)

    2011-03-01

    The discovery of a pattern of X-ray "stripes" in the remains of an exploded star may provide the first direct evidence that a cosmic event can accelerate particles to energies a hundred times higher than achieved by the most powerful particle accelerator on Earth. This result comes from a very long observation of the Tycho supernova remnant with NASA's Chandra X-ray Observatory. It could explain how some of the extremely energetic particles bombarding the Earth, called cosmic rays, are produced. "We've seen lots of intriguing structures in supernova remnants, but we've never seen stripes before," said Kristoffer Eriksen, a postdoctoral researcher at Rutgers University who led the study. "This made us think very hard about what's happening in the blast wave of this powerful explosion." This latest study from Chandra provides support for a theory about how magnetic fields can be dramatically amplified in such blast waves. In this theory, the magnetic fields become highly tangled and the motions of the particles very turbulent near the expanding supernova shock wave at the front edge of the supernova remnant. High-energy charged particles can bounce back and forth across the shock wave repeatedly, gaining energy with each crossing. Theoretical models of the motion of the most energetic particles -- which are mostly protons -- are predicted to leave a messy network of holes and dense walls corresponding to weak and strong regions of magnetic fields, respectively. The X-ray stripes discovered by the Chandra researchers are thought to be regions where the turbulence is greater and the magnetic fields more tangled than surrounding areas, and may be the walls predicted by the theory. Electrons become trapped in these regions and emit X-rays as they spiral around the magnetic field lines. However, the regular and almost periodic pattern of the X-ray stripes was not predicted by the theory. "It was a big surprise to find such a neatly arranged set of stripes," said co

  11. Rolling in their Graves: White Dwarf Rotation as a Function of Mass

    Science.gov (United States)

    Hermes, JJ

    2018-01-01

    We have more than doubled the number of rotation rates measured for isolated pulsating white dwarfs thanks to extensive space-based photometry from Kepler and K2. Using follow-up optical spectroscopy to measure masses, we have put the first constraints on white dwarf rotation as a function of mass, constraining the endpoints of angular momentum evolution in stars. We find that 0.51-to-0.73-solar-mass white dwarfs, which evolved from 0.9-to-3.0-solar-mass ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for higher-mass white dwarfs. Our raw and reduced data, still growing every K2 Campaign field, are available for the community to (re-)analyze at http://www.k2wd.org.

  12. Rx J0648.0-4418: the Fastest-Spinning White Dwarf

    Science.gov (United States)

    Mereghetti, S.

    2015-01-01

    HD 49798/RX J0648.0-4418 is a post common-envelope X-ray binary composed of a hot subdwarf and one of the most massive white dwarfs with a dynamical mass measurement (1.28±0.05 M⊙). This white dwarf, with a spin period of 13.2 s, rotates more than twice faster than the white dwarf in the cataclysmic variable AE Aqr. The current properties of these two binaries, as well as their future evolution, are quite different, despite both contain a fast-spinning white dwarf. HD 49798/RX J0648.0-4418 could be the progenitor of either a Type Ia supernova or of a non-recycled millisecond pulsars.

  13. White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Line Widths: Kepler Observations of 27 Pulsating DA White Dwarfs through K2 Campaign 8

    Science.gov (United States)

    Hermes, J. J.; Gänsicke, B. T.; Kawaler, Steven D.; Greiss, S.; Tremblay, P.-E.; Gentile Fusillo, N. P.; Raddi, R.; Fanale, S. M.; Bell, Keaton J.; Dennihy, E.; Fuchs, J. T.; Dunlap, B. H.; Clemens, J. C.; Montgomery, M. H.; Winget, D. E.; Chote, P.; Marsh, T. R.; Redfield, S.

    2017-10-01

    We present photometry and spectroscopy for 27 pulsating hydrogen-atmosphere white dwarfs (DAVs; a.k.a. ZZ Ceti stars) observed by the Kepler space telescope up to K2 Campaign 8, an extensive compilation of observations with unprecedented duration (>75 days) and duty cycle (>90%). The space-based photometry reveals pulsation properties previously inaccessible to ground-based observations. We observe a sharp dichotomy in oscillation mode line widths at roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s have substantially broader mode line widths, more reminiscent of a damped harmonic oscillator than a heat-driven pulsator. Extended Kepler coverage also permits extensive mode identification: we identify the spherical degree of 87 out of 201 unique radial orders, providing direct constraints of the rotation period for 20 of these 27 DAVs, more than doubling the number of white dwarfs with rotation periods determined via asteroseismology. We also obtain spectroscopy from 4 m-class telescopes for all DAVs with Kepler photometry. Using these homogeneously analyzed spectra, we estimate the overall mass of all 27 DAVs, which allows us to measure white dwarf rotation as a function of mass, constraining the endpoints of angular momentum in low- and intermediate-mass stars. We find that 0.51-0.73 M ⊙ white dwarfs, which evolved from 1.7-3.0 M ⊙ ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for higher-mass white dwarfs. Finally, we announce an online repository for our Kepler data and follow-up spectroscopy, which we collect at http://k2wd.org.

  14. White Dwarf/M Dwarf Binaries as Single Degenerate Progenitors of Type Ia Supernovae

    Science.gov (United States)

    Wheeler, J. Craig

    2012-10-01

    Limits on the companions of white dwarfs in the single-degenerate scenario for the origin of Type Ia supernovae (SNe Ia) have gotten increasingly tight, yet igniting a nearly Chandrasekhar mass C/O white dwarf from a condition of near hydrostatic equilibrium provides compelling agreement with observed spectral evolution. The only type of non-degenerate stars that survive the tight limits, MV >~ 8.4 on the SN Ia in SNR 0509-67.5 and MV >~ 9.5 in the remnant of SN 1572, are M dwarfs. While M dwarfs are observed in cataclysmic variables, they have special properties that have not been considered in most work on the progenitors of SNe Ia: they have small but finite magnetic fields and they flare frequently. These properties are explored in the context of SN Ia progenitors. White dwarf/M dwarf pairs may be sufficiently plentiful to provide, in principle, an adequate rate of explosions even with slow orbital evolution due to magnetic braking or gravitational radiation. Even modest magnetic fields on the white dwarf and M dwarf will yield adequate torques to lock the two stars together, resulting in a slowly rotating white dwarf, with the magnetic poles pointing at one another in the orbital plane. The mass loss will be channeled by a "magnetic bottle" connecting the two stars, landing on a concentrated polar area on the white dwarf. This enhances the effective rate of accretion compared to spherical accretion. Luminosity from accretion and hydrogen burning on the surface of the white dwarf may induce self-excited mass transfer. The combined effects of self-excited mass loss, polar accretion, and magnetic inhibition of mixing of accretion layers give possible means to beat the "nova limit" and grow the white dwarf to the Chandrasekhar mass even at rather moderate mass accretion rates.

  15. WHITE DWARF/M DWARF BINARIES AS SINGLE DEGENERATE PROGENITORS OF TYPE Ia SUPERNOVAE

    Energy Technology Data Exchange (ETDEWEB)

    Wheeler, J. Craig, E-mail: wheel@astro.as.utexas.edu [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States)

    2012-10-20

    Limits on the companions of white dwarfs in the single-degenerate scenario for the origin of Type Ia supernovae (SNe Ia) have gotten increasingly tight, yet igniting a nearly Chandrasekhar mass C/O white dwarf from a condition of near hydrostatic equilibrium provides compelling agreement with observed spectral evolution. The only type of non-degenerate stars that survive the tight limits, M{sub V} {approx}> 8.4 on the SN Ia in SNR 0509-67.5 and M{sub V} {approx}> 9.5 in the remnant of SN 1572, are M dwarfs. While M dwarfs are observed in cataclysmic variables, they have special properties that have not been considered in most work on the progenitors of SNe Ia: they have small but finite magnetic fields and they flare frequently. These properties are explored in the context of SN Ia progenitors. White dwarf/M dwarf pairs may be sufficiently plentiful to provide, in principle, an adequate rate of explosions even with slow orbital evolution due to magnetic braking or gravitational radiation. Even modest magnetic fields on the white dwarf and M dwarf will yield adequate torques to lock the two stars together, resulting in a slowly rotating white dwarf, with the magnetic poles pointing at one another in the orbital plane. The mass loss will be channeled by a 'magnetic bottle' connecting the two stars, landing on a concentrated polar area on the white dwarf. This enhances the effective rate of accretion compared to spherical accretion. Luminosity from accretion and hydrogen burning on the surface of the white dwarf may induce self-excited mass transfer. The combined effects of self-excited mass loss, polar accretion, and magnetic inhibition of mixing of accretion layers give possible means to beat the 'nova limit' and grow the white dwarf to the Chandrasekhar mass even at rather moderate mass accretion rates.

  16. X-Ray Emissions from Accreting White Dwarfs: A Review

    Science.gov (United States)

    Mukai, K.

    2017-01-01

    Interacting binaries in which a white dwarf accretes material from a companion-cataclysmic variables (CVs) in which the mass donor is a Roche-lobe filling star on or near the main sequence, and symbiotic stars in which the mass donor is a late type giant-are relatively commonplace. They display a wide range of behaviors in the optical, X-rays, and other wavelengths, which still often baffle observers and theorists alike. Here I review the existing body of research on X-ray emissions from these objects for the benefits of both experts and newcomers to the field. I provide introductions to the past and current X-ray observatories, the types of known X-ray emissions from these objects, and the data analysis techniques relevant to this field. I then summarize of our knowledge regarding the X-ray emissions from magnetic CVs, non-magnetic CVs and symbiotic stars, and novae in eruption. I also discuss space density and the X-ray luminosity functions of these binaries and their contribution to the integrated X-ray emission from the Galaxy. I then discuss open questions and future prospects.

  17. THE HABITABILITY AND DETECTION OF EARTH-LIKE PLANETS ORBITING COOL WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Fossati, L.; Haswell, C. A.; Patel, M. R.; Busuttil, R. [Department of Physical Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA (United Kingdom); Bagnulo, S. [Armagh Observatory, College Hill, Armagh BT61 9DG (United Kingdom); Kowalski, P. M. [GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam (Germany); Shulyak, D. V. [Institute of Astrophysics, Georg-August-University, Friedrich-Hund-Platz 1, D-37077 Goettingen (Germany); Sterzik, M. F., E-mail: l.fossati@open.ac.uk, E-mail: C.A.Haswell@open.ac.uk, E-mail: M.R.Patel@open.ac.uk, E-mail: r.busuttil@open.ac.uk, E-mail: sba@arm.ac.uk, E-mail: kowalski@gfz-potsdam.de, E-mail: denis.shulyak@gmail.com, E-mail: msterzik@eso.org [European Southern Observatory, Casilla 19001, Santiago 19 (Chile)

    2012-09-20

    Since there are several ways planets can survive the giant phase of the host star, we examine the habitability and detection of planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU would remain in the continuous habitable zone (CHZ) for {approx}8 Gyr. We show that photosynthetic processes can be sustained on such planets. The DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, and hence non-magnetic white dwarfs are compatible with the persistence of complex life. Polarization due to a terrestrial planet in the CHZ of a cool white dwarf (CWD) is 10{sup 2} (10{sup 4}) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a viable way to detect close-in rocky planets around white dwarfs. Multi-band polarimetry would also allow us to reveal the presence of a planet atmosphere, providing a first characterization. Planets in the CHZ of a 0.6 M{sub Sun} white dwarf will be distorted by Roche geometry, and a Kepler-11d analog would overfill its Roche lobe. With current facilities a super-Earth-sized atmosphereless planet is detectable with polarimetry around the brightest known CWD. Planned future facilities render smaller planets detectable, in particular by increasing the instrumental sensitivity in the blue.

  18. The DB gap and a new class of pulsating white dwarfs

    Directory of Open Access Journals (Sweden)

    Shibahashi H.

    2013-03-01

    Full Text Available The recent systematic surveys providing enormously massive datasets of white dwarfs show that there is still a deficit of a factor of 2.5 in the DA/non-DA ratio within the temperature range of 30 000 K < Teff < 45 000 K, which has been regarded as the “DB gap” meaning a range with almost no helium atmosphere white dwarfs. Since all white dwarfs have to evolve through this temperature range along almost the identical sequence on the color-magnitude diagram, this implies that most of the helium atmosphere DO stars once evolve into hydrogen atmosphere hot DA stars in the temperature range of the DB gap and then back to helium atmosphere DB stars. Possible scenarios for this chameleon-like disguises of white dwarfs with helium dominant atmospheres are described and a new class of pulsating white dwarfs, named the hot-DAV stars, is predicted from these scenarios. One pulsating DA white dwarf, being consistent with the prediction, has been discovered indeed.

  19. The critical binary star separation for a planetary system origin of white dwarf pollution

    Science.gov (United States)

    Veras, Dimitri; Xu, Siyi; Rebassa-Mansergas, Alberto

    2018-01-01

    The atmospheres of between one quarter and one half of observed single white dwarfs in the Milky Way contain heavy element pollution from planetary debris. The pollution observed in white dwarfs in binary star systems is, however, less clear, because companion star winds can generate a stream of matter which is accreted by the white dwarf. Here, we (i) discuss the necessity or lack thereof of a major planet in order to pollute a white dwarf with orbiting minor planets in both single and binary systems, and (ii) determine the critical binary separation beyond which the accretion source is from a planetary system. We hence obtain user-friendly functions relating this distance to the masses and radii of both stars, the companion wind, and the accretion rate on to the white dwarf, for a wide variety of published accretion prescriptions. We find that for the majority of white dwarfs in known binaries, if pollution is detected, then that pollution should originate from planetary material.

  20. An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate.

    Science.gov (United States)

    Gilfanov, Marat; Bogdán, Akos

    2010-02-18

    There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 10(7) years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of approximately 30-50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate.

  1. Stellar equilibrium configurations of white dwarfs in the f( R, T) gravity

    Science.gov (United States)

    Carvalho, G. A.; Lobato, R. V.; Moraes, P. H. R. S.; Arbañil, José D. V.; Otoniel, E.; Marinho, R. M.; Malheiro, M.

    2017-12-01

    In this work we investigate the equilibrium configurations of white dwarfs in a modified gravity theory, namely, f( R, T) gravity, for which R and T stand for the Ricci scalar and trace of the energy-momentum tensor, respectively. Considering the functional form f(R,T)=R+2λ T, with λ being a constant, we obtain the hydrostatic equilibrium equation for the theory. Some physical properties of white dwarfs, such as: mass, radius, pressure and energy density, as well as their dependence on the parameter λ are derived. More massive and larger white dwarfs are found for negative values of λ when it decreases. The equilibrium configurations predict a maximum mass limit for white dwarfs slightly above the Chandrasekhar limit, with larger radii and lower central densities when compared to standard gravity outcomes. The most important effect of f( R, T) theory for massive white dwarfs is the increase of the radius in comparison with GR and also f( R) results. By comparing our results with some observational data of massive white dwarfs we also find a lower limit for λ , namely, λ >- 3× 10^{-4}.

  2. Unstable Helium Shell Burning on Accreting White Dwarfs

    Science.gov (United States)

    Shen, Ken J.; Bildsten, Lars

    2009-07-01

    AM Canum Venaticorum (AM CVn) binaries consist of a degenerate helium donor and a helium, C/O, or O/Ne white dwarf accretor, with accretion rates of \\dot{M} = 10^{-13}\\--10^{-5} \\, M_\\odot \\; yr^{-1}. For accretion rates thermonuclear supernovae. In this paper, we study the evolution of the He-burning shells in more detail. We calculate maximum achievable temperatures as well as the minimum envelope masses that achieve dynamical burning conditions, finding that AM CVn systems with accretors gsim0.8 M sun will undergo dynamical burning. Triple-α reactions during the hydrostatic evolution set a lower limit to the 12C mass fraction of 0.001-0.05 when dynamical burning occurs, but core dredge-up may yield 12C, 16O, and/or 20Ne mass fractions of ~0.1. Accreted 14N will likely remain 14N during the accretion and convective phases, but regardless of 14N's fate, the neutron-to-proton ratio at the beginning of convection is fixed until the onset of dynamical burning. During explosive burning, the 14N will undergo 14N(α, γ)18F(α, p)21Ne, liberating a proton for the subsequent 12C(p, γ)13N(α, p)16O reaction, which bypasses the relatively slow α-capture onto 12C. Future hydrodynamic simulations must include these isotopes, as the additional reactions will reduce the Zel'dovich-von Neumann-Döring length, making the propagation of the detonation wave more likely.

  3. Magnetically gated accretion in an accreting ‘non-magnetic’ white dwarf

    Science.gov (United States)

    Scaringi, S.; Maccarone, T. J.; D’Angelo, C.; Knigge, C.; Groot, P. J.

    2017-12-01

    White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15 per cent of these binaries, the magnetic field of the white dwarf is strong enough (at 106 gauss or more) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as ‘non-magnetic’, because until now there has been no evidence that they have a magnetic field that is strong enough to affect the accretion dynamics. Here we report an analysis of archival optical observations of the ‘non-magnetic’ accreting white dwarf in the binary system MV Lyrae, whose light curve displays quasi-periodic bursts of about 30 minutes duration roughly every 2 hours. The timescale and amplitude of these bursts indicate the presence of an unstable, magnetically regulated accretion mode, which in turn implies the existence of magnetically gated accretion, in which disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyrae of between 2 × 104 gauss and 1 × 105 gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cycles have been identified.

  4. The instability strip of ZZ Ceti white dwarfs. I. Introduction of time-dependent convection

    Science.gov (United States)

    Van Grootel, V.; Dupret, M.-A.; Fontaine, G.; Brassard, P.; Grigahcène, A.; Quirion, P.-O.

    2012-03-01

    Aims: The determination of the location of the theoretical ZZ Ceti instability strip in the log g - Teff diagram has remained a challenge over the years due to the lack of a suitable treatment for convection in these stars. For the first time, a full nonadiabatic approach including time-dependent convection is applied to ZZ Ceti pulsators, and we provide the appropriate details related to the inner workings of the driving mechanism. Methods: We used the nonadiabatic pulsation code MAD with a representative evolutionary sequence of a 0.6 M⊙ DA white dwarf. This sequence is made of state-of-the-art models that include a detailed modeling of the feedback of convection on the atmospheric structure. The assumed convective efficiency in these models is the so-called ML2/α = 1.0 version. We also carried out, for comparison purposes, nonadiabatic computations within the frozen convection approximation, as well as calculations based on models with standard grey atmospheres. Results: We find that pulsational driving in ZZ Ceti stars is concentrated at the base of the superficial H convection zone, but at depths, near the blue edge of the instability strip, somewhat larger than those obtained with the frozen convection approach. Despite the fact that this approach is formally invalid in such stars, particularly near the blue edge of the instability strip, the predicted boundaries are not dramatically different in both cases. The revised blue edge for a 0.6 M⊙ model is found to be around Teff = 11 970 K, some 240 K hotter than the value predicted within the frozen convection approximation, in rather good agreement with the empirical value. On the other hand, our predicted red edge temperature for the same stellar mass is only about 5600 K (80 K hotter than with the frozen convection approach), much lower than the observed value. Conclusions: We correctly understand the development of pulsational instabilities of a white dwarf as it cools at the blue edge of the ZZ Ceti

  5. White dwarfs in cataclysmic variables - Low state IUE observations of V794 Aquilae, MR Serpentis, and AN Ursae Majoris

    Science.gov (United States)

    Szkody, Paula; Mateo, Mario; Downes, Ronald

    1988-01-01

    IUE low-dispersion observations of one novalike system (V794 Aql) and two AM Her systems (MR Ser and AN UMa) during low or intermediate-low accretion-rate states are reported. The decreased UV emission during these times was used to place limits on the white dwarf temperatures. When these results are added to the existing data on white dwarf temperatures, it appears that the white dwarfs in systems with normal high accretion rates and only occasional low-accretion-rate states remain hotter than the white dwarfs in systems with low average accretion rates.

  6. FORMATION OF THE GALACTIC MILLISECOND PULSAR TRIPLE SYSTEM PSR J0337+1715—A NEUTRON STAR WITH TWO ORBITING WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Tauris, T. M. [Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn (Germany); Van den Heuvel, E. P. J., E-mail: tauris@astro.uni-bonn.de [Astronomical Institute Anton Pannekoek, University of Amsterdam, P.O. Box 94249, 1090 GE Amsterdam (Netherlands)

    2014-01-20

    The millisecond pulsar in a triple system (PSR J0337+1715, recently discovered by Ransom et al.) is an unusual neutron star with two orbiting white dwarfs. The existence of such a system in the Galactic field poses new challenges to stellar astrophysics for understanding evolution, interactions, and mass transfer in close multiple stellar systems. In addition, this system provides the first precise confirmation for a very wide-orbit system of the white dwarf mass-orbital period relation. Here, we present a self-consistent, semi-analytical solution to the formation of PSR J0337+1715. Our model constrains the peculiar velocity of the system to be less than 160 km s{sup –1} and brings novel insight to, for example, common envelope evolution in a triple system, for which we find evidence for in-spiral of both outer stars. Finally, we briefly discuss our scenario in relation to alternative models.

  7. Physics of the Compact Debris Disks Around Metal Rich White Dwarfs

    Science.gov (United States)

    Rafikov, Roman

    2017-06-01

    Significant fraction of the white dwarfs are known to have metal polluted atmospheres, with a much smaller fraction (of order several per cent) also exhibiting near-infrared excesses, indicative of the presence of the dense debris disks in their immediate vicinity. Such disks are believed to originate from tidal disruption of the minor objects originally orbiting these white dwarfs, thus shedding light on the properties of the planetary systems hosted by these stellar remnants. They also provide an obvious mass reservoir to explain the observed metal contamination of the white dwarf atmospheres. In my talk I'll focus on the physical processes affecting the evolution of the debris disks and affecting the rate, at which they accrete onto their central stars. I will demonstrate the key role of the Poynting-Robertson drag in setting the minimum metal accretion rate onto the white dwarf. I will also show how the coupling between the particulate and gaseous debris disks can naturally give rise to much higher accretion rates of metals. Better understanding of these processes will help us illuminate accreton history of minor planets by the white dwarfs.

  8. Post-main Sequence Evolution of Icy Minor Planets: Implications for Water Retention and White Dwarf Pollution

    Science.gov (United States)

    Malamud, Uri; Perets, Hagai B.

    2016-12-01

    Most observations of polluted white dwarf atmospheres are consistent with accretion of water-depleted planetary material. Among tens of known cases, merely two involve accretion of objects that contain a considerable mass fraction of water. The purpose of this study is to investigate the relative scarcity of these detections. Based on a new and highly detailed model, we evaluate the retention of water inside icy minor planets during the high-luminosity stellar evolution that follows the main sequence. Our model fully considers the thermal, physical, and chemical evolution of icy bodies, following their internal differentiation as well as water depletion, from the moment of their birth and through all stellar evolution phases preceding the formation of the white dwarf. We also account for different initial compositions and formation times. Our results differ from previous studies, which have either underestimated or overestimated water retention. We show that water can survive in a variety of circumstances and in great quantities, and therefore other possibilities are discussed in order to explain the infrequency of water detection. We predict that the sequence of accretion is such that water accretes earlier, and more rapidly, than the rest of the silicate disk, considerably reducing the chance of its detection in H-dominated atmospheres. In He-dominated atmospheres, the scarcity of water detections could be observationally biased. It implies that the accreted material is typically intrinsically dry, which may be the result of the inside-out depopulation sequence of minor planets.

  9. Pressure Distortion of the H2-He Collision-induced Absorption at the Photosphere of Cool White Dwarf Stars

    Science.gov (United States)

    Blouin, S.; Kowalski, P. M.; Dufour, P.

    2017-10-01

    Collision-induced absorption (CIA) from molecular hydrogen is a dominant opacity source in the atmosphere of cool white dwarfs. It results in a significant flux depletion in the near-IR and IR parts of their spectra. Because of the extreme conditions of helium-rich atmospheres (where the density can be as high as a few g cm-3), this opacity source is expected to undergo strong pressure distortion and the currently used opacities have not been validated at such extreme conditions. To check the distortion of the CIA opacity, we applied state-of-the-art ab initio methods of computational quantum chemistry to simulate the CIA opacity at high densities. The results show that the CIA profiles are significantly distorted above densities of 0.1 {{g}} {{cm}}-3 in a way that is not captured by the existing models. The roto-translational band is enhanced and shifted to higher frequencies as an effect of the decrease of the interatomic separation of the H2 molecule. The vibrational band is blueward shifted and split into Q R and Q P branches, separated by a pronounced interference dip. Its intensity is also substantially reduced. The distortions result in a shift of the maximum of the absorption from 2.3 μm to 3-7 μm, which could potentially explain the spectra of some very cool, helium-rich white dwarfs.

  10. A Deep Proper Motion Catalog Within the Sloan Digital Sky Survey Footprint. II. The White Dwarf Luminosity Function

    Science.gov (United States)

    Munn, Jeffrey A.; Harris, Hugh C.; von Hippel, Ted; Kilic, Mukremin; Liebert, James W.; Williams, Kurtis A.; DeGennaro, Steven; Jeffery, Elizabeth; Dame, Kyra; Gianninas, A.; Brown, Warren R.

    2017-01-01

    A catalog of 8472 white dwarf (WD) candidates is presented, selected using reduced proper motions from the deep proper motion catalog of Munn et al. Candidates are selected in the magnitude range 16Digital Sky Survey (SDSS) imaging footprint. Distances, bolometric luminosities, and atmospheric compositions are derived by fitting SDSS ugriz photometry to pure hydrogen and helium model atmospheres (assuming surface gravities {log} {\\text{}}g=8). The disk white dwarf luminosity function (WDLF) is constructed using a sample of 2839 stars with 5.5< {M}{bol}< 17, with statistically significant numbers of stars cooler than the turnover in the luminosity function. The WDLF for the halo is also constructed, using a sample of 135 halo WDs with 5< {M}{bol}< 16. We find space densities of disk and halo WDs in the solar neighborhood of 5.5+/- 0.1× {10}-3 {{pc}}-3 and 3.5+/- 0.7× {10}-5 {{pc}}-3, respectively. We resolve the bump in the disk WDLF due to the onset of fully convective envelopes in WDs, and see indications of it in the halo WDLF as well.

  11. Source-receiver two-way wave extrapolation for prestack exploding-reflector modelling and migration

    KAUST Repository

    Alkhalifah, Tariq Ali

    2014-10-08

    Most modern seismic imaging methods separate input data into parts (shot gathers). We develop a formulation that is able to incorporate all available data at once while numerically propagating the recorded multidimensional wavefield forward or backward in time. This approach has the potential for generating accurate images free of artiefacts associated with conventional approaches. We derive novel high-order partial differential equations in the source-receiver time domain. The fourth-order nature of the extrapolation in time leads to four solutions, two of which correspond to the incoming and outgoing P-waves and reduce to the zero-offset exploding-reflector solutions when the source coincides with the receiver. A challenge for implementing two-way time extrapolation is an essential singularity for horizontally travelling waves. This singularity can be avoided by limiting the range of wavenumbers treated in a spectral-based extrapolation. Using spectral methods based on the low-rank approximation of the propagation symbol, we extrapolate only the desired solutions in an accurate and efficient manner with reduced dispersion artiefacts. Applications to synthetic data demonstrate the accuracy of the new prestack modelling and migration approach.

  12. From Accretion to Explosion and Beyond: Transforming White Dwarfs to Neutron Stars and Black Holes

    Science.gov (United States)

    Di Stefano, Rosanne; Harris, R.

    2010-03-01

    White dwarfs accreting at high rates can grow in mass, exhibiting episodes of supersoft-source activity. Some can achieve the Chandrasekhar mass and will either become Type Ia supernovae or else will collapse, becoming neutron stars. We consider white dwarfs with giant donors, computing the rates of both supernovae and collapses. For the collapses, we follow each system to the end of accretion. Some of these systems will appear as ultraluminous x-ray sources and some will go on to become low-mass black holes. This scenario should be fairly common in young stellar populations and links a wide range of astrophysical phenomena. Indeed, it is a veritable cornucopia for the high-energy astrophysicist, offering accreting white dwarfs, neutron stars, and black holes, Type Ia supernovae, gamma-ray bursts, supersoft sources, ultraluminous sources, and neutron star and black hole binaries in globular clusters.

  13. New Observations of the Mysterious Metal-Polluted White Dwarf GD 394

    Science.gov (United States)

    Wilson, D. J.; Gänsicke, B. T.; Koester, D.; Holberg, J.; Burleigh, M. R.; Belardi, C.

    2017-03-01

    GD394 is a hot, extremely metal-polluted white dwarf. EUVE observations in the early 1990s revealed a 1.15 day periodicity with a 25 percent amplitude. This was hypothesised to be due to an accretion spot on the white dwarf, rotating into and out of view on the white dwarf spin period. We have obtained phase-resolved HST/STIS high-resolution FUV spectra of GD394 that sample the entire spin period. The spectra contain dozens of strong metal absorption lines, which we use to test the hypothesis of a rotating spot. No variation is seen in the STIS spectra, nor between them and a 1992 GHRS spectrum. Furthermore, archive SuperWASP observations do not show the optical variability predicted by the spot hypothesis. The STIS spectra also show mysterious C IV lines that may be an indicator of circumstellar material.

  14. 3D hydrodynamic simulations of tidal disruption of terrestrial planets around white dwarfs

    Science.gov (United States)

    Liu, Shangfei; Zhang, Jinsu; Lin, Douglas N. C.

    2018-01-01

    Recent K2 mission spotted striking variability due to a group of minor bodies transiting white dwarf WD 1145+017 with periods ranging from 4.5 hours to 4.9 hours. One of the formation scenarios is that those transiting objects are the debris of a tidally disrupted minor planet. This scenario is consistent with fact that the white dwarf also hosts a dusty disk and displays strong metal atmospheric pollution. In this work, we perform state-of-the-art three-dimensional hydrodynamic simulations to study the consequences of tidal disruption of planets with various differentiated compositions by a white dwarf. We study the general outcomes of tidal disruption including partially disruption and total disruption. We also apply our results to the WD 1145+017 system to infer the physical and orbital properties of the progenitor.

  15. Post main sequence evolution of icy minor planets: water retention and white dwarf pollution

    Science.gov (United States)

    Malamud, Uri; Perets, Hagai

    2017-06-01

    We investigate the evolution of icy minor planets from the moment of their birth and through the all evolutionary stages of their host stars, including the main sequence, red giant branch and asymptotic giant branch phases. We then asses the degree of water retention in planetary systems around white dwarf, as a function of various parameters. We consider progenitor stars of different masses and metallicities. We also consider minor planets of various sizes, initial orbital distances, compositions and formation times. Our results indicate that water can survive to the white dwarf stage in a variety of circumstances, especially around G, F, A and even some B type stars. We discuss the significance of water retention with respect to white dwarf pollution and also for planet habitability.

  16. Ultra-high energy cosmic rays from white dwarf pulsars and the Hillas criterion

    Science.gov (United States)

    Lobato, Ronaldo V.; Coelho, Jaziel G.; Malheiro, M.

    2017-06-01

    The origins of ultra-high-energy cosmic rays (E ≳ 1019 eV) are a mystery and still under debate in astroparticle physics. In recent years some efforts were made to understand their nature. In this contribution we consider the possibility of Some Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) beeing white dwarf pulsars, and show that these sources can achieve large electromagnetic potentials on their surface that accelerate particle almost at the speed of light, with energies E ~ 1020-21 eV. The sources SGRs/AXPs considered as highly magnetized white dwarfs are well described in the Hillas diagram, lying close to the AR Sorpii and AE Aquarii which are understood as white dwarf pulsars.

  17. A library of IUE white dwarf spectra for stellar population analyses.

    Science.gov (United States)

    Bica, E.; Bonatto, C.; Giovannini, O.

    1996-10-01

    We present high Signal to Noise ratio IUE spectra of different classes of white dwarfs, to be used as templates for stellar population analyses in the ultraviolet region. We present average stellar parameters associated to each group. The library contains 6 groups for DA's, 2 for DO's and 5 for DB's. We also present equivalent widths of spectral features, and continuum measurements. We call attention to the spectral characteristics which are promising indicators of the presence of white dwarfs in the spectra of composite stellar populations.

  18. Mixing by Non-linear Gravity Wave Breaking on a White Dwarf Surface

    Science.gov (United States)

    Calder, A. C.; Alexakis, A.; Dursi, L. J.; Rosner, R.; Truran, J. W.; Fryxell, B.; Ricker, P.; Zingale, M.; Olson, K.; Timmes, F. X.; MacNeice, P.

    2002-11-01

    We present the results of a simulation of a wind-driven non-linear gravity wave breaking on the surface of a white dwarf. The ``wind'' consists of H/He from an accreted envelope, and the simulation demonstrates that this breaking wave mechanism can produce a well-mixed layer of H/He with C/O from the white dwarf above the surface. Material from this mixed layer may then be transported throughout the accreted envelope by convection, which would enrich the C/O abundance of the envelope as is expected from observations of novae.

  19. DOUBLE DEGENERATE MERGERS AS PROGENITORS OF HIGH-FIELD MAGNETIC WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Garcia-Berro, Enrique; Loren-Aguilar, Pablo; Aznar-Siguan, Gabriela; Torres, Santiago; Camacho, Judit [Departament de Fisica Aplicada, Universitat Politecnica de Catalunya, c/Esteve Terrades, 5, 08860 Castelldefels (Spain); Althaus, Leandro G.; Corsico, Alejandro H. [Facultad de Ciencias Astronomicas y Geofisicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, (1900) La Plata (Argentina); Kuelebi, Baybars; Isern, Jordi, E-mail: garcia@fa.upc.edu [Institute for Space Studies of Catalonia, c/Gran Capita 2-4, Edif. Nexus 104, 08034 Barcelona (Spain)

    2012-04-10

    High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mechanism that produces the magnetic field are still unknown. Here, we show that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores can produce magnetic fields of the required strength that do not decay for long timescales. Using a state-of-the-art Monte Carlo simulator, we also show that the expected number of high-field magnetic white dwarfs produced in this way is consistent with that found in the solar neighborhood.

  20. Significantly Super-Chandrasekhar Limiting Mass White Dwarfs and their Consequences

    Science.gov (United States)

    Mukhopadhyay, B.; Das, U.; Rao, A. R.; Subramanian, S.; Bhattacharya, M.; Mukerjee, S.; Bhatia, T. S.; Sutradhar, J.

    2017-03-01

    Since 2012, we have initiated a new idea showing that the mass of highly magnetized or modified Einstein's gravity induced white dwarfs could be significantly super-Chandrasekhar with a different mass-limit. This discovery has several important consequences, including explanation of peculiar, over-luminous type Ia supernovae, soft gamma-ray repeaters and anomalous X-ray pulsars without invoking extraordinarily strong, yet unobserved, magnetic fields. It further argues for a possible second standard candle. Based on simpler calculations, these white dwarfs are also shown to be much less luminous than their standard counter-parts (of low magnetic fields). This discovery altogether initiates a new field of research.

  1. The Identification of the White Dwarf Companion to the Millisecond Pulsar J2317+1439

    Science.gov (United States)

    Dai, S.; Smith, M. C.; Wang, S.; Okamoto, S.; Xu, R. X.; Yue, Y. L.; Liu, J. F.

    2017-06-01

    We report the identification of the optical counterpart to the companion of the millisecond pulsar J2317+1439. At the timing position of the pulsar, we find an object with g=22.96+/- 0.05, r=22.86+/- 0.04, and i=22.82+/- 0.05. The magnitudes and colors of the object are consistent with a white dwarf (WD). Compared with WD cooling models, we estimate that it has a mass of {0.39}-0.10+0.13 {M}⊙ , an effective temperature of {8077}-470+550 K, and a cooling age of 10.9 ± 0.3 Gyr. Combining our results with published constraints on the orbital parameters obtained through pulsar timing, we estimate the pulsar mass to be {3.4}-1.1+1.4 {M}⊙ . Although the constraint on the pulsar mass is still weak, there is a significant possibility that the pulsar could be more massive than two solar masses.

  2. The population of white dwarf-main sequence binaries in the SDSS DR 12

    Science.gov (United States)

    Cojocaru, R.; Rebassa-Mansergas, A.; Torres, S.; García-Berro, E.

    2017-09-01

    We present a Monte Carlo population synthesis study of white dwarf-main sequence (WD+MS) binaries in the Galactic disc aimed at reproducing the ensemble properties of the entire population observed by the Sloan Digital Sky Survey (SDSS) Data Release 12. Our simulations take into account all known observational biases and use the most up-to-date stellar evolutionary models. This allows us to perform a sound comparison between the simulations and the observational data. We find that the properties of the simulated and observed parameter distributions agree best when assuming low values of the common envelope efficiency (0.2-0.3), a result that is in agreement with previous findings obtained by observational and population synthesis studies of close SDSS WD+MS binaries. We also show that all synthetic populations that result from adopting an initial mass ratio distribution with a positive slope are excluded by observations. Finally, we confirm that the properties of the simulated WD+MS binary populations are nearly independent of the age adopted for the thin disc, on the contribution of WD+MS binaries from the thick disc (0-17 per cent of the total population) and on the assumed fraction of the internal energy that is used to eject the envelope during the common envelope phase (0.1-0.5).

  3. He-accreting carbon-oxygen white dwarfs and Type Ia supernovae

    Science.gov (United States)

    Wang, Bo; Podsiadlowski, Philipp; Han, Zhanwen

    2017-12-01

    He accretion on to carbon-oxygen white dwarfs (CO WDs) plays a fundamental role when studying the formation of Type Ia supernovae (SNe Ia). Employing the mesa stellar evolution code, we calculated the long-term evolution of He-accreting CO WDs. Previous studies usually supposed that a WD can grow in mass to the Chandrasekhar limit in the stable He burning region and finally produce an SN Ia. However, in this study, we find that off-centre carbon ignition occurs in the stable He burning region if the accretion rate is above a critical value (˜2.05 × 10-6 M⊙ yr-1), resulting in accretion-induced collapse rather than an SN Ia. If the accretion rate is below the critical value, explosive carbon ignition will eventually happen in the centre producing an SN Ia. Taking into account the possibility of off-centre carbon ignition, we have re-determined the initial parameter space that produces SNe Ia in the He star donor channel, one of the promising channels to produce SNe Ia in young populations. Since this parameter space is smaller than was found in the previous study of Wang et al. (2009), the SN Ia rates are also correspondingly smaller. We also determined the chemical abundance profile of the He-accreting WDs at the moment of explosive carbon ignition, which can be used as initial input for SN Ia explosion models.

  4. Simulating the tidal disruption of the asteroid orbiting white dwarf WD 1145+017

    Science.gov (United States)

    Veras, Dimitri; Carter, Philip; Leinhardt, Zoe; Gänsicke, Boris

    2017-06-01

    Post-main-sequence planetary science has been galvanised by the striking variability, depth and shape of the photometric transit curves due to objects orbiting white dwarf WD 1145+017, a star which also hosts a dusty debris disc and circumstellar gas, and displays strong metal atmospheric pollution. However, the physical properties of the likely asteroid which is discharging disintegrating fragments remain largely unconstrained from the observations. This process has not yet been modelled numerically. Here, we use the N-body code PKDGRAV to compute dissipation properties for asteroids of different spins, densities, masses, and eccentricities. We simulate both homogeneous and differentiated asteroids, for up to two years, and find that the disruption timescale is strongly dependent on density and eccentricity, but weakly dependent on mass and spin. We find that primarily rocky differentiated bodies with moderate (~3-4 g/cm^3) bulk densities on near-circular (e <~ 0.1) orbits can remain intact while occasionally shedding mass from their mantles. These results suggest that the asteroid orbiting WD 1145+017 is differentiated, resides just outside of the Roche radius for bulk density but just inside the Roche radius for mantle density, and is more akin physically to an asteroid like Vesta instead of one like Itokawa.

  5. Probing the Accretion Induced Collapse of White Dwarfs in the Binary Millisecond Pulsars Population

    Science.gov (United States)

    Taani, Ali

    The recycling process suggested that low-mass X-ray binaries (LMXBs) could evolve into binary Millisecond Pulsars (MSPs). I will discuss another possible channel involving the Accretion Induced Collapse (AIC) of a white dwarf (WD) in binaries. I will investigate the progenitors of MSPs with a distribution of long orbital periods, to show the link between WD binaries and long orbits for some binary MSPs. For this task, I present a model that attempts to turn binary MSPs into wide binaries (P_orb > 2 d) with high eccentricities (e > 0.1). in the Galactic disk, since the AIC process in a close binary will impart a kick velocity caused by asymmetric collapse to the thus formed neutron star, and the binding energy plus the mass loss (0.2~Msun) not expected to exceed a few tens of km/s. An appropriate kick can disrupt the binary system and result in the birth of isolated MSPs. Otherwise, the binary survives and an eccentric binary MSP is formed. The circularity of the orbit implies that the companion is a WD. In addition AIC can retain pulsars in globular clusters due to the small momentum kick expected to be associated with the implosion.

  6. WIRED for EC: New White Dwarfs with WISE Infrared Excesses and New Classification Schemes from the Edinburgh-Cape Blue Object Survey

    Science.gov (United States)

    Dennihy, E.; Clemens, J. C.; Debes, John H.; Dunlap, B. H.; Kilkenny, D.; O'Brien, P. C.; Fuchs, J. T.

    2017-11-01

    We present a simple method for identifying candidate white dwarf systems with dusty exoplanetary debris based on a single temperature blackbody model fit to the infrared excess. We apply this technique to a sample of Southern Hemisphere white dwarfs from the recently completed Edinburgh-Cape Blue Object Survey and identify four new promising dusty debris disk candidates. We demonstrate the efficacy of our selection method by recovering three of the four Spitzer confirmed dusty debris disk systems in our sample. Further investigation using archival high-resolution imaging shows that Spitzer data of the unrecovered fourth object is likely contaminated by a line-of-sight object that either led to a misclassification as a dusty disk in the literature or is confounding our method. Finally, in our diagnostic plot, we show that dusty white dwarfs, which also host gaseous debris, lie along a boundary of our dusty debris disk region, providing clues to the origin and evolution of these especially interesting systems.

  7. Stripped Red Giants - Helium Core White Dwarf Progenitors and their sdB Siblings

    Science.gov (United States)

    Heber, U.

    2017-03-01

    Some gaps in the mosaic of binary star evolution have recently been filled by the discoveries of helium-core white dwarf progenitors (often called extremely low mass (ELM) white dwarfs) as stripped cores of first-giant branch objects. Two varieties can be distinguished. One class is made up by SB1 binaries, companions being white dwarfs as well. Another class, the so-called EL CVn stars, are composite spectrum binaries, with A-Type companions. Pulsating stars are found among both classes. A riddle is posed by the apparently single objects. There is a one-to-one correspondence of the phenomena found for these new classes of star to those observed for sdB stars. In fact, standard evolutionary scenarios explain the origin of sdB stars as red giants that have been stripped close to the tip of first red giant branch. A subgroup of subluminous B stars can also be identified as stripped helium-cores of red giants. They form an extension of the ELM sequence to higher temperatures. Hence low mass white dwarfs of helium cores and sdB stars in binaries are close relatives in terms of stellar evolution.

  8. Comparing the white dwarf cooling sequences in 47 Tuc and NGC 6397

    Energy Technology Data Exchange (ETDEWEB)

    Richer, Harvey B.; Goldsbury, Ryan; Heyl, Jeremy [Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1 (Canada); Hurley, Jarrod [Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122 (Australia); Dotter, Aaron [Research School of Astronomy and Astrophysics, Australian National University, Canberra (Australia); Kalirai, Jason S. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Woodley, Kristin A. [Department of Astronomy and Astrophysics, University of California at Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 (United States); Fahlman, Gregory G. [National Research Council, Herzberg Institute of Astrophysics, Victoria, BC, V9E 2E7 (Canada); Rich, R. Michael [Division of Astronomy and Astrophysics, University of California at Los Angeles, Los Angeles, CA 90095 (United States); Shara, Michael M., E-mail: richer@astro.ubc.ca, E-mail: rgoldsb@phas.ubc.ca, E-mail: heyl@phas.ubc.ca, E-mail: jhurley@swin.edu.au, E-mail: dotter@mso.anu.edu.au, E-mail: jkalirai@stsci.edu, E-mail: kwoodley@ucolick.org, E-mail: greg.fahlman@nrc-cnrc.gc.ca, E-mail: rmr@astro.ucla.edu, E-mail: mshara@amnh.org [Department of Astrophysics, American Museum of Natural History, Central Park West and 79th Street, New York, NY 10024 (United States)

    2013-12-01

    Using deep Hubble Space Telescope imaging, color-magnitude diagrams are constructed for the globular clusters 47 Tuc and NGC 6397. As expected, because of its lower metal abundance, the main sequence of NGC 6397 lies well to the blue of that of 47 Tuc. A comparison of the white dwarf cooling sequences of the two clusters, however, demonstrates that these sequences are indistinguishable over most of their loci—a consequence of the settling out of heavy elements in the dense white dwarf atmosphere and the near equality of their masses. Lower quality data on M4 continues this trend to a third cluster whose metallicity is intermediate between these two. While the path of the white dwarfs in the color-magnitude diagram is nearly identical in 47 Tuc and NGC 6397, the numbers of white dwarfs along the path are not. This results from the relatively rapid relaxation in NGC 6397 compared to 47 Tuc and provides a cautionary note that simply counting objects in star clusters in random locations as a method of testing stellar evolutionary theory is likely dangerous unless dynamical considerations are included.

  9. Multidimensional simulations of the accretion-induced collapse of white dwarfs to neutron stars

    NARCIS (Netherlands)

    Dessart, L.|info:eu-repo/dai/nl/304846074; Burrows, A.; Ott, C.D.; Livne, E.; Yoon, S.C.|info:eu-repo/dai/nl/266576753; Langer, N.|info:eu-repo/dai/nl/304829498

    2006-01-01

    We present 2.5-dimensional radiation-hydrodynamics simulations of the accretion-induced collapse (AIC) of white dwarfs, starting from two-dimensional rotational equilibrium configurations, thereby accounting consistently for the effects of rotation prior to and after core collapse.We focus our study

  10. VizieR Online Data Catalog: SDSS magnetic white dwarf stars (Kepler+, 2013)

    Science.gov (United States)

    Kepler, S. O.; Pelisoli, I.; Jordan, S.; Kleinman, S. J.; Koester, D.; Kulebi, B.; Pecanha, V.; Castanheira, B. G.; Nitta, A.; Costa, J. E. S.; Winget, D. E.; Kanaan, A.; Fraga, L.

    2017-07-01

    We classified more than 48000 spectra, selected as possible white dwarf stars from the SDSS DR7 by their colours, through visual inspection and detected Zeeman splittings in 521 DA stars. Table 1 shows the estimated values for the magnetic fields for the 521 spectra we measured. (1 data file).

  11. Consequence of total lepton number violation in strongly magnetized iron white dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Belyaev, V.B. [Bogolyubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Dubna 141980 (Russian Federation); Ricci, P. [Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, I-50019 Sesto Fiorentino (Firenze) (Italy); Šimkovic, F. [Department of Nuclear Physics and Biophysics, Comenius University, Mlynská dolina F1, SK-842 15, Bratislava (Slovakia); Bogolyubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Dubna 141980 (Russian Federation); Adam, J.; Tater, M. [Institute of Nuclear Physics ASCR, CZ-250 68 Řež (Czech Republic); Truhlík, E., E-mail: truhlik@ujf.cas.cz [Institute of Nuclear Physics ASCR, CZ-250 68 Řež (Czech Republic)

    2015-05-15

    The influence of a neutrinoless electron to positron conversion on a cooling of strongly magnetized iron white dwarfs is studied. It is shown that they can be good candidates for soft gamma-ray repeaters and anomalous X-ray pulsars.

  12. Hydrodynamic Simulations of Classical Novae: Accretion onto CO White Dwarfs as SN Ia Progenitors

    Science.gov (United States)

    Starrfield, Sumner; Bose, Maitrayee; Iliadis, Christian; Hix, William R.; José, Jordi; Hernanz, Margarita

    2017-06-01

    We have continued our studies of accretion onto white dwarfs by following the evolution of thermonuclear runaways on Carbon Oxygen (CO) white dwarfs. We have varied the mass of the white dwarf and the composition of the accreted material but chosen to keep the mass accretion rate at 2 x 10^{-10} solar masses per year to obtain the largest amount of accreted material possible with rates near to those observed. We assume either 25% core material or 50% core material has been mixed into the accreting material prior to the explosion. We use our 1D, lagrangian, hydrodynamic code: NOVA. We will report on the results of these simulations and compare the ejecta abundances to those measured in pre-solar grains that are thought to arise from classical nova explosions. These results will also be compared to recent results with SHIVA (Jose and Hernanz). We find that in all cases and for all white dwarf masses that less mass is ejected than accreted and, therefore, the white dwarf is growing in mass as a result of the accretion and resulting explosion.This work was supported in part by NASA under the Astrophysics Theory Program grant 14-ATP14-0007 and the U.S. DOE under Contract No. DE-FG02- 97ER41041. SS acknowledges partial support from NASA, NSF, and HST grants to ASU and WRH is supported by the U.S. Department of Energy, Office of Nuclear Physics. The results reported herein benefitted from collaborations and/or information exchange within NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate.

  13. The importance of Urca-process cooling in accreting ONe white dwarfs

    Science.gov (United States)

    Schwab, Josiah; Bildsten, Lars; Quataert, Eliot

    2017-12-01

    We study the evolution of accreting oxygen-neon (ONe) white dwarfs (WDs), with a particular emphasis on the effects of the presence of the carbon-burning products 23Na and 25Mg. These isotopes lead to substantial cooling of the WD via the 25Mg-25Na, 23Na-23Ne and 25Na-25Ne Urca pairs. We derive an analytic formula for the peak Urca-process cooling rate and use it to obtain a simple expression for the temperature to which the Urca process cools the WD. Our estimates are equally applicable to accreting carbon-oxygen WDs. We use the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code to evolve a suite of models that confirm these analytic results and demonstrate that Urca-process cooling substantially modifies the thermal evolution of accreting ONe WDs. Most importantly, we show that MESA models with lower temperatures at the onset of the 24Mg and 24Na electron captures develop convectively unstable regions, even when using the Ledoux criterion. We discuss the difficulties that we encounter in modelling these convective regions and outline the potential effects of this convection on the subsequent WD evolution. For models in which we do not allow convection to operate, we find that oxygen ignites around a density of log(ρc/g cm-3) ≈ 9.95, very similar to the value without Urca cooling. Nonetheless, the inclusion of the effects of Urca-process cooling is an important step in producing progenitor models with more realistic temperature and composition profiles which are needed for the evolution of the subsequent oxygen deflagration and hence for studies of the signature of accretion-induced collapse.

  14. SN 2010LP—A TYPE IA SUPERNOVA FROM A VIOLENT MERGER OF TWO CARBON-OXYGEN WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Kromer, M.; Taubenberger, S.; Seitenzahl, I. R.; Hillebrandt, W. [Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching (Germany); Pakmor, R. [Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, D-69118 Heidelberg (Germany); Pignata, G. [Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago (Chile); Fink, M.; Röpke, F. K. [Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Str. 31, D-97074 Würzburg (Germany); Sim, S. A. [Astrophysics Research Centre, School of Mathematics and Physics, Queen' s University Belfast, Belfast BT7 1NN (United Kingdom)

    2013-11-20

    SN 2010lp is a subluminous Type Ia supernova (SN Ia) with slowly evolving lightcurves. Moreover, it is the only subluminous SN Ia observed so far that shows narrow emission lines of [O I] in late-time spectra, indicating unburned oxygen close to the center of the ejecta. Most explosion models for SNe Ia cannot explain the narrow [O I] emission. Here, we present hydrodynamic explosion and radiative transfer calculations showing that the violent merger of two carbon-oxygen white dwarfs of 0.9 and 0.76 M {sub ☉} adequately reproduces the early-time observables of SN 2010lp. Moreover, our model predicts oxygen close to the center of the explosion ejecta, a pre-requisite for narrow [O I] emission in nebular spectra as observed in SN 2010lp.

  15. X-ray reflection from cold white dwarfs in magnetic cataclysmic variables

    Science.gov (United States)

    Hayashi, Takayuki; Kitaguchi, Takao; Ishida, Manabu

    2018-02-01

    We model X-ray reflection from white dwarfs (WDs) in magnetic cataclysmic variables (mCVs) using a Monte Carlo simulation. A point source with a power-law spectrum or a realistic post-shock accretion column (PSAC) source irradiates a cool and spherical WD. The PSAC source emits thermal spectra of various temperatures stratified along the column according to the PSAC model. In the point-source simulation, we confirm the following: a source harder and nearer to the WD enhances the reflection; higher iron abundance enhances the equivalent widths (EWs) of fluorescent iron Kα1, 2 lines and their Compton shoulder, and increases the cut-off energy of a Compton hump; significant reflection appears from an area that is more than 90° apart from the position right under the point X-ray source because of the WD curvature. The PSAC simulation reveals the following: a more massive WD basically enhances the intensities of the fluorescent iron Kα1, 2 lines and the Compton hump, except for some specific accretion rate, because the more massive WD makes a hotter PSAC from which higher-energy X-rays are preferentially emitted; a larger specific accretion rate monotonically enhances the reflection because it makes a hotter and shorter PSAC; the intrinsic thermal component hardens by occultation of the cool base of the PSAC by the WD. We quantitatively estimate the influences of the parameters on the EWs and the Compton hump with both types of source. We also calculate X-ray modulation profiles brought about by the WD spin. These depend on the angles of the spin axis from the line of sight and from the PSAC, and on whether the two PSACs can be seen. The reflection spectral model and the modulation model involve the fluorescent lines and the Compton hump and can directly be compared to the data, which allows us to estimate these geometrical parameters with unprecedented accuracy.

  16. The Next Generation Virgo Cluster Survey. XXVIII. Characterization of the Galactic White Dwarf Population

    Energy Technology Data Exchange (ETDEWEB)

    Fantin, Nicholas J. [Department of Physics and Astronomy,University of Victoria, Victoria, BC, V8P 1A1 (Canada); Côté, Patrick; Gwyn, S. D. J.; Ferrarese, Laura; McConnachie, Alan [National Research Council of Canada, Herzberg Astronomy and Astrophysics Program, 5071 W. Saanich Road, Victoria, BC, V9E 2E7 (Canada); Hanes, David A. [Queen’s University, Department of Physics, Engineering Physics and Astronomy, Kingston, Ontario (Canada); Bianchi, Luciana [Department of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218 (United States); Cuillandre, Jean-Charles [CEA/IRFU/SAp, Laboratoire AIM Paris-Saclay, CNRS/INSU, Université Paris Diderot, Observatoire de Paris, PSL Research University, F-91191 Gif-sur-Yvette Cedex (France); Starkenburg, Else, E-mail: nfantin@uvic.ca [Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam (Germany)

    2017-07-01

    We use three different techniques to identify hundreds of white dwarf (WD) candidates in the Next Generation Virgo Cluster Survey (NGVS) based on photometry from the NGVS and GUViCS, and proper motions derived from the NGVS and the Sloan Digital Sky Survey (SDSS). Photometric distances for these candidates are calculated using theoretical color–absolute magnitude relations, while effective temperatures are measured by fitting their spectral energy distributions. Disk and halo WD candidates are separated using a tangential velocity cut of 200 km s{sup −1} in a reduced proper motion diagram, which leads to a sample of six halo WD candidates. Cooling ages, calculated for an assumed WD mass of 0.6 M {sub ⊙}, range between 60 Myr and 6 Gyr, although these estimates depend sensitively on the adopted mass. Luminosity functions for the disk and halo subsamples are constructed and compared to previous results from the SDSS and SuperCOSMOS survey. We compute a number density of (2.81 ± 0.52) × 10{sup −3} pc{sup −3} for the disk WD population—consistent with previous measurements. We find (7.85 ± 4.55) × 10{sup −6} pc{sup −3} for the halo, or 0.3% of the disk. Observed stellar counts are also compared to predictions made by the TRILEGAL and Besançon stellar population synthesis models. The comparison suggests that the TRILEGAL model overpredicts the total number of WDs. The WD counts predicted by the Besançon model agree with the observations, although a discrepancy arises when comparing the predicted and observed halo WD populations; the difference is likely due to the WD masses in the adopted model halo.

  17. Timing of a young mildly recycled pulsar with a massive white dwarf companion

    Science.gov (United States)

    Lazarus, P.; Tauris, T. M.; Knispel, B.; Freire, P. C. C.; Deneva, J. S.; Kaspi, V. M.; Allen, B.; Bogdanov, S.; Chatterjee, S.; Stairs, I. H.; Zhu, W. W.

    2014-01-01

    We report on timing observations of the recently discovered binary pulsar PSR J1952+2630 using the Arecibo Observatory. The mildly recycled 20.7-ms pulsar is in a 9.4-h orbit with a massive, MWD > 0.93 M⊙, white dwarf (WD) companion. We present, for the first time, a phase-coherent timing solution, with precise spin, astrometric and Keplerian orbital parameters. This shows that the characteristic age of PSR J1952+2630 is 77 Myr, younger by one order of magnitude than any other recycled pulsar-massive WD system. We derive an upper limit on the true age of the system of 150 Myr. We investigate the formation of PSR J1952+2630 using detailed modelling of the mass-transfer process from a naked helium star on to the neutron star following a common-envelope phase (Case BB Roche lobe overflow). From our modelling of the progenitor system, we constrain the accretion efficiency of the neutron star, which suggests a value between 100 and 300 per cent of the Eddington accretion limit. We present numerical models of the chemical structure of a possible oxygen-neon-magnesium WD companion. Furthermore, we calculate the past and the future spin evolution of PSR J1952+2630, until the system merges in about 3.4 Gyr due to gravitational wave emission. Although we detect no relativistic effects in our timing analysis, we show that several such effects will become measurable with continued observations over the next 10 yr; thus, PSR J1952+2630 has potential as a testbed for gravitational theories.

  18. Making the Standard Candle: A study of how the progenitor white dwarf modulates the peak luminosity of type Ia supernovae

    Energy Technology Data Exchange (ETDEWEB)

    Brown, Edward F [Michigan State University

    2010-01-21

    The goals of the proposed research as stated in the proposal were to: • Build a suite of one-dimensional initial models of different metallicities and central densities. • Using the improved flame capturing scheme, simulate the explosion of a white dwarf with embedded Lagrangian tracer particles, and post-process the thermal histories of the tracers to reconstruct the nucleosynthesis of the explosion. • Survey the effects of a changing progenitor metallicity on the isotopic yields. Of particular interest is 1) whether the linear relation between the mass of 56Ni synthesized and the pro- genitor metallicity is moderated by the effect of electron captures in the core; and 2) how a varying central density alters the relation between metallicity and 56Ni mass. • Using these results, examine how the observed metallicity distribution would affect the brightness distribution of SNe Ia and the isotopic ratios about the Fe-peak.

  19. KOI-256's Magnetic Activity Under the Influence of the White Dwarf

    Science.gov (United States)

    Yoldaş, Ezgi; Dal, Hasan Ali

    2017-11-01

    We present the findings about chromospheric activity nature of KOI-256 obtained from the Kepler Mission data. First, it was found that there are some sinusoidal variations out-of-eclipses due to cool spot activity. The sinusoidal variations modelled by the spotmodel program indicate that the active component has two different active regions. Their longitudinal variation revealed that one of them has a migration period of 3.95 yrs, while the other has a migration period of 8.37 yrs. Second, 225 flares were detected from the short cadence data in total. The parameters, such as increase (T r) and decay (T d) times, total flare time (T t), equivalent durations (P), were calculated for each flare. The distribution of equivalent durations versus total flare times in logarithmic scale is modelled to find flare activity level. The Plateau value known as the saturation level of the active component was calculated to be 2.3121 ± 0.0964 s, and the Half-life value, which is required flare total time to reach the saturation, was computed to be 2233.6 s. In addition, the frequency of N 1, which is the number of flares per an hour in the system, was found to be 0.05087 h-1, while the flare frequency N 2 that the flare-equivalent duration emitting per an hour was found to be 0.00051. Contrary to the spot activity, it has been found that the flares are in tends to appear at specific phases due to the white dwarf component.

  20. HS 2231+2441: an HW Vir system composed of a low-mass white dwarf and a brown dwarf★

    Science.gov (United States)

    Almeida, L. A.; Damineli, A.; Rodrigues, C. V.; Pereira, M. G.; Jablonski, F.

    2017-12-01

    HW Vir systems are rare evolved eclipsing binaries composed of a hot compact star and a low-mass main sequence star in a close orbit. These systems provide a direct way to measure the fundamental properties, e.g. masses and radii, of their components, hence they are crucial in studying the formation of subdwarf B stars and low-mass white dwarfs, the common-envelope phase and the pre-phase of cataclysmic variables. Here, we present a detailed study of HS 2231+2441, an HW Vir type system, by analysing BVRCIC photometry and phase-resolved optical spectroscopy. The spectra of this system, which are dominated by the primary component features, were fitted using non-local thermodynamic equilibrium models providing an effective temperature Teff = 28 500 ± 500 K, surface gravity log g = 5.40 ± 0.05 cm s-2 and helium abundance log (n(He)/n(H)) = -2.52 ± 0.07. The geometrical orbit and physical parameters were derived by simultaneously modelling the photometric and spectroscopic data using the Wilson-Devinney code. We derive two possible solutions for HS 2231+2441 that provide the component masses: M1 = 0.19 M⊙ and M2 = 0.036 M⊙ or M1 = 0.288 M⊙ and M2 = 0.046 M⊙. Considering the possible evolutionary channels for forming a compact hot star, the primary of HS 2231+2441 probably evolved through the red-giant branch scenario and does not have a helium-burning core, which is consistent with a low-mass white dwarf. Both solutions are consistent with a brown dwarf as the secondary.

  1. THE CRITICAL MASS RATIO OF DOUBLE WHITE DWARF BINARIES FOR VIOLENT MERGER-INDUCED TYPE IA SUPERNOVA EXPLOSIONS

    Energy Technology Data Exchange (ETDEWEB)

    Sato, Yushi [Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan); Nakasato, Naohito [Department of Computer Science and Engineering, University of Aizu, Tsuruga Ikki-machi Aizu-Wakamatsu, Fukushima 965-8580 (Japan); Tanikawa, Ataru; Hachisu, Izumi [Department of Earth Science and Astronomy, College of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902 (Japan); Nomoto, Ken’ichi [Kavli Institute for the Physics and Mathematics of the universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583 (Japan); Maeda, Keiichi, E-mail: sato@ea.c.u-tokyo.ac.jp [Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502 (Japan)

    2016-04-10

    Mergers of carbon–oxygen (CO) white dwarfs (WDs) are considered to be one of the potential progenitors of type Ia supernovae (SNe Ia). Recent hydrodynamical simulations showed that the less massive (secondary) WD violently accretes onto the more massive (primary) one, carbon detonation occurs, the detonation wave propagates through the primary, and the primary finally explodes as a sub-Chandrasekhar mass SN Ia. Such an explosion mechanism is called the violent merger scenario. Based on the smoothed particle hydrodynamics simulations of merging CO WDs, we derived a critical mass ratio (q{sub cr}) leading to the violent merger scenario that is more stringent than previous results. We conclude that this difference mainly comes from the differences in the initial condition of whether or not the WDs are synchronously spinning. Using our new results, we estimated the brightness distribution of SNe Ia in the violent merger scenario and compared it with previous studies. We found that our new q{sub cr} does not significantly affect the brightness distribution. We present the direct outcome immediately following CO WD mergers for various primary masses and mass ratios. We also discussed the final fate of the central system of the bipolar planetary nebula Henize 2-428, which was recently suggested to be a double CO WD system whose total mass exceeds the Chandrasekhar-limiting mass, merging within the Hubble time. Even considering the uncertainties in the proposed binary parameters, we concluded that the final fate of this system is almost certainly a sub-Chandrasekhar mass SN Ia in the violent merger scenario.

  2. An Eccentric Binary Millisecond Pulsar with a Helium White Dwarf Companion in the Galactic field

    Science.gov (United States)

    Antoniadis, John; Kaplan, David L.; Stovall, Kevin; Freire, Paulo C. C.; Deneva, Julia S.; Koester, Detlev; Jenet, Fredrick; Martinez, Jose G.

    2016-10-01

    Low-mass white dwarfs (LMWDs) are believed to be exclusive products of binary evolution, as the universe is not old enough to produce them from single stars. Because of the strong tidal forces operating during the binary interaction phase, the remnant systems observed today are expected to have negligible eccentricities. Here, we report on the first unambiguous identification of an LMWD in an eccentric (e = 0.13) orbit around the millisecond pulsar PSR J2234+0511, which directly contradicts this picture. We use our spectra and radio-timing solution (derived elsewhere) to infer the WD temperature ({T}{{eff}}=8600+/- 190 K), and peculiar systemic velocity relative to the local standard of rest (≃ 31 km s-1). We also place model-independent constraints on the WD radius ({R}{{WD}}={0.024}-0.002+0.004 {R}⊙ ) and surface gravity ({log} g={7.11}-0.16+0.08 dex). The WD and kinematic properties are consistent with the expectations for low-mass X-ray binary evolution and disfavor a dynamic three-body formation channel. In the case of the high eccentricity being the result of a spontaneous phase transition, we infer a mass of ˜1.60 M ⊙ for the pulsar progenitor, which is too low for the quark-nova mechanism proposed by Jiang et al., and too high for the scenario of Freire & Tauris, in which a WD collapses into a neutron star via a rotationally delayed accretion-induced collapse. We find that eccentricity pumping via interaction with a circumbinary disk is consistent with our inferred parameters. Finally, we report tentative evidence for pulsations that, if confirmed, would transform the star into an unprecedented laboratory for WD physics.

  3. Is the DO-type White Dwarf RE 0503-289 a Unique Object?

    Science.gov (United States)

    Hoyer, D.; Rauch, T.; Werner, K.

    2017-03-01

    Trans-iron group elements (atomic numbers Z > 28) are produced by s-process nucleosynthesis on the asymptotic giant branch (AGB). Flash induced mixing during a late thermal pulse (LTP) should strongly enrich their photospheric abundances. While they are so far not found in the hydrogen-deficient post-LTP [WC] -type Wolf-Rayet stars and PG 1159-type stars, they were identified in a small number of DO-type white dwarfs (WDs). Abundance determinations have shown that these are indeed up to about 35000 times solar. The detection of Ge (Z = 32) in some DA-type WDs yields about solar abundances. Thus, an LTP scenario may be a pre-requisite for very high trans-iron group element abundances. Recently, a very high number of lines of trans-iron group elements were discovered in the ultraviolet (UV) spectrum of the DO-type WD RE 0503-289. Abundance analyses by means of non-local thermodynamic equilibrium (NLTE) model-atmosphere techniques have shown that these elements are strongly overabundant (up to about 4 dex) compared to solar values. To prove the suggestion that an LTP is necessary to yield such overabundances, we performed UV spectroscopy with HST/COS of the PG 1159-type star PG 1707+427 and of two DO-type stars, namely PG 0109+111 and WD 0111+002. These three stars are located in the Hertzsprung-Russell diagram close before and after the so-called PG 1159 wind limit. At about this limit, PG 1159-type stars evolve into DO-type WDs. These observations will show whether the high abundances of trans-iron group elements are a common phenomenon and establish constraints for AGB nucleosynthesis and post-LTP evolution. To clarify this, we performed an advanced NLTE spectral analysis and present our preliminary results.

  4. Mass retention efficiencies of He accretion onto carbon-oxygen white dwarfs and type Ia supernovae

    Science.gov (United States)

    Wu, C.; Wang, B.; Liu, D.; Han, Z.

    2017-07-01

    Context. Type Ia supernovae (SNe Ia) play a crucial role in studying cosmology and galactic chemical evolution. They are thought to be thermonuclear explosions of carbon-oxygen white dwarfs (CO WDs) when their masses reach the Chandrasekar mass limit in binaries. Previous studies have suggested that He novae may be progenitor candidates of SNe Ia. However, the mass retention efficiencies during He nova outbursts are still uncertain. Aims: In this article, we aim to study the mass retention efficiencies of He nova outbursts and to investigate whether SNe Ia can be produced through He nova outbursts. Methods: Using the stellar evolution code Modules for Experiments in Stellar Astrophysics, we simulated a series of multicycle He-layer flashes, in which the initial WD masses range from 0.7 to 1.35 M⊙ with various accretion rates. Results: We obtained the mass retention efficiencies of He nova outbursts for various initial WD masses, which can be used in the binary population synthesis studies. In our simulations, He nova outbursts can increase the mass of the WD to the Chandrasekar mass limit and the explosive carbon burning can be triggered in the center of the WD; this suggests that He nova outbursts can produce SNe Ia. Meanwhile, the mass retention efficiencies in the present work are lower than those of previous studies, which leads to a lower birthrates of SNe Ia through the WD + He star channel. Furthermore, we obtained the elemental abundances distribution at the moment of explosive carbon burning, which can be used as the initial input parameters in studying explosion models of SNe Ia.

  5. The binary fraction, separation distribution, and merger rate of white dwarfs from SPY

    Science.gov (United States)

    Maoz, Dan; Hallakoun, Na'ama

    2017-05-01

    From a sample of spectra of 439 white dwarfs (WDs) from the ESO-VLT Supernova-Ia Progenitor Survey (SPY), we measure the maximal changes in radial velocity (ΔRVmax) between epochs (generally two epochs, separated by up to 470 d), and model the observed ΔRVmax statistics via Monte Carlo simulations, to constrain the population characteristics of double WDs (DWDs). The DWD fraction among WDs is fbin = 0.10 ± 0.02 (1σ, random) +0.02 (systematic), in the separation range ≲4 au within which the data are sensitive to binarity. Assuming the distribution of binary separation, a, is a power law, dN/da ∝ aα, at the end of the last common-envelope phase and the start of solely gravitational-wave-driven binary evolution, the constraint by the data is α = -1.3 ± 0.2 (1σ) ±0.2 (systematic). If these parameters extend to small separations, the implied Galactic WD merger rate per unit stellar mass is Rmerge = (1-80) × 10-13 yr^{-1} M_{⊙}^{-1} (2σ), with a likelihood-weighted mean of Rmerge = (7 ± 2) × 10-13 yr^{-1} M_{⊙}^{-1} (1σ). The Milky Way's specific Type Ia supernova (SN Ia) rate is likely RIa ≈ 1.1 × 10-13 yr^{-1} M_{⊙}^{-1} and therefore, in terms of rates, a possibly small fraction of all merging DWDs (e.g. those with massive-enough primary WDs) could suffice to produce most or all SNe Ia.

  6. Compact Objects in Astrophysics White Dwarfs, Neutron Stars and Black Holes

    CERN Document Server

    Camenzind, Max

    2007-01-01

    Compact objects are an important class of astronomical objects in current research. Supermassive black holes play an important role in the understanding of the formation of galaxies in the early Universe. Old white dwarfs are nowadays used to calibrate the age of the Universe. Mergers of neutron stars and black holes are the sources of intense gravitational waves which will be measured in the next ten years by gravitational wave detectors. Camenzind's Compact Objects in Astrophysics gives a comprehensive introduction and up-to-date overview about the physical processes behind these objects, covering the field from the beginning to most recent results, including all relevant observations. After a presentation of the taxonomy of compact objects, the basic principles of general relativity are given. The author then discusses in detail the physics and observations of white dwarfs and neutron stars (including the most recent equations of state for neutron star matter), the gravitational field of rapidly rotating c...

  7. A faint type of supernova from a white dwarf with a helium-rich companion

    OpenAIRE

    Perets, H.B.; Gal-Yam, A; Mazzali, P.; Arnett, D.; Kagan, D.; Filippenko, A. V.; Li, W.; Arcavi, I.; Cenko, S. B.; Fox, D. B.; Leonard, D. C.; Moon, D. -S.; Sand, D. J.; Soderberg, A. M.; Foley, R. J.

    2009-01-01

    Supernovae (SNe) are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as as type Ib/c and II SNe, and are associated with young stellar populations. A type Ia SN is thought to arise from the thermonuclear detonation of a white dwarf star composed mainly of carbon and oxygen, whose mass approaches the Chandrasekhar limit. ...

  8. New Insights into the Problem of the Surface Gravity Distribution of Cool DA White Dwarfs

    Science.gov (United States)

    Tremblay, P.-E.; Bergeron, P.; Kalirai, J. S.; Gianninas, A.

    2010-04-01

    We review at length the longstanding problem in the spectroscopic analysis of cool hydrogen-line (DA) white dwarfs (T effTremblay & Bergeron and find that the gravity distribution of cool objects remains suspiciously high. Finally, we find that photometric masses are, on average, in agreement with expected values, and that the high-log g problem is so far unique to the spectroscopic approach.

  9. Stellar Archeology: What White Dwarf Stars Tell Us About the History of the Galaxy

    Directory of Open Access Journals (Sweden)

    Terry D. Oswalt

    2012-06-01

    Full Text Available White dwarf stars have played important roles in rather diverse areas of astrophysics. This paper outlines how these stellar remnants, especially those in widely separated “fragile” binaries, have provided unique leverage on difficult astrophysical problems such as the ages of stars, the structure and evolution of the Galaxy, the nature of dark matter and even the discovery of dark energy.

  10. Normal Type Ia Supernovae from Violent Mergers of White Dwarf Binaries

    Science.gov (United States)

    Pakmor, R.; Kromer, M.; Taubenberger, S.; Sim, S. A.; Röpke, F. K.; Hillebrandt, W.

    2012-03-01

    One of the most important questions regarding the progenitor systems of Type Ia supernovae (SNe Ia) is whether mergers of two white dwarfs can lead to explosions that reproduce observations of normal events. Here we present a fully three-dimensional simulation of a violent merger of two carbon-oxygen white dwarfs with masses of 0.9 M ⊙ and 1.1 M ⊙ combining very high resolution and exact initial conditions. A well-tested combination of codes is used to study the system. We start with the dynamical inspiral phase and follow the subsequent thermonuclear explosion under the plausible assumption that a detonation forms in the process of merging. We then perform detailed nucleosynthesis calculations and radiative transfer simulations to predict synthetic observables from the homologously expanding supernova ejecta. We find that synthetic color light curves of our merger, which produces about 0.62 M ⊙ of 56Ni, show good agreement with those observed for normal SNe Ia in all wave bands from U to K. Line velocities in synthetic spectra around maximum light also agree well with observations. We conclude that violent mergers of massive white dwarfs can closely resemble normal SNe Ia. Therefore, depending on the number of such massive systems available these mergers may contribute at least a small fraction to the observed population of normal SNe Ia.

  11. NORMAL TYPE Ia SUPERNOVAE FROM VIOLENT MERGERS OF WHITE DWARF BINARIES

    Energy Technology Data Exchange (ETDEWEB)

    Pakmor, R. [Heidelberger Institut fuer Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg (Germany); Kromer, M.; Taubenberger, S.; Hillebrandt, W. [Max-Planck-Institut fuer Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching (Germany); Sim, S. A. [Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611 (Australia); Roepke, F. K. [Institut Fuer Theoretische Physik und Astrophysik, Universitaet Wuerzburg, Emil-Fischer-Str. 31, 97074 Wuerzburg (Germany)

    2012-03-15

    One of the most important questions regarding the progenitor systems of Type Ia supernovae (SNe Ia) is whether mergers of two white dwarfs can lead to explosions that reproduce observations of normal events. Here we present a fully three-dimensional simulation of a violent merger of two carbon-oxygen white dwarfs with masses of 0.9 M{sub Sun} and 1.1 M{sub Sun} combining very high resolution and exact initial conditions. A well-tested combination of codes is used to study the system. We start with the dynamical inspiral phase and follow the subsequent thermonuclear explosion under the plausible assumption that a detonation forms in the process of merging. We then perform detailed nucleosynthesis calculations and radiative transfer simulations to predict synthetic observables from the homologously expanding supernova ejecta. We find that synthetic color light curves of our merger, which produces about 0.62 M{sub Sun} of {sup 56}Ni, show good agreement with those observed for normal SNe Ia in all wave bands from U to K. Line velocities in synthetic spectra around maximum light also agree well with observations. We conclude that violent mergers of massive white dwarfs can closely resemble normal SNe Ia. Therefore, depending on the number of such massive systems available these mergers may contribute at least a small fraction to the observed population of normal SNe Ia.

  12. Direct formation of millisecond pulsars from rotationally delayed accretion-induced collapse of massive white dwarfs

    Science.gov (United States)

    Freire, Paulo C. C.; Tauris, Thomas M.

    2014-02-01

    Millisecond pulsars (MSPs) are believed to be old neutron stars, formed via Type Ib/c core-collapse supernovae, which have subsequently been spun up to high rotation rates via accretion from a companion star in a highly circularized low-mass X-ray binary. The recent discoveries of Galactic field binary MSPs in eccentric orbits, and mass functions compatible with that expected for helium white dwarf companions, PSR J2234+06 and PSR J1946+3417, therefore challenge this picture. Here, we present a hypothesis for producing this new class of systems, where the MSPs are formed directly from a rotationally delayed accretion-induced collapse of a super-Chandrasekhar mass white dwarf. We compute the orbital properties of the MSPs formed in such events and demonstrate that our hypothesis can reproduce the observed eccentricities, masses and orbital periods of the white dwarfs, as well as forecasting the pulsar masses and velocities. Finally, we compare this hypothesis to a triple-star scenario.

  13. Metal abundances in hot white dwarfs with signatures of a superionized wind

    Science.gov (United States)

    Werner, K.; Rauch, T.; Kruk, J. W.

    2018-01-01

    About a dozen hot white dwarfs with effective temperatures Teff = 65 000-120 000 K exhibit unusual absorption features in their optical spectra. These objects were tentatively identified as Rydberg lines of ultra-high excited metals in ionization stages v-x, indicating line formation in a dense environment with temperatures near 106 K. Since some features show blueward extensions, it was argued that they stem from a superionized wind. A unique assignment of the lines to particular elements is not possible, although they probably stem from C, N, O, and Ne. To further investigate this phenomenon, we analyzed the ultraviolet spectra available from only three stars of this group; that is, two helium-rich white dwarfs, HE 0504-2408 and HS 0713+3958 with spectral type DO, and a hydrogen-rich white dwarf, HS 2115+1148 with spectral type DAO. We identified light metals (C, N, O, Si, P, and S) with generally subsolar abundances and heavy elements from the iron group (Cr, Mn, Fe, Co, Ni) with solar or oversolar abundance. The abundance patterns are not unusual for hot WDs and can be interpreted as the result of gravitational settling and radiative levitation of elements. As to the origin of the ultra-high ionized metals lines, we discuss the possible presence of a multicomponent radiatively driven wind that is frictionally heated.

  14. Do weak magnetic fields prevent hydrogen from accreting onto metal-line white dwarf stars?

    Science.gov (United States)

    Friedrich, S.; Jordan, S.; Koester, D.

    2004-09-01

    The widely accepted assumption is that metals detected in the spectra of a few cool helium-rich white dwarfs cannot be of primordial origin and therefore must be accreted from the interstellar medium. However, the observed abundances of hydrogen are much too low to be compatible with the high accretion rates inferred from metal accretion. Hydrogen accretion is therefore suppressed compared to metal accretion. The hypothesis most widely discussed as cause for this ``hydrogen screening'' is the propeller mechanism: Metals are accreted in the form of grains onto a slowly rotating, weakly magnetized white dwarf, whereas ionized hydrogen is repelled at the Alfvén radius. We have obtained circular polarization spectra of the helium-rich white dwarfs GD 40 (WD0300-013) and L745-46A (WD0738-172) - which both show strong metal lines as well as hydrogen - in order to search for signatures of a weak magnetic field. The magnetic field strengths necessary for the propeller mechanism to work in these stars are at least 144 000 G and 3000 G, respectively. Whereas L745-46A might have a magnetic field of about -6900 G no magnetic field could be found with an upper limit for the field strength of 4000 G (with 99% confidence) for GD 40. Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO Programme 66.D-0541).

  15. Rapid Evolution of the Gaseous Exoplanetary Debris around the White Dwarf Star HE 1349–2305

    Science.gov (United States)

    Dennihy, E.; Clemens, J. C.; Dunlap, B. H.; Fanale, S. M.; Fuchs, J. T.; Hermes, J. J.

    2018-02-01

    Observations of heavy metal pollution in white dwarf stars indicate that metal-rich planetesimals are frequently scattered into star-grazing orbits, tidally disrupted, and accreted onto the white dwarf surface, offering direct insight into the dynamical evolution of post-main-sequence exoplanetary systems. Emission lines from the gaseous debris in the accretion disks of some of these systems show variations on timescales of decades, and have been interpreted as the general relativistic precession of a recently formed, elliptical disk. Here we present a comprehensive spectroscopic monitoring campaign of the calcium infrared triplet emission in one system, HE 1349–2305, which shows morphological emission profile variations suggestive of a precessing, asymmetric intensity pattern. The emission profiles are shown to vary on a timescale of one to two years, which is an order of magnitude shorter than what has been observed in other similar systems. We demonstrate that this timescale is likely incompatible with general relativistic precession, and consider alternative explanations for the rapid evolution, including the propagation of density waves within the gaseous debris. We conclude with recommendations for follow-up observations, and discuss how the rapid evolution of the gaseous debris in HE 1349–2305 could be leveraged to test theories of exoplanetary debris disk evolution around white dwarf stars.

  16. Polarimetric Evidence of the First White Dwarf Pulsar: The Binary System AR Scorpii

    Directory of Open Access Journals (Sweden)

    David A.H. Buckley

    2018-01-01

    Full Text Available The binary star AR Scorpii was recently discovered to exhibit high amplitude coherent variability across the electromagnetic spectrum (ultraviolet to radio at two closely spaced ∼2 min periods, attributed to the spin period of a white dwarf and the beat period. There is strong evidence (low X-ray luminosity, lack of flickering and absense of broad emission lines that AR Sco is a detached non-accreting system whose luminosity is dominated by the spin-down power of a white dwarf, due to magnetohydrodynamical (MHD interactions with its M5 companion. Optical polarimetry has revealed highly pulsed linear polarization on the same periods, reaching a maximum of 40%, consistent with a pulsar-like dipole, with the Stokes Q and U variations reminiscent of the Crab pulsar. These observations, coupled with the spectral energy distribution (SED which is dominated by non-thermal emission, characteristic of synchrotron emission, support the notion that a strongly magnetic (∼200 MG white dwarf is behaving like a pulsar, whose magnetic field interacts with the secondary star’s photosphere and magnetosphere. Radio synchrotron emission is produced from the pumping action of the white dwarf’s magnetic field on coronal loops from the M-star companion, while emission at high frequencies (UV/optical/X-ray comes from the particle wind, driven by large electric potential, again reminiscent of processes seen in neutron star pulsars.

  17. Transit detection limits for sub-stellar and terrestrial companions to white dwarfs

    Science.gov (United States)

    Faedi, F.; West, R.; Burleigh, M. R.; Goad, M. R.; Hebb, L.

    2009-06-01

    The SuperWASP project is a ground-based ultra wide angle search for extra-solar planetary transits that has successfully detected 15 previously unknown planets in the last two years. We have used SuperWASP photometric data to investigate the transit characteristics of and detection limits for brown dwarfs, gas giants and terrestrial companions in orbit around white dwarfs. The relatively small size of a white dwarf host star (approximately 1 Earth radius), implies that any sub-stellar or gas giant companion will completely eclipse it, while terrestrial bodies smaller than the Moon will produce relatively large (> 1%) transits, detectable in good S/N light-curves. We performed extensive simulations using SuperWASP photometric data and we found that for Gaussian random noise we are sensitive to companions as small as the Moon. Our sensitivity drops in the presence of co-variant noise structure, nevertheless Earth-size bodies remain readily detectable in relatively low S/N data. We searched for eclipses and transit signals in a sample of 174 WASP targets, resulting from a cross-correlation of the McCook & Sion catalogue and the SuperWASP data archive. This study found no evidence for sub-stellar or planetary companions in close orbits around our sample of white dwarfs.

  18. A WHITE DWARF MERGER AS PROGENITOR OF THE ANOMALOUS X-RAY PULSAR 4U 0142+61?

    Energy Technology Data Exchange (ETDEWEB)

    Rueda, J. A.; Boshkayev, K.; Izzo, L.; Ruffini, R. [Dipartimento di Fisica and ICRA, Sapienza Universita di Roma, P.le Aldo Moro 5, I-00185 Rome (Italy); Loren-Aguilar, P. [School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL (United Kingdom); Kuelebi, B. [Institut de Ciencies de l' Espai (CSIC), Facultat de Ciencies, Campus UAB, Torre C5-parell, E-08193 Bellaterra (Spain); Aznar-Siguan, G.; Garcia-Berro, E., E-mail: jorge.rueda@icra.it, E-mail: enrique.garcia-berro@upc.edu [Institute for Space Studies of Catalonia, c/Gran Capita 2-4, Edif. Nexus 104, E-08034 Barcelona (Spain)

    2013-08-01

    It has been recently proposed that massive, fast-rotating, highly magnetized white dwarfs could describe the observational properties of some of soft gamma-ray repeaters and anomalous X-ray pulsars (AXPs). Moreover, it has also been shown that high-field magnetic white dwarfs can be the outcome of white dwarf binary mergers. The products of these mergers consist of a hot central white dwarf surrounded by a rapidly rotating disk. Here we show that the merger of a double degenerate system can explain the characteristics of the peculiar AXP 4U 0142+61. This scenario accounts for the observed infrared excess. We also show that the observed properties of 4U 0142+6 are consistent with an approximately 1.2 M{sub Sun} white dwarf, remnant of the coalescence of an original system made of two white dwarfs of masses 0.6 M{sub Sun} and 1.0 M{sub Sun }. Finally, we infer a post-merging age {tau}{sub WD} Almost-Equal-To 64 kyr and a magnetic field B Almost-Equal-To 2 Multiplication-Sign 10{sup 8} G. Evidence for such a magnetic field may come from the possible detection of the electron cyclotron absorption feature observed between the B and V bands at Almost-Equal-To 10{sup 15} Hz in the spectrum of 4U 0142+61.

  19. The Carnegie Supernova Project. I. Third Photometry Data Release of Low-redshift Type Ia Supernovae and Other White Dwarf Explosions

    DEFF Research Database (Denmark)

    Krisciunas, Kevin; Contreras, Carlos; Burns, Christopher R.

    2017-01-01

    We present final natural-system optical (ugriBV) and near-infrared (YJH) photometry of 134 supernovae (SNe) with probable white dwarf progenitors that were observed in 2004-2009 as part of the first stage of the Carnegie Supernova Project (CSP-I). The sample consists of 123 Type. Ia SNe, 5 Type...... optical extinction coefficients and color terms are derived and demonstrated to be stable during the five CSP-I observing campaigns. Measurements of the CSP-I near-infrared bandpasses are also described, and near-infrared color terms are estimated through synthetic photometry of stellar atmosphere models...

  20. Stellar Laboratories . [VI. New Mo IV - VII Oscillator Strengths and the Molybdenum Abundance in the Hot White Dwarfs G191-B2B and RE 0503-289

    Science.gov (United States)

    Rauch, T.; Quinet, T.; Hoyer, D.; Werner, K.; Demleitner, M.; Kruk, J. W.

    2016-01-01

    For the spectral analysis of high-resolution and high signal-to-noise (SN) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims: To identify molybdenum lines in the ultraviolet (UV) spectra of the DA-type white dwarf G191B2B and the DO-type white dwarf RE 0503289 and, to determine their photospheric Mo abundances, reliable Mo iv-vii oscillator strengths are used. Methods: We newly calculated Mo iv-vii oscillator strengths to consider their radiative and collisional bound-bound transitions indetail in our NLTE stellar-atmosphere models for the analysis of Mo lines exhibited in high-resolution and high SN UV observations of RE 0503289.Results. We identified 12 Mo v and nine Mo vi lines in the UV spectrum of RE 0503289 and measured a photospheric Mo abundance of 1.2 3.0 104(mass fraction, 22 500 56 400 times the solar abundance). In addition, from the As v and Sn iv resonance lines,we measured mass fractions of arsenic (0.51.3 105, about 300 1200 times solar) and tin (1.33.2 104, about 14 300 35 200 times solar). For G191B2B, upper limits were determined for the abundances of Mo (5.3 107, 100 times solar) and, in addition, for Kr (1.1106, 10 times solar) and Xe (1.7107, 10 times solar). The arsenic abundance was determined (2.35.9 107, about 21 53 times solar). A new, registered German Astrophysical Virtual Observatory (GAVO) service, TOSS, has been constructed to provide weighted oscillator strengths and transition probabilities.Conclusions. Reliable measurements and calculations of atomic data are a prerequisite for stellar-atmosphere modeling. Observed Mo v-vi line profiles in the UV spectrum of the white dwarf RE 0503289 were well reproduced with our newly calculated oscillator strengths. For the first time, this allowed the photospheric Mo abundance in a white dwarf to be determined.

  1. High Speed White Dwarf Asteroseismology with the Herty Hall Cluster

    Science.gov (United States)

    Gray, Aaron; Kim, A.

    2012-01-01

    Asteroseismology is the process of using observed oscillations of stars to infer their interior structure. In high speed asteroseismology, we complete that by quickly computing hundreds of thousands of models to match the observed period spectra. Each model on a single processor takes five to ten seconds to run. Therefore, we use a cluster of sixteen Dell Workstations with dual-core processors. The computers use the Ubuntu operating system and Apache Hadoop software to manage workloads.

  2. An Accreting White Dwarf near the Chandrasekhar Limit in the Andromeda Galaxy

    Science.gov (United States)

    Tang, Sumin; Bildsten, Lars; Wolf, William M.; Li, K. L.; Kong, Albert K. H.; Cao, Yi; Cenko, S. Bradley; De Cia, Annalisa; Kasliwal, Mansi M.; Kulkarni, Shrinivas R.; hide

    2014-01-01

    The iPTF (Intermediate Palomar Transient Factory) detection of the most recent outburst of the recurrent nova system RX J0045.4+4154 in the Andromeda Galaxy has enabled the unprecedented study of a massive (mass is greater than 1.3 solar masses) accreting white dwarf (WD). We detected this nova as part of the near daily iPTF monitoring of M31 to a depth of R (red band-pass filter) approximately equal to magnitude 21 and triggered optical photometry, spectroscopy and soft X-ray monitoring of the outburst. Peaking at an absolute magnitude of MR (red, mid-infrared band-pass filter) equals magnitude -6.6, and with a decay time of 1 magnitude per day, it is a faint and very fast nova. It shows optical emission lines of He/N and expansion velocities of 1900 to 2600 kilometers per second 1-4 days after the optical peak. The Swift monitoring of the X-ray evolution revealed a supersoft source (SSS) with kT (energy: Boltzmann constant times temperature) (sub eff (effective)) approximately equal to 90-110 electronvolts that appeared within 5 days after the optical peak, and lasted only 12 days. Most remarkably, this is not the first event from this system, rather it is a recurrent nova with a time between outbursts of approximately 1 year, the shortest known. Recurrent X-ray emission from this binary was detected by ROSAT in 1992 and 1993, and the source was well characterized as a mass greater than 1.3 solar masses WD SSS. Based on the observed recurrence time between different outbursts, the duration and effective temperature of the SS phase, MESA models of accreting WDs allow us to constrain the accretion rate to mass greater than 1.7x10 (sup -7) solar masses per year and WD mass greater than 1.30 solar masses. If the WD keeps 30 percent of the accreted material, it will take less than a million years to reach core densities high enough for carbon ignition (if made of C/O) or electron capture (if made of O/Ne) to end the binary evolution.

  3. CALIBRATION OF THE MIXING-LENGTH THEORY FOR CONVECTIVE WHITE DWARF ENVELOPES

    Energy Technology Data Exchange (ETDEWEB)

    Tremblay, P.-E. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Ludwig, H.-G. [Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, D-69117 Heidelberg (Germany); Freytag, B. [Department of Physics and Astronomy at Uppsala University, Regementsvägen 1, Box 516, SE-75120 Uppsala (Sweden); Fontaine, G.; Brassard, P. [Département de Physique, Université de Montréal, C. P. 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7 (Canada); Steffen, M., E-mail: tremblay@stsci.edu [Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, D-14482 Potsdam (Germany)

    2015-02-01

    A calibration of the mixing-length parameter in the local mixing-length theory (MLT) is presented for the lower part of the convection zone in pure-hydrogen-atmosphere white dwarfs. The parameterization is performed from a comparison of three-dimensional (3D) CO5BOLD simulations with a grid of one-dimensional (1D) envelopes with a varying mixing-length parameter. In many instances, the 3D simulations are restricted to the upper part of the convection zone. The hydrodynamical calculations suggest, in those cases, that the entropy of the upflows does not change significantly from the bottom of the convection zone to regions immediately below the photosphere. We rely on this asymptotic entropy value, characteristic of the deep and adiabatically stratified layers, to calibrate 1D envelopes. The calibration encompasses the convective hydrogen-line (DA) white dwarfs in the effective temperature range 6000 ≤ T {sub eff} (K) ≤15, 000 and the surface gravity range 7.0 ≤ log g ≤ 9.0. It is established that the local MLT is unable to reproduce simultaneously the thermodynamical, flux, and dynamical properties of the 3D simulations. We therefore propose three different parameterizations for these quantities. The resulting calibration can be applied to structure and envelope calculations, in particular for pulsation, chemical diffusion, and convective mixing studies. On the other hand, convection has no effect on the white dwarf cooling rates until there is a convective coupling with the degenerate core below T {sub eff} ∼ 5000 K. In this regime, the 1D structures are insensitive to the MLT parameterization and converge to the mean 3D results, hence they remain fully appropriate for age determinations.

  4. Search for Weak Magnetic Fields in DBZ and DBAZ White Dwarfs

    Science.gov (United States)

    Friedrich, S.; Jordan, S.; Koester, D.

    It is widely accepted that metals detected in the spectra of a few cool white dwarfs cannot be of primordial origin and therefore must be accreted from the interstellar medium. However the observed abundances of hydrogen in the atmospheres of these stars are much too low to be compatible with the high accretion rates inferred from metal accretion if solar abundances are assumed. It was therefore proposed that metals are accreted in the form of grains onto a slowly rotating, weakly magnetized white dwarf, whereas ionized hydrogen is repelled at the Alfven radius. In order to test this hypothesis we obtained circular polarization spectra of two metal line white dwarfs (GD40 and L745-46A) with the VLT-UT1 and FORS1 to search for such magnetic fields. Within the errors (±0.1% and ±0.3% for L745-46A and GD40, respectively) we could not find signatures of a magnetic field in the spectra of any of the two stars. If we exclude the possibility, that we are looking on the magnetic equator of a pure magnetic dipole, in which case the components of the magnetic field along the line of sight completely cancel and no circular polarization can be detected, we conclude, that the field strength of the magnetic field on both stars must be well below the 105 Gauss required by theory. We could confirm an Hα line in the flux spectrum of GD40, which was found by Greenstein & Liebert (ApJ 360, 662) and determine the hydrogen abundance in the stellar atmosphere which is a factor of 100 to 1000 below the value expected from accretion with solar abundances.

  5. Dynamical instability of white dwarfs and breaking of spherical symmetry under the presence of extreme magnetic fields

    Energy Technology Data Exchange (ETDEWEB)

    Coelho, J. G.; Cáceres, D. L.; Rueda, J. A.; Ruffini, R. [Dipartimento di Fisica and ICRA, Sapienza Università di Roma, P.le Aldo Moro 5, I-00185 Rome (Italy); Marinho, R. M.; Malheiro, M. [Departamento de Física, Instituto Tecnológico de Aeronáutica, ITA, São José dos Campos, 12228-900 SP (Brazil); Negreiros, R., E-mail: jaziel.coelho@icranet.org, E-mail: jorge.rueda@icra.it, E-mail: m.malheiro@ita.br [Instituto de Física, Universidade Federal Fluminense, UFF, Niterói, 24210-346 RJ (Brazil)

    2014-10-10

    Massive, highly magnetized white dwarfs with fields up to 10{sup 9} G have been observed and theoretically used for the description of a variety of astrophysical phenomena. Ultramagnetized white dwarfs with uniform interior fields up to 10{sup 18} G have been recently purported to obey a new maximum mass limit, M {sub max} ≈ 2.58 M {sub ☉}, which largely overcomes the traditional Chandrasekhar value, M {sub Ch} ≈ 1.44 M {sub ☉}. Such a larger limit would make these astrophysical objects viable candidates for the explanation of the superluminous population of Type Ia supernovae. We show that several macro and micro physical aspects such as gravitational, dynamical stability, breaking of spherical symmetry, general relativity, inverse β decay, and pycnonuclear fusion reactions are of most relevance for the self-consistent description of the structure and assessment of stability of these objects. It is shown in this work that the first family of magnetized white dwarfs indeed satisfy all the criteria of stability, while the ultramagnetized white dwarfs are very unlikely to exist in nature since they violate minimal requests of stability. Therefore, the canonical Chandrasekhar mass limit of white dwarfs still has to be applied.

  6. An independent constraint on the secular rate of variation of the gravitational constant from pulsating white dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Córsico, Alejandro H.; Althaus, Leandro G. [Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, (1900) La Plata (Argentina); García-Berro, Enrique [Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades, 5, 08860 Castelldefels (Spain); Romero, Alejandra D., E-mail: acorsico@fcaglp.unlp.edu.ar, E-mail: althaus@fcaglp.unlp.edu.ar, E-mail: enrique.garcia-berro@upc.edu, E-mail: alejandra.romero@ufrgs.br [Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, Porto Alegre 91501-970, RS (Brazil)

    2013-06-01

    A secular variation of the gravitational constant modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code and an up-to-date pulsational code we compute the effects of a secularly varying G on the pulsational properties of variable white dwarfs. Comparing the the theoretical results obtained taking into account the effects of a running G with the observed periods and measured rates of change of the periods of two well studied pulsating white dwarfs, G117-B15A and R548, we place constraints on the rate of variation of Newton's constant. We derive an upper bound Ġ/G ∼ −1.8 × 10{sup −10} yr{sup −1} using the variable white dwarf G117-B15A, and Ġ/G ∼ −1.3 × 10{sup −10} yr{sup −1} using R548. Although these upper limits are currently less restrictive than those obtained using other techniques, they can be improved in a future measuring the rate of change of the period of massive white dwarfs.

  7. Angular momentum exchange in white dwarf binaries accreting through direct impact

    Energy Technology Data Exchange (ETDEWEB)

    Sepinsky, J. F. [Department of Physics and Electrical Engineering, The University of Scranton, Scranton, PA 18510 (United States); Kalogera, V., E-mail: jeremy.sepinsky@scranton.edu, E-mail: vicky@northwestern.edu [Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States)

    2014-04-20

    We examine the exchange of angular momentum between the component spins and the orbit in semi-detached double white dwarf binaries undergoing mass transfer through direct impact of the transfer stream. We approximate the stream as a series of discrete massive particles ejected in the ballistic limit at the inner Lagrangian point of the donor toward the accretor. This work improves upon similar earlier studies in a number of ways. First, we self-consistently calculate the total angular momentum of the orbit at all times. This includes changes in the orbital angular momentum during the ballistic trajectory of the ejected mass, as well as changes during the ejection/accretion due to the radial component of the particle's velocity. Second, we calculate the particle's ballistic trajectory for each system, which allows us to determine the precise position and velocity of the particle upon accretion. We can then include specific information about the radius of the accretor as well as the angle of impact. Finally, we ensure that the total angular momentum is conserved, which requires the donor star spin to vary self-consistently. With these improvements, we calculate the angular momentum change of the orbit and each binary component across the entire parameter space of direct impact double white dwarf binary systems. We find a significant decrease in the amount of angular momentum removed from the orbit during mass transfer, as well as cases where this process increases the angular momentum of the orbit at the expense of the spin angular momentum of the donor. We conclude that, unlike earlier claims in the literature, mass transfer through direct impact need not destabilize the binary and that the quantity and sign of the orbital angular momentum transfer depends on the binary properties, particularly the masses of the double white dwarf binary component stars. This stabilization may significantly impact the population synthesis calculations of the expected

  8. The heavily polluted atmosphere of the DAZ white dwarf GALEX J193156.8+011745

    Czech Academy of Sciences Publication Activity Database

    Vennes, Stephane; Kawka, Adela; Németh, Péter

    2010-01-01

    Roč. 404, č. 1 (2010), L40-L44 ISSN 0035-8711 R&D Projects: GA AV ČR(CZ) IAA300030908; GA ČR GAP209/10/0967; GA MŠk(CZ) LC06014 Grant - others:GA AV ČR(CZ) IAA301630901 Program:IA Institutional research plan: CEZ:AV0Z10030501 Keywords : GALEX J193156.8+011745 * white dwarfs Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 4.888, year: 2010

  9. White Dwarf/M Dwarf Binaries as Single Degenerate Progenitors of Type Ia Supernovae

    OpenAIRE

    Wheeler, J. Craig

    2012-01-01

    Limits on the companions of white dwarfs in the single degenerate scenario for the origin of Type Ia supernovae (SNIa) have gotten increasingly tight. The only type of non-degenerate stars that survive the limits on the companions of SNIa in SNR 0509-67.5 and SN1572 are M dwarfs. M dwarfs have special properties that have not been considered in most work on the progenitors of SNIa: they have small but finite magnetic fields, and they flare frequently. These properties are explored in the cont...

  10. The White Dwarf Mass and the Accretion Rate of Recurrent Novae: An X-ray Perspective

    Science.gov (United States)

    Mukai, Koji; Sokoloski, Jennifer L.; Nelson, Thomas; Luna, Gerardo J. M.

    2011-01-01

    We present recent results of quiescent X-ray observations of recurrent novae (RNe) and related objects. Several RNe are luminous hard X-ray sources in quiescence, consistent with accretion onto a near Chandrasekhar mass white dwarf. Detection of similar hard X-ray emissions in old novae and other cataclysmic variables may lead to identification of additional RN candidates. On the other hand, other RNe are found to be comparatively hard X-ray faint. We present several scenarios that may explain this dichotomy, which should be explored further.

  11. Exploded views for volume data.

    Science.gov (United States)

    Bruckner, Stefan; Gröiller, M Eduard

    2006-01-01

    Exploded views are an illustration technique where an object is partitioned into several segments. These segments are displaced to reveal otherwise hidden detail. In this paper we apply the concept of exploded views to volumetric data in order to solve the general problem of occlusion. In many cases an object of interest is occluded by other structures. While transparency or cutaways can be used to reveal a focus object, these techniques remove parts of the context information. Exploded views, on the other hand, do not suffer from this drawback. Our approach employs a force-based model: the volume is divided into a part configuration controlled by a number of forces and constraints. The focus object exerts an explosion force causing the parts to arrange according to the given constraints. We show that this novel and flexible approach allows for a wide variety of explosion-based visualizations including view-dependent explosions. Furthermore, we present a high-quality GPU-based volume ray casting algorithm for exploded views which allows rendering and interaction at several frames per second.

  12. Quasisoft X-Ray Sources: White Dwarfs? Neutron Stars? Black Holes?

    Science.gov (United States)

    Di Stefano, Rosanne

    establish that these systems are intermediate-mass black holes would be difficult: they are so far from us that it is challenging to collect enough photons from them. The solution to this problem may come from QSSs, which have temperatures and luminosities consistent with what is expected from the inner disk of a black hole of somewhat higher than stellar mass. To test whether QSSs are accreting black holes, we need to discover some closer to us. This is what we propose to do. What are the progenitors of Type Ia supernovae? Nuclear-burning white dwarfs (NBWDs) are implicated in all progenitor models, during some stages of the progenitor's evolution. While NBWDs could be observable as supersoft X-ray sources (SSSs), sources with temperatures a few times lower than that of QSSs, we have found that there are too few SSSs in other galaxies to support the hypothesis that Type Ia SNe progenitors pass through a long supersoft phase. We now understand that the key issue in testing models of the progenitors is understanding the range of observable properties of NBWDs. Even though there are too few QSSs to make up the deficit, identifying and studying those QSSs which may be NBWDs is important. For every luminous soft source we identify, there will be several less luminous sources, some of which could be accreting or isolated neutron stars. We have been able to find such a small fraction of the neutron stars in our Galaxy that the search for dim neutron stars is important. Given the rich trove of science to be generated by the archival work we propose, this is a quintessential NASA program: we will use archived data from many missions to make progress toward achieving NASA's science objectives. Our work will help to answer the question: Do intermediate-mass black holes exist? It will also shed light on the nature of the progenitors of Type Ia supernovae, thereby helping to reduce the systematic uncertainties now inherent in an important cosmological tool.

  13. White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Line Widths: Kepler  Observations of 27 Pulsating DA White Dwarfs through K2 Campaign 8

    Energy Technology Data Exchange (ETDEWEB)

    Hermes, J. J.; Fanale, S. M.; Dennihy, E.; Fuchs, J. T.; Dunlap, B. H.; Clemens, J. C. [Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599 (United States); Gänsicke, B. T.; Greiss, S.; Tremblay, P.-E.; Fusillo, N. P. Gentile; Raddi, R.; Chote, P.; Marsh, T. R. [Department of Physics, University of Warwick, Coventry CV4 7AL (United Kingdom); Kawaler, Steven D. [Department of Physics and Astronomy, Iowa State University, Ames, IA 50011 (United States); Bell, Keaton J.; Montgomery, M. H.; Winget, D. E. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Redfield, S., E-mail: jjhermes@unc.edu [Wesleyan University Astronomy Department, Van Vleck Observatory, 96 Foss Hill Drive, Middletown, CT 06459 (United States)

    2017-10-01

    We present photometry and spectroscopy for 27 pulsating hydrogen-atmosphere white dwarfs (DAVs; a.k.a. ZZ Ceti stars) observed by the Kepler space telescope up to K2 Campaign 8, an extensive compilation of observations with unprecedented duration (>75 days) and duty cycle (>90%). The space-based photometry reveals pulsation properties previously inaccessible to ground-based observations. We observe a sharp dichotomy in oscillation mode line widths at roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s have substantially broader mode line widths, more reminiscent of a damped harmonic oscillator than a heat-driven pulsator. Extended Kepler coverage also permits extensive mode identification: we identify the spherical degree of 87 out of 201 unique radial orders, providing direct constraints of the rotation period for 20 of these 27 DAVs, more than doubling the number of white dwarfs with rotation periods determined via asteroseismology. We also obtain spectroscopy from 4 m-class telescopes for all DAVs with Kepler photometry. Using these homogeneously analyzed spectra, we estimate the overall mass of all 27 DAVs, which allows us to measure white dwarf rotation as a function of mass, constraining the endpoints of angular momentum in low- and intermediate-mass stars. We find that 0.51–0.73 M {sub ⊙} white dwarfs, which evolved from 1.7–3.0 M {sub ⊙} ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for higher-mass white dwarfs. Finally, we announce an online repository for our Kepler data and follow-up spectroscopy, which we collect at http://k2wd.org.

  14. The interstellar medium and the highly ionized species observed in the spectrum of the nearby white dwarf G191-B2B

    Science.gov (United States)

    Bruhweiler, F. C.; Kondo, Y.

    1981-01-01

    High-resolution spectra of the nearby (48 pc) white dwarf G191-B2B, obtained with the International Ultraviolet Explorer, reveal sharp resonance lines of N V, C IV, and Si IV. The origin of these features is most likely linked to the white dwarf, possibly being formed in an expanding halo around the star. Interstellar lines of C II, N I, Mg II, Si II, and Fe II are also seen in the spectrum. Analysis of these features indicates an average neutral hydrogen number density of 0.064 for this line of sight. In combination with the recent EUV and soft X-ray results, this is interpreted to mean that the interstellar medium in the most immediate solar vicinity is of the normal density n approximately equal to 0.1/cu cm of lower ionization, while just beyond it, at least in some directions, is a hot lower density plasma. These results are apparently in conflict with the model of the interstellar medium by McKee and Ostriker (1977) in its present form.

  15. The discovery of Ni V in the photospheres of the hot DA white dwarfs RE 2214-492 and G191-B2B

    Science.gov (United States)

    Holberg, J. B.; Hubeny, I.; Barstow, M. A.; Lanz, T.; Sion, E. M.; Tweedy, R. W.

    1994-01-01

    We have co-added six recently obtained International Ultraviolet Explorer (IUE) echelle spectra of the hot DA white dwarf RE 2214-492 and 10 existing archive spectra of the well-known hot DA, G191-B2B. We find that both stars contain numerous weak features due to Ni V. Nickel is thus the second iron-group element to be found in the spectra of the very hottest DA white dwarfs. In addition to Ni V, we also observe Al III in both stars and present evidence for the possible presence of Ni IV and Fe IV in RE 2214-492. The presence of Ni and Al, together with previously reported elements, will contribute significantly to both the EUV opacity and to the apparent complexity of the UV spectra of these stars. Using Non-Local Thermodynamic Equilibrium (NLTE) model atmospheres we estimate the Ni abundances in RE 2214-492 the G191-B2B to be log(Ni/H) = -5.5 +/- 0.3 and -6.0 +/- 0.3, respectively.

  16. A faint type of supernova from a white dwarf with a helium-rich companion.

    Science.gov (United States)

    Perets, H B; Gal-Yam, A; Mazzali, P A; Arnett, D; Kagan, D; Filippenko, A V; Li, W; Arcavi, I; Cenko, S B; Fox, D B; Leonard, D C; Moon, D-S; Sand, D J; Soderberg, A M; Anderson, J P; James, P A; Foley, R J; Ganeshalingam, M; Ofek, E O; Bildsten, L; Nelemans, G; Shen, K J; Weinberg, N N; Metzger, B D; Piro, A L; Quataert, E; Kiewe, M; Poznanski, D

    2010-05-20

    Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The 'old' environment near the supernova location, and the very low derived ejected mass ( approximately 0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive (44)Ti.

  17. Importance of tides for periastron precession in eccentric neutron star-white dwarf binaries

    Energy Technology Data Exchange (ETDEWEB)

    Sravan, N.; Valsecchi, F.; Kalogera, V. [Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), and Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States); Althaus, L. G., E-mail: niharika.sravan@gmail.com [Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Argentina Instituto de Astrofísica La Plata, CONICET-UNLP, Paseo del Bosque s/n, (1900) La Plata (Argentina)

    2014-09-10

    Although not nearly as numerous as binaries with two white dwarfs, eccentric neutron star-white dwarf (NS-WD) binaries are important gravitational-wave (GW) sources for the next generation of space-based detectors sensitive to low frequency waves. Here we investigate periastron precession in these sources as a result of general relativistic, tidal, and rotational effects; such precession is expected to be detectable for at least some of the detected binaries of this type. Currently, two eccentric NS-WD binaries are known in the galactic field, PSR J1141–6545 and PSR B2303+46, both of which have orbits too wide to be relevant in their current state to GW observations. However, population synthesis studies predict the existence of a significant Galactic population of such systems. Though small in most of these systems, we find that tidally induced periastron precession becomes important when tides contribute to more than 3% of the total precession rate. For these systems, accounting for tides when analyzing periastron precession rate measurements can improve estimates of the inferred WD component mass and, in some cases, will prevent us from misclassifying the object. However, such systems are rare, due to rapid orbital decay. To aid the inclusion of tidal effects when using periastron precession as a mass measurement tool, we derive a function that relates the WD radius and periastron precession constant to the WD mass.

  18. Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

    Science.gov (United States)

    Shipman, Harry; Barnhill, Maurice; Provencal, Judi; Roby, Scott; Bues, Irmela; Cordova, France; Hammond, Gordon; Hintzen, Paul; Koester, Detlev; Liebert, James

    1995-01-01

    Observations of cool white dwarf stars with the Hubble Space Telescope (HST) has uncovered a number of spectral features from previouslly unobserved species. In this paper we present the data on four cool white dwarfs. We present identifications, equivalent width measurements, and brief summaries of the significance of our findings. The four stars observed are GD 40 (DBZ3, G 74-7 (DAZ), L 745-46A (DZ), and LDS 749B (DBA). Many additional species of heavey elements were detected in GD 40 and G 74-7. In L 745-46A, while the detections are limited to Fe 1, Fe II, and Mg II, the quality of the Mg II h and K line profiles should permit a test of the line broadening theories, which are so crucial to abundance determinations. The clear detection of Mg II h and k in LDS 749 B should, once an abundance determination is made, provide a clear test of the hypothesis that the DBA stars are the result of accretion from the interstellar medium. This star contains no other clear features other than a tantalizing hint of C II 1335 with a P Cygni profile, and some expected He 1 lines.

  19. High-Time-Resolution Photometry of the White Dwarf Pulsar AR Scorpii

    Science.gov (United States)

    Stiller, Robert A.; Littlefield, Colin; Garnavich, Peter

    2018-01-01

    The cataclysmic variable AR Sco was recently discovered to be the first-ever white dwarf pulsar by Marsh et al. (2016) and Buckley et al. (2017). AR Sco has a 3.56-hour orbital period, a beat period of 1.97 minutes, and a spin period of 1.95 minutes. The flux varies by up to a factor of four during the beat period. It is believed that there is little to no accretion because of the weak X-ray emissions from the system. The white dwarf pulsar is believed to be spin-powered and is in an ejector state (Beskrovnaya et. al 2017) which is further evidence of little to no accretion. 24 hours of high-time-resolution photometry was taken using the 0.8 meter Sarah L. Krizmanich Telescope at the University of Notre Dame. We used our own observations and previous observations to calculate a new spin down timescale. In our data, AR Sco is brightest at an orbital phase of approximately 0.4 which suggests that if the orbital modulation is a reflection effect, the inner hemisphere in not uniformly irradiated. We establish that the amplitude and waveform of the beat pulse changes as function of orbital phase and that this can be attributed to the beat and spin pulses constructively and destructively interfering with one another.

  20. Extreme abundance ratios in the polluted atmosphere of the cool white dwarf NLTT 19868

    Science.gov (United States)

    Kawka, Adela; Vennes, Stéphane

    2016-05-01

    We present an analysis of intermediate-dispersion spectra and photometric data of the newly identified cool, polluted white dwarf NLTT 19868. The spectra obtained with X-shooter on the Very Large Telescope-Melipal show strong lines of calcium, and several lines of magnesium, aluminium and iron. We use these spectra and the optical-to-near-infrared spectral energy distribution to constrain the atmospheric parameters of NLTT 19868. Our analysis shows that NLTT 19868 is iron poor with respect to aluminium and calcium. A comparison with other cool, polluted white dwarfs shows that the Fe to Ca abundance ratio (Fe/Ca) varies by up to approximately two orders of magnitudes over a narrow temperature range with NLTT 19868 at one extremum in the Fe/Ca ratio and, in contrast, NLTT 888 at the other extremum. The sample shows evidence of extreme diversity in the composition of the accreted material: in the case of NLTT 888, the inferred composition of the accreted matter is akin to iron-rich planetary core composition, while in the case of NLTT 19868 it is close to mantle composition depleted by subsequent chemical separation at the bottom of the convection zone.

  1. Extinct Stars and Eviscerated Planets: Using Observations of White Dwarf Pollution to Understand the Formation, Composition and Evolution of Planetary Systems

    Science.gov (United States)

    Payne, M. J.

    2017-05-01

    I will discuss the work that will required for the white-dwarf community to be able to extract the unique data that only white dwarf systems can supply on the fundamental processes governing the formation and evolution of planetary systems.

  2. Radio Observations as a Tool to Investigate Shocks and Asymmetries in Accreting White Dwarf Binaries

    Science.gov (United States)

    Weston, Jennifer H. S.

    2016-07-01

    This dissertation uses radio observations with the Karl G. Jansky Very Large Array (VLA) to investigate the mechanisms that power and shape accreting white dwarfs (WD) and their ejecta. We test the predictions of both simple spherical and steady-state radio emission models by examining nova V1723 Aql, nova V5589 Sgr, symbiotic CH Cyg, and two small surveys of symbiotic binaries. First, we highlight classical nova V1723 Aql with three years of radio observations alongside optical and X-ray observations. We use these observations to show that multiple outflows from the system collided to create early non-thermal shocks with a brightness temperature of ≥106 K. While the late-time radio light curve is roughly consistent an expanding thermal shell of mass 2x10-4 M⊙ solar masses, resolved images of V1723 Aql show elongated material that apparently rotates its major axis over the course of 15 months, much like what is seen in gamma-ray producing nova V959 Mon, suggesting similar structures in the two systems. Next, we examine nova V5589 Sgr, where we find that the early radio emission is dominated by a shock-powered non-thermal flare that produces strong (kTx > 33 keV) X-rays. We additionally find roughly 10-5 M⊙ solar masses of thermal bremsstrahlung emitting material, all at a distance of ~4 kpc. The similarities in the evolution of both V1723 Aql and V5589 Sgr to that of nova V959 Mon suggest that these systems may all have dense equatorial tori shaping faster flows at their poles. Turning our focus to symbiotic binaries, we first use our radio observations of CH Cyg to link the ejection of a collimated jet to a change of state in the accretion disk. We additionally estimate the amount of mass ejected during this period (10-7 M⊙ masses), and improve measurements of the period of jet precession (P=12013 ± 74 days). We then use our survey of eleven accretion-driven symbiotic systems to determine that the radio brightness of a symbiotic system could potentially

  3. A 1.05 M{sub ☉} companion to PSR J2222–0137: The coolest known white dwarf?

    Energy Technology Data Exchange (ETDEWEB)

    Kaplan, David L. [Department of Physics, University of Wisconsin-Milwaukee, 1900 East Kenwood Boulevard, Milwaukee, WI 53211 (United States); Boyles, Jason; McLaughlin, Maura A.; Lorimer, Duncan R. [Department of Physics and Astronomy, West Virginia University, White Hall, Box 6315, Morgantown, WV 26506-6315 (United States); Dunlap, Bart H. [Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255 (United States); Tendulkar, Shriharsh P. [Space Radiation Laboratory, California Institute of Technology, 1200 East California Boulevard, MC 249-17, Pasadena, CA 91125 (United States); Deller, Adam T. [ASTRON, P.O. Box 2, 7990 AA Dwingeloo (Netherlands); Ransom, Scott M. [National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475 (United States); Stairs, Ingrid H., E-mail: kaplan@uwm.edu [Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia V6T 1Z1 (Canada)

    2014-07-10

    The recycled pulsar PSR J2222–0137 is one of the closest known neutron stars (NSs) with a parallax distance of 267{sub −0.9}{sup +1.2} pc and an edge-on orbit. We measure the Shapiro delay in the system through pulsar timing with the Green Bank Telescope, deriving a low pulsar mass (1.20 ± 0.14 M{sub ☉}) and a high companion mass (1.05 ± 0.06 M{sub ☉}) consistent with either a low-mass NS or a high-mass white dwarf. We can largely reject the NS hypothesis on the basis of the system's extremely low eccentricity (3 × 10{sup –4})—too low to have been the product of two supernovae under normal circumstances. However, despite deep optical and near-infrared searches with Southern Astrophysical Research and the Keck telescopes we have not discovered the optical counterpart of the system. This is consistent with the white dwarf hypothesis only if the effective temperature is <3000 K, a limit that is robust to distance, mass, and atmosphere uncertainties. This would make the companion to PSR J2222–0137 one of the coolest white dwarfs ever observed. For the implied age to be consistent with the age of the Milky Way requires the white dwarf to have already crystallized and entered the faster Debye-cooling regime.

  4. A 1.05 M ⊙ Companion to PSR J2222-0137: The Coolest Known White Dwarf?

    Science.gov (United States)

    Kaplan, David L.; Boyles, Jason; Dunlap, Bart H.; Tendulkar, Shriharsh P.; Deller, Adam T.; Ransom, Scott M.; McLaughlin, Maura A.; Lorimer, Duncan R.; Stairs, Ingrid H.

    2014-07-01

    The recycled pulsar PSR J2222-0137 is one of the closest known neutron stars (NSs) with a parallax distance of 267_{-0.9}^{+1.2} pc and an edge-on orbit. We measure the Shapiro delay in the system through pulsar timing with the Green Bank Telescope, deriving a low pulsar mass (1.20 ± 0.14 M ⊙) and a high companion mass (1.05 ± 0.06 M ⊙) consistent with either a low-mass NS or a high-mass white dwarf. We can largely reject the NS hypothesis on the basis of the system's extremely low eccentricity (3 × 10-4)—too low to have been the product of two supernovae under normal circumstances. However, despite deep optical and near-infrared searches with Southern Astrophysical Research and the Keck telescopes we have not discovered the optical counterpart of the system. This is consistent with the white dwarf hypothesis only if the effective temperature is <3000 K, a limit that is robust to distance, mass, and atmosphere uncertainties. This would make the companion to PSR J2222-0137 one of the coolest white dwarfs ever observed. For the implied age to be consistent with the age of the Milky Way requires the white dwarf to have already crystallized and entered the faster Debye-cooling regime.

  5. A BAYESIAN APPROACH TO DERIVING AGES OF INDIVIDUAL FIELD WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    O' Malley, Erin M. [Department of Physics and Astronomy, Siena College, Loudonville, NY 12211 (United States); Von Hippel, Ted [Department of Physical Sciences, Embry-Riddle Aeronautical University, Daytona Beach, FL 32114 (United States); Van Dyk, David A., E-mail: ted.vonhippel@erau.edu, E-mail: dvandyke@imperial.ac.uk [Statistics Section, Department of Mathematics, Imperial College London, SW7 2AZ (United Kingdom)

    2013-09-20

    We apply a self-consistent and robust Bayesian statistical approach to determine the ages, distances, and zero-age main sequence (ZAMS) masses of 28 field DA white dwarfs (WDs) with ages of approximately 4-8 Gyr. Our technique requires only quality optical and near-infrared photometry to derive ages with <15% uncertainties, generally with little sensitivity to our choice of modern initial-final mass relation. We find that age, distance, and ZAMS mass are correlated in a manner that is too complex to be captured by traditional error propagation techniques. We further find that the posterior distributions of age are often asymmetric, indicating that the standard approach to deriving WD ages can yield misleading results.

  6. Effect of Generalized Uncertainty Principle on Main-Sequence Stars and White Dwarfs

    Directory of Open Access Journals (Sweden)

    Mohamed Moussa

    2015-01-01

    Full Text Available This paper addresses the effect of generalized uncertainty principle, emerged from different approaches of quantum gravity within Planck scale, on thermodynamic properties of photon, nonrelativistic ideal gases, and degenerate fermions. A modification in pressure, particle number, and energy density are calculated. Astrophysical objects such as main-sequence stars and white dwarfs are examined and discussed as an application. A modification in Lane-Emden equation due to a change in a polytropic relation caused by the presence of quantum gravity is investigated. The applicable range of quantum gravity parameters is estimated. The bounds in the perturbed parameters are relatively large but they may be considered reasonable values in the astrophysical regime.

  7. Ultrahigh-energy cosmic rays from tidally-ignited white dwarfs

    Science.gov (United States)

    Alves Batista, Rafael; Silk, Joseph

    2017-11-01

    Ultrahigh-energy cosmic rays (UHECRs) can be accelerated by tidal disruption events of stars by black holes. We suggest a novel mechanism for UHECR acceleration wherein white dwarfs (WDs) are tidally compressed by intermediate-mass black holes (IMBHs), leading to their ignition and subsequent explosion as a supernova. Cosmic rays accelerated by the supernova may receive an energy boost when crossing the accretion-powered jet. The rate of encounters between WDs and IMBHs can be relatively high, as the number of IMBHs may be substantially augmented once account is taken of their likely presence in dwarf galaxies. Here we show that this kind of tidal disruption event naturally provides an intermediate composition for the observed UHECRs, and suggest that dwarf galaxies and globular clusters are suitable sites for particle acceleration to ultrahigh energies.

  8. Vii. New Kr IV - VII Oscillator Strengths and an Improved Spectral Analysis of the Hot, Hydrogen-deficient Do-type White Dwarf RE 0503-289

    Science.gov (United States)

    Rauch, T.; Quinet, P.; Hoyer, D.; Werner, K.; Richter, P.; Kruk, J. W.; Demleitner, M.

    2016-01-01

    For the spectral analysis of high-resolution and high signal-to-noise (SN) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims. New Krivvii oscillator strengths for a large number of lines enable us to construct more detailed model atoms for our NLTEmodel-atmosphere calculations. This enables us to search for additional Kr lines in observed spectra and to improve Kr abundance determinations. Methods. We calculated Krivvii oscillator strengths to consider radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Kr lines that are exhibited in high-resolution and high SN ultraviolet (UV)observations of the hot white dwarf RE 0503.

  9. THE CIRCUMSTELLAR ENVIRONMENT OF R CORONAE BOREALIS: WHITE DWARF MERGER OR FINAL-HELIUM-SHELL FLASH?

    Energy Technology Data Exchange (ETDEWEB)

    Clayton, Geoffrey C.; Andrews, J. E. [Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803 (United States); Sugerman, Ben E. K. [Department of Physics and Astronomy, Goucher College, 1021 Dulaney Valley Rd., Baltimore, MD 21204 (United States); Adam Stanford, S. [IGPP, Lawrence Livermore National Laboratory, Livermore, CA 94551 (United States); Whitney, B. A. [Space Science Institute, 4750 Walnut St. Suite 205, Boulder, CO 80301 (United States); Honor, J.; Babler, B. [Department of Astronomy, 475 North Charter St., University of Wisconsin, Madison, WI 53706 (United States); Barlow, M. J. [Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT (United Kingdom); Gordon, K. D.; Bond, Howard E.; Matsuura, M. [STScI, 3700 San Martin Dr., Baltimore, MD 21218 (United States); Geballe, T. R. [Gemini Observatory, 670 N. A' ohoku Place, Hilo, HI 96720 (United States); De Marco, O. [Department of Physics, Macquarie University, Sydney, NSW 2109 (Australia); Lawson, W. A. [School of PEMS, University of New South Wales, ADFA, P.O. Box 7916, Canberra, ACT 2610 (Australia); Sibthorpe, B. [Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany); Olofsson, G. [Department of Astronomy, Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, SE-10691 Stockholm (Sweden); Polehampton, E. [Space Science and Technology Department, Rutherford Appleton Laboratory, Didcot OX11 0QX (United Kingdom); Gomez, H. L.; Hargrave, P. C. [School of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff, Wales CF24 3YB (United Kingdom); Ivison, R. J., E-mail: gclayton@phys.lsu.edu, E-mail: jandrews@phys.lsu.edu, E-mail: ben.sugerman@goucher.edu, E-mail: stanford@physics.ucdavis.edu, E-mail: bwhitney@spacescience.org, E-mail: jhonor@astro.wisc.edu, E-mail: brian@astro.wisc.edu, E-mail: mjb@star.ucl.ac.uk [UK Astronomy Technology Centre, ROE, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom); and others

    2011-12-10

    In 2007, R Coronae Borealis (R CrB) went into a historically deep and long decline. In this state, the dust acts like a natural coronagraph at visible wavelengths, allowing faint nebulosity around the star to be seen. Imaging has been obtained from 0.5 to 500 {mu}m with Gemini/GMOS, Hubble Space Telescope/WFPC2, Spitzer/MIPS, and Herschel/SPIRE. Several of the structures around R CrB are cometary globules caused by wind from the star streaming past dense blobs. The estimated dust mass of the knots is consistent with their being responsible for the R CrB declines if they form along the line of sight to the star. In addition, there is a large diffuse shell extending up to 4 pc away from the star containing cool 25 K dust that is detected all the way out to 500 {mu}m. The spectral energy distribution of R CrB can be well fitted by a 150 AU disk surrounded by a very large diffuse envelope which corresponds to the size of the observed nebulosity. The total masses of the disk and envelope are 10{sup -4} and 2 M{sub Sun }, respectively, assuming a gas-to-dust ratio of 100. The evidence pointing toward a white dwarf merger or a final-helium-shell flash origin for R CrB is contradictory. The shell and the cometary knots are consistent with a fossil planetary nebula. Along with the fact that R CrB shows significant lithium in its atmosphere, this supports the final-helium-shell flash. However, the relatively high inferred mass of R CrB and its high fluorine abundance support a white dwarf merger.

  10. Data exploder for variable shaped beam exposure

    Science.gov (United States)

    Nogatch, John; Kirsch, Hartmut; Shi, Jun

    2007-05-01

    As the industry moves to 45nm and beyond, rapidly increasing file sizes are an obstacle to achieving fast turnaround time for mask manufacturing. Conventional Mask Data Preparation (MDP) requires the production of large files, in a format specific to each make and model of E-beam tool. An alternative approach extracts the data from a data file already present in the MDP flow, and provides it directly to the E-beam tool. This extraction is called a "Data Exploder", because the output data volume can be larger than the input data. Exploding the data in real-time saves the time required to write and then read large disk files. The Data Exploder is compatible with multithread and multiprocess parallel reading. The practical application and limitations of the Data Exploder are described, including throughput performance, requirements for disk storage, network interconnect, and CPU configurations.

  11. Diffusive settling of neon-22 in isolated carbon-oxygen white dwarfs, and, Arbitrarily degenerate donors in ultracompact stellar binaries

    Science.gov (United States)

    Deloye, Christopher J.

    I detail my work, conducted under Professor Lars Bildsten concerning several aspects of white dwarf (WD) physics. In Part I, I describe our study of the impact of 22Ne diffusion in liquid WD interiors. The neutron excess of 22Ne compared to 12C and 16O allows it to sink towards the center, releasing gravitational binding energy. This energy slows WD cooling by an amount that depends on the mass of the WD, the 22Ne diffusion rate, and metallicity. A gradient in 22Ne also affects the internal buoyancy of the WD, altering the star's g-mode frequencies at a detectable level. In Part II, I move onto the donors in ultracompact binary systems (UCBs). These systems represent extreme examples of mass-transferring binaries, having orbital periods less than 70 min. I detail our new model set for the donors in these systems, which allows consideration of donors with arbitrary composition and entropy. This is a major improvement over the models available prior to our work as we can now consider constraining internal donor properties from observational data. We apply these models to UCB systems that contain either a neutron star (NS) accretor or a WD accretor, placing constraints on members of both classes of systems. In particular, we find that the composition of the donors in certain NS accretor systems can be strongly constrained, while we can provide weak limits on the entropy of the donors in the WD accretor systems. This latter fact may help distinguish between possible formation channels for these systems. Finally, I conclude by considering several outstanding questions in both classes of systems and outline how our models can be used or modified to address each of them.

  12. A stable implementation of the prestack exploding reflector modeling and migration

    KAUST Repository

    Alkhalifah, Tariq Ali

    2011-01-01

    The double square-root DSR equation is known to image data by downward continuation using one-way depth extrapolation methods. A two-way time extrapolation of the DSR-derived phase operator suffers from an essential singularity for horizontally traveling waves. This singularity can be avoided by limiting the range of wavenumbers treated in a spectral-based extrapolation referred to as the low-rank method. However, the region around the singularity will still induce instability. We derive an approximation to the DSR formulation based on a high frequency approximation. The resulting equation is both highly accurate and free of singularities. Applications to synthetic data including imaging of the Marmousi dataset demonstrate the accuracy of the new prestack modeling and migration approach. © 2011 Society of Exploration Geophysicists.

  13. Maximum mass ratio of AM CVn-type binary systems and maximum white dwarf mass in ultra-compact X-ray binaries

    Directory of Open Access Journals (Sweden)

    Arbutina Bojan

    2011-01-01

    Full Text Available AM CVn-type stars and ultra-compact X-ray binaries are extremely interesting semi-detached close binary systems in which the Roche lobe filling component is a white dwarf transferring mass to another white dwarf, neutron star or a black hole. Earlier theoretical considerations show that there is a maximum mass ratio of AM CVn-type binary systems (qmax ≈ 2/3 below which the mass transfer is stable. In this paper we derive slightly different value for qmax and more interestingly, by applying the same procedure, we find the maximum expected white dwarf mass in ultra-compact X-ray binaries.

  14. The accretion of solar material onto white dwarfs: No mixing with core material implies that the mass of the white dwarf is increasing

    Directory of Open Access Journals (Sweden)

    Sumner Starrfield

    2014-02-01

    Full Text Available Cataclysmic Variables (CVs are close binary star systems with one component a white dwarf (WD and the other a larger cooler star that fills its Roche Lobe. The cooler star is losing mass through the inner Lagrangian point of the binary and some unknown fraction of this material is accreted by the WD. One consequence of the WDs accreting material, is the possibility that they are growing in mass and will eventually reach the Chandrasekhar Limit. This evolution could result in a Supernova Ia (SN Ia explosion and is designated the Single Degenerate Progenitor (SD scenario. This paper is concerned with the SD scenario for SN Ia progenitors. One problem with the single degenerate scenario is that it is generally assumed that the accreting material mixes with WD core material at some time during the accretion phase of evolution and, since the typical WD has a carbon-oxygen CO core, the mixing results in large amounts of carbon and oxygen being brought up into the accreted layers. The presence of enriched carbon causes enhanced nuclear fusion and a Classical Nova explosion. Both observations and theoretical studies of these explosions imply that more mass is ejected than is accreted. Thus, the WD in a Classical Nova system is losing mass and cannot be a SN Ia progenitor. However, the composition in the nuclear burning region is important and, in new calculations reported here, the consequences to the WD of no mixing of accreted material with core material have been investigated so that the material involved in the explosion has only a Solar composition. WDs with a large range in initial masses and mass accretion rates have been evolved. I find that once sufficient material has been accreted, nuclear burning occurs in all evolutionary sequences and continues until a thermonuclear runaway (TNR occurs and the WD either ejects a small amount of material or its radius grows to about 1012 cm and the evolution is ended. In all cases where mass ejection occurs

  15. Does Explosive Nuclear Burning Occur in Tidal Disruption Events of White Dwarfs by Intermediate-mass Black Holes?

    Energy Technology Data Exchange (ETDEWEB)

    Tanikawa, Ataru; Sato, Yushi; Hachisu, Izumi [Department of Earth Science and Astronomy, College of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902 (Japan); Nomoto, Ken’ichi; Maeda, Keiichi [Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583 (Japan); Nakasato, Naohito, E-mail: tanikawa@ea.c.u-tokyo.ac.jp [Department of Computer Science and Engineering, University of Aizu, Tsuruga Ikki-machi Aizu-Wakamatsu, Fukushima 965-8580 (Japan)

    2017-04-20

    We investigate nucleosynthesis in tidal disruption events (TDEs) of white dwarfs (WDs) by intermediate-mass black holes. We consider various types of WDs with different masses and compositions by means of three-dimensional (3D) smoothed particle hydrodynamics (SPH) simulations. We model these WDs with different numbers of SPH particles, N , from a few 10{sup 4} to a few 10{sup 7} in order to check mass resolution convergence, where SPH simulations with N > 10{sup 7} (or a space resolution of several 10{sup 6} cm) have unprecedentedly high resolution in this kind of simulation. We find that nuclear reactions become less active with increasing N and that these nuclear reactions are excited by spurious heating due to low resolution. Moreover, we find no shock wave generation. In order to investigate the reason for the absence of a shock wave, we additionally perform one-dimensional (1D) SPH and mesh-based simulations with a space resolution ranging from 10{sup 4} to 10{sup 7} cm, using a characteristic flow structure extracted from the 3D SPH simulations. We find shock waves in these 1D high-resolution simulations, one of which triggers a detonation wave. However, we must be careful of the fact that, if the shock wave emerged in an outer region, it could not trigger the detonation wave due to low density. Note that the 1D initial conditions lack accuracy to precisely determine where a shock wave emerges. We need to perform 3D simulations with ≲10{sup 6} cm space resolution in order to conclude that WD TDEs become optical transients powered by radioactive nuclei.

  16. Local Ignition in Carbon-Oxygen White Dwarfs. I. One-Zone Ignition and Spherical Shock Ignition of Detonations

    Science.gov (United States)

    Dursi, L. Jonathan; Timmes, F. X.

    2006-04-01

    The details of ignition of Type Ia supernovae remain fuzzy, despite the importance of this input for any large-scale model of the final explosion. Here, we begin a process of understanding the ignition of these hot spots by examining the burning of one zone of material, and then we investigate the ignition of a detonation due to rapid heating at single point. We numerically measure the ignition delay time for onset of burning in mixtures of degenerate material and provide fitting formulae for conditions of relevance in the Type Ia problem. Using the neon abundance as a proxy for the white dwarf progenitor's metallicity, we then find that ignition times can decrease by ~20% with the addition of even 5% of neon by mass. When temperature fluctuations that successfully kindle a region are very rare, such a reduction in ignition time can increase the probability of ignition by orders of magnitude. If the neon comes largely at the expense of carbon, a similar decrease in the ignition time can occur. We then consider the ignition of a detonation by an explosive energy input in one localized zone, for example, a Sedov blast wave leading to a shock-ignited detonation. Building on previous work on curved detonations, we confirm that surprisingly large inputs of energy are required to successfully launch a detonation, leading to required match heads of ~4500 detonation thicknesses-tens of centimeters to hundreds of meters-which is orders of magnitude larger than naive considerations might suggest. This is a very difficult constraint to meet for some pictures of a deflagration-to-detonation transition, such as a Zel'dovich gradient mechanism ignition in the distributed burning regime.

  17. Linking long-term planetary N-body simulations with periodic orbits: application to white dwarf pollution

    Science.gov (United States)

    Antoniadou, Kyriaki I.; Veras, Dimitri

    2016-12-01

    Mounting discoveries of debris discs orbiting newly formed stars and white dwarfs (WDs) showcase the importance of modelling the long-term evolution of small bodies in exosystems. WD debris discs are, in particular, thought to form from very long-term (0.1-5.0 Gyr) instability between planets and asteroids. However, the time-consuming nature of N-body integrators which accurately simulate motion over Gyrs necessitates a judicious choice of initial conditions. The analytical tools known as periodic orbits can circumvent the guesswork. Here, we begin a comprehensive analysis directly linking periodic orbits with N-body integration outcomes with an extensive exploration of the planar circular restricted three-body problem (CRTBP) with an outer planet and inner asteroid near or inside of the 2:1 mean motion resonance. We run nearly 1000 focused simulations for the entire age of the Universe (14 Gyr) with initial conditions mapped to the phase space locations surrounding the unstable and stable periodic orbits for that commensurability. In none of our simulations did the planar CRTBP architecture yield a long-time-scale (≳0.25 per cent of the age of the Universe) asteroid-star collision. The pericentre distance of asteroids which survived beyond this time-scale (≈35 Myr) varied by at most about 60 per cent. These results help affirm that collisions occur too quickly to explain WD pollution in the planar CRTBP 2:1 regime, and highlight the need for further periodic orbit studies with the eccentric and inclined TBP architectures and other significant orbital period commensurabilities.

  18. Accretion onto Carbon-Oxygen White Dwarfs as a possible mechanism for growth to the Chandrasekhar Limit

    Science.gov (United States)

    Starrfield, Sumner; Bose, Maitrayee; Iliadis, Christian; Hix, William R.; José, Jordi; Hernanz, Margarita

    2017-08-01

    We have continued our studies of accretion onto white dwarfs by following the evolution of thermonuclear runaways (TNRs) on Carbon Oxygen (CO) white dwarfs. We have varied the mass of the white dwarf and the composition of the accreted material. We use the results of the multi-dimensional studies of TNRs in white dwarfs, accreting only Solar matter, which show that sufficient core material is dredged-up by the TNR and then ejected by the explosion to agree with the observations of the ejecta abundances. We have also found that the initial 12C abundance is inversely proportional to the amount of material accreted prior to the TNR. Therefore, we first accrete Solar material and follow the evolution until a TNR occurs. Because the 12C abundance is significantly smaller then if we had initially mixed the accreting gas with the carbon-oxygen core, more matter takes part in the explosion than if we had begun the evolution with the mixed composition. We then instantaneously switch the composition to a mixture with either 25% core material or 50% core material (plus accreted material) and follow the resulting evolution of the TNR. We use our 1D, Lagrangian, hydrodynamic code: NOVA. We report on the results of these new simulations and compare the ejecta abundances to those measured in pre-solar grains that are thought to arise from classical nova explosions. These results will also be compared to recent results with SHIVA (Josè and Hernanz). We find that there are some white dwarf masses where significantly less mass is ejected than accreted during the Classical Nova event and, therefore, the white dwarf is growing in mass as a result of the accretion and in spite of the resulting explosion.This work was supported in part by NASA under the Astrophysics Theory Program grant 14-ATP14-0007 and the U.S. DOE under Contract No. DE-FG02- 97ER41041. SS acknowledges partial support from NASA, NSF, and HST grants to ASU and WRH is supported by the U.S. Department of Energy, Office

  19. Kinematic, Photometric, and Spectroscopic Properties of Faint White Dwarf Stars Discovered in the HALO7D Survey of the Milky Way Galaxy

    Science.gov (United States)

    Harris, Madison; Cunningham, Emily; Guhathakurta, Puragra; Cheshire, Ishani; Gupta, Nandita

    2018-01-01

    White dwarf (WD) stars represent the final phase in the life of solar-mass stars. The extreme low luminosity of WDs means that most detailed measurements of such stars are limited to samples in the immediate neighborhood of the Sun in the thin disk of the Milky Way galaxy. We present spectra, line-of-sight (LOS) velocities, and proper motions (PMs) of a sample of faint (m_V ~ 19.0–24.5) white dwarfs (WDs) from the HALO7D survey. HALO7D is a Keck II/DEIMOS spectroscopic survey of unprecedented depth (8–24 hour integrations) in the CANDELS fields of main sequence turnoff stars in the Milky Way's outer halo. Faint WD stars are rare but useful by-products of this survey. We identify the sample of WDs based on their characteristic broad spectral Balmer absorption features, and present a Bayesian method for measuring their LOS velocities. Using their broadband colors, LOS velocities and PMs measured with the Hubble Space Telescope, we identify candidate halo members among the WDs based on the predicted velocity distributions from the Besançon numerical model of stellar populations in the Milky Way galaxy. The WDs found in the HALO7D survey will yield new insights on the old stellar population associated with the Milky Way's thick disk and halo. Funding for this research was provided by the National Science Foundation and NASA/STScI. NG and IC's participation in this research was under the auspices of the Science Internship Program at the University of California Santa Cruz.

  20. A STRANGE STAR SCENARIO FOR THE FORMATION OF ECCENTRIC MILLISECOND PULSAR/HELIUM WHITE DWARF BINARIES

    Energy Technology Data Exchange (ETDEWEB)

    Jiang, Long; Li, Xiang-Dong [Department of Astronomy, Nanjing University, Nanjing 210046 (China); Dey, Jishnu; Dey, Mira, E-mail: lixd@nju.edu.cn [Department of Physics, Presidency University, 86/1, College Street, Kolkata 700 073 (India)

    2015-07-01

    According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during binary evolution, as observed in most binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are the PSRs J2234+06, J1946+3417, and J1950+2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf (WD) companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive WDs, or the eccentricity may be induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars (NSs) in LMXBs may reach the density of quark deconfinement, which can lead to phase transition from NSs to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitational mass of the NS during the transition. The eccentricities can be reproduced with a reasonable estimate of the mass loss. This scenario might also account for the formation of the youngest known X-ray binary Cir X–1, which also possesses a low-field compact star in an eccentric orbit.

  1. Formation of millisecond pulsars with low-mass helium white dwarf companions in very compact binaries

    Energy Technology Data Exchange (ETDEWEB)

    Jia, Kun; Li, X.-D., E-mail: lixd@nju.edu.cn [Department of Astronomy, Nanjing University, Nanjing 210093 (China)

    2014-08-20

    Binary millisecond pulsars (BMSPs) are thought to have evolved from low-mass X-ray binaries (LMXBs). If the mass transfer in LMXBs is driven by nuclear evolution of the donor star, the final orbital period is predicted to be well correlated with the mass of the white dwarf (WD), which is the degenerate He core of the donor. Here we show that this relation can be extended to very small WD mass (∼0.14-0.17 M {sub ☉}) and narrow orbital period (about a few hours), depending mainly on the metallicities of the donor stars. There is also discontinuity in the relation, which is due to the temporary contraction of the donor when the H-burning shell crosses the hydrogen discontinuity. BMSPs with low-mass He WD companions in very compact binaries can be accounted for if the progenitor binary experienced very late Case A mass transfer. The WD companion of PSR J1738+0333 is likely to evolve from a Pop II star. For PSR J0348+0432, to explain its extreme compact orbit in the Roche-lobe-decoupling phase, even lower metallicity (Z = 0.0001) is required.

  2. ON THE FORMATION OF ECCENTRIC MILLISECOND PULSARS WITH HELIUM WHITE-DWARF COMPANIONS

    Energy Technology Data Exchange (ETDEWEB)

    Antoniadis, John, E-mail: antoniadis@dunlap.utoronto.ca [Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn (Germany); Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4 (Canada)

    2014-12-20

    Millisecond pulsars (MSPs) orbiting helium white dwarfs (WDs) in eccentric orbits challenge the established binary-evolution paradigm that predicts efficient orbital circularization during the mass-transfer episode that spins up the pulsar. Freire and Tauris recently proposed that these binary MSPs may instead form from the rotationally delayed accretion-induced collapse of a massive WD. However, their hypothesis predicts that eccentric systems preferably host low-mass pulsars and travel with small systemic velocities—in tension with new observational constraints. Here, I show that a substantial growth in eccentricity may alternatively arise from the dynamical interaction of the binary with a circumbinary disk. Such a disk may form from ejected donor material during hydrogen flash episodes, when the neutron star is already an active radio pulsar and tidal forces can no longer circularize the binary. I demonstrate that a short-lived (10{sup 4}-10{sup 5} yr) disk can result in eccentricities of e ≅ 0.01-0.15 for orbital periods between 15 and 50 days. Finally, I propose that, more generally, the disk hypothesis may explain the lack of circular binary pulsars for the aforementioned orbital-period range.

  3. The Massive CO White Dwarf in the Symbiotic Recurrent Nova RS Ophiuchi

    Science.gov (United States)

    Mikołajewska, Joanna; Shara, Michael M.

    2017-10-01

    If accreting white dwarfs (WDs) in binary systems are to produce type Ia supernovae (SNe Ia), they must grow to nearly the Chandrasekhar mass and ignite carbon burning. Proving conclusively that a WD has grown substantially since its birth is a challenging task. Slow accretion of hydrogen inevitably leads to the erosion, rather than the growth of WDs. Rapid hydrogen accretion does lead to growth of a helium layer, due to both decreased degeneracy and the inhibition of mixing of the accreted hydrogen with the underlying WD. However, until recently, simulations of helium-accreting WDs all claimed to show the explosive ejection of a helium envelope once it exceeded ˜ {10}-1 {M}⊙ . Because CO WDs cannot be born with masses in excess of ˜ 1.1 {M}⊙ , any such object in excess of ˜ 1.2 {M}⊙ must have grown substantially. We demonstrate that the WD in the symbiotic nova RS Oph is in the mass range 1.2-1.4 M ⊙. We compare UV spectra of RS Oph with those of novae with ONe WDs and with novae erupting on CO WDs. The RS Oph WD is clearly made of CO, demonstrating that it has grown substantially since birth. It is a prime candidate to eventually produce an SN Ia.

  4. Prompt emission from tidal disruptions of white dwarfs by intermediate mass black holes

    Directory of Open Access Journals (Sweden)

    Laguna P.

    2012-12-01

    Full Text Available We present a qualitative picture of prompt emission from tidal disruptions of white dwarfs (WD by intermediate mass black holes (IMBH. The smaller size of an IMBH compared to a supermassive black hole and a smaller tidal radius of a WD disruption lead to a very fast event with high peak luminosity. Magnetic field is generated in situ following the tidal disruption, which leads to effective accretion. Since large-scale magnetic field is also produced, geometrically thick super-Eddington inflow leads to a relativistic jet. The dense jet possesses a photosphere, which emits quasi-thermal radiation in soft X-rays. The source can be classified as a long low-luminosity gamma-ray burst (ll-GRB. Tidal compression of a WD causes nuclear ignition, which is observable as an accompanying supernova. We suggest that GRB060218 and SN2006aj is such a pair of ll-GRB and supernova. We argue that in a flux-limited sample the disruptions of WDs by IMBHs are more frequent then the disruptions of other stars by IMBHs.

  5. A Massive-born Neutron Star with a Massive White Dwarf Companion

    Science.gov (United States)

    Cognard, Ismaël; Freire, Paulo C. C.; Guillemot, Lucas; Theureau, Gilles; Tauris, Thomas M.; Wex, Norbert; Graikou, Eleni; Kramer, Michael; Stappers, Benjamin; Lyne, Andrew G.; Bassa, Cees; Desvignes, Gregory; Lazarus, Patrick

    2017-08-01

    We report on the results of a 4 year timing campaign of PSR J2222-0137, a 2.44 day binary pulsar with a massive white dwarf (WD) companion, with the Nançay, Effelsberg, and Lovell radio telescopes. Using the Shapiro delay for this system, we find a pulsar mass m p = 1.76 ± 0.06 M ⊙ and a WD mass m c = 1.293 ± 0.025 M ⊙. We also measure the rate of advance of periastron for this system, which is marginally consistent with the general relativity prediction for these masses. The short lifetime of the massive WD progenitor star led to a rapid X-ray binary phase with little (< 10-2 M ⊙) mass accretion onto the neutron star; hence, the current pulsar mass is, within uncertainties, its birth mass, which is the largest measured to date. We discuss the discrepancy with previous mass measurements for this system; we conclude that the measurements presented here are likely to be more accurate. Finally, we highlight the usefulness of this system for testing alternative theories of gravity by tightly constraining the presence of dipolar radiation. This is of particular importance for certain aspects of strong-field gravity, like spontaneous scalarization, since the mass of PSR J2222-0137 puts that system into a poorly tested parameter range.

  6. A Massive-born Neutron Star with a Massive White Dwarf Companion

    Energy Technology Data Exchange (ETDEWEB)

    Cognard, Ismaël; Guillemot, Lucas; Theureau, Gilles [Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, Université d’Orléans/CNRS, F-45071 Orléans Cedex 02 (France); Freire, Paulo C. C. [Station de radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, F-18330 Nançay (France); Tauris, Thomas M.; Wex, Norbert; Graikou, Eleni; Kramer, Michael; Desvignes, Gregory; Lazarus, Patrick [Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn (Germany); Stappers, Benjamin; Lyne, Andrew G. [Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, M13 9PL (United Kingdom); Bassa, Cees [ASTRON, The Netherlands Institute for Radioastronomy, Postbus 2, 7900 AA, Dwingeloo (Netherlands)

    2017-08-01

    We report on the results of a 4 year timing campaign of PSR J2222−0137, a 2.44 day binary pulsar with a massive white dwarf (WD) companion, with the Nançay, Effelsberg, and Lovell radio telescopes. Using the Shapiro delay for this system, we find a pulsar mass m {sub p} = 1.76 ± 0.06 M {sub ⊙} and a WD mass m {sub c} = 1.293 ± 0.025 M {sub ⊙}. We also measure the rate of advance of periastron for this system, which is marginally consistent with the general relativity prediction for these masses. The short lifetime of the massive WD progenitor star led to a rapid X-ray binary phase with little (< 10{sup −2} M {sub ⊙}) mass accretion onto the neutron star; hence, the current pulsar mass is, within uncertainties, its birth mass, which is the largest measured to date. We discuss the discrepancy with previous mass measurements for this system; we conclude that the measurements presented here are likely to be more accurate. Finally, we highlight the usefulness of this system for testing alternative theories of gravity by tightly constraining the presence of dipolar radiation. This is of particular importance for certain aspects of strong-field gravity, like spontaneous scalarization, since the mass of PSR J2222−0137 puts that system into a poorly tested parameter range.

  7. Periodic optical variability and debris accretion in white dwarfs: a test for a causal connection★

    Science.gov (United States)

    Hallakoun, Na'ama; Maoz, Dan; Agol, Eric; Brown, Warren R.; Dufour, Patrick; Farihi, Jay; Gänsicke, Boris T.; Kilic, Mukremin; Kosakowski, Alekzander; Loeb, Abraham; Mazeh, Tsevi; Mullally, Fergal

    2018-01-01

    Recent Kepler photometry has revealed that about half of white dwarfs (WDs) have periodic, low-level (˜10-4 - 10-3), optical variations. Hubble Space Telescope (HST) ultraviolet spectroscopy has shown that up to about one half of WDs are actively accreting rocky planetary debris, as evidenced by the presence of photospheric metal absorption lines. We have obtained HST ultraviolet spectra of seven WDs that have been monitored for periodic variations, to test the hypothesis that these two phenomena are causally connected, i.e. that the optical periodic modulation is caused by WD rotation coupled with an inhomogeneous surface distribution of accreted metals. We detect photospheric metals in four out of the seven WDs. However, we find no significant correspondence between the existence of optical periodic variability and the detection of photospheric ultraviolet absorption lines. Thus the null hypothesis stands, that the two phenomena are not directly related. Some other source of WD surface inhomogeneity, perhaps related to magnetic field strength, combined with the WD rotation, or alternatively effects due to close binary companions, may be behind the observed optical modulation. We report the marginal detection of molecular hydrogen in WD J1949+4734, only the fourth known WD with detected H2 lines. We also re-classify J1926+4219 as a carbon-rich He-sdO subdwarf.

  8. Accretion-induced variability links young stellar objects, white dwarfs, and black holes.

    Science.gov (United States)

    Scaringi, Simone; Maccarone, Thomas J; Körding, Elmar; Knigge, Christian; Vaughan, Simon; Marsh, Thomas R; Aranzana, Ester; Dhillon, Vikram S; Barros, Susana C C

    2015-10-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies.

  9. A Strange Star Scenario for the Formation of Eccentric Millisecond Pulsar/Helium White Dwarf Binaries

    Science.gov (United States)

    Jiang, Long; Li, Xiang-Dong; Dey, Jishnu; Dey, Mira

    2015-07-01

    According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during binary evolution, as observed in most binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are the PSRs J2234+06, J1946+3417, and J1950+2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf (WD) companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive WDs, or the eccentricity may be induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars (NSs) in LMXBs may reach the density of quark deconfinement, which can lead to phase transition from NSs to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitational mass of the NS during the transition. The eccentricities can be reproduced with a reasonable estimate of the mass loss. This scenario might also account for the formation of the youngest known X-ray binary Cir X-1, which also possesses a low-field compact star in an eccentric orbit.

  10. The age-metallicity relation in the solar neighbourhood from a pilot sample of white dwarf-main sequence binaries

    Science.gov (United States)

    Rebassa-Mansergas, A.; Anguiano, B.; García-Berro, E.; Freeman, K. C.; Cojocaru, R.; Manser, C. J.; Pala, A. F.; Gänsicke, B. T.; Liu, X.-W.

    2016-12-01

    The age-metallicity relation (AMR) is a fundamental observational constraint for understanding how the Galactic disc formed and evolved chemically in time. However, there is not yet an agreement on the observational properties of the AMR for the solar neighbourhood, primarily due to the difficulty in obtaining accurate stellar ages for individual field stars. We have started an observational campaign for providing the much needed observational input by using wide white-dwarf-main-sequence (WDMS) binaries. White dwarfs are `natural' clocks and can be used to derive accurate ages. Metallicities can be obtained from the main-sequence companions. Since the progenitors of white dwarfs and the main-sequence stars were born at the same time, WDMS binaries provide a unique opportunity to observationally constrain in a robust way the properties of the AMR. In this work we present the AMR derived from analysing a pilot sample of 23 WDMS binaries and provide clear observational evidence for the lack of correlation between age and metallicity at young and intermediate ages (0-7 Gyr).

  11. Solar abundances of rock-forming elements, extreme oxygen and hydrogen in a young polluted white dwarf

    Science.gov (United States)

    Farihi, J.; Koester, D.; Zuckerman, B.; Vican, L.; Gänsicke, B. T.; Smith, N.; Walth, G.; Breedt, E.

    2016-12-01

    The Teff = 20 800 K white dwarf WD 1536+520 is shown to have broadly solar abundances of the major rock-forming elements O, Mg, Al, Si, Ca, and Fe, together with a strong relative depletion in the volatile elements C and S. In addition to the highest metal abundances observed to date, including log (O/He) = -3.4, the helium-dominated atmosphere has an exceptional hydrogen abundance at log (H/He) = -1.7. Within the uncertainties, the metal-to-metal ratios are consistent with the accretion of an H2O-rich and rocky parent body, an interpretation supported by the anomalously high trace hydrogen. The mixed atmosphere yields unusually short diffusion time-scales for a helium atmosphere white dwarf, of no more than a few hundred years, and equivalent to those in a much cooler, hydrogen-rich star. The overall heavy element abundances of the disrupted parent body deviate modestly from a bulk Earth pattern, and suggest the deposition of some core-like material. The total inferred accretion rate is 4.2 × 109 g s-1, and at least four times higher than for any white dwarf with a comparable diffusion time-scale. Notably, when accretion is exhausted in this system, both metals and hydrogen will become undetectable within roughly 300 Myr, thus supporting a scenario where the trace hydrogen is related to the ongoing accretion of planetary debris.

  12. Importance of fingering convection for accreting white dwarfs in the framework of full evolutionary calculations: the case of the hydrogen-rich white dwarfs GD 133 and G 29-38

    Science.gov (United States)

    Wachlin, F. C.; Vauclair, G.; Vauclair, S.; Althaus, L. G.

    2017-05-01

    Context. A large fraction of white dwarfs show photospheric chemical composition that is polluted by heavy elements accreted from a debris disk. Such debris disks result from the tidal disruption of rocky planetesimals that have survived to whole stellar evolution from the main sequence to the final white dwarf stage. Determining the accretion rate of this material is an important step toward estimating the mass of the planetesimals and understanding the ultimate fate of the planetary systems. Aims: The accretion of heavy material with a mean molecular weight, μ, higher than the mean molecular weight of the white dwarf outer layers, induces a double-diffusive instability producing the fingering convection and an extra-mixing. As a result, the accreted material is diluted deep into the star. We explore the effect of this extra-mixing on the abundance evolution of Mg, O, Ca, Fe and Si in the cases of the two well-studied polluted DAZ white dwarfs: GD 133 and G 29-38. Methods: We performed numerical simulations of the accretion of material that has a chemical composition similar to the bulk Earth composition. We assumed a continuous and uniform accretion and considered a range of accretion rates from 104 g/s to 1010 g/s. Two cases are simulated, one using the standard mixing length theory (MLT) and one including the double-diffusive instability (fingering convection). Results: The double-diffusive instability develops on a very short timescale. The surface abundance rapidly reaches a stationary value while the depth of the zone mixed by the fingering convection increases. In the case of GD 133, the accretion rate needed to reproduce the observed abundances exceeds by more than two orders of magnitude the rate estimated by neglecting the fingering convection. In the case of G 29-38 the needed accretion rate is increased by approximately 1.7 dex. Conclusions: Our numerical simulations of the accretion of heavy elements on the hydrogen-rich white dwarf GD 133 and G 29

  13. Stellar Laboratories: 3. New Ba 5, Ba 6, and Ba 7 Oscillator Strengths and the Barium Abundance in the Hot White Dwarfs G191-B2B and RE 0503-289

    Science.gov (United States)

    Rauch, T.; Werner, K.; Quinet, P.; Kruk, Jeffrey Walter

    2014-01-01

    Context. For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims. Reliable Ba 5-7 oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE 0503-289 and to determine their photospheric Ba abundances. Methods. We newly calculated Ba v-vii oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE 0503-289. Results. For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba vi and Ba vii lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE 0503-289. The Ba vi/Ba vii ionization equilibrium is well reproduced with the previously determined effective temperature of 70 000 K and surface gravity of log g=7.5. The Ba abundance is 3.5 +/- 0.5 × 10(exp-4) (mass fraction, about 23 000 times the solar value). In the FUSE spectrum of G191-B2B, we identified the strongest Ba vii line (at 993.41 Å) only, and determined a Ba abundance of 4.0 +/- 0.5 × 10(exp-6) (about 265 times solar). Conclusions. Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba vi-vii line profiles in two white dwarfs' (G191-B2B and RE 0503-289) far-ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed to determine the photospheric Ba abundance of these two stars precisely.

  14. Optical Identification of He White Dwarfs Orbiting Four Millisecond Pulsars in the Globular Cluster 47 Tucanae

    Science.gov (United States)

    Cadelano, M.; Pallanca, C.; Ferraro, F. R.; Salaris, M.; Dalessandro, E.; Lanzoni, B.; Freire, P. C. C.

    2015-10-01

    We used ultra-deep UV observations obtained with the Hubble Space Telescope to search for optical companions to binary millisecond pulsars (MSPs) in the globular cluster 47 Tucanae. We identified four new counterparts (to MSPs 47TucQ, 47TucS, 47TucT, and 47TucY) and confirmed those already known (to MSPs 47TucU and 47TucW). In the color-magnitude diagram, the detected companions are located in a region between the main sequence and the CO white dwarf (WD) cooling sequences, consistent with the cooling tracks of He WDs with masses between 0.15 M⊙ and 0.20 M⊙. For each identified companion, mass, cooling age, temperature, and pulsar mass (as a function of the inclination angle) have been derived and discussed. For 47TucU we also found that the past accretion history likely proceeded at a sub-Eddington rate. The companion to the redback 47TucW is confirmed to be a non-degenerate star, with properties particularly similar to those observed for black widow systems. Two stars have been identified within the 2σ astrometric uncertainty from the radio positions of 47TucH and 47TucI, but the available data prevent us from firmly assessing whether they are the true companions of these two MSPs. Based on observations collected with the NASA/ESA HST (Prop. 12950), obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.

  15. The end of the White Dwarf Cooling Sequence of NGC 6752

    Science.gov (United States)

    Bedin, Luigi

    2017-08-01

    We propose to study the last HST-accessible white dwarf (WD) cooling sequence (CS) for a nearby globular cluster (GC), the chemically complex, extreme blue horizontal branch cluster NGC 6752. Over 97% of stars end their lives as WDs, and the WD CS provides constraints not only on the age, but also potentially the star formation history of a GC. The CS of WDs also lies in the least-explored region of the color-magnitude diagram of old stellar populations. Recent deep imaging with HST has successfully reached the end of the WD CS in only three classical old GCs, M4, NGC 6397 and 47 Tuc, and reveals an unexpectedly complex, and double-peaked, WD CS in the metal rich old open cluster NGC 6791. One more investigation is in progress on the massive globular Omega Centauri, where over 14 sub-populations are known to exist.While almost every cluster is known to host multiple populations, every single cluster is unique. NGC 6752 is a bridge between the relatively simple globular clusters, and Omega Cen, the most complex globular cluster known. NGC 6752 has an extended blue horizontal branch, a collapsed core and 3 chemically distinct populations. It is our last chance to add diversity to our very limited sample of WD CS, so far containing only 3 globular clusters, one old open cluster, and the complex Omega Cen system. We need to undertake this investigation while HST is still operational, as there is no foreseeable opportunity in the post-HST era to have one extra WD CS in the homogeneus optical photometric system of HST.

  16. Far-UV Spectroscopy of Two Extremely Hot, Helium-Rich White Dwarfs

    Science.gov (United States)

    Werner, K.; Rauch, T.; Kruk, J. W.

    2017-01-01

    A large proportion of hot post-asymptotic giant branch stars and white dwarfs (WDs) are hydrogen-deficient. Two distinct evolutionary sequences have been identified. One of them comprises stars of spectral type [WC] and PG1159, and it originates from a late helium-shell flash, creating helium-rich stellar atmospheres with significant admixtures of carbon (up to about 50, mass fraction). The other sequence comprises stars of spectral type O(He) and luminous subdwarf O stars which possibly are descendants of RCrB stars and extreme helium stars. Their carbon abundances are significantly lower (of the order of 1 or less) and it is thought that they originate from binary-star evolution (through merger or common-envelope evolution). Here we investigate two of the three hottest known helium-rich (DO) WDs (PG 1034+001 and PG 0038+199). They are the only ones for which spectra were recorded with the Far Ultraviolet Spectroscopic Explorer and the Hubble Space Telescope, allowing a comprehensive ultraviolet spectral analysis. We find effective temperatures of T(eff) =115000 +/- 5000 K and 125000 +/- 5000 K, respectively, and a surface gravity of log g = 7 +/-0.5. In both stars, nitrogen is strongly oversolar while C and O are significantly subsolar. For all other assessed metals (Ne, Si, P, S, Ar, Fe, and Ni) we find abundances close to solar. We conclude that these WDs are immediate descendants of O(He) stars and, hence, result from close-binary evolution.

  17. THE EFFECTS OF CURVATURE AND EXPANSION ON HELIUM DETONATIONS ON WHITE DWARF SURFACES

    Energy Technology Data Exchange (ETDEWEB)

    Moore, Kevin; Bildsten, Lars [Department of Physics, University of California, Santa Barbara, CA (United States); Townsley, Dean M. [Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL (United States)

    2013-10-20

    Accreted helium layers on white dwarfs have been highlighted for many decades as a possible site for a detonation triggered by a thermonuclear runaway. In this paper, we find the minimum helium layer thickness that will sustain a steady laterally propagating detonation and show that it depends on the density and composition of the helium layer, specifically {sup 12}C and {sup 16}O. Detonations in these thin helium layers have speeds slower than the Chapman-Jouget (CJ) speed from complete helium burning, v{sub CJ} = 1.5 × 10{sup 9} cm s{sup –1}. Though gravitationally unbound, the ashes still have unburned helium (≈80% in the thinnest cases) and only reach up to heavy elements such as {sup 40}Ca, {sup 44}Ti, {sup 48}Cr, and {sup 52}Fe. It is rare for these thin shells to generate large amounts of {sup 56}Ni. We also find a new set of solutions that can propagate in even thinner helium layers when {sup 16}O is present at a minimum mass fraction of ≈0.07. Driven by energy release from α captures on {sup 16}O and subsequent elements, these slow detonations only create ashes up to {sup 28}Si in the outer detonated He shell. We close by discussing how the unbound helium burning ashes may create faint and fast 'Ia' supernovae as well as events with virtually no radioactivity, and speculate on how the slower helium detonation velocities impact the off-center ignition of a carbon detonation that could cause a Type Ia supernova in the double detonation scenario.

  18. The White-Dwarf Mass-Radius Relation from 40 Eridani B and Other Nearby Visual Binaries

    Science.gov (United States)

    Bond, Howard E.; Bergeron, P.; Bedard, A.

    2018-01-01

    The bright, nearby DA-type white dwarf (WD) 40 Eridani B is orbited by the M dwarf 40 Eri C, allowing determination of the WD's mass. Until recently, however, the mass depended on orbital elements determined four decades ago, and that mass was so low that it created several astrophysical puzzles. Using new astrometric measurements, the binary-star group at the U.S. Naval Observatory has revised the dynamical mass upward, to 0.573 ± 0.018 M⊙. We have used model-atmosphere analysis to update other parameters of the WD, including effective temperature, surface gravity, radius, and luminosity. We then comparethese results with WD interior models.Within the observational uncertainties, theoretical cooling tracks for CO-core WDs of its measured mass are consistent with the position of 40 Eri B in the H-R diagram; equivalently, the theoretical mass-radius relation (MRR) is consistent with the star's location in the mass-radius plane. This consistency is, however, achieved only if we assume a "thin'' outer hydrogen layer, with qH = MH/MWD ∼ 10–10.We discuss other evidence that a significant fraction of DA WDs have such thin H layers, in spite of expectation from canonical stellar-evolution theory of "thick'' H layers with qH ∼ 10–4 . The cooling age of 40 Eri B is ~122 Myr, and its total age is ~1.8 Gyr. We present the MRRs for 40 Eri B and three other nearby WDs in visual binaries with precise mass determinations, and show that the agreement of current theory with observation is excellent in all cases.However, astrophysical puzzles remain. The eccentricity of the BC orbit has remained high (0.43), even though the progenitor of B ought to have interacted tidally with C when it was an AGB star. This puzzle exists also for the Sirius and Procyon systems. If thin hydrogen layers are common among WDs, the mass scale will need to be shifted downwards by a few hundredths of a solar mass.

  19. CRYSTAL CHEMISTRY OF THREE-COMPONENT WHITE DWARFS AND NEUTRON STAR CRUSTS: PHASE STABILITY, PHASE STRATIFICATION, AND PHYSICAL PROPERTIES

    Energy Technology Data Exchange (ETDEWEB)

    Engstrom, T. A.; Yoder, N. C.; Crespi, V. H., E-mail: tae146@psu.edu, E-mail: ncy5007@psu.edu, E-mail: vhc2@psu.edu [Department of Physics, The Pennsylvania State University, University Park, PA 16802 (United States)

    2016-02-20

    A systematic search for multicomponent crystal structures is carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are “bred” by a genetic algorithm and optimized at constant pressure under the assumption of linear response (Thomas–Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the T = 0 bulk phase diagrams, five of which are complicated multinary structures not previously predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferences of systems with pairwise Yukawa interactions, including and extending to the regime of low-density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravitating, hydrostatically stable white dwarf, with fixed overall composition. To our knowledge, this is the first attempt to incorporate complex multinary phases into the equilibrium phase-layering diagram and mass–radius-composition dependence, both of which are reported for He–C–O and C–O–Ne white dwarfs. Finite thickness interfacial phases (“interphases”) show up at the boundaries between single-component body-centered cubic (bcc) crystalline regions, some of which have lower lattice symmetry than cubic. A second application—quasi-static settling of heavy nuclei in white dwarfs—builds on our equilibrium phase-layering method. Tests of this nonequilibrium method reveal extra phases that play the role of transient host phases for the settling species.

  20. Surprising Rapid Collapse of Sirius B from Red Giant to White Dwarf Through Mass Transfer to Sirius a

    Science.gov (United States)

    Yousef, Shahinaz; Ali, Ola

    2013-03-01

    Sirius was observed in antiquity as a red star. In his famous astronomy textbook the Almagest written 140 AD, Ptolemy described the star Sirius as fiery red. He curiously depicted it as one of six red-colored stars. The other five are class M and K stars, such as Arcturus and Betelgeuse. Apparent confirmation in ancient Greek and Roman sources are found and Sirius was also reported red in Europe about 1400 years ago. Sirius must have changed to a white dwarf in the night of Ascension. The star chapter in the Quran started with "by the star as it collapsed (1) your companion have not gone astray nor being misled (2), and in verse 49 which is the rotation period of the companion Sirius B around Sirius A, it is said" He is the Lord of Sirius (49). If Sirius actually was red what could have caused it to change into the brilliant bluish-white star we see today? What the naked eye perceives as a single star is actually a binary star system, consisting of a white main sequence star of spectral type A1V, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The red color indicates that the star seen then was a red giant. It looks that what they have seen in antiquity was Sirius B which was then a red giant and it collapsed to form a white dwarf. Since there is no evidence of a planetary nebula, then the red Sirius paradox can be solved in terms of stellar evolution with mass transfer. Sirius B was the most massive star which evolved to a red giant and filled the Roche lobe. Mass transfer to Sirius A occurred through the Lagrangian point. Sirius A then became more massive while Sirius B lost mass and shrank. Sirius B then collapsed abruptly into a white dwarf. In the case of Algol, Ptolmy observed it as white star but it was red at the time of El sufi. At present it is white. The rate of mass transfer from Sirius B to Sirius A, and from Algol B to A is estimated from observational data of colour change from red to bullish white to be 0

  1. ARE ULTRA-LONG GAMMA-RAY BURSTS CAUSED BY BLUE SUPERGIANT COLLAPSARS, NEWBORN MAGNETARS, OR WHITE DWARF TIDAL DISRUPTION EVENTS?

    Energy Technology Data Exchange (ETDEWEB)

    Ioka, Kunihito [Center for Gravitational Physics, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502 (Japan); Hotokezaka, Kenta; Piran, Tsvi, E-mail: kunihito.ioka@yukawa.kyoto-u.ac.jp [Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904 (Israel)

    2016-12-10

    Ultra-long gamma-ray bursts (ulGRBs) are a new population of GRBs with extreme durations of ∼10{sup 4} s. Leading candidates for their origin are blue supergiant collapsars, magnetars, and white dwarf tidal disruption events (WD-TDEs) caused by massive black holes (BHs). Recent observations of supernova-like (SN-like) bumps associated with ulGRBs challenged both the WD-TDE and the blue supergiant models because of the detection of SNe and the absence of hydrogen lines, respectively. We propose that WD-TDEs can accommodate the observed SN-like bumps if the fallback WD matter releases energy into the unbound WD ejecta. The observed ejecta energy, luminosity, and velocity are explained by the gravitational energy, Eddington luminosity, and escape velocity of the formed accretion disk, respectively. We also show that the observed X-rays can ionize the ejecta, eliminating lines. The SN-like light curves (SN 2011kl) for the ulGRB 111209A are consistent with all three models, although a magnetar model is unnatural because the spin-down time required to power the SN-like bump is a hundred times longer than the GRB. Our results imply that TDEs are a possible energy source for SN-like events in general and for ulGRBs in particular.

  2. Down the Tubes: Vetting the Apparent Water-rich Parent Body being Accreted by the White Dwarf GD 16

    Science.gov (United States)

    Melis, Carl

    2015-10-01

    How water is distributed in a planetary system critically affects the formation, evolution, and habitability of its constituent rocky bodies. White dwarf stars provide a unique method to probe the prevalence of water-rich rocky bodies outside of our Solar system and where they preferentially reside in a planetary system. However, as evidenced by the case of GD 362, some parent bodies that at first glance might appear to be water-rich can actually be quite water-scarce. At this time there are only a small number of plausibly water-rich rocky bodies that are being actively accreted by their host white dwarf star. Given such a sample size it is crucial to characterize each one in sufficient detail to remove interlopers like GD 362 that might otherwise affect future statistical analyses. In this proposal we seek to vet GD 16, a water-rich candidate yet to be observed with HST-COS that is the brightest remaining such target in the UV.

  3. FUSE observations of PG1342+444: new insights into the nature of the hottest DA white dwarfs

    Science.gov (United States)

    Barstow, M. A.; Good, S. A.; Holberg, J. B.; Burleigh, M. R.; Bannister, N. P.; Hubeny, I.; Napiwotzki, R.

    2002-02-01

    We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the very hot (Teff~60000K) DA white dwarf PG1342+444, describing our data reduction and analysis techniques. The spectrum reveals a number of photospheric absorption lines from high ionization species along with numerous interstellar features. The photospheric detections include the 1031.9- and 1037.0-Å OVI lines which are seen for the first time in a hot DA atmosphere and are usually associated with the much hotter PG1159 stars and so-called OVI central stars of planetary nebulae. Estimates of the stellar effective temperature made independently using both the Balmer and Lyman series lines are in disagreement (Teff~67000 and ~54000K respectively), when taking into account just the statistical uncertainties in the analyses. However, the presence of weak absorption from the CIII multiplet near 1176Å, which is predicted to be much stronger if the star were as cool as the Lyman measurement suggests, leads us to favour the higher temperature. PG1342+444 appears to have enhanced C, Fe and Ni abundances in its atmosphere compared with all the other G191-B2B-like DA white dwarfs, which might affect the temperature structure of the atmosphere if not homogeneously distributed, as assumed in this study.

  4. ESO 439-162/163 - a common proper motion binary formed by a magnetic DQ and a DC type white dwarf

    Energy Technology Data Exchange (ETDEWEB)

    Ruiz, M.T.; Maza, J.

    1988-12-01

    In the course of a search for faint large proper motion stars, a common proper motion pair was identified having a mu = 0.38 + or - 0.03 arcsec/yr in the direction theta = 233 deg. The stars are separated by 23 arcsec and have apparent visual magnitudes 18.77 and 19.84, respectively. Spectrophotometry of the stars established that the fainter component is a cold DC white dwarf, while the brighter one is a magnetic white dwarf with strong Swan bands of C2 shifted and broadened by an about 10 to the 8th G magnetic field. 6 references.

  5. The Carnegie Supernova Project. I. Third Photometry Data Release of Low-redshift Type Ia Supernovae and Other White Dwarf Explosions

    Science.gov (United States)

    Krisciunas, Kevin; Contreras, Carlos; Burns, Christopher R.; Phillips, M. M.; Stritzinger, Maximilian D.; Morrell, Nidia; Hamuy, Mario; Anais, Jorge; Boldt, Luis; Busta, Luis; Campillay, Abdo; Castellón, Sergio; Folatelli, Gastón; Freedman, Wendy L.; González, Consuelo; Hsiao, Eric Y.; Krzeminski, Wojtek; Persson, Sven Eric; Roth, Miguel; Salgado, Francisco; Serón, Jacqueline; Suntzeff, Nicholas B.; Torres, Simón; Filippenko, Alexei V.; Li, Weidong; Madore, Barry F.; DePoy, D. L.; Marshall, Jennifer L.; Rheault, Jean-Philippe; Villanueva, Steven

    2017-11-01

    We present final natural-system optical (ugriBV) and near-infrared (YJH) photometry of 134 supernovae (SNe) with probable white dwarf progenitors that were observed in 2004-2009 as part of the first stage of the Carnegie Supernova Project (CSP-I). The sample consists of 123 Type Ia SNe, 5 Type Iax SNe, 2 super-Chandrasekhar SN candidates, 2 Type Ia SNe interacting with circumstellar matter, and 2 SN 2006bt-like events. The redshifts of the objects range from z=0.0037 to 0.0835; the median redshift is 0.0241. For 120 (90%) of these SNe, near-infrared photometry was obtained. Average optical extinction coefficients and color terms are derived and demonstrated to be stable during the five CSP-I observing campaigns. Measurements of the CSP-I near-infrared bandpasses are also described, and near-infrared color terms are estimated through synthetic photometry of stellar atmosphere models. Optical and near-infrared magnitudes of local sequences of tertiary standard stars for each supernova are given, and a new calibration of Y-band magnitudes of the Persson et al. standards in the CSP-I natural system is presented.

  6. The Virtual Observatory Service TheoSSA: Establishing a Database of Synthetic Stellar Flux Standards I. NLTE Spectral Analysis of the DA-Type White Dwarf G191-B2B *,**,***,****

    Science.gov (United States)

    Rauch, T.; Werner, K.; Bohlin, R.; Kruk, J. W.

    2013-01-01

    Hydrogen-rich, DA-type white dwarfs are particularly suited as primary standard stars for flux calibration. State-of-the-art NLTE models consider opacities of species up to trans-iron elements and provide reliable synthetic stellar-atmosphere spectra to compare with observations. Aims. We will establish a database of theoretical spectra of stellar flux standards that are easily accessible via a web interface. Methods. In the framework of the Virtual Observatory, the German Astrophysical Virtual Observatory developed the registered service TheoSSA. It provides easy access to stellar spectral energy distributions (SEDs) and is intended to ingest SEDs calculated by any model-atmosphere code. In case of the DA white dwarf G191-B2B, we demonstrate that the model reproduces not only its overall continuum shape but also the numerous metal lines exhibited in its ultraviolet spectrum. Results. TheoSSA is in operation and contains presently a variety of SEDs for DA-type white dwarfs. It will be extended in the near future and can host SEDs of all primary and secondary flux standards. The spectral analysis of G191-B2B has shown that our hydrostatic models reproduce the observations best at Teff =60 000 +/- 2000K and log g=7.60 +/- 0.05.We newly identified Fe vi, Ni vi, and Zn iv lines. For the first time, we determined the photospheric zinc abundance with a logarithmic mass fraction of -4.89 (7.5 × solar). The abundances of He (upper limit), C, N, O, Al, Si, O, P, S, Fe, Ni, Ge, and Sn were precisely determined. Upper abundance limits of about 10% solar were derived for Ti, Cr, Mn, and Co. Conclusions. The TheoSSA database of theoretical SEDs of stellar flux standards guarantees that the flux calibration of all astronomical data and cross-calibration between different instruments can be based on the same models and SEDs calculated with different model-atmosphere codes and are easy to compare.

  7. Double Osbnd Ne-Mg white dwarfs merging as the source of the powerfull gravitational waves for LIGO/VIRGO type interferometers

    Science.gov (United States)

    Lipunov, V. M.

    2017-10-01

    New strong non-spiralling-in gravitational wave (GW) source for LIGO/VIRGO detectors are proposed. Double Osbnd Ne-Mg white dwarf mergers can produce strong gravitational waves with frequencies in the several hundreds Hz range. Such events can be followed by a Super Nova type Ia.

  8. Stellar Laboratories II. New Zn Iv and Zn v Oscillator Strengths and Their Validation in the Hot White Dwarfs G191-B2B and RE0503-289

    Science.gov (United States)

    Rauch, T.; Werner, K.; Quinet, P.; Kruk, J. W.

    2014-01-01

    Context. For the spectral analysis of high-resolution and high-signal-to-noise (SN) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. In a recent analysis of the ultraviolet (UV) spectrum of the DA-type white dwarf G191B2B,21 Zn iv lines were newly identified. Because of the lack of Zn iv data, transition probabilities of the isoelectronic Ge vi were adapted for a first, coarse determination of the photospheric Zn abundance.Aims. Reliable Zn iv and Zn v oscillator strengths are used to improve the Zn abundance determination and to identify more Zn lines in the spectra of G191B2B and the DO-type white dwarf RE 0503289. Methods. We performed new calculations of Zn iv and Zn v oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of the Zn iv v spectrum exhibited in high-resolution and high-SN UV observations of G191B2B and RE 0503289. Results. In the UV spectrum of G191B2B, we identify 31 Zn iv and 16 Zn v lines. Most of these are identified for the first time in any star. We can reproduce well almost all of them at log Zn 5.52 0.2 (mass fraction, about 1.7 times solar). In particular, the Zn iv Zn v ionization equilibrium, which is a very sensitive Teff indicator, is well reproduced with the previously determined Teff 60 000 2000 K and log g 7.60 0.05. In the spectrum of RE 0503289, we identified 128 Zn v lines for the first time and determined log Zn 3.57 0.2 (155 times solar). Conclusions. Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Zn iv and Zn v line profiles in two white dwarf (G191B2B and RE 0503289) ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed us to determine the

  9. A 1.05Msun Companion to PSR J2222-0137: The Coolest Known White Dwarf?

    Science.gov (United States)

    Kaplan, David

    2014-10-01

    The recycled pulsar PSR J2222-0137 is one of the closest known neutron stars, with a parallax distance of 267+/-1 pc. Using radio Shapiro delay measurements, we determine a companion mass (1.05+/-0.06 Msun) consistent with either a low-mass neutron star or a high-mass white dwarf (WD). However, the orbital eccentricity is too low to be the product of two supernovae and we are forced to conclude that the companion is a WD. And yet, despite deep optical and near-infrared searches with SOAR and the Keck telescopes we have not discovered the optical counterpart of the system and can limit its effective temperature to pulsars.

  10. Numerical simulation of exploding pusher targets

    Science.gov (United States)

    Atzeni, S.; Rosenberg, M. J.; Gatu Johnson, M.; Petrasso, R. D.

    2017-10-01

    Exploding pusher targets, i.e. gas-filled large aspect-ratio glass or plastic shells, driven by a strong laser-generated shock, are widely used as pulsed sources of neutrons and fast charged particles. Recent experiments on exploding pushers provided evidence for the transition from a purely fluid behavior to a kinetic one. Indeed, fluid models largely overpredict yield and temperature as the Knudsen number Kn (ratio of ion mean-free path to compressed gas radius) is comparable or larger than one. At Kn = 0.3 - 1, fluid codes reasonably estimate integral quantities as yield and neutron-averaged temperatures, but do not reproduce burn radii, burn profiles and DD/DHe3 yield ratio. This motivated a detailed simulation study of intermediate-Kn exploding pushers. We will show how simulation results depend on models for laser-interaction, electron conductivity (flux-limited local vs nonlocal), viscosity (physical vs artificial), and ion mixing. Work partially supported by Sapienza Project C26A15YTMA, Sapienza 2016 (n. 257584), and Eurofusion Project AWP17-ENR-IFE-CEA-01.

  11. A comprehensive near- and far-ultraviolet spectroscopic study of the hot DA white dwarf G191-B2B

    Science.gov (United States)

    Preval, S. P.; Barstow, M. A.; Holberg, J. B.; Dickinson, N. J.

    2013-11-01

    We present a detailed spectroscopic analysis of the hot DA white dwarf G191-B2B, using the best signal-to-noise ratio, high-resolution near- and far-UV spectrum obtained to date. This is constructed from co-added Hubble Space Telescope (HST) Space Telescope Imaging Spectrometer (STIS) E140H, E230H and FUSE observations, covering the spectral ranges of 1150-3145 Å and 910-1185 Å, respectively. With the aid of recently published atomic data, we have been able to identify previously undetected absorption features down to equivalent widths of only a few mÅ. In total, 976 absorption features have been detected to 3σ confidence or greater, with 947 of these lines now possessing an identification, the majority of which are attributed to Fe and Ni transitions. In our survey, we have also potentially identified an additional source of circumstellar material originating from Si III. While we confirm the presence of Ge detected by Vennes et al., we do not detect any other species. Furthermore, we have calculated updated abundances for C, N, O, Si, P, S, Fe and Ni, while also calculating, for the first time, a non-local thermodynamic equilibrium abundance for Al, deriving Al III/H=1.60_{-0.08}^{+0.07}× {10}^{-7}. Our analysis constitutes what is the most complete spectroscopic survey of any white dwarf. All observed absorption features in the FUSE spectrum have now been identified, and relatively few remain elusive in the STIS spectrum.

  12. Once in a blue moon: detection of `bluing' during debris transits in the white dwarf WD 1145+017

    Science.gov (United States)

    Hallakoun, N.; Xu (许偲艺), S.; Maoz, D.; Marsh, T. R.; Ivanov, V. D.; Dhillon, V. S.; Bours, M. C. P.; Parsons, S. G.; Kerry, P.; Sharma, S.; Su, K.; Rengaswamy, S.; Pravec, P.; Kušnirák, P.; Kučáková, H.; Armstrong, J. D.; Arnold, C.; Gerard, N.; Vanzi, L.

    2017-08-01

    The first transiting planetesimal orbiting a white dwarf was recently detected in K2 data of WD 1145+017 and has been followed up intensively. The multiple, long and variable transits suggest the transiting objects are dust clouds, probably produced by a disintegrating asteroid. In addition, the system contains circumstellar gas, evident by broad absorption lines, mostly in the u΄ band, and a dust disc, indicated by an infrared excess. Here we present the first detection of a change in colour of WD 1145+017 during transits, using simultaneous multiband fast-photometry ULTRACAM measurements over the u΄g΄r΄i΄ bands. The observations reveal what appears to be 'bluing' during transits; transits are deeper in the redder bands, with a u΄ - r΄ colour difference of up to ∼-0.05 mag. We explore various possible explanations for the bluing, including limb darkening or peculiar dust properties. 'Spectral' photometry obtained by integrating over bandpasses in the spectroscopic data in and out of transit, compared to the photometric data, shows that the observed colour difference is most likely the result of reduced circumstellar absorption in the spectrum during transits. This indicates that the transiting objects and the gas share the same line of sight and that the gas covers the white dwarf only partially, as would be expected if the gas, the transiting debris and the dust emitting the infrared excess are part of the same general disc structure (although possibly at different radii). In addition, we present the results of a week-long monitoring campaign of the system using a global network of telescopes.

  13. VizieR Online Data Catalog: Solar neighborhood. XXXIX. Nearby white dwarfs (Subasavage+, 2017)

    Science.gov (United States)

    Subasavage, J. P.; Jao, W.-C.; Henry, T. J.; Harris, H. C.; Dahn, C. C.; Bergeron, P.; Dufour, P.; Dunlap, B. H.; Barlow, B. N.; Ianna, P. A.; Lepine, S.; Margheim, S. J.

    2017-10-01

    Standardized photometric observations were carried out at three separate telescopes. The Small & Moderate Aperture Research Telescope System (SMARTS) 0.9m telescope at Cerro Tololo Inter-American Observatory (CTIO) was used during Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI) observing runs when conditions were photometric. A Tektronics 2K*2K detector was used in region-of-interest mode centered on the central quarter of the full CCD producing a Field Of View (FOV) of 6.8'*6.8'. The SMARTS 1.0m telescope at CTIO was used with the Y4KCam 4K*4K imager, producing a 19.7'*19.7' FOV. Finally, the Ritchey 40-in telescope at USNO Flagstaff Station (NOFS) was used with a Tektronics 2K*2K detector with a 20.0'*20.0' FOV. Near-infrared JHKs photometry was collected for WD0851-246, at the CTIO 4.0 m Blanco telescope using the NEWFIRM during an engineering night on 2011.27 UT. National Optical Astronomy Observatory (NOAO) Extremely Wide-field Infrared Image (NEWFIRM) is a 4K*4K InSb mosaic that provides a 28'*28' FOV on the Blanco telescope. Additional photometry values were extracted from the Sloan Digital Sky Survey (SDSS) DR12 (Alam et al. 2015, Cat. V/147), 2MASS, and the United Kingdom Infra-Red Telescope (UKIRT) Infrared Sky Survey (UKIDSS) DR9 Large Area Survey (LAS; see Lawrence et al. 2012, Cat. II/319), when available. Two White Dwarfs (WDs) presented here (WD1743-545 and WD2057-493) are newly discovered nearby WDs identified during a spectroscopic survey of WD candidates in the southern hemisphere (J. Subasavage et al. 2017, in preparation) taken from the SUPERBLINK catalog (Lepine & Shara 2015ASPC..493..455S). A third WD included here (WD2307-691) was previously unclassified, yet is a common proper-motion companion to a Hipparcos star within 25pc (HIP114416). A fourth WD (WD2028-171) was suspected to be a WD by the authors based on a trawl of the New Luyten Two Tenths (NLTT) catalog (Luyten 1979, Cat. I/98). Finally, a fifth WD (WD1241

  14. A Detailed Model Atmosphere Analysis of Cool White Dwarfs in the Sloan Digital Sky Survey

    Science.gov (United States)

    2010-09-01

    J09194811+4356216, Figure 7) in the Harris et al. (2006) Vtan 30 km s−1 and Mbol > 14.6 mag sample is actually a subdwarf, which corresponds to a...Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil), and Ministerio de Ciencia , Tecnologı́a e Innovación Productiva (Argentina

  15. Stellar laboratories. IV. New Ga iv, Ga v, and Ga vi oscillator strengths and the gallium abundance in the hot white dwarfs G191-B2B and RE 0503-289

    Science.gov (United States)

    Rauch, T.; Werner, K.; Quinet, P.; Kruk, J. W.

    2015-05-01

    Context. For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, advanced non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These atmospheres are strongly dependent on the reliability of the atomic data that are used to calculate them. Aims: Reliable Ga iv-vi oscillator strengths are used to identify Ga lines in the spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE 0503-289 and to determine their photospheric Ga abundances. Methods: We newly calculated Ga iv-vi oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for analyzing of Ga lines exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE 0503-289. Results: We unambiguously detected 20 isolated and 6 blended (with lines of other species) Ga v lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE 0503-289. The identification of Ga iv and Ga vi lines is uncertain because they are weak and partly blended by other lines. The determined Ga abundance is 3.5 ± 0.5 × 10-5 (mass fraction, about 625 times the solar value). The Ga iv/Ga v ionization equilibrium, which is a very sensitive indicator for the effective temperature, is well reproduced in RE 0503-289. We identified the strongest Ga iv lines (at 1258.801, 1338.129 Å) in the HST/STIS spectrum of G191-B2B and measured a Ga abundance of 2.0 ± 0.5 × 10-6 (about 22 times solar). Conclusions: Reliable measurements and calculations of atomic data are a prerequisite for stellar-atmosphere modeling. The observed Ga iv-v line profiles in two white dwarf (G191-B2B and RE 0503-289) ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. For the first time, this allowed us to determine the photospheric Ga abundance in white dwarfs. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space

  16. Stellar laboratories. III. New Ba v, Ba vi, and Ba vii oscillator strengths and the barium abundance in the hot white dwarfs G191-B2B and RE 0503-289

    Science.gov (United States)

    Rauch, T.; Werner, K.; Quinet, P.; Kruk, J. W.

    2014-06-01

    Context. For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims: Reliable Ba v-vii oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE 0503-289 and to determine their photospheric Ba abundances. Methods: We newly calculated Ba v-vii oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE 0503-289. Results: For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba vi and Ba vii lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE 0503-289. The Ba vi/Ba vii ionization equilibrium is well reproduced with the previously determined effective temperature of 70 000 K and surface gravity of log g = 7.5. The Ba abundance is 3.5 ± 0.5 × 10-4 (mass fraction, about 23 000 times the solar value). In the FUSE spectrum of G191-B2B, we identified the strongest Ba vii line (at 993.41 Å) only, and determined a Ba abundance of 4.0 ± 0.5 × 10-6 (about 265 times solar). Conclusions: Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba vi-vii line profiles in two white dwarfs' (G191-B2B and RE 0503-289) far-ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed to determine the photospheric Ba abundance of these two stars precisely. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for

  17. Stellar laboratories. VI. New Mo iv-vii oscillator strengths and the molybdenum abundance in the hot white dwarfs G191-B2B and RE 0503-289

    Science.gov (United States)

    Rauch, T.; Quinet, P.; Hoyer, D.; Werner, K.; Demleitner, M.; Kruk, J. W.

    2016-03-01

    Context. For the spectral analysis of high-resolution and high signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims: To identify molybdenum lines in the ultraviolet (UV) spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE 0503-289 and, to determine their photospheric Mo abundances, reliable Mo iv-vii oscillator strengths are used. Methods: We newly calculated Mo iv-vii oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Mo lines exhibited in high-resolution and high S/N UV observations of RE 0503-289. Results: We identified 12 Mo v and 9 Mo vi lines in the UV spectrum of RE 0503-289 and measured a photospheric Mo abundance of 1.2-3.0 × 10-4 (mass fraction, 22 500-56 400 times the solar abundance). In addition, from the As v and Sn iv resonance lines, we measured mass fractions of arsenic (0.5-1.3 × 10-5, about 300-1200 times solar) and tin (1.3-3.2 × 10-4, about 14 300-35 200 times solar). For G191-B2B, upper limits were determined for the abundances of Mo (5.3 × 10-7, 100 times solar) and, in addition, for Kr (1.1 × 10-6, 10 times solar) and Xe (1.7 × 10-7, 10 times solar). The arsenic abundance was determined (2.3-5.9 × 10-7, about 21-53 times solar). A new, registered German Astrophysical Virtual Observatory (GAVO) service, TOSS, has been constructed to provide weighted oscillator strengths and transition probabilities. Conclusions: Reliable measurements and calculations of atomic data are a prerequisite for stellar-atmosphere modeling. Observed Mo v-vi line profiles in the UV spectrum of the white dwarf RE 0503-289 were well reproduced with our newly calculated oscillator strengths. For the first time, this allowed the photospheric Mo

  18. A Multiwavelength Study of Nearby Millisecond Pulsar PSR J1400-1431: Improved Astrometry and an Optical Detection of Its Cool White Dwarf Companion

    Science.gov (United States)

    Swiggum, J. K.; Kaplan, D. L.; McLaughlin, M. A.; Lorimer, D. R.; Bogdanov, S.; Ray, P. S.; Lynch, R.; Gentile, P.; Rosen, R.; Heatherly, S. A.; Barlow, B. N.; Hegedus, R. J.; Vasquez Soto, A.; Clancy, P.; Kondratiev, V. I.; Stovall, K.; Istrate, A.; Penprase, B.; Bellm, E. C.

    2017-09-01

    In 2012, five high-school students involved in the Pulsar Search Collaboratory discovered the millisecond pulsar (MSP) PSR J1400-1431, and initial timing parameters were published in Rosen et al. a year later. Since then, we have obtained a phase-connected timing solution spanning five years, resolving a significant position discrepancy and measuring \\dot{P}, proper motion, parallax, and a monotonic slope in dispersion measure over time. Due to PSR J1400-1431’s proximity and significant proper motion, we use the Shklovskii effect and other priors to determine a 95% confidence interval for PSR J1400-1431’s distance, d={270}-80+130 pc. With an improved timing position, we present the first detection of the pulsar’s low-mass white dwarf (WD) companion using the Goodman Spectrograph on the 4.1 m SOAR telescope. Deeper imaging suggests that it is a cool DA-type WD with {T}{eff}=3000+/- 100 K and R/{R}⊙ =(2.19+/- 0.03)× {10}-2 (d/270 {pc}). We show a convincing association between PSR J1400-1431 and a γ-ray point source, 3FGL J1400.5-1437, but only weak (3.3σ) evidence of pulsations after folding γ-ray photons using our radio timing model. We detect an X-ray counterpart with XMM-Newton, but the measured X-ray luminosity (1×1029 erg s-1) makes PSR J1400-1431 the least X-ray luminous rotation-powered MSP detected to date. Together, our findings present a consistent picture of a nearby (d≈ 230 pc) MSP in a 9.5-day orbit around a cool ˜0.3 M ⊙ WD companion, with orbital inclination I≳ 60^\\circ .

  19. Three new barium dwarfs with white dwarf companions: BD+68°1027, RE J0702+129 and BD+80°670

    Science.gov (United States)

    Kong, X. M.; Bharat Kumar, Y.; Zhao, G.; Zhao, J. K.; Fang, X. S.; Shi, J. R.; Wang, L.; Zhang, J. B.; Yan, H. L.

    2018-02-01

    We report three new barium (Ba) dwarfs lying in Sirius-like systems. They provide direct evidence that Ba dwarfs are companions to white dwarfs (WDs). Atmospheric parameters, stellar masses and the chemical abundances of 25 elements, including light, α, Fe-peak and s-process elements, are derived from high-resolution and high S/N spectra. The enhancement of s-process elements with [s/Fe] ratios between 0.4 and 0.6 confirm them as mild barium stars. The estimated metallicities (-0.31, -0.06 and 0.13) of BD+68°1027, RE J0702+129 and BD+80°670 are in the range of known Ba dwarfs and giants. As expected, the observed indices [hs/ls], [s/Fe] and [C/Fe] show an anti-correlation with metallicity. Asymptotic giant branch (AGB) progenitor masses are estimated for the WD companions of RE J0702+129 (1.47 M⊙) and BD+80°670 (3.59 M⊙). These confirm the predicted range of progenitor AGB masses (1.5-4 M⊙) for unseen WDs around Ba dwarfs. The surface abundances of s-process elements in RE J0702+129 and BD+80°670 are compared with AGB models and they are in close agreement, within the predicted accretion efficiencies and pollution factors for Ba stars. These results support that the origin of s-process overabundances in Ba dwarfs is similar to those of Ba giants via the McClure hypothesis in which Ba stars accumulate s-process elements through mass transfer from their host companions during the AGB phase.

  20. An estimation of the white dwarf mass in the Dwarf Nova GK Persei with NuSTAR observations of two states

    Science.gov (United States)

    Wada, Yuuki; Yuasa, Takayuki; Nakazawa, Kazuhiro; Makishima, Kazuo; Hayashi, Takayuki; Ishida, Manabu

    2018-02-01

    We report on X-ray observations of the Dwarf Nova GK Persei performed by NuSTAR in 2015. GK Persei, behaving also as an Intermediate Polar, exhibited a Dwarf Nova outburst in 2015 March-April. The object was observed with NuSTAR during the outburst state, and again in a quiescent state wherein the 15-50 keV flux was 33 times lower. Using a multitemperature plasma emission and reflection model, the highest plasma temperature in the accretion column was measured as 19.7^{+1.3}_{-1.0} keV in outburst and 36.2^{+3.5}_{-3.2} keV in quiescence. The significant change of the maximum temperature is considered to reflect an accretion-induced decrease of the inner-disc radius Rin, where accreting gas is captured by the magnetosphere. Assuming this radius scales as R_in ∝ \\dot{M}^{-2/7}, where \\dot{M} is the mass accretion rate, we obtain R_in = 1.9 ^{+0.4}_{-0.2} R_WD and R_in = 7.4^{+2.1}_{-1.2} R_WD in outburst and quiescence, respectively, where RWD is the white-dwarf (WD) radius of this system. Utilizing the measured temperatures and fluxes, as well as the standard mass-radius relation of WDs, we estimate the WD mass as MWD = 0.87 ± 0.08 M⊙ including typical systematic uncertainties by 7 per cent. The surface magnetic field is also measured as B ˜ 5 × 105 G. These results exemplify a new X-ray method of estimating MWD and B of WDs by using large changes in \\dot{M}.

  1. Massive Star Burps, Then Explodes

    Science.gov (United States)

    2007-04-01

    evolved stars that have shed their outer envelopes. Swift XRT Image Swift XRT Image (Credit: NASA / GSFC / CXC /S.Immler) Most astronomers did not expect that a massive star would explode so soon after a major outburst, or that a Wolf-Rayet star would produce such a luminous eruption, so SN 2006jc represents a puzzle for theorists. "It challenges some aspects of our current model of stellar evolution," says Foley. "We really don't know what caused this star to have such a large eruption so soon before it went supernova." "SN 2006jc provides us with an important clue that LBV-style eruptions may be related to the deaths of massive stars, perhaps more closely than we used to think," adds coauthor and UC Berkeley astronomer Nathan Smith. "The fact that we have no well-established theory for what actually causes these outbursts is the elephant in the living room that nobody is talking about." SN 2006jc occurred in galaxy UGC 4904, located 77 million light years from Earth in the constellation Lynx. The supernova explosion, a peculiar variant of a Type Ib, was first sighted by Itagaki, American amateur astronomer Tim Puckett and Italian amateur astronomer Roberto Gorelli. See also NASA Goddard press release at: http://www.nasa.gov/centers/goddard/news/topstory/ 2007/supernova_imposter.html

  2. FORMATION OF BINARY MILLISECOND PULSARS BY ACCRETION-INDUCED COLLAPSE OF WHITE DWARFS UNDER WIND-DRIVEN EVOLUTION

    Energy Technology Data Exchange (ETDEWEB)

    Ablimit, Iminhaji; Li, Xiang-Dong, E-mail: lixd@nju.edu.cn [Department of Astronomy, Nanjing University, Nanjing 210046 (China)

    2015-02-20

    Accretion-induced collapse (AIC) of massive white dwarfs (WDs) has been proposed to be an important channel to form binary millisecond pulsars (MSPs). Recent investigations on thermal timescale mass transfer in WD binaries demonstrate that the resultant MSPs are likely to have relatively wide orbit periods (≳ 10 days). Here we calculate the evolution of WD binaries taking into account the excited wind from the companion star induced by X-ray irradiation of the accreting WD, which may drive rapid mass transfer even when the companion star is less massive than the WD. This scenario can naturally explain the formation of the strong-field neutron star in the low-mass X-ray binary 4U 1822–37. After AIC the mass transfer resumes when the companion star refills its Roche lobe, and the neutron star is recycled owing to mass accretion. A large fraction of the binaries will evolve to become binary MSPs with an He WD companion, with the orbital periods distributed between ≳ 0.1 days and ≲ 30 days, while some of them may follow the cataclysmic variable-like evolution toward very short orbits. If we instead assume that the newborn neutron star appears as an MSP and that part of its rotational energy is used to ablate its companion star, the binaries may also evolve to be the redback-like systems.

  3. Formation of Binary Millisecond Pulsars by Accretion-induced Collapse of White Dwarfs under Wind-driven Evolution

    Science.gov (United States)

    Ablimit, Iminhaji; Li, Xiang-Dong

    2015-02-01

    Accretion-induced collapse (AIC) of massive white dwarfs (WDs) has been proposed to be an important channel to form binary millisecond pulsars (MSPs). Recent investigations on thermal timescale mass transfer in WD binaries demonstrate that the resultant MSPs are likely to have relatively wide orbit periods (gsim 10 days). Here we calculate the evolution of WD binaries taking into account the excited wind from the companion star induced by X-ray irradiation of the accreting WD, which may drive rapid mass transfer even when the companion star is less massive than the WD. This scenario can naturally explain the formation of the strong-field neutron star in the low-mass X-ray binary 4U 1822-37. After AIC the mass transfer resumes when the companion star refills its Roche lobe, and the neutron star is recycled owing to mass accretion. A large fraction of the binaries will evolve to become binary MSPs with an He WD companion, with the orbital periods distributed between >~ 0.1 days and <~ 30 days, while some of them may follow the cataclysmic variable-like evolution toward very short orbits. If we instead assume that the newborn neutron star appears as an MSP and that part of its rotational energy is used to ablate its companion star, the binaries may also evolve to be the redback-like systems.

  4. Post-main-sequence Evolution of Icy Minor Planets. II. Water Retention and White Dwarf Pollution around Massive Progenitor Stars

    Science.gov (United States)

    Malamud, Uri; Perets, Hagai B.

    2017-06-01

    Most studies suggest that the pollution of white dwarf (WD) atmospheres arises from the accretion of minor planets, but the exact properties of polluting material, and in particular the evidence for water in some cases, are not yet understood. Here we study the water retention of small icy bodies in exo-solar planetary systems, as their respective host stars evolve through and off the main sequence and eventually become WDs. We explore, for the first time, a wide range of star masses and metallicities. We find that the mass of the WD progenitor star is of crucial importance for the retention of water, while its metallicity is relatively unimportant. We predict that minor planets around lower-mass WD progenitors would generally retain more water and would do so at closer distances from the WD than compared with high-mass progenitors. The dependence of water retention on progenitor mass and other parameters has direct implications for the origin of observed WD pollution, and we discuss how our results and predictions might be tested in the future as more observations of WDs with long cooling ages become available.

  5. Exploded view diagrams of mathematical surfaces

    KAUST Repository

    Karpenko, Olga A.

    2010-11-01

    We present a technique for visualizing complicated mathematical surfaces that is inspired by hand-designed topological illustrations. Our approach generates exploded views that expose the internal structure of such a surface by partitioning it into parallel slices, which are separated from each other along a single linear explosion axis. Our contributions include a set of simple, prescriptive design rules for choosing an explosion axis and placing cutting planes, as well as automatic algorithms for applying these rules. First we analyze the input shape to select the explosion axis based on the detected rotational and reflective symmetries of the input model. We then partition the shape into slices that are designed to help viewers better understand how the shape of the surface and its cross-sections vary along the explosion axis. Our algorithms work directly on triangle meshes, and do not depend on any specific parameterization of the surface. We generate exploded views for a variety of mathematical surfaces using our system. © 2006 IEEE.

  6. Stellar Laboratories: New GeV and Ge VI Oscillator Strengths and their Validation in the Hot White Dwarf RE0503-289

    Science.gov (United States)

    Rauch, T.; Werner, K.; Biemont, E.; Quinet, P.; Kruk, J. W.

    2013-01-01

    prerequisite for stellar-atmosphere modeling. Our oscillator-strength calculations have allowed, for the first time, Ge V and Ge VI lines to be successfully reproduced in a white dwarf s (RE 0503-289) spectrum and to determine its photospheric Ge abundance.

  7. Low-mass Pre-He White Dwarf Stars in Kepler Eclipsing Binaries with Multi-periodic Pulsations

    Science.gov (United States)

    Zhang, X. B.; Fu, J. N.; Liu, N.; Luo, C. Q.; Ren, A. B.

    2017-12-01

    We report the discovery of two thermally bloated low-mass pre-He white dwarfs (WDs) in two eclipsing binaries, KIC 10989032 and KIC 8087799. Based on the Kepler long-cadence photometry, we determined comprehensive photometric solutions of the two binary systems. The light curve analysis reveals that KIC 10989032 is a partially eclipsed detached binary system containing a probable low-mass WD with the temperature of about 10,300 K. Having a WD with the temperature of about 13,300, KKIC 8087799 is typical of an EL CVn system. By utilizing radial velocity measurements available for the A-type primary star of KIC 10989032, the mass and radius of the WD component are determined to be 0.24+/- 0.02 {M}⊙ and 0.50+/- 0.01 {R}⊙ , respectively. The values of mass and radius of the WD in KIC 8087799 are estimated as 0.16 ± 0.02 M ⊙ and 0.21 ± 0.01 R ⊙, respectively, according to the effective temperature and mean density of the A-type star derived from the photometric solution. We therefore introduce KIC 10989032 and KIC 8087799 as the eleventh and twelfth dA+WD eclipsing binaries in the Kepler field. Moreover, both binaries display marked multi-periodic pulsations superimposed on binary effects. A preliminary frequency analysis is applied to the light residuals when subtracting the synthetic eclipsing light curves from the observations, revealing that the light pulsations of the two systems are both due to the δ Sct-type primaries. We hence classify KIC 10989032 and KIC 8087799 as two WD+δ Sct binaries.

  8. The formation of low-mass helium white dwarfs orbiting pulsars . Evolution of low-mass X-ray binaries below the bifurcation period

    Science.gov (United States)

    Istrate, A. G.; Tauris, T. M.; Langer, N.

    2014-11-01

    Context. Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs) that have been spun up to high rotation rates via accretion of matter from a companion star in a low-mass X-ray binary (LMXB). This scenario has been strongly supported by various pieces of observational evidence. However, many details of this recycling scenario remain to be understood. Aims: Here we investigate binary evolution in close LMXBs to study the formation of radio MSPs with low-mass helium white dwarf companions (He WDs) in tight binaries with orbital periods Porb ≃ 2-9h. In particular, we examine i) if the observed systems can be reproduced by theoretical modelling using standard prescriptions of orbital angular momentum losses (i.e. with respect to the nature and the strength of magnetic braking), ii) if our computations of the Roche-lobe detachments can match the observed orbital periods, and iii) if the correlation between WD mass and orbital period (MWD, Porb) is valid for systems with Porb< 2 days. Methods: Numerical calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in ~400 close LMXB systems with different initial values of donor star mass, NS mass, orbital period, and the so-called γ-index of magnetic braking. Subsequently, we followed the orbital and the interior evolution of the detached low-mass (proto) He WDs, including stages with residual shell hydrogen burning. Results: We find that severe fine-tuning is necessary to reproduce the observed MSPs in tight binaries with He WD companions of mass <0.20 M⊙, which suggests that something needs to be modified or is missing in the standard input physics of LMXB modelling. Results from previous independent studies support this conclusion. We demonstrate that the theoretically calculated (MWD, Porb)-relation is in general also valid for systems with Porb< 2 days, although with a large scatter in He WD masses between 0.15-0.20 M⊙. The results of the thermal

  9. The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. I. NLTE spectral analysis of the DA-type white dwarf G191-B2B

    Science.gov (United States)

    Rauch, T.; Werner, K.; Bohlin, R.; Kruk, J. W.

    2013-12-01

    Context. Hydrogen-rich, DA-type white dwarfs are particularly suited as primary standard stars for flux calibration. State-of-the-art NLTE models consider opacities of species up to trans-iron elements and provide reliable synthetic stellar-atmosphere spectra to compare with observations. Aims: We will establish a database of theoretical spectra of stellar flux standards that are easily accessible via a web interface. Methods: In the framework of the Virtual Observatory, the German Astrophysical Virtual Observatory developed the registered service TheoSSA. It provides easy access to stellar spectral energy distributions (SEDs) and is intended to ingest SEDs calculated by any model-atmosphere code. In case of the DA white dwarf G191-B2B, we demonstrate that the model reproduces not only its overall continuum shape but also the numerous metal lines exhibited in its ultraviolet spectrum. Results: TheoSSA is in operation and contains presently a variety of SEDs for DA-type white dwarfs. It will be extended in the near future and can host SEDs of all primary and secondary flux standards. The spectral analysis of G191-B2B has shown that our hydrostatic models reproduce the observations best at and log g = 7.60 ± 0.05. We newly identified Fe vi, Ni vi, and Zn iv lines. For the first time, we determined the photospheric zinc abundance with a logarithmic mass fraction of -4.89 (7.5 × solar). The abundances of He (upper limit), C, N, O, Al, Si, O, P, S, Fe, Ni, Ge, and Sn were precisely determined. Upper abundance limits of about 10% solar were derived for Ti, Cr, Mn, and Co. Conclusions: The TheoSSA database of theoretical SEDs of stellar flux standards guarantees that the flux calibration of all astronomical data and cross-calibration between different instruments can be based on the same models and SEDs calculated with different model-atmosphere codes and are easy to compare. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope

  10. A Dynamic Density Functional Theory Approach to Diffusion in White Dwarfs and Neutron Star Envelopes

    Science.gov (United States)

    Diaw, A.; Murillo, M. S.

    2016-09-01

    We develop a multicomponent hydrodynamic model based on moments of the Born-Bogolyubov-Green-Kirkwood-Yvon hierarchy equations for physical conditions relevant to astrophysical plasmas. These equations incorporate strong correlations through a density functional theory closure, while transport enters through a relaxation approximation. This approach enables the introduction of Coulomb coupling correction terms into the standard Burgers equations. The diffusive currents for these strongly coupled plasmas is self-consistently derived. The settling of impurities and its impact on cooling can be greatly affected by strong Coulomb coupling, which we show can be quantified using the direct correlation function.

  11. Stellar laboratories. II. New Zn iv and Zn v oscillator strengths and their validation in the hot white dwarfs G191-B2B and RE 0503-289

    Science.gov (United States)

    Rauch, T.; Werner, K.; Quinet, P.; Kruk, J. W.

    2014-04-01

    Context. For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. In a recent analysis of the ultraviolet (UV) spectrum of the DA-type white dwarf G191-B2B, 21 Zn iv lines were newly identified. Because of the lack of Zn iv data, transition probabilities of the isoelectronic Ge vi were adapted for a first, coarse determination of the photospheric Zn abundance. Aims: Reliable Zn iv and Zn v oscillator strengths are used to improve the Zn abundance determination and to identify more Zn lines in the spectra of G191-B2B and the DO-type white dwarf RE 0503-289. Methods: We performed new calculations of Zn iv and Zn v oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of the Zn iv - v spectrum exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE 0503-289. Results: In the UV spectrum of G191-B2B, we identify 31 Zn iv and 16 Zn v lines. Most of these are identified for the first time in any star. We can reproduce well almost all of them at log Zn = -5.52 ± 0.2 (mass fraction, about 1.7 times solar). In particular, the Zn iv / Zn v ionization equilibrium, which is a very sensitive Teff indicator, is well reproduced with the previously determined and log g = 7.60 ± 0.05. In the spectrum of RE 0503-289, we identified 128 Zn v lines for the first time and determined log Zn = -3.57 ± 0.2 (155 times solar). Conclusions: Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Zn iv and Zn v line profiles in two white dwarf (G191-B2B and RE 0503-289) ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed us to

  12. A New Take on Exploding Carts

    Science.gov (United States)

    Broder, Darren; Burleigh, James; Christian, Matthew; Mowry, Shawn; Hassel, George E.

    2017-01-01

    The Exploding Carts is a popular introductory physics activity in which a one-dimensional explosion is simulated utilizing two dynamics carts that are pushed apart by a spring-loaded plunger released from one of the carts. Traditional treatments of the Exploding Carts usually involve multiple trials where the mass of one or both of the carts is…

  13. Turbulent Nuclear Burning of Carbon Fuel in Double-Degenerate White Dwarfs

    Science.gov (United States)

    Mozumdar, Pritom; Fisher, Robert

    2018-01-01

    Type Ia supernovae (SNe Ia) are of interest as standardizable cosmological candles, though their stellar progenitors are still poorly understood. The double-degenerate (DD) channel is promising, but the mechanism for the explosion remains a matter of active investigation. A long-standing problem in modeling SNe Ia is the fact that 3D simulations leave the length scales crucial for a possible detonation unresolved. In this work, we have performed local 3D hydrodynamical adaptive mesh refinement simulations of driven turbulence for various initial conditions characteristic of the DD scenario, which are capable of capturing length scales relevant to the Zel’dovich gradient mechanism. Because the carbon burning rate is highly sensitive to temperature in this regime, we demonstrate that turbulence can dramatically enhance the nuclear burning rate, and we investigate the connection to a possible detonation.

  14. A Spectroscopic Search for White Dwarf Companions to 101 Nearby M Dwarfs

    Science.gov (United States)

    Bar, Ira; Vreeswijk, Paul; Gal-Yam, Avishay; Ofek, Eran O.; Nelemans, Gijs

    2017-11-01

    Recent studies of the stellar population in the solar neighborhood (explosive phenomena. In an attempt to uncover these hidden WDs, we present intermediate resolution spectroscopy over the wavelength range of 3000–25000 Å of 101 nearby M dwarfs (dMs), observed with the Very Large Telescope X-Shooter spectrograph. For each star we search for a hot component superimposed on the dM spectrum. X-Shooter has excellent blue sensitivity and thus can reveal a faint hot WD despite the brightness of its red companion. Visual examination shows no clear evidence of a WD in any of the spectra. We place upper limits on the effective temperatures of WDs that may still be hiding by fitting dM templates to the spectra and modeling the WD spectra. On average our survey is sensitive to WDs hotter than about 5300 K. This suggests that the frequency of WD companions of {T}{eff}≳ 5300 {{K}} with separation of the order of ≲50 au among the local dM population is <3% at the 95% confidence level. The reduced spectra are made available via the WISeREP3 repository. Based on observations collected in service mode using the Very Large Telescope (VLT) under program IDs 095_D-0949(A) and 096_D-0963(A).

  15. Numerical simulations of high-energy flows in accreting magnetic white dwarfs

    Science.gov (United States)

    Van Box Som, Lucile; Falize, É.; Bonnet-Bidaud, J.-M.; Mouchet, M.; Busschaert, C.; Ciardi, A.

    2018-01-01

    Some polars show quasi-periodic oscillations (QPOs) in their optical light curves that have been interpreted as the result of shock oscillations driven by the cooling instability. Although numerical simulations can recover this physics, they wrongly predict QPOs in the X-ray luminosity and have also failed to reproduce the observed frequencies, at least for the limited range of parameters explored so far. Given the uncertainties on the observed polar parameters, it is still unclear whether simulations can reproduce the observations. The aim of this work is to study QPOs covering all relevant polars showing QPOs. We perform numerical simulations including gravity, cyclotron and bremsstrahlung radiative losses, for a wide range of polar parameters, and compare our results with the astronomical data using synthetic X-ray and optical luminosities. We show that shock oscillations are the result of complex shock dynamics triggered by the interplay of two radiative instabilities. The secondary shock forms at the acoustic horizon in the post-shock region in agreement with our estimates from steady-state solutions. We also demonstrate that the secondary shock is essential to sustain the accretion shock oscillations at the average height predicted by our steady-state accretion model. Finally, in spite of the large explored parameter space, matching the observed QPO parameters requires a combination of parameters inconsistent with the observed ones. This difficulty highlights the limits of one-dimensional simulations, suggesting that multi-dimensional effects are needed to understand the non-linear dynamics of accretion columns in polars and the origins of QPOs.

  16. The effects of close binaries on the magnetic activity of M dwarfs as probed using close white dwarf companions

    Science.gov (United States)

    Morgan, D. P.

    2017-01-01

    I present a study of close white dwarf (WD) and M dwarf (dM) binary systems (WD+dM) to examine the effects that close companions have on magnetic field generation in dMs. Using the Sloan Digital Sky Survey (SDSS) Data Release 8 spectroscopic database, I constructed a sample of 1756 WD+dM high-quality pairs. I show that early-type dMs (M4), where stars become fully convective, the activity fraction and activity lifetimes of WD+dM binary systems become more comparable to those of the field dMs. The implications of having a close binary companion may include: increased stellar rotation through disk disruption, tidal effects, and/or angular momentum exchange. Thus, the similarity in activity between late-type field dMs and late-type dMs with close companions is likely due to the mechanism generating magnetic fields being less sensitive to the effects caused by a close companion; namely, increased stellar rotation. Using a subset of 181 close WD+dM pairs, matched to the time-domain SDSS Stripe 82 catalog, I show that enhanced magnetic activity extends to the flaring behavior of dMs in close binaries. Specifically, early spectral type dMs (M0-M4), in close WD+dM pairs, are two orders of magnitude more likely to flare than field dMs, whereas late-type dMs (M4-M6) in close WD+dM pairs flare as frequently or less than the late-type field dM sample. To test whether the presence of a close companion leads to star-star interactions, I searched for correlations between the WD occultations and flares from the dM member in KOI-256, an eclipsing WD+dM system. I find no correlations between the flaring activity of the dM and the WD occultations, indicating the there are no obvious signs of star-star interactions at work. In addition, the dM member of KOI-256 flares more than any other dM observed by Kepler and shows evidence for solar-like magnetic activity cycles, a feature not seen in many dMs to date.

  17. Evolution towards and beyond accretion-induced collapse of massive white dwarfs and formation of millisecond pulsars

    Science.gov (United States)

    Tauris, T. M.; Sanyal, D.; Yoon, S.-C.; Langer, N.

    2013-10-01

    Context. Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs), formed via type Ib/c core-collapse supernovae (SNe), which have been spun up to high rotation rates via accretion from a companion star in a low-mass X-ray binary (LMXB). In an alternative formation channel, NSs are produced via the accretion-induced collapse (AIC) of a massive white dwarf (WD) in a close binary. Aims: Here we investigate binary evolution leading to AIC and examine if NSs formed in this way can subsequently be recycled to form MSPs and, if so, how they can observationally be distinguished from pulsars formed via the standard core-collapse SN channel in terms of their masses, spins, orbital periods and space velocities. Methods: Numerical calculations with a detailed stellar evolution code were used for the first time to study the combined pre- and post-AIC evolution of close binaries. We investigated the mass transfer onto a massive WD (treated as a point mass) in 240 systems with three different types of non-degenerate donor stars: main-sequence stars, red giants, and helium stars. When the WD is able to accrete sufficient mass (depending on the mass-transfer rate and the duration of the accretion phase) we assumed it collapses to form a NS and we studied the dynamical effects of this implosion on the binary orbit. Subsequently, we followed the mass-transfer epoch which resumes once the donor star refills its Roche lobe and calculated the continued LMXB evolution until the end. Results: We show that recycled pulsars may form via AIC from all three types of progenitor systems investigated and find that the final properties of the resulting MSPs are, in general, remarkably similar to those of MSPs formed via the standard core-collapse SN channel. However, as a consequence of the fine-tuned mass-transfer rate necessary to make the WD grow in mass, the resultant MSPs created via the AIC channel preferentially form in certain orbital period intervals. In addition

  18. Maximum mass ratio of am CVn-type binary systems and maximum white dwarf mass in ultra-compact x-ray binaries (addendum - Serb. Astron. J. No. 183 (2011, 63

    Directory of Open Access Journals (Sweden)

    Arbutina B.

    2012-01-01

    Full Text Available We recalculated the maximum white dwarf mass in ultra-compact X-ray binaries obtained in an earlier paper (Arbutina 2011, by taking the effects of super-Eddington accretion rate on the stability of mass transfer into account. It is found that, although the value formally remains the same (under the assumed approximations, for white dwarf masses M2 >~0.1MCh mass ratios are extremely low, implying that the result for Mmax is likely to have little if any practical relevance.

  19. Exploded representation of a refracting surface

    Directory of Open Access Journals (Sweden)

    W.H. Heath

    2005-01-01

    Full Text Available The concept of the exploded refracting sur-face is useful in the optics of contact lenses and vision underwater. The purpose of this paper is to show how to represent a refracting surface as an exploded pair of surfaces separated by a gap of zero width.  The analysis is in terms of linear optics and allows for astigmatic and non-coaxial cases.

  20. SUPER-CHANDRASEKHAR-MASS LIGHT CURVE MODELS FOR THE HIGHLY LUMINOUS TYPE Ia SUPERNOVA 2009dc

    Energy Technology Data Exchange (ETDEWEB)

    Kamiya, Yasuomi [Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan); Tanaka, Masaomi [National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan); Nomoto, Ken' ichi [Kavli Institute for the Physics and Mathematics of the Universe, Todai Institutes for Advanced Study, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583 (Japan); Blinnikov, Sergei I. [Institute for Theoretical and Experimental Physics, 117218 Moscow (Russian Federation); Sorokina, Elena I. [Sternberg Astronomical Institute, Lomonosov Moscow State University, 119992 Moscow (Russian Federation); Suzuki, Tomoharu, E-mail: yasuomi.kamiya@ipmu.jp [College of Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501 (Japan)

    2012-09-10

    Several highly luminous Type Ia supernovae (SNe Ia) have been discovered. Their high luminosities are difficult to explain with the thermonuclear explosions of Chandrasekhar-mass white dwarfs (WDs). In the present study, we estimate the progenitor mass of SN 2009dc, one of the extremely luminous SNe Ia, using the hydrodynamical models as follows. Explosion models of super-Chandrasekhar-mass (super-Ch-mass) WDs are constructed, and multi-color light curves (LCs) are calculated. The comparison between our calculations and the observations of SN 2009dc suggests that the exploding WD has a super-Ch mass of 2.2-2.4 M{sub Sun }, producing 1.2-1.4 M{sub Sun} of {sup 56}Ni, if the extinction by its host galaxy is negligible. If the extinction is significant, the exploding WD is as massive as {approx}2.8 M{sub Sun }, and {approx}1.8 M{sub Sun} of {sup 56}Ni is necessary to account for the observations. Whether the host-galaxy extinction is significant or not, the progenitor WD must have a thick carbon-oxygen layer in the outermost zone (20%-30% of the WD mass), which explains the observed low expansion velocity of the ejecta and the presence of carbon. Our estimate of the mass of the progenitor WD, especially for the extinction-corrected case, is challenging to the current scenarios of SNe Ia. Implications for the progenitor scenarios are also discussed.

  1. Stellar laboratories. VII. New Kr iv - vii oscillator strengths and an improved spectral analysis of the hot, hydrogen-deficient DO-type white dwarf RE 0503-289

    Science.gov (United States)

    Rauch, T.; Quinet, P.; Hoyer, D.; Werner, K.; Richter, P.; Kruk, J. W.; Demleitner, M.

    2016-05-01

    Context. For the spectral analysis of high-resolution and high signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims: New Kr iv-vii oscillator strengths for a large number of lines enable us to construct more detailed model atoms for our NLTE model-atmosphere calculations. This enables us to search for additional Kr lines in observed spectra and to improve Kr abundance determinations. Methods: We calculated Kr iv-vii oscillator strengths to consider radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Kr lines that are exhibited in high-resolution and high S/N ultraviolet (UV) observations of the hot white dwarf RE 0503-289. Results: We reanalyzed the effective temperature and surface gravity and determined Teff = 70000 ± 2000 K and log (g/ cm s-2) = 7.5 ± 0.1. We newly identified ten Kr v lines and one Kr vi line in the spectrum of RE 0503-289. We measured a Kr abundance of -3.3 ± 0.3 (logarithmic mass fraction). We discovered that the interstellar absorption toward RE 0503-289 has a multi-velocity structure within a radial-velocity interval of -40 km s-1National Commission for Astronomy.Tables A.9-A.12 are only available via the German Astrophysical Virtual Observatory (GAVO) service TOSS (http://dc.g-vo.org/TOSS).

  2. A New Take on Exploding Carts

    Science.gov (United States)

    Broder, Darren; Burleigh, James; Christian, Matthew; Mowry, Shawn; Hassel, George E.

    2017-11-01

    The Exploding Carts is a popular introductory physics activity in which a one-dimensional explosion is simulated utilizing two dynamics carts that are pushed apart by a spring-loaded plunger released from one of the carts. Traditional treatments of the Exploding Carts usually involve multiple trials where the mass of one or both of the carts is increased, producing explosions with varying post-explosion speeds (the speed at the instant contact is lost between the carts). The subsequent analysis normally entails comparing post-explosion speeds within a given trial, which leads to the concept of momentum and its conservation. Though generally not emphasized in traditional treatments of the Exploding Carts, it is also an interesting endeavor (and the focus of this paper) to compare post-explosion speeds between different trials. Specifically, the post-explosion speed of a constant mass cart in various trials can be compared to the post-explosion speed of the equal mass explosion (no additional mass added to the carts) as the mass of the other cart is changed.

  3. A Strange Dwarf Scenario for the Formation of the Peculiar Double White Dwarf Binary SDSS J125733.63+542850.5

    Science.gov (United States)

    Jiang, Long; Chen, Wen-Cong; Li, Xiang-Dong

    2018-01-01

    The Hubble Space Telescope observation of the double white dwarf (WD) binary SDSS J125733.63+542850.5 reveal that the massive WD has a surface gravity logg1 ˜ 8.7 (which implies a mass of M1 ˜ 1.06 M⊙) and an effective temperature T1 ˜ 13000 K, while the effective temperature of the low-mass WD (M2 double WD binary. We assume that the massive WD is a strange dwarf originating from a phase transition in a ˜1.1 M⊙ WD, which has experienced accretion and spin-down processes. Its high effective temperature could arise from the heating process during the phase transition. Our simulations suggest that the progenitor of SDSS J125733.63+542850.5 can be a binary system consisting of a 0.65 M_{⊙} WD and a 1.5 M_{⊙} main sequence star in a 1.492 day orbit. Especially, the secondary star (i.e., the progenitor of the low mass WD) is likely to have an ultra-low metallicity of Z = 0.0001.

  4. The Solar Neighborhood. XXXIX. Parallax Results from the CTIOPI and NOFS Programs: 50 New Members of the 25 parsec White Dwarf Sample

    Science.gov (United States)

    Subasavage, John P.; Jao, Wei-Chun; Henry, Todd J.; Harris, Hugh C.; Dahn, Conard C.; Bergeron, P.; Dufour, P.; Dunlap, Bart H.; Barlow, Brad N.; Ianna, Philip A.; Lépine, Sébastien; Margheim, Steven J.

    2017-07-01

    We present 114 trigonometric parallaxes for 107 nearby white dwarf (WD) systems from both the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI) and the U. S. Naval Observatory Flagstaff Station (NOFS) parallax programs. Of these, 76 parallaxes for 69 systems were measured by the CTIOPI program and 38 parallaxes for as many systems were measured by the NOFS program. A total of 50 systems are confirmed to be within the 25-pc horizon of interest. Coupled with a spectroscopic confirmation of a common proper-motion companion to a Hipparcos star within 25 pc as well as confirmation parallax determinations for two WD systems included in the recently released Tycho Gaia Astrometric Solution catalog, we add 53 new systems to the 25-pc WD sample—a 42% increase. Our sample presented here includes four strong candidate halo systems, a new metal-rich DAZ WD, a confirmation of a recently discovered nearby short-period (P = 2.85 hr) double degenerate, a WD with a new astrometric perturbation (long period, unconstrained with our data), and a new triple system where the WD companion main-sequence star has an astrometric perturbation (P ˜ 1.6 year).

  5. BEER Analysis of Kepler and CoRoT Light Curves. IV. Discovery of Four New Low-mass White-Dwarf Companions in the Kepler Data

    Science.gov (United States)

    Faigler, S.; Kull, I.; Mazeh, T.; Kiefer, F.; Latham, D. W.; Bloemen, S.

    2015-12-01

    We report the discovery of four short-period eclipsing systems in the Kepler light curves, consisting of an A-star primary and a low-mass white dwarf (WD) secondary (dA+WD)—KIC 4169521, KOI-3818, KIC 2851474, and KIC 9285587. The systems show BEaming, Ellipsoidal and Reflection (BEER) phase modulations together with primary and secondary eclipses. These add to the 6 Kepler and 18 WASP short-period eclipsing dA+WD binaries that were previously known. The light curves, together with follow-up spectroscopic observations, allow us to derive the masses, radii, and effective temperatures of the two components of the four systems. The orbital periods, of 1.17-3.82 days, and WD masses, of 0.19-0.22 M⊙, are similar to those of the previously known systems. The WD radii of KOI-3818, KIC 2851474, and KIC 9285587 are 0.026, 0.035, and 0.026 R⊙, respectively, the smallest WD radii derived so far for short-period eclipsing dA+WD binaries. These three binaries extend the previously known population to older systems with cooler and smaller WD secondaries. KOI-3818 displays evidence for a fast-rotating primary and a minute but significant eccentricity, ˜1.5 × 10-3. These features are probably the outcome of the mass-transfer process.

  6. The Solar Neighborhood. XXXIX. Parallax Results from the CTIOPI and NOFS Programs: 50 New Members of the 25 parsec White Dwarf Sample

    Energy Technology Data Exchange (ETDEWEB)

    Subasavage, John P.; Harris, Hugh C.; Dahn, Conard C. [U.S. Naval Observatory, 10391 West Naval Observatory Road, Flagstaff, AZ 86005-8521 (United States); Jao, Wei-Chun; Lépine, Sébastien [Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30302-4106 (United States); Henry, Todd J.; Ianna, Philip A. [RECONS Institute, Chambersburg, PA 17201 (United States); Bergeron, P.; Dufour, P. [Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7 (Canada); Dunlap, Bart H. [University of North Carolina at Chapel Hill, Dept. of Physics and Astronomy, Chapel Hill, NC 27599 (United States); Barlow, Brad N. [Department of Physics, High Point University, One University Parkway, High Point, NC 27268 (United States); Margheim, Steven J., E-mail: jsubasavage@nofs.navy.mil [Gemini Observatory, Southern Operations Center, Casilla 603, La Serena (Chile)

    2017-07-01

    We present 114 trigonometric parallaxes for 107 nearby white dwarf (WD) systems from both the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI) and the U. S. Naval Observatory Flagstaff Station (NOFS) parallax programs. Of these, 76 parallaxes for 69 systems were measured by the CTIOPI program and 38 parallaxes for as many systems were measured by the NOFS program. A total of 50 systems are confirmed to be within the 25-pc horizon of interest. Coupled with a spectroscopic confirmation of a common proper-motion companion to a Hipparcos star within 25 pc as well as confirmation parallax determinations for two WD systems included in the recently released Tycho Gaia Astrometric Solution catalog, we add 53 new systems to the 25-pc WD sample—a 42% increase. Our sample presented here includes four strong candidate halo systems, a new metal-rich DAZ WD, a confirmation of a recently discovered nearby short-period ( P  = 2.85 hr) double degenerate, a WD with a new astrometric perturbation (long period, unconstrained with our data), and a new triple system where the WD companion main-sequence star has an astrometric perturbation ( P  ∼ 1.6 year).

  7. The High Time Resolution Universe survey - XI. Discovery of five recycled pulsars and the optical detectability of survey white dwarf companions

    Science.gov (United States)

    Bates, S. D.; Thornton, D.; Bailes, M.; Barr, E.; Bassa, C. G.; Bhat, N. D. R.; Burgay, M.; Burke-Spolaor, S.; Champion, D. J.; Flynn, C. M. L.; Jameson, A.; Johnston, S.; Keith, M. J.; Kramer, M.; Levin, L.; Lyne, A.; Milia, S.; Ng, C.; Petroff, E.; Possenti, A.; Stappers, B. W.; van Straten, W.; Tiburzi, C.

    2015-02-01

    We present the discovery of a further five recycled pulsar systems in the mid-Galactic latitude portion of the High Time Resolution Universe survey. The pulsars have rotational periods ranging from 2 to 66 ms, and four are in binary systems with orbital periods between 10.8 h and 9 d. Three of these binary systems are particularly interesting; PSR J1227-6208 has a pulse period of 34.5 ms and the highest mass function of all pulsars with near-circular orbits. The circular orbit suggests that the companion is not another neutron star, so future timing experiments may reveal one of the heaviest white dwarfs ever found (>1.3 M⊙). Timing observations of PSR J1431-4715 indicate that it is eclipsed by its companion which has a mass indicating it belongs to the redback class of eclipsing millisecond pulsars. PSR J1653-2054 has a companion with a minimum mass of only 0.08 M⊙, placing it among the class of pulsars with low-mass companions. Unlike the majority of such systems, however, no evidence of eclipses is seen at 1.4 GHz.

  8. Chromospheric impact of an exploding solar granule

    Science.gov (United States)

    Fischer, C. E.; Bello González, N.; Rezaei, R.

    2017-06-01

    Context. Observations of multi-wavelength and therefore height-dependent information following events throughout the solar atmosphere and unambiguously assigning a relation between these rapidly evolving layers are rare and difficult to obtain. Yet, they are crucial for our understanding of the physical processes that couple the different regimes in the solar atmosphere. Aims: We characterize the exploding granule event with simultaneous observations of Hinode spectroplarimetric data in the solar photosphere and Hinode broadband Ca II H images combined with Interface Region Imaging Spectrograph (IRIS) slit spectra. We follow the evolution of an exploding granule and its connectivity throughout the atmosphere and analyze the dynamics of a magnetic element that has been affected by the abnormal granule. Methods: In addition to magnetic flux maps we use a local correlation tracking method to infer the horizontal velocity flows in the photosphere and apply a wavelet analysis on several IRIS chromospheric emission features such as Mg II k2v and Mg II k3 to detect oscillatory phenomena indicating wave propagation. Results: During the vigorous expansion of the abnormal granule we detect radially outward horizontal flows, causing, together with the horizontal flows from the surrounding granules, the magnetic elements in the bordering intergranular lanes to be squeezed and elongated. In reaction to the squeezing, we detect a chromospheric intensity and velocity oscillation pulse which we identify as an upward traveling hot shock front propagating clearly through the IRIS spectral line diagnostics of Mg II h&k. Conclusions: Exploding granules can trigger upward-propagating shock fronts that dissipate in the chromosphere. Movies associated to Figs. A.1 and A.2 are available in electronic form at http://www.aanda.org

  9. Stellar laboratories . IX. New Se v, Sr iv-vii, Te vi, and I vi oscillator strengths and the Se, Sr, Te, and I abundances in the hot white dwarfs G191-B2B and RE 0503-289

    Science.gov (United States)

    Rauch, T.; Quinet, P.; Knörzer, M.; Hoyer, D.; Werner, K.; Kruk, J. W.; Demleitner, M.

    2017-10-01

    Context. To analyze spectra of hot stars, advanced non-local thermodynamic equilibrium (NLTE) model-atmosphere techniques are mandatory. Reliable atomic data is crucial for the calculation of such model atmospheres. Aims: We aim to calculate new Sr iv-vii oscillator strengths to identify for the first time Sr spectral lines in hot white dwarf (WD) stars and to determine the photospheric Sr abundances. To measure the abundances of Se, Te, and I in hot WDs, we aim to compute new Se v, Te vi, and I vi oscillator strengths. Methods: To consider radiative and collisional bound-bound transitions of Se v, Sr iv - vii, Te vi, and I vi in our NLTE atmosphere models, we calculated oscillator strengths for these ions. Results: We newly identified four Se v, 23 Sr v, 1 Te vi, and three I vi lines in the ultraviolet (UV) spectrum of RE 0503-289. We measured a photospheric Sr abundance of 6.5+ 3.8-2.4× 10-4 (mass fraction, 9500-23 800 times solar). We determined the abundances of Se (1.6+ 0.9-0.6× 10-3, 8000-20 000), Te (2.5+ 1.5-0.9× 10-4, 11 000-28 000), and I (1.4+ 0.8-0.5× 10-5, 2700-6700). No Se, Sr, Te, and I line was found in the UV spectra of G191-B2B and we could determine only upper abundance limits of approximately 100 times solar. Conclusions: All identified Se v, Sr v, Te vi, and I vi lines in the UV spectrum of RE 0503-289 were simultaneously well reproduced with our newly calculated oscillator strengths. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26666. Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. Full Tables A.15 to A.21 are only available via the German Astrophysical Virtual Observatory (GAVO) service TOSS (http://dc.g-vo.org/TOSS).

  10. Nucleosynthesis in delayed detonation models of Type IA supernovae

    Science.gov (United States)

    Khokhlov, A. M.

    1991-05-01

    The preliminary results of detailed nucleosynthesis computations are presented for delayed detonation (DD) models of Type Ia supernovae. The main parameters of the models considered are the initial density at the center, the ratio of the deflagration wave velocity and the velocity of sound ahead of the wave, and the density ahead of the deflagration wave at which the transition from a deflagration to a detonation occurs. For thermonuclear models of Type Ia supernovae, the limitations on the deflagration wave velocity near the center, Ddef is less than approximately 0.03 a(s), and on the central density of an exploding white dwarf, rho(c) is less than approximately 2-2.5 10 to the 9th g/cu cm, are obtained. It is suggested that the transition from a deflagration to a detonation in the DD models occurs due to both the turbulence in the vicinity of the flame front and the strong pulsation of the dwarf during the explosion.

  11. THE C-FLAME QUENCHING BY CONVECTIVE BOUNDARY MIXING IN SUPER-AGB STARS AND THE FORMATION OF HYBRID C/O/Ne WHITE DWARFS AND SN PROGENITORS

    Energy Technology Data Exchange (ETDEWEB)

    Denissenkov, P. A.; Herwig, F. [Department of Physics and Astronomy, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6 (Canada); Truran, J. W. [The Joint Institute for Nuclear Astrophysics, Notre Dame, IN 46556 (United States); Paxton, B., E-mail: pavelden@uvic.ca, E-mail: fherwig@uvic.ca [Kavli Institute for Theoretical Physics and Department of Physics, Kohn Hall, University of California, Santa Barbara, CA 93106 (United States)

    2013-07-20

    After off-center C ignition in the cores of super asymptotic giant branch (SAGB) stars, the C flame propagates all the way down to the center, trailing behind it the C-shell convective zone, and thus building a degenerate ONe core. This standard picture is obtained in stellar evolution simulations if the bottom C-shell convection boundary is assumed to be a discontinuity associated with a strict interpretation of the Schwarzschild condition for convective instability. However, this boundary is prone to additional mixing processes, such as thermohaline convection and convective boundary mixing. Using hydrodynamic simulations, we show that contrary to previous results, thermohaline mixing is too inefficient to interfere with the C-flame propagation. However, even a small amount of convective boundary mixing removes the physical conditions required for the C-flame propagation all the way to the center. This result holds even if we allow for some turbulent heat transport in the CBM region. As a result, SAGB stars build in their interiors hybrid C-O-Ne degenerate cores composed of a relatively large CO core (M{sub CO} Almost-Equal-To 0.2 M{sub Sun }) surrounded by a thick ONe zone ({Delta}M{sub ONe} {approx}> 0.85 M{sub Sun }) with another thin CO layer above. If exposed by mass loss, these cores will become hybrid C-O-Ne white dwarfs. Otherwise, the ignition of C-rich material in the central core, surrounded by the thick ONe zone, may trigger a thermonuclear supernova (SN) explosion. The quenching of the C-flame may have implications for the ignition mechanism of SN Ia in the double-degenerate merger scenario.

  12. Post-main-sequence Evolution of Icy Minor Planets. III. Water Retention in Dwarf Planets and Exomoons and Implications for White Dwarf Pollution

    Science.gov (United States)

    Malamud, Uri; Perets, Hagai B.

    2017-11-01

    Studies suggest that the pollution of white dwarf (WD) atmospheres arises from the accretion of minor planets, but the exact properties of polluting material, and in particular the evidence for water in some cases are not yet understood. Several previous works studied the possibility of water surviving inside minor planets around evolving stars. However, they all focused on small, comet-sized to moonlet-sized minor planets, when the inferred mass inside the convection zones of He-dominated WDs could actually be compatible with much more massive minor planets. Here we explore for the first time, the water retention inside exoplanetary dwarf planets, or moderate-sized moons, with radii of the order of hundreds of kilometers. This paper concludes a series of papers that has now covered nearly the entire potential mass range of minor planets, in addition to the full mass range of their host stars. We find that water retention is (a) affected by the mass of the WD progenitor, and (b) it is on average at least 5%, irrespective of the assumed initial water composition, if it came from a single accretion event of an icy dwarf planet or moon. The latter prediction strengthens the possibility of habitability in WD planetary systems, and it may also be used in order to distinguish between pollution originating from multiple small accretion events and singular large accretion events. To conclude our work, we provide a code that calculates ice and water retention by interpolation and may be freely used as a service to the community.

  13. Progress in Modeling Classical Nova Outbursts

    Science.gov (United States)

    Calder, A. C.; Alexakis, A.; Dursi, L. J.; Mignone, A.; Timmes, F. X.; Truran, J. W.; Rosner, R.; Lamb, D. Q.; Brown, E.; Fryxell, B.; Zingale, M.; Ricker, P.; Olson, K.

    2003-03-01

    We report on progress in modeling many facets of Classical Novae. These include magnetohydrodynamical simulations of the accretion phase (for the case of magnetic white dwarfs) and hydrodynamical simulations of the mixing of white dwarf material into the hydrogen-rich envelope by resonant gravity wave breaking at the surface of the white dwarf (See also Alexakis, et al.). We also report on initial efforts at the development of a sub-grid enrichment model based on these results as well as results of one-dimensional simulations with mixing length convection of the enrichment process exploring the long-term behavior of the enriched region. Finally, we present two-dimensional simulations of the onset and development of convection in nova precursor models and during the runaway. This work was supported by the DOE ASCI/Alliances program at the University of Chicago under grant No. B341495.

  14. A novel and sensitive method for measuring very weak magnetic fields of DA white dwarfs. A search for a magnetic field at the 250 G level in 40 Eridani B

    Science.gov (United States)

    Landstreet, J. D.; Bagnulo, S.; Valyavin, G. G.; Gadelshin, D.; Martin, A. J.; Galazutdinov, G.; Semenko, E.

    2015-08-01

    Context. Searches for magnetic fields in white dwarfs have clarified both the frequency of occurrence and the global structure of the fields found down to field strengths of the order of 500 kG. Below this level, the situation is still very unclear. Aims: We are engaged in a project to find and study the weakest magnetic fields that are detectable in white dwarfs, in order to empirically determine how the frequency of occurrence and the structure of fields present changes with field strength. In this paper we report the successful testing of a very sensitive method of longitudinal field detection in DA white dwarfs. We use this method to carry out an extremely sensitive search for magnetism in the bright white dwarf 40 Eri B. Methods: The method of field measurement we use is to measure, at high spectral resolution, the polarisation signal V/I of the narrow non-LTE line core in Hα in DA stars. This small feature provides a much higher amplitude polarisation signal than the broad Balmer line wings. We test the usefulness of this technique by searching for a weak magnetic field in 40 Eri B. Results: One hour of observation of I and V Stokes components of the white dwarf 40 Eri B using ESPaDOnS at the CFHT is found to provide a standard error of measurement of the mean longitudinal magnetic field ⟨ Bz ⟩ of about 85 G. This is the smallest standard error of field measurement ever obtained for a white dwarf. The non-detections obtained are generally consistent with slightly less accurate measurements of 40 Eri B obtained with ISIS at the WHT and the Main Stellar Spectrograph at SAO, in order to provide comparison standards for the new method. These further measurements allow us to make a quantitative comparison of the relative efficiencies of low-resolution spectropolarimetery (using most or all of the Balmer lines) with the new method (using only the core of Hα). Conclusions: The new method of field detection reaches the level of sensitivity that was expected. It

  15. Exploding Head Syndrome:A Case Report

    Directory of Open Access Journals (Sweden)

    Gautam Ganguly

    2013-01-01

    Full Text Available Introduction: Exploding head syndrome (EHS is a rare parasomnia in which affected individuals awaken from sleep with the sensation of a loud bang. The etiology is unknown, but other conditions including primary and secondary headache disorders and nocturnal seizures need to be excluded. Case Presentation: A 57-year-old Indian male presented with four separate episodes of awakening from sleep at night after hearing a flashing sound on the right side of his head over the last 2 years. These events were described ‘as if there are explosions in my head’. A neurologic examination, imaging studies, and a polysomnogram ensued, and the results led to the diagnosis of EHS. Conclusion: EHS is a benign, uncommon, predominately nocturnal disorder that is self-limited. No treatment is generally required. Reassurance to the patient is often all that is needed.

  16. Stochastic and Higher-Order Effects on Exploding Pulses

    Directory of Open Access Journals (Sweden)

    Orazio Descalzi

    2017-08-01

    Full Text Available The influence of additive noise, multiplicative noise, and higher-order effects on exploding solitons in the framework of the prototype complex cubic-quintic Ginzburg-Landau equation is studied. Transitions from explosions to filling-in to the noisy spatially homogeneous finite amplitude solution, collapse (zero solution, and periodic exploding dissipative solitons are reported.

  17. Stellar laboratories . VIII. New Zr iv-vii, Xe iv-v, and Xe vii oscillator strengths and the Al, Zr, and Xe abundances in the hot white dwarfs G191-B2B and RE 0503-289

    Science.gov (United States)

    Rauch, T.; Gamrath, S.; Quinet, P.; Löbling, L.; Hoyer, D.; Werner, K.; Kruk, J. W.; Demleitner, M.

    2017-03-01

    Context. For the spectral analysis of high-resolution and high-signal-to-noise spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims: To search for zirconium and xenon lines in the ultraviolet (UV) spectra of G191-B2B and RE 0503-289, new Zr iv-vii, Xe iv-v, and Xe vii oscillator strengths were calculated. This allows, for the first time, determination of the Zr abundance in white dwarf (WD) stars and improvement of the Xe abundance determinations. Methods: We calculated Zr iv-vii, Xe iv-v, and Xe vii oscillator strengths to consider radiative and collisional bound-bound transitions of Zr and Xe in our NLTE stellar-atmosphere models for the analysis of their lines exhibited in UV observations of the hot WDs G191-B2B and RE 0503-289. Results: We identified one new Zr iv, 14 new Zr v, and ten new Zr vi lines in the spectrum of RE 0503-289. Zr was detected for the first time in a WD. We measured a Zr abundance of -3.5 ± 0.2 (logarithmic mass fraction, approx. 11 500 times solar). We identified five new Xe vi lines and determined a Xe abundance of -3.9 ± 0.2 (approx. 7500 times solar). We determined a preliminary photospheric Al abundance of -4.3 ± 0.2 (solar) in RE 0503-289. In the spectra of G191-B2B, no Zr line was identified. The strongest Zr iv line (1598.948 Å) in our model gave an upper limit of -5.6 ± 0.3 (approx. 100 times solar). No Xe line was identified in the UV spectrum of G191-B2B and we confirmed the previously determined upper limit of -6.8 ± 0.3 (ten times solar). Conclusions: Precise measurements and calculations of atomic data are a prerequisite for advanced NLTE stellar-atmosphere modeling. Observed Zr iv-vi and Xe vi-vii line profiles in the UV spectrum of RE 0503-289 were simultaneously well reproduced with our newly calculated oscillator strengths. Based on observations

  18. Delayed detonations in full-star models of type Ia supernova explosions

    Science.gov (United States)

    Röpke, F. K.; Niemeyer, J. C.

    2007-03-01

    Aims:We present the first full-star three-dimensional explosion simulations of thermonuclear supernovae including parameterized deflagration-to-detonation transitions that occur once the flame enters the distributed burning regime. Methods: Treating the propagation of both the deflagration and the detonation waves in a common front-tracking approach, the detonation is prevented from crossing ash regions. Results: Our criterion triggers the detonation wave at the outer edge of the deflagration flame and consequently it has to sweep around the complex structure and to compete with expansion. Despite the impeded detonation propagation, the obtained explosions show reasonable agreement with global quantities of observed type Ia supernovae. By igniting the flame in different numbers of kernels around the center of the exploding white dwarf, we set up three different models shifting the emphasis from the deflagration phase to the detonation phase. The resulting explosion energies and iron group element productions cover a large part of the diversity of type Ia supernovae. Conclusions: .Flame-driven deflagration-to-detonation transitions, if hypothetical, remain a possibility deserving further investigation.

  19. Investigations on stratification structure parameters formed from electrical exploding wires in vacuum

    Science.gov (United States)

    Wu, Jian; Lu, Yihan; Li, Xingwen; Zhang, Daoyuan; Qiu, Aici

    2017-11-01

    The density distribution and the energy deposition in dark and light transverse layers from stratification structures were measured and analyzed from electrical exploding wire experiments in vacuum. It is demonstrated that the differences in the areal density of the dark layer and light layer regions were about 2 × 1017/cm2 at 296 ns after voltage collapsed. This difference corresponded to an energy deposition difference of ˜2 eV/atom between the dark and light layers, which likely arises from a ±10% resistivity perturbation during the explosion, as suggested by combining a gas column expansion model and a thermodynamic model.

  20. Starting Models in FLASH for Calculations of Type Ia Supernovae

    Science.gov (United States)

    Lamb, D. Q.; Caceres, A.; Calder, A. C.; Dursi, L. J.; Fryxell, B.; MacNeice, P.; Olson, K.; Plewa, T.; Ricker, P.; Riley, K.; Rosner, R.; Siegel, A.; Timmes, F. X.; Truran, J. W.; Vladimirova, N.; Wiers, G.; Zingale, M.

    2003-05-01

    Type Ia supernovae are thought to be the result of a thermonuclear explosion in a white dwarf that is approaching the Chandrasekhar mass limit. The properties of the supernova explosion, including its energy, depends significantly on the way in which the thermonuclear runaway begins. Where in the white dwarf ignition takes place, and how many ignition points there are, are important unsolved questions. We discuss the challenges of modeling Type Ia supernova during the several hours before thermonuclear runaway using the FLASH code. In three-dimensional hydrodynamic codes, the pre-supernova white dwarf can exhibit ``ringing'' at the fundamental frequency of the star that is driven by numerical noise. These solutions manifest themselves as undamped velocity waves (the white dwarf "breathes in and out") that reach peak amplitudes of about 200 km s-1. We show the results of several methods aimed at reducing the amplitude of these undamped waves in FLASH. We also discuss some of our experiments in mapping spherically symmetric models, which suggest large scale convective motions of 50 km s-1 a few hours prior to ignition, onto a three-dimensional mesh. This work was supported in part by the DOE under the ASCI/Alliance program.

  1. Study of exploding type pumping x-ray laser

    Energy Technology Data Exchange (ETDEWEB)

    Yashiro, Hidehiko; Tomie, Toshihisa [Electrotechnical Lab., Tsukuba, Ibaraki (Japan)

    2000-03-01

    We are proposing a new x-ray pumping scheme named 'Exploding type pumping scheme'. A plasma is produced from a very thin membrane which is irradiated by a short pulse laser. Effective heating and ionization of a plasma from membrane is evaluated from Si spectral lines and theoretically hydrodynamic simulation. Effect of ASE from KrF amplifiers is experimentally evaluated as a negligible level when a short pulse laser is well-synchronized. (author)

  2. Delayed Diagnosis of Pharyngeal Perforation following Exploding Tyre Blast Barotrauma

    OpenAIRE

    Field, Samantha M.; Manjaly, Joseph G.; S. Krishan Ramdoo; Jones, Huw A. S.; Tatla, Taran S.

    2014-01-01

    Introduction. Pharyngoesophageal perforation secondary to barotrauma is a rare phenomenon that can have serious complications if identified late. It is challenging to detect due to nonspecific symptoms. We present a case in which detection proved difficult leading to delayed diagnosis. Case Report. A 27-year-old mechanic presented with haemoptysis, dysphonia, and odynophagia after a car tyre exploded in his face. Flexible nasoendoscopy (FNE) revealed blood in the pharynx, thought to represent...

  3. Magnetic properties of iron nanoparticles prepared by exploding wire technique

    OpenAIRE

    Alqudami, Abdullah; Annapoorni, S.; Lamba, Subhalakshmi; Kothari, P C; Kotnala, R K

    2006-01-01

    Nanoparticles of iron were prepared in distilled water using very thin iron wires and sheets, by the electro-exploding wire technique. Transmission electron microscopy reveals the size of the nanoparticles to be in the range 10 to 50 nm. However, particles of different sizes can be segregated by using ultrahigh centrifuge. X-ray diffraction studies confirm the presence of the cubic phase of iron. These iron nanoparticles were found to exhibit fluorescence in the visible region in contrast to ...

  4. Asteroseismological study of massive ZZ Ceti stars with fully evolutionary models

    Energy Technology Data Exchange (ETDEWEB)

    Romero, A. D.; Kepler, S. O. [Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, Porto Alegre 91501-970, RS (Brazil); Córsico, A. H.; Althaus, L. G. [Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, (1900) La Plata (Argentina); Fraga, L., E-mail: alejandra.romero@ufrgs.br [Southern Observatory for Astrophysical Research, Casilla 603, La Serena (Chile)

    2013-12-10

    We present the first asteroseismological study for 42 massive ZZ Ceti stars based on a large set of fully evolutionary carbon-oxygen core DA white dwarf models characterized by a detailed and consistent chemical inner profile for the core and the envelope. Our sample comprises all of the ZZ Ceti stars with spectroscopic stellar masses between 0.72 and 1.05 M {sub ☉} known to date. The asteroseismological analysis of a set of 42 stars enables study of the ensemble properties of the massive, pulsating white dwarf stars with carbon-oxygen cores, in particular the thickness of the hydrogen envelope and the stellar mass. A significant fraction of stars in our sample have stellar mass that is high enough to crystallize at the effective temperatures of the ZZ Ceti instability strip, which enables us to study the effects of crystallization on the pulsation properties of these stars. Our results show that the phase diagram presented in Horowitz et al. seems to be a good representation of the crystallization process inside white dwarf stars, in agreement with the results from white dwarf luminosity function in globular clusters.

  5. Exploding superstars understanding supernovae and gamma-ray bursts

    CERN Document Server

    Mazure, Alain

    2009-01-01

    The exceptional cosmic history and the fabulous destinies of exploding stars – supernovae and gamma-ray bursters – are highly fertile areas of research and are also very special tools to further our understanding of the universe. In this book, cosmologists Dr Alain Mazure and Dr Stéphane Basa throw light on the assemblage of facts, hypotheses and cosmological conclusions and show how these ‘beacons’ illuminate their immediate surroundings and allow us to study the vast cosmos, like searchlights revealing the matter comprising our universe.

  6. Optical probing of exploding wires using schlieren and interferometric imaging.

    Science.gov (United States)

    Romanova, V. M.; Pikuz, S. A.; Shelkovenko, T. A.; Hu, Min; Sinars, D. B.; Kusse, B. R.; Dimant, Ya. S.; Greenly, J. B.; Hummer, D. A.

    1999-11-01

    Optical diagnostics using an Nd:YAG laser (l=532 nm) were used to probe exploding fine wires (7.5-20 mm initial diameter) driven by a sinusoidal 4.5/,kA current pulse (350 ns risetime). Phase transitions from vapor to plasma have been directly observed during the explosion process of various wire materials (Al, Au, etc.). Three simultaneous schlieren channels and 1--3 interferometry channels were used. For some experiments, the schlieren channels were made up of a bright-field iris, a dark-field knife edge, and a dark-field strip block. For other experiments, three bright-field images were made at 10 ns intervals. Interferometry was performed using a new astigmatism-free shearing interferometer based on a double-prism air wedge. The images were made with a 4 ns exposure time at 50--2000 ns after the start of the wire current. Simultaneous X-ray backlighting [1] of the wires enabled reliable interpretation of the optical images, and with a step wedge [2] was used to measure the ion density. Combining the ion density and electron density measurements allowed us to estimate the ionization state of the exploded wire. 1. T.A.Shelkovenko, S.A.Pikuz, A.R.Mingaleev, D.A.Hammer, Rev. Sci. Instrum., 70, 667 (1999). 2. S.A.Pikuz, T.A.Shelkovenko, A.R.Mingaleev, H.Neves, D.A.Hammer, Phys. Plasmas (in press).

  7. Ballistic trauma from an exploding electronic cigarette: Case report

    Directory of Open Access Journals (Sweden)

    Christopher Ban, DMD

    2017-09-01

    Full Text Available Electronic cigarettes (e-cigarettes first became available in the United States in 2007, and since that time, the number of e-cigarette users in the US has grown to over 2.5 million. During the period from 2010–2013 alone, the percentage of Americans who reported that they had ever used electronic cigarettes more than doubled from 3.3% to 8.5%. This number will continue to grow, as the use of electronic cigarettes as an alternative to smoking and in smoking cessation is being explored by the public and medical professionals alike. This article presents a case report involving a patient who was injured when the electronic cigarette he was smoking exploded in his face, causing a ballistic injury to his maxilla, as well as a series of other associated injuries. There have been several recent reports in the literature of exploding electronic cigarettes. This article presents a case of avulsive injury due to ballistic trauma with associated impaction of the vaporizing device.

  8. Long-term Photometric Variations in the Candidate White-dwarf Pulsar AR Scorpii from K2 , CRTS, and ASAS-SN Observations

    Energy Technology Data Exchange (ETDEWEB)

    Littlefield, Colin; Garnavich, Peter; Kennedy, Mark [Department of Physics, University of Notre Dame, Notre Dame, IN (United States); Callanan, Paul [Department of Physics, University College Cork, Cork (Ireland); Shappee, Benjamin [Carnegie Observatories, Pasadena, CA (United States); Holoien, Thomas [Department of Astronomy, The Ohio State University, Columbus, OH (United States)

    2017-08-10

    We analyze long-cadence Kepler K2 observations of AR Sco from 2014, along with survey photometry obtained between 2005 and 2016 by the Catalina Real-Time Sky Survey and the All-Sky Automated Survey for Supernovae. The K2 data show the orbital modulation to have been fairly stable during the 78 days of observations, but we detect aperiodic deviations from the average waveform with an amplitude of ∼2% on a timescale of a few days. A comparison of the K2 data with the survey photometry reveals that the orbital waveform gradually changed between 2005 and 2010, with the orbital maximum shifting to earlier phases. We compare these photometric variations with proposed models of this unusual system.

  9. The Effect of Surface Heterogeneities in Exploding Metallic Foils

    Science.gov (United States)

    Neal, William; Sanchez, Nathaniel; Jensen, Brian; Gibson, John; Martinez, Mike; Romero, Jonathon; Owens, Charles; Jaramillo, Denis; Iverson, Adam; Carlson, Carl; Derry, Alex; Rigg, Paulo

    2017-06-01

    During the electrical explosion of bridge-wires and bridge-foils, the metal bridge undergoes rapid resistive-heating. The metal is rapidly expanded through solid, liquid, vapour and plasma phases. This study uses ALEGRA MHD, a Sandia National Laboratory magneto-hydrocode, to predict the formation of these metallic phases during the explosion process and determine the effects of surface heterogeneities on the spatial distribution of these phases. The simulations are compared against x-ray phase contrast radiographs of electrically exploded bridge-foils. From comparison of these data, it is evident that the meso-structure of the metallic foil dominates the explosion process and is something that should be controlled during the manufacturing processes for detonator designs.

  10. Dynamics of exploding plasmas in a large magnetized plasma

    Energy Technology Data Exchange (ETDEWEB)

    Niemann, C.; Gekelman, W.; Constantin, C. G.; Everson, E. T.; Schaeffer, D. B.; Clark, S. E.; Zylstra, A. B.; Pribyl, P.; Tripathi, S. K. P.; Bondarenko, A. S. [Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095 (United States); Winske, D. [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Larson, D.; Glenzer, S. H. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)

    2013-01-15

    The dynamics of an exploding laser-produced plasma in a large ambient magneto-plasma was investigated with magnetic flux probes and Langmuir probes. Debris-ions expanding at super-Alfvenic velocity (up to M{sub A}=1.5) expel the ambient magnetic field, creating a large (>20 cm) diamagnetic cavity. We observe a field compression of up to B/B{sub 0}=1.5 as well as localized electron heating at the edge of the bubble. Two-dimensional hybrid simulations reproduce these measurements well and show that the majority of the ambient ions are energized by the magnetic piston and swept outside the bubble volume. Nonlinear shear-Alfven waves ({delta}B/B{sub 0}>25%) are radiated from the cavity with a coupling efficiency of 70% from magnetic energy in the bubble to the wave.

  11. The two promising scenarios to explode core collapse supernovae

    Science.gov (United States)

    Soker, Noam

    2017-10-01

    I compare to each other what I consider to be the two most promising scenarios to explode core-collapse supernovae (CCSNe). Both are based on the negative jet feedback mechanism (JFM). In the jittering jets scenario a collapsing core of a single slowly-rotating star can launch jets. The accretion disk or belt (a sub-Keplerian accretion flow concentrated toward the equatorial plane) that launches the jets is intermittent with varying directions of the axis. Instabilities, such as the standing accretion shock instability (SASI), lead to stochastic angular momentum variations that allow the formation of the intermittent accretion disks/belts. According to this scenario no failed CCSNe exist. According to the fixed axis scenario, the core of the progenitor star must be spun up during its late evolutionary phases, and hence all CCSNe are descendants of strongly interacting binary systems, most likely through a common envelope evolution (whether the companion survives or not). Due to the strong binary interaction, the axis of the accretion disk that is formed around the newly born neutron star has a more or less fixed direction. According to the fixed axis scenario, accretion disks/belts are not formed around the newly born neutron star of single stars; they rather end in failed CCSNe. I also raise the possibility that the jittering jets scenario operates for progenitors with initial mass of 8 {M}ȯ ≲ {M}{ZAMS}≲ 18 {M}ȯ , while the fixed axis scenario operates for {M}{ZAMS}≳ 18 {M}ȯ . For the first time these two scenarios are compared to each other, as well as to some aspects of the neutrino-driven explosion mechanism. These new comparisons further suggest that the JFM plays a major role in exploding massive stars.

  12. Population synthesis studies of white dwarf binaries

    OpenAIRE

    Kolb, U.; Willems, B.

    2004-01-01

    Presentamos estudios de s ntesis de poblaci on de binarias enana blanca { estrella de la secuencia principal, de variables catacl smicas que son conducidas por discos circumbinarios y de binarias eclipsantes, en la b usqueda del tr ansito de exoplanetas SuperWASP.

  13. A platform for exploding wires in different media

    Science.gov (United States)

    Han, Ruoyu; Wu, Jiawei; Qiu, Aici; Zhou, Haibin; Wang, Yanan; Yan, Jiaqi; Ding, Weidong

    2017-10-01

    A platform SWE-2 used for single wire explosion experiments has been designed, established, and commissioned. This paper describes the design and initial experiments of SWE-2. In summary, two pulsed current sources based on pulse capacitors and spark gaps are adopted to drive sub-microsecond and microsecond time scale wire explosions in a gaseous/liquid medium, respectively. In the initial experiments, a single copper wire was exploded in air, helium, and argon with a 0.1-0.3 MPa ambient pressure as well as tap water with a 283-323 K temperature, 184-11 000 μ S/cm conductivity, or 0.1-0.9 MPa hydrostatic pressure. In addition, the diagnostic system is introduced in detail. Energy deposition, optical emission, and shock wave characteristics are briefly discussed based on experimental results. The platform was demonstrated to operate successfully with a single wire load. These results provide the potential for further applications of this platform, such as plasma-matter interactions, shock wave effects, and reservoir simulations.

  14. Delayed Diagnosis of Pharyngeal Perforation following Exploding Tyre Blast Barotrauma.

    Science.gov (United States)

    Field, Samantha M; Manjaly, Joseph G; Ramdoo, S Krishan; Jones, Huw A S; Tatla, Taran S

    2014-01-01

    Introduction. Pharyngoesophageal perforation secondary to barotrauma is a rare phenomenon that can have serious complications if identified late. It is challenging to detect due to nonspecific symptoms. We present a case in which detection proved difficult leading to delayed diagnosis. Case Report. A 27-year-old mechanic presented with haemoptysis, dysphonia, and odynophagia after a car tyre exploded in his face. Flexible nasoendoscopy (FNE) revealed blood in the pharynx, thought to represent mucosal haemorrhage. Initial treatment consisted of IV dexamethasone and antibiotics. After 3 days, odynophagia persisted prompting a CT scan. This revealed a defect in the posterior hypopharynx and surgical emphysema in the deep neck tissues. Contrast swallow confirmed posterior hypopharyngeal leak. NG feeding was commenced until repeated contrast swallow confirmed resolution of the defect. Discussion. Prompt nonsurgical management of pharyngoesophageal perforation has good outcomes but untreated perforation can have serious complications. FNE should be performed routinely, but only a contrast swallow can diagnose a functional perforation. Clinicians should have a high index of clinical suspicion when patients present with barotrauma and odynophagia. Patients should be kept nil by mouth until perforation has been excluded. Conclusion. When faced with cases of facial barotrauma, clinicians should have a low threshold for further imaging to exclude pharyngoesophageal perforation.

  15. Delayed Diagnosis of Pharyngeal Perforation following Exploding Tyre Blast Barotrauma

    Directory of Open Access Journals (Sweden)

    Samantha M. Field

    2014-01-01

    Full Text Available Introduction. Pharyngoesophageal perforation secondary to barotrauma is a rare phenomenon that can have serious complications if identified late. It is challenging to detect due to nonspecific symptoms. We present a case in which detection proved difficult leading to delayed diagnosis. Case Report. A 27-year-old mechanic presented with haemoptysis, dysphonia, and odynophagia after a car tyre exploded in his face. Flexible nasoendoscopy (FNE revealed blood in the pharynx, thought to represent mucosal haemorrhage. Initial treatment consisted of IV dexamethasone and antibiotics. After 3 days, odynophagia persisted prompting a CT scan. This revealed a defect in the posterior hypopharynx and surgical emphysema in the deep neck tissues. Contrast swallow confirmed posterior hypopharyngeal leak. NG feeding was commenced until repeated contrast swallow confirmed resolution of the defect. Discussion. Prompt nonsurgical management of pharyngoesophageal perforation has good outcomes but untreated perforation can have serious complications. FNE should be performed routinely, but only a contrast swallow can diagnose a functional perforation. Clinicians should have a high index of clinical suspicion when patients present with barotrauma and odynophagia. Patients should be kept nil by mouth until perforation has been excluded. Conclusion. When faced with cases of facial barotrauma, clinicians should have a low threshold for further imaging to exclude pharyngoesophageal perforation.

  16. Helium in double-detonation models of type Ia supernovae

    Science.gov (United States)

    Boyle, Aoife; Sim, Stuart A.; Hachinger, Stephan; Kerzendorf, Wolfgang

    2017-03-01

    The double-detonation explosion model has been considered a candidate for explaining astrophysical transients with a wide range of luminosities. In this model, a carbon-oxygen white dwarf star explodes following detonation of a surface layer of helium. One potential signature of this explosion mechanism is the presence of unburned helium in the outer ejecta, left over from the surface helium layer. In this paper we present simple approximations to estimate the optical depths of important He i lines in the ejecta of double-detonation models. We use these approximations to compute synthetic spectra, including the He i lines, for double-detonation models obtained from hydrodynamical explosion simulations. Specifically, we focus on photospheric-phase predictions for the near-infrared 10 830 Å and 2 μm lines of He i. We first consider a double detonation model with a luminosity corresponding roughly to normal SNe Ia. This model has a post-explosion unburned He mass of 0.03 M⊙ and our calculations suggest that the 2 μm feature is expected to be very weak but that the 10 830 Å feature may have modest opacity in the outer ejecta. Consequently, we suggest that a moderate-to-weak He i 10 830 Å feature may be expected to form in double-detonation explosions at epochs around maximum light. However, the high velocities of unburned helium predicted by the model ( 19 000 km s-1) mean that the He i 10 830 Å feature may be confused or blended with the C i 10 690 Å line forming at lower velocities. We also present calculations for the He i 10 830 Å and 2 μm lines for a lower mass (low luminosity) double detonation model, which has a post-explosion He mass of 0.077 M⊙. In this case, both the He i features we consider are strong and can provide a clear observational signature of the double-detonation mechanism.

  17. PUSHing Core-Collapse Supernovae to Explosions in Spherical Symmetry: Explodability and Nucleosynthesis Yields

    Science.gov (United States)

    Sinha, Sanjana; Ebinger, Kevin; Frohlich, Carla; Perego, Albino; Hempel, Matthias; Liebendoerfer, Matthias; Thielemann, F.-K.

    2017-01-01

    Core-collapse supernovae (CCSNe) are the highly energetic deaths of massive stars. They play a vital role in the synthesis and dissemination of many chemical elements. CCSN nucleosynthesis calculations have previously relied on artificial explosion methods that do not adequately capture the physics of the innermost stellar layers. Multidimensional simulations currently being performed to fully unravel the explosion mechanism of CCSNe are very computationally expensive. The PUSH method, calibrated against SN1987A, provides parametrized spherically symmetric models that follow the consistent evolution of the proto-neutron star as well as the electron fraction of the ejecta. This method is computationally affordable and captures the physics relevant for nucleosynthesis calculations. Here, we present the results of a broad study that investigates the explodability and nucleosynthesis yields of progenitors covering a wide range of ZAMS masses. Comparisons of the predicted explosion properties and yields with observational CCSNe and metal-poor star data will also be presented. The complete set of nucleosynthesis yields will be a valuable input to models of galactic chemical evolution. United States Department of Energy (DOE Grant No. SC0010263).

  18. Neutrino and gravitational wave signal of a delayed-detonation model of Type Ia supernovae

    OpenAIRE

    Seitenzahl, Ivo R.; Herzog, Matthias; Ruiter, Ashley J.; Marquardt, Kai; Ohlmann, Sebastian T.; Roepke, Friedrich K.

    2015-01-01

    The progenitor system(s) and the explosion mechanism(s) of Type Ia supernovae (SNe Ia) are still under debate. Non-electromagnetic observables, in particular gravitational waves and neutrino emission, of thermonuclear supernovae are a complementary window to light curves and spectra for studying these enigmatic objects. A leading model for SNe Ia is the thermonuclear incineration of a near-Chandrasekhar mass carbon-oxygen white dwarf star in a "delayed-detonation". We calculate a three-dimens...

  19. Nucleosynthesis in Type-I Supernovae - Carbon Deflagration and Helium Detonation Models

    Science.gov (United States)

    Nomoto, K.

    The evolution of accreting white dwarfs in close binary systems is studied from the onset of accretion through the thermonuclear explosion. Relatively rapid accretion onto C+O white dwarfs leads to a carbon deflagration supernova which disrupts the star completely. Explosive nucleosynthesis in the deflagration wave produces 0.5 - 0.6 M_sun; 56Ni in the inner part of the star. In the outer layers of the star, the deflagration wave synthesizes appreciable amount of intermediate mass elements such as Ca, Ar, S, Si, Mg, and O. The carbon deflagration model can account for many of the observed features. On the other hand, slow accretion results in the detonation supernova explosion. This type of explosion produces almost exclusively iron peak elements and thus cannot account for the observed features of Type I supernovae. Possible progenitors of Type I supernovae are discussed.

  20. Bioethanol production from steam-exploded rice husk by recombinant Escherichia coli KO11.

    Science.gov (United States)

    Tabata, Takamitsu; Yoshiba, Yusuke; Takashina, Tomonori; Hieda, Kazuo; Shimizu, Norio

    2017-03-01

    Rice husk is one of the most abundant types of lignocellulosic biomass. Because of its significant amount of sugars, such as cellulose and hemicellulose, it can be used for the production of biofuels such as bioethanol. However, the complex structure of lignocellulosic biomass, consisting of cellulose, hemicellulose and lignin, is resistant to degradation, which limits biomass utilization for ethanol production. The protection of cellulose by lignin contributes to the recalcitrance of lignocelluloses to hydrolysis. Therefore, we conducted steam-explosion treatment as pretreatment of rice husk. However, recombinant Escherichia coli KO11 did not ferment the reducing sugar solution obtained by enzymatic saccharification of steam-exploded rice husk. When the steam-exploded rice husk was washed with hot water to remove inhibitory substances and M9 medium (without glucose) was used as a fermentation medium, E. coli KO11 completely fermented the reducing sugar solution obtained by enzymatic saccharification of hot water washing-treated steam-exploded rice husk to ethanol. We report here the efficient production of bioethanol using steam-exploded rice husk.

  1. Ocular injuries from exploding glass-bottled Coca-Cola® drinks in Port Harcourt, Nigeria

    Directory of Open Access Journals (Sweden)

    Pedro-Egbe CN

    2011-05-01

    Full Text Available Chinyere Nnenne Pedro-Egbe, Chibuike Sydney Ejimadu, Henrietta NwachukwuDepartment of Ophthalmology, University of Port Harcourt Teaching Hospital, Port Harcourt, NigeriaBackground: Eye injuries and subsequent loss of vision from the glass and caps of exploding pressurized bottled drinks have been well reported, and as a result most developed countries now use mainly plastic bottles. In Nigeria, however, most drinks are still sold in glass bottles and ocular injuries from this source are therefore not uncommon.Aim: To retrospectively analyze ocular injuries resulting from exploding glass-bottled Coca-Cola® and propose ways of eliminating such injuries in future.Setting: Eye Clinic, University of Port Harcourt Teaching Hospital, Port Harcourt, Nigeria.Materials and methods: The medical records of all cases of ocular injury that presented at the Eye Clinic of the University of Port Harcourt Teaching Hospital over a 5-year period (January 2006 to December 2010 were retrieved and relevant data including age, sex, occupation, events surrounding bottle explosion, and type of ocular injury sustained were extracted.Results: A total of 426 cases of ocular injuries was seen during the period under review. There were 335 (78.6% males and 91 (21.4% females. Six patients had ocular injury from exploding glass-bottled Coca-Cola®, giving an incidence of 1.4%. The presenting visual acuities (VA were light perception (2 cases, counting fingers (2 cases, and 1 VA of 6/24 and 1 VA of 6/12. There were 4 (66.7% cases of corneoscleral laceration with uveal prolapse and 1 case of total hyphema.Conclusion: Because pressurized glass-bottles can explode with normal handling, legislation to ban the use of glass containers for bottling carbonated drinks will go a long way to reducing ocular morbidity from this source. Plastic bottles should be introduced as an alternative.Keywords: ocular injuries, exploding glass-bottled drink

  2. A Semi-Analytic Model of a Buoyant Flame Bubble Propagation During the Deflagration Phase of a Type Ia Supernova

    Science.gov (United States)

    Jumper, Kevin; Fisher, Robert

    2012-03-01

    Type Ia supernovae are astronomical events in which a white dwarf, the cold remnant of a star that has exhausted its hydrogen fuel, detonates and briefly produces an explosion brighter than most galaxies. Many researchers think that they could occur as the white dwarf approaches a critical mass of 1.4 solar masses by accreting matter from a companion main sequence star, a scenario that is referred to as the single-degenerate channel. Assuming such a progenitor, we construct a semi-analytic model of the propagation of a flame bubble ignited at a single off-center point within the white dwarf. The bubble then rises under the influences of buoyancy and drag, burning the surrounding fuel material in a process called deflagration. We contrast the behavior of the deflagration phase in the presence of a physically high Reynolds number regime with the low Reynolds number regimes inherent to three-dimensional simulations, which are a consequence of numerical viscosity. Our work may help validate three-dimensional deflagration results over a range of initial conditions.

  3. Imploding and Exploding Migraine Headaches: Comparison of Methods to Diagnose Pain Directionality

    Science.gov (United States)

    Files, Julia A.; Schwedt, Todd J.; Mayer, Anita P.; David, Paru S.; Vargas, Bert B.; Chang, Yu-Hui; Hunt, Megan; Patel, Salma; Ko, Marcia G.; Tozer, Beverly S.; Burstein, Rami; Dodick, David W.

    2015-01-01

    Background The study aims to compare methods of determining headache directionality (imploding, exploding, and/or ocular headaches) in women with migraine, investigate the concordance between physician assignment and patient self-assignment of pain directionality, and evaluate whether patients assigned their headaches to the same direction when queried using different methods. Directionality of migraine headache pain (imploding, exploding, or ocular) may reflect differences in the underlying pathogenesis of individual migraine attacks among and within individuals. Emerging evidence suggests that directionality of pain in migraine sufferers may predict response to onabotulinumtoxin A. The best method of determining headache directionality in migraine sufferers has not been systematically explored. Methods We conducted a prospective cross-sectional survey study of 198 female patients with migraine presenting to a Women’s Health Clinic. Patients determined the directionality (imploding, exploding, and/or ocular) of their own migraine pain by choosing among 3 pictures graphically representing directionality and also by responding to a written question regarding directionality. Clinicians then classified directionality of migraine pain using structured interviews. Concordance between clinician assignment of directionality and patient self-assignment was determined with Kappa coefficients. Results Subjects were females between the ages of 18 and 77 years (mean 48 years). According to patient selection of representative pictures, 62 (31.6%) had imploding headaches with or without ocular pain, 36 (18.4%) had exploding headaches with or without ocular pain, 78 (39.8%) had ocular pain only, and 20 (10.2%) had imploding and exploding headaches with or without ocular pain. Two subjects did not respond. According to patient responses to a written question, 80 (41.0%) had imploding headaches with or without ocular pain, 53 (27.2%) had exploding headaches with or without ocular

  4. Effect of thickness of insulation coating on temperature of electrically exploded tungsten wires in vacuum

    Science.gov (United States)

    Shi, Huantong; Zou, Xiaobing; Wang, Xinxin

    2017-07-01

    This paper reports an interesting observation of great differences in the temperature of exploded wires with insulation coating of different thicknesses. Two kinds of polyimide-coated tungsten wires were used with the same conductive diameter 12.5 μm but a different thickness of coating, 0.75-2.25 μm and 2.25-4.25 μm, respectively. The specific energy reconstructed from the current and voltage signals was quite close for the tested wires. However, the exploding scenario, obtained from Mach-Zehnder interferograms, showed great differences: a neutral outer-layer was observed around the thick-coated wire, which was absent for the thin-coated wire; and the calculated electron density and local thermal equilibrium temperature were much higher for thick-coated wires. The heat-preserving neutral layer formed by the decomposition of the insulation was supposed to be the cause of this phenomenon.

  5. Ocular injuries from exploding glass-bottled Coca-Cola® drinks in Port Harcourt, Nigeria.

    Science.gov (United States)

    Pedro-Egbe, Chinyere Nnenne; Ejimadu, Chibuike Sydney; Nwachukwu, Henrietta

    2011-01-01

    Eye injuries and subsequent loss of vision from the glass and caps of exploding pressurized bottled drinks have been well reported, and as a result most developed countries now use mainly plastic bottles. In Nigeria, however, most drinks are still sold in glass bottles and ocular injuries from this source are therefore not uncommon. To retrospectively analyze ocular injuries resulting from exploding glass-bottled Coca-Cola® and propose ways of eliminating such injuries in future. Eye Clinic, University of Port Harcourt Teaching Hospital, Port Harcourt, Nigeria. The medical records of all cases of ocular injury that presented at the Eye Clinic of the University of Port Harcourt Teaching Hospital over a 5-year period (January 2006 to December 2010) were retrieved and relevant data including age, sex, occupation, events surrounding bottle explosion, and type of ocular injury sustained were extracted. A total of 426 cases of ocular injuries was seen during the period under review. There were 335 (78.6%) males and 91 (21.4%) females. Six patients had ocular injury from exploding glass-bottled Coca-Cola®, giving an incidence of 1.4%. The presenting visual acuities (VA) were light perception (2 cases), counting fingers (2 cases), and 1 VA of 6/24 and 1 VA of 6/12. There were 4 (66.7%) cases of corneoscleral laceration with uveal prolapse and 1 case of total hyphema. Because pressurized glass-bottles can explode with normal handling, legislation to ban the use of glass containers for bottling carbonated drinks will go a long way to reducing ocular morbidity from this source. Plastic bottles should be introduced as an alternative.

  6. The Fermi Gamma-Ray Space Telescope, Exploding Stars, Neutron Stars, and Black Holes

    Science.gov (United States)

    Thompson, David J.

    2010-01-01

    Since August, 2008, the Fermi Gamma-ray Space Telescope has been scanning the sky, producing a full-sky image every three hours. These cosmic gamma-rays come from extreme astrophysical phenomena, many related to exploding stars (supernovae) or what these explosions leave behind: supernova remnants, neutron stars, and black holes. This talk uses sample Fermi results, plus simple demonstrations, to illustrate the exotic properties of these endpoints of stellar evolution.

  7. Observation of 23 Supernovae That Exploded <300 pc from Earth during the past 300 kyr

    Science.gov (United States)

    Firestone, R. B.

    2014-07-01

    Four supernovae (SNe), exploding recorded 44, 37, 32, and 22 kyr ago in the radiocarbon (14C) record during the past 50 kyr. Each SN left a nearly identical signature in the record, beginning with an initial sudden increase in atmospheric radiocarbon, when the SN exploded, followed by a hiatus of 1500 yr, and concluding with a sustained 2000 yr increase in global radiocarbon due to γ-rays produced by diffusive shock in the SN remnant (SNR). For the past 18 kyr excess radiocarbon has decayed with the 14C half-life. SN22kyrBP, is identified as the Vela SN that exploded 250 ± 30 pc from Earth. These SN are confirmed in the 10Be, 26Al, 36Cl, and NO_3^- geologic records. The rate of near-Earth SNe is consistent with the observed rate of historical SNe giving a galactic rate of 14 ± 3 kyr-1 assuming the Chandra Galactic Catalog SNR distribution. The Earth has been used as a calorimeter to determine that ≈2 × 1049 erg were released as γ-rays at the time of each SN explosion and ≈1050 erg in γ-rays following each SN. The background rate of 14C production by cosmic rays has been determined as 1.61 atoms cm-2 s-1. Approximately 1/3 of the cosmic ray energy produced by diffusive shock in the SNR was observed to be emitted as high-energy γ-rays. Analysis of the 10Be/9Be ratio in marine sediment identified 19 additional near-Earth SNe that exploded 50-300 kyr ago. Comparison of the radiocarbon record with global temperature variations indicated that each SN explosion is correlated with a concurrent global warming of ≈3°C-4°C.

  8. Gold nanoparticles prepared by electro-exploding wire technique in aqueous solutions

    Science.gov (United States)

    Kumar, Lalit; Kapoor, Akanksha; Meghwal, Mayank; Annapoorni, S.

    2016-05-01

    This article presents an effective approach for the synthesis of Au nanoparticles via an environmentally benevolent electro-exploding wire (EEW) technique. In this process, Au nanoparticles evolve through the plasma generated from the parent Au metal. Compared to other typical chemical methods, electro-exploding wire technique is a simple and economical technique which normally operates in water or organic liquids under ambient conditions. Efficient size control was achieved using different aqueous medium like (1mM) NaCl, deionized water and aqueous solution of sodium hydroxide (NaOH, pH 9.5) using identical electro-exploding conditions. The gold nanoparticles exhibited the UV-vis absorption spectrum with a maximum absorption band at 530 nm, similar to that of gold nanoparticles chemically prepared in a solution. The mechanism of size variation of Au nanoparticles is also proposed. The results obtained help to develop methodologies for the control of EEW based nanoparticle growth and the functionalization of nanoparticle surfaces by specific interactions.

  9. Assessment of Proton Deflectometry for Exploding Wire Experiments

    Energy Technology Data Exchange (ETDEWEB)

    Beg, Farhat Nadeem [University of California San Diego

    2013-09-25

    This project provides the first demonstration of the application of proton deflectometry for the diagnosis of electromagnetic field topology and current-carrying regions in Z-pinch plasma experiments. Over the course of this project several milestones were achieved. High-energy proton beam generation was demonstrated on the short-pulse high-intensity Leopard laser, (10 Joules in ~350 femtoseconds, and the proton beam generation was shown to be reproducible. Next, protons were used to probe the electromagnetic field structure of short circuit loads in order to benchmark the two numerical codes, the resistive-magnetohydrodynamics (MHD) code, Gorgon, and the hybrid particle-in-cell code, LSP for the interpretation of results. Lastly, the proton deflectometry technique was used to map the magnetic field structure of pulsed-power-driven plasma loads including wires and supersonic jets formed with metallic foils. Good agreement between the modeling and experiments has been obtained. The demonstrated technique holds great promise to significantly improve the understanding of current flow and electromagnetic field topology in pulsed power driven high energy density plasmas. Proton probing with a high intensity laser was for the first time implemented in the presence of the harsh debris and x-ray producing z-pinch environment driven by a mega-ampere-scale pulsed-power machine. The intellectual merit of the program was that it investigated strongly driven MHD systems and the influence of magnetic field topology on plasma evolution in pulsed power driven plasmas. The experimental program involved intense field-matter interaction in the generation of the proton probe, as well as the generation of plasma subjected to 1 MegaGauss scale magnetic fields. The computational aspect included two well-documented codes, in combination for the first time to provide accurate interpretation of the experimental results. The broader impact included the support of 2 graduate students, one at

  10. Valorization of lignin and cellulose in acid-steam-exploded corn stover by a moderate alkaline ethanol post-treatment based on an integrated biorefinery concept.

    Science.gov (United States)

    Yang, Sheng; Zhang, Yue; Yue, Wen; Wang, Wei; Wang, Yun-Yan; Yuan, Tong-Qi; Sun, Run-Cang

    2016-01-01

    Due to the unsustainable consumption of fossil resources, great efforts have been made to convert lignocellulose into bioethanol and commodity organic compounds through biological methods. The conversion of cellulose is impeded by the compactness of plant cell wall matrix and crystalline structure of the native cellulose. Therefore, appropriate pretreatment and even post-treatment are indispensable to overcome this problem. Additionally, an adequate utilization of coproduct lignin will be important for improving the economic viability of modern biorefinery industries. The effectiveness of moderate alkaline ethanol post-treatment on the bioconversion efficiency of cellulose in the acid-steam-exploded corn stover was investigated in this study. Results showed that an increase of the alcoholic sodium hydroxide (NaOH) concentration from 0.05 to 4% led to a decrease in the lignin content in the post-treated samples from 32.8 to 10.7%, while the cellulose digestibility consequently increased. The cellulose conversion of the 4% alcoholic NaOH integrally treated corn stover reached up to 99.3% after 72 h, which was significantly higher than that of the acid steam exploded corn stover without post-treatment (57.3%). In addition to the decrease in lignin content, an expansion of cellulose I lattice induced by the 4% alcoholic NaOH post-treatment played a significant role in promoting the enzymatic hydrolysis of corn stover. More importantly, the lignin fraction (AL) released during the 4% alcoholic NaOH post-treatment and the lignin-rich residue (EHR) remained after the enzymatic hydrolysis of the 4% alcoholic NaOH post-treated acid-steam-exploded corn stover were employed to synthesize lignin-phenol-formaldehyde (LPF) resins. The plywoods prepared with the resins exhibit satisfactory performances. An alkaline ethanol system with an appropriate NaOH concentration could improve the removal of lignin and modification of the crystalline structure of cellulose in acid-steam-exploded

  11. Physiological function of insoluble dietary fiber prepared from exploded oak wood (Quercus mongolica).

    Science.gov (United States)

    Yang, Jae-Kyung; Choi, Myung-Suk; Kim, Chang-Joon; Shin, Yong-Seung; Han, Dae-Yong; Han, Sang-Woo; Lim, Bu-Kug; Lee, Jong-Yoon; Rhee, Soon-Jae; Kim, Eun-Hee; Kim, Gon-Sup

    2006-01-01

    This study investigated the production of insoluble dietary fiber using exploded and chemically treated oak wood (Quercus mongolica) and the physiological functions of prepared insoluble dietary fiber in laboratory animals. To produce high quality insoluble dietary fiber, the steam explosion treatment was performed at 25 kgf/cm2 pressure for 6 minutes. In the chemical analysis of insoluble dietary fiber, exploded oak wood was pretreated by 1% sodium hydroxide solution. The insoluble dietary fiber contained 7.6% residual lignin and 61.7% of alpha-cellulose. In order to compare the physiological functions of prepared insoluble dietary fiber with those of commercial insoluble dietary fiber, Sprague-Dawley male rats weighing 100 +/- 10 g were randomly assigned to one normal diet and five high cholesterol diets, containing 1% cholesterol. The high cholesterol diet groups were classified as the fiber-free diet (FF group), 5% commercial alpha-cellulose diet group (5C group), 10% commercial alpha-cellulose group (10C group), 5% insoluble dietary fiber group (5M group) and 10% insoluble dietary fiber group (10M group). Food intake, weight gain and food efficiency ratio in high cholesterol groups were significantly higher than those of the normal group, but there were no significant differences among the high cholesterol diet groups. In addition, there were no significant differences in the weights of liver, kidney and small intestine in insoluble dietary fiber-supplemented groups. Cecum weights in all insoluble dietary fiber groups were significantly higher than those of the FF group. There were no significant differences in the activities of the glutamic pyruvic transaminase (GPT) and glutamic oxaloacetic transaminase (GOT) among the insoluble dietary fiber-supplemented groups. In conclusion, the prepared insoluble dietary fiber and the commercially available insoluble fiber showed the same physiological effects. Moreover, the preparation method for the insoluble dietary

  12. Observation of 23 supernovae that exploded <300 pc from Earth during the past 300 kyr

    Energy Technology Data Exchange (ETDEWEB)

    Firestone, R. B., E-mail: rbfirestone@lbl.gov [Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (United States)

    2014-07-01

    Four supernovae (SNe), exploding ≤300 pc from Earth, were recorded 44, 37, 32, and 22 kyr ago in the radiocarbon ({sup 14}C) record during the past 50 kyr. Each SN left a nearly identical signature in the record, beginning with an initial sudden increase in atmospheric radiocarbon, when the SN exploded, followed by a hiatus of 1500 yr, and concluding with a sustained 2000 yr increase in global radiocarbon due to γ-rays produced by diffusive shock in the SN remnant (SNR). For the past 18 kyr excess radiocarbon has decayed with the {sup 14}C half-life. SN22kyrBP, is identified as the Vela SN that exploded 250 ± 30 pc from Earth. These SN are confirmed in the {sup 10}Be, {sup 26}Al, {sup 36}Cl, and NO{sub 3}{sup −} geologic records. The rate of near-Earth SNe is consistent with the observed rate of historical SNe giving a galactic rate of 14 ± 3 kyr{sup –1} assuming the Chandra Galactic Catalog SNR distribution. The Earth has been used as a calorimeter to determine that ≈2 × 10{sup 49} erg were released as γ-rays at the time of each SN explosion and ≈10{sup 50} erg in γ-rays following each SN. The background rate of {sup 14}C production by cosmic rays has been determined as 1.61 atoms cm{sup –2} s{sup –1}. Approximately 1/3 of the cosmic ray energy produced by diffusive shock in the SNR was observed to be emitted as high-energy γ-rays. Analysis of the {sup 10}Be/{sup 9}Be ratio in marine sediment identified 19 additional near-Earth SNe that exploded 50-300 kyr ago. Comparison of the radiocarbon record with global temperature variations indicated that each SN explosion is correlated with a concurrent global warming of ≈3°C-4°C.

  13. Performance characterization of a miniaturized exploding foil initiator via modified VISAR interferometer and shock wave analysis

    Science.gov (United States)

    Yu, Hyeonju; Kim, Bohoon; Jang, Seung-gyo; Kim, Kyu-Hyoung; Yoh, Jack J.

    2017-06-01

    A pyrotechnic device that consists of a donor/acceptor pair separated by a gap or a bulkhead relies on the shock attenuation characteristics of the gap material and the shock sensitivity of the donor and acceptor explosives. In this study, a miniaturized exploding foil initiator, based on high pulsed electrical power generator, was designed to launch a micro Kapton® flyer for impact initiation of a high explosive in order to understand its performance characteristics. Here, the explosive substance was replaced with a witness plate because the flyer poses various flight motions of rotation, bend, and fragmentation due to its extreme thinness. By using a Velocity Interferometer System for Any Reflector and ANSYS Explicit Dynamics, the averaged velocity of a flyer is measured, which then allows for the calculation of the shock pressure and the duration imparted to the explosive for an initiation. Subsequently, the relationship between the flyer velocity, the amplitude, and the width of impact loading can be used to assess the performance of the designed exploding foil initiator of a micro pyro-mechanical device.

  14. Three-stage enzymatic hydrolysis of steam-exploded corn stover at high substrate concentration.

    Science.gov (United States)

    Yang, Jing; Zhang, Xiaoping; Yong, Qiang; Yu, Shiyuan

    2011-04-01

    The feasibility of three-stage hydrolysis of steam-exploded corn stover at high-substrate concentration was investigated. When substrate concentration was 30% and enzyme loading was 15-30 FPU/g cellulose, three-stage (9+9+12 h) hydrolysis could reach a hydrolysis yield of 59.9-81.4% in 30 h. Compared with one-stage hydrolysis for 72 h, an increase of 34-37% in hydrolysis yield could be achieved. When steam-exploded corn stover was used as the substrate for enzyme synthesis and hydrolysis was conducted at a substrate concentration of 25% with an enzyme loading of 20 FPU/g cellulose, a hydrolysis yield of 85.1% was obtained, 19% higher than that the commercial cellulase could reach under the same conditions. The removal of end products was suggested to improve the adsorption of cellulase on the substrate and enhance the productivity of enzymatic hydrolysis. Copyright © 2010 Elsevier Ltd. All rights reserved.

  15. A comparison study of exploding a Cu wire in air, water, and solid powders

    Science.gov (United States)

    Han, Ruoyu; Wu, Jiawei; Ding, Weidong; Zhou, Haibin; Qiu, Aici; Wang, Yanan

    2017-11-01

    In this paper, an experimental study on exploding a copper wire in air, water, incombustible powders, and energetic materials is performed. We examined the effects of the surrounding media on the explosion process and its related phenomena. Experiments were first carried out with copper wire explosions driven by microsecond timescale pulsed currents in air, water, and the half-half case. Then, the copper wires were exploded in air, water, SiO2 powders, quartz sand, NaCl powders, and energetic-material cylinders, respectively. Our experimental results indicated that the explosion process was significantly influenced by the surrounding media, resulting in noticeable differences in energy deposition, optical emission, and shock waves. In particular, incombustible powders could throttle the current flow completely when a fine wire was adopted. We also found that an air or incombustible-powder layer could drastically attenuate the shock wave generated by a wire explosion. As for energetic-material loads, obvious discrepancies were found in voltage/current waveforms from vaporization when compared with a wire explosion in air/water, which meant the metal vapor/liquid drops play a significant role in the ignition process.

  16. Effects of load voltage on voltage breakdown modes of electrical exploding aluminum wires in air

    Science.gov (United States)

    Wu, Jian; Li, Xingwen; Yang, Zefeng; Wang, Kun; Chao, Youchuang; Shi, Zongqian; Jia, Shenli; Qiu, Aici

    2015-06-01

    The effects of the load voltage on the breakdown modes are investigated in exploding aluminum wires driven by a 1 kA, 0.1 kA/ns pulsed current in air. From laser probing images taken by laser shadowgraphy, schlieren imaging, and interferometry, the position of the shockwave front, the plasma channel, and the wire core edge of the exploding product can be determined. The breakdown mode makes a transition from the internal mode, which involves breakdown inside the wire core, to the shunting mode, which involves breakdown in the compressed air, with decreasing charging voltage. The breakdown electrical field for a gaseous aluminum wire core of nearly solid density is estimated to be more than 20 kV/cm, while the value for gaseous aluminum of approximately 0.2% solid density decreases to 15-20 kV/cm. The breakdown field in shunting mode is less than 20 kV/cm and is strongly affected by the vaporized aluminum, the desorbed gas, and the electrons emitted from the wire core during the current pause. Ohmic heating during voltage collapses will induce further energy deposition in the current channel and thus will result in different expansion speeds for both the wire core and the shockwave front in the different modes.

  17. Lignin isolated from steam-exploded eucalyptus wood chips by phase separation and its affinity to Trichoderma reesei cellulase.

    Science.gov (United States)

    Nonaka, Hiroshi; Kobayashi, Ai; Funaoka, Masamitsu

    2013-07-01

    Steam-exploded eucalyptus wood chips were treated with p-cresol and 72% sulfuric acid at ambient temperature. Steam-exploded lignin was isolated as acetone-soluble and diethyl ether-insoluble compounds from the cresol layer. The lignin extraction yield was only 47%, and the amount of cresol grafted to lignin was much less than that in the case of eucalyptus lignin without steam explosion. Clearly, the steam explosion process depolymerized native lignin, and simultaneously, promoted polymerization via labile benzyl positions. The steam-exploded eucalyptus lignin adsorbed more Trichoderma reesei cellulase; however, its enzymatic activity was less than that of eucalyptus lignin that did not undergo steam explosion. It is evident that pretreatment potentially affects the affinity between lignin and cellulase and the resultant saccharification efficiency. Copyright © 2013 Elsevier Ltd. All rights reserved.

  18. A Multi-Survey Approach to White Dwarf Discovery

    Science.gov (United States)

    2012-04-01

    magnitude-limited astrometric survey is conducted, such as Gaia or LSST . Only a subset of our WD candidates were able to be observed because of telescope...Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium

  19. The White Dwarf in EM Cygni: Beyond the Veil

    Science.gov (United States)

    2009-07-10

    Cyg is an eclipsing system as well as a double-lined spectroscopic binary, it has been extensively studied (e.g., Robinson 1974; Nevo & Sadeh 1978...166, 429 Nevo , I., & Sadeh, D. 1978, MNRAS, 182, 595 North, R. C., Marsh, T. R., Moran, C. K. J., Kolb, U., Smith, R. C., & Stehle, R. 2000, MNRAS, 313

  20. Searching For Infrared Excesses Around White Dwarf Stars

    Science.gov (United States)

    Deeb Wilson, Elin; Rebull, Luisa M.; Debes, John H.; Stark, Chris

    2017-01-01

    Many WDs have been found to be “polluted,” meaning they contain heavier elements in their atmospheres. Either an active process that counters gravitational settling is taking place, or an external mechanism is the cause. One proposed external mechanism for atmospheric pollution of WDs is the disintegration and accretion of rocky bodies, which would result in a circumstellar (CS) disk. As CS disks are heated, they emit excess infrared (IR) emission. WDs with IR excesses indicative of a CS disk are known as dusty WDs. Statistical studies are still needed to determine how numerous dusty, polluted WDs are, along with trends and correlations regarding rate of planetary accretion, the lifetimes of CS disks, and the structure and evolution of CS disks. These findings will allow for a better understanding of the fates of planets along with potential habitability of surviving planets.In this work, we are trying to confirm IR excesses around a sample of 69 WD stars selected as part of the WISE InfraRed Excesses around Degenerates (WIRED) Survey (Debes et al. 2011). We have archival data from WISE, Spitzer, 2MASS, DENIS, and SDSS. The targets were initially selected from the Sloan Digital Sky Survey (SDSS), and identified as containing IR excesses based on WISE data. We also have data from the Four Star Infrared Camera array, which is part of Carnegie Institution’s Magellan 6.5 meter Baade Telescope located at Las Campanas Observatory in Chile. These Four Star data are much higher spatial resolution than the WISE data that were used to determine if each WD has an IR excess. There are often not many bands delineating the IR excess portion of the SED; therefore, we are using the Four Star data to check if there is another source in the WISE beam affecting the IR excess.

  1. Records for conversion of laser energy to nuclear energy in exploding nanostructures

    Science.gov (United States)

    Jortner, Joshua; Last, Isidore

    2017-09-01

    Table-top nuclear fusion reactions in the chemical physics laboratory can be driven by high-energy dynamics of Coulomb exploding, multicharged, deuterium containing nanostructures generated by ultraintense, femtosecond, near-infrared laser pulses. Theoretical-computational studies of table-top laser-driven nuclear fusion of high-energy (up to 15 MeV) deuterons with 7Li, 6Li and D nuclei demonstrate the attainment of high fusion yields within a source-target reaction design, which constitutes the highest table-top fusion efficiencies obtained up to date. The conversion efficiency of laser energy to nuclear energy (0.1-1.0%) for table-top fusion is comparable to that for DT fusion currently accomplished for 'big science' inertial fusion setups.

  2. Characterization of the steam-exploded spent Shiitake mushroom medium and its efficient conversion to ethanol.

    Science.gov (United States)

    Asada, Chikako; Asakawa, Ai; Sasaki, Chizuru; Nakamura, Yoshitoshi

    2011-11-01

    Spent Shiitake mushroom medium was subjected to steam explosion followed by simultaneous saccharification and fermentation (SSF) using Meicelase and Saccahromyces cerevisiae AM12. Water extraction of the medium exposed to steam at 20 atm for 5 min enhanced the saccharification rate by about 20% compared to steam-exploded medium before water extraction and resulted in the production of 23.8 g/l ethanol from a substrate concentration of 100g/l. This corresponded to 87.6% of the theoretical ethanol yield, i.e., 15.9 g ethanol was obtained from 100g of spent Shiitake mushroom medium. Spent Shiitake mushroom medium subjected to steam explosion and then water extraction appears to be a candidate for efficient bioconversion to ethanol. Copyright © 2011 Elsevier Ltd. All rights reserved.

  3. An exploding foil shockwave technique for magnetic flux compression and high voltage pulse generation

    CERN Document Server

    Goh, S E

    2002-01-01

    This thesis describes a novel electromagnetic shockwave technique for use in compressing magnetic flux and to serve as the basis for a new approach to producing fast-rising voltage pulses with amplitudes of several hundred kV. The shockwave is produced by an exploding foil driven electric gun that accelerates a Mylar flyer to impact with a sample of aluminium powder. Both Japanese and Russian researchers have previously published experimental results for shockwave magnetic flux compression using an explosive driver. The present research considers replacing the explosive energy of this driver by the electrostatic energy stored in a capacitor bank, thereby enabling experiments to be performed in a laboratory environment. Differences in performance that arise from the use of explosive and electrical driver are examined. A conventional electric gun system in planar geometry is developed to study the insulator-to-metallic transition in shock-compressed aluminium powder. This provides data on the conducting shock f...

  4. MHD instabilities developing in a conductor exploding in the skin effect mode

    Science.gov (United States)

    Oreshkin, V. I.; Chaikovsky, S. A.; Datsko, I. M.; Labetskaya, N. A.; Mesyats, G. A.; Oreshkin, E. V.; Ratakhin, N. A.; Rybka, D. V.

    2016-12-01

    The results of experiments with exploding copper conductors, performed on the MIG facility (providing currents of amplitude of about 2.5 MA and rise time of 100 ns), are analyzed. With an frame optical camera, large-scale instabilities of wavelength 0.2-0.5 mm were detected on the conductor surface. The instabilities show up as plasma "tongues" expanding with a sound velocity in the opposite direction to the magnetic field gradient. Analysis performed using a two-dimensional MHD code has shown that the structures observed in the experiments were formed most probably due to flute instabilities. The growth of flute instabilities is predetermined by the development of thermal instabilities near the conductor surface. The thermal instabilities arise behind the front of the nonlinear magnetic diffusion wave propagating through the conductor. The wavefront on its own is not subject to thermal instabilities.

  5. Misconceptions in the Exploding Flask Demonstration Resolved through Students' Critical Thinking.

    Science.gov (United States)

    Spierenburg, Rick; Jacobse, Leon; de Bruin, Iris; van den Bos, Daan J; Vis, Dominique M; Juurlink, Ludo B F

    2017-09-12

    As it connects to a large set of important fundamental ideas in chemistry and analytical techniques discussed in high school chemistry curricula, we review the exploding flask demonstration. In this demonstration, methanol vapor is catalytically oxidized by a Pt wire catalyst in an open container. The exothermicity of reactions occurring at the catalytic surface heats the metal to the extent that it glows. When restricting reactant and product gas flow, conditions may favor repetitive occurrence of a small explosion. We show how mass spectrometry and infrared spectroscopy allow for unravelling the chemical background of this demonstration and discuss various ideas on how to use it in a classroom setting to engage students' critical thinking about chemical research. Along the way, we show that two commonly published ideas about the chemical background of this demonstration are incorrect, and we suggest simple tests that may be performed in a high school setting either as an addition to the demonstration or as a student research project.

  6. Effect of poultry litter biochar on Saccharomyces cerevisiae growth and ethanol production from steam-exploded poplar and corn stover

    Science.gov (United States)

    Diallo, Oumou

    The use of ethanol produced from lignocellulosic biomass for transportation fuel offers solutions in reducing environmental emission and the use of non-renewable fuels. However, lignocellulosic ethanol production is still hampered by economic and technical obstacles. For instance, the inhibitory effect of toxic compounds produced during biomass pretreatment was reported to inhibit the fermenting microorganisms, hence there was a decrease in ethanol yield and productivity. Thus, there is a need to improve the bioconversion of lignocellulosic biomass to ethanol in order to promote its commercialization. The research reported here investigated the use of poultry litter biochar to improve the ethanol production from steam-exploded poplar and corn stover. The effect of poultry litter biochar was first studied on Saccharomyces cerevisiae ATCC 204508/S288C growth, and second on the enzyme hydrolysis and fermentation of two steam-exploded biomasses: (poplar and corn stover). The third part of the study investigated optimal process parameters (biochar loading, biomass loading, and enzyme loading) on the reducing sugars production, and ethanol yield from steam-exploded corn stover. In this study, it has been shown that poultry litter biochar improved the S. cerevisiae growth and ethanol productivity; therefore poultry litter biochar could potentially be used to improve the ethanol production from steam-exploded lignocellulosic biomass.

  7. NASA's Swift Satellite Catches First Supernova in The Act of Exploding

    Science.gov (United States)

    2008-05-01

    GREENBELT, Md.- Thanks to a fortuitous observation with NASA’s Swift satellite, astronomers for the first time have caught a star in the act of exploding. Astronomers have previously observed thousands of stellar explosions, known as supernovae, but they have always seen them after the fireworks were well underway. "For years we have dreamed of seeing a star just as it was exploding, but actually finding one is a once in a lifetime event," says team leader Alicia Soderberg, a Hubble and Carnegie-Princeton Fellow at Princeton University in Princeton, N.J. "This newly born supernova is going to be the Rosetta stone of supernova studies for years to come." A typical supernova occurs when the core of a massive star runs out of nuclear fuel and collapses under its own gravity to form an ultradense object known as a neutron star. The newborn neutron star compresses and then rebounds, triggering a shock wave that plows through the star’s gaseous outer layers and blows the star to smithereens. Astronomers thought for nearly four decades that this shock "break-out" will produce bright X-ray emission lasting a few minutes. X-ray Image X-ray Images But until this discovery, astronomers have never observed this signal. Instead, they have observed supernovae brightening days or weeks later, when the expanding shell of debris is energized by the decay of radioactive elements forged in the explosion. "Seeing the shock break-out in X-rays can give a direct view of the exploding star in the last minutes of its life and also provide a signpost to which astronomers can quickly point their telescopes to watch the explosion unfold," says Edo Berger, a Carnegie-Princeton Fellow at Princeton University. Soderberg's discovery of the first shock breakout can be attributed to luck and Swift's unique design. On January 9, 2008, Soderberg and Berger were using Swift to observe a supernova known as SN 2007uy in the spiral galaxy NGC 2770, located 90 million light-years from Earth in the

  8. Exploding and Being Swallowed: Cannibalism in Toni Morrison’s Beloved

    Directory of Open Access Journals (Sweden)

    Lay Sion Ng

    2017-01-01

    Full Text Available Cannibalism is a meta-discourse in Toni Morrison’s Beloved. In Alan Rice’s “Who’s Eating Whom,” Beloved’s dream of “exploding and being swallowed” has been critically linked to the cruel practices of slavery, yet it is important to note the way in which the dream of “being swallowed” is largely unexplored. This paper concentrates on the latter aspect, stating that in Beloved, cannibalism and slavery relate not only to the domination of black slaves by white masters, but also to the black mother-child relationships between Sethe and Beloved, Sethe and Denver, and the black sister-sister relationship between Denver and Beloved. This paper argues that the whites designate themselves as the ones who represent civilization through implanting the image of cannibalism into the black Other. Ironically, the system of slavery precisely deconstructs the images that they have built of themselves, making them something no more than cannibals.

  9. Review of effects of dielectric coatings on electrical exploding wires and Z pinches

    Science.gov (United States)

    Wu, Jian; Li, Xingwen; Li, Mo; Li, Yang; Qiu, Aici

    2017-10-01

    As the most powerful x-ray source in the laboratories, the wire array Z pinches have been of great relevance to inertial confinement fusions, laboratory astrophysics, and other high-energy density applications. In order to produce x-ray with greater power and higher efficiency, the dynamics of wire array has been investigated extensively, and various methods have been proposed to improve the implosion quality of the wire array. This review focuses on the experimental and theoretical investigations regarding the effects of the dielectric coatings on electrical exploding wires and Z pinches. Since the early 2000, the electrical wire explosion related to the first stage of the wire array Z pinches has been studied extensively, and the results indicated that the dielectric coatings can significantly increase the joule energy deposition into a wire in the initial stage, and even the corona free explosion of tungsten wires can be achieved. Recently, there is an increasing interest in the dynamics of insulated wire array Z pinches. By applying dielectric coatings, the ablation process is suppressed, the x-ray start time is delayed, and the possibility of multi-peak radiation is decreased. This review is organized by the evolution dynamics of wire array Z pinches, and a broad introduction to relevant scientific concepts and various other applications are presented. According to the current research status, the challenges, opportunities and further developments of Z pinch loads using dielectric coatings are proposed to further promote the researches and their applications.

  10. Different SEP recovery cycle in adolescent migraineurs with exploding or imploding pain.

    Science.gov (United States)

    Iacovelli, Elisa; Tarantino, Samuela; Capuano, Alessandro; De Luca, Massimiliano; De Ranieri, Cristiana; Vigevano, Federico; Arendt-Nielsen, Lars; Valeriani, Massimiliano

    2013-01-01

    Our aim was to investigate whether migraine adolescents with pain directed inside (imploding pain--IP) and outside (exploding pain--EP) the head may have different levels of cortical excitability underlying their migraineous syndrome. Ten migraine children referring prevalent EP (mean age 14.5 ± 1.4 years, 3 girls, 7 boys), 10 patients with IP (mean age 14.1 ± 2.2 years, 4 girls, 6 boys), and 13 control subjects (mean age 13 ± 1.8 years, 6 males, 7 females) participated to the study. The recovery cycle of the somatosensory evoked potentials to electrical median nerve stimuli at interstimulus intervals of 5, 20, and 40 ms was measured. Anger expression, anxiety, and somatic concerns were investigated in migraine patients. Overall, SEP recovery cycle was shorter in migraineurs than in healthy controls. The recovery cycle of the frontal N30 SEP component was significantly shorter in IP than in EP patients. While among the EP patients those with faster N30 recovery cycle had higher Trait-Anger score, the opposite was found among the IP patients. Our results suggest that the inhibitory mechanisms within the somatosensory cortex are more impaired in IP than in EP migraine adolescents. The pathophysiological difference between IP and EP migraineurs was strengthened also by the opposite correlations between the brain excitability and the anger expression. Copyright © 2013 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.

  11. Fermentation of wet-exploded corn stover for the production of volatile fatty acids.

    Science.gov (United States)

    Murali, Nanditha; Fernandez, Sebastian; Ahring, Birgitte Kiaer

    2017-03-01

    Volatile fatty acids (VFA) have been used as platform molecules for production of biofuels and bioproducts. In the current study, we examine the VFA production from wet-exploded corn stover through anaerobic fermentation using rumen bacteria. The total VFA yield (acetic acid equivalents) was found to increase from 22.8g/L at 2.5% total solids (TS) to 40.8g/L at 5% TS. It was found that the acetic acid concentration increased from 10g/L to 22g/L at 2.5% and 5% TS, respectively. An increased propionic acid production was seen between day 10 and 20 at 5% TS. Valeric acid (4g/L) was produced at 5% TS and not at 2.5% TS. Composition analysis showed that 50% of the carbohydrates were converted to VFA at 5% TS and 33% at 2.5% TS. Our results show that rumen fermentation of lignocellulosic biomass after wet explosion can produce high concentrations of VFA without addition of external enzymes of importance for the process economics of lignocellulosic biorefineries. Copyright © 2016 Elsevier Ltd. All rights reserved.

  12. Sexy males and choosy females on exploded leks: correlates of male attractiveness in the Little Bustard.

    Science.gov (United States)

    Jiguet, Frédéric; Bretagnolle, Vincent

    2014-03-01

    In their choice of mates, females may use alternative tactics, including a comparative assessment of males in a population, using one or several relative preference criteria. Traits involved in female choice should presumably be variable between, but not within males, thus potentially providing reliable cues of male identity and quality for prospecting females. In lekking species, sexual selection is usually intense, and females can freely choose mates. Studying the Little Bustard Tetrax tetrax, a bird with an exploded lek mating system, we first identified male phenotypic traits that showed higher among, than within variation (plumage pattern, display rates and call structure). Among those and other traits (ornaments and their symmetry, body condition, lek spatial organization and territory quality), we identified phenotypic traits that correlated with male attractiveness toward females. At least four phenotypic male traits were correlated with female attraction, i.e. body condition, lek attendance, ornamental symmetry and display rates. Traits related to the initial female attraction on male territory seem to differ from traits related to the decision of females to stay in the territory of attractive males. Copyright © 2014 Elsevier B.V. All rights reserved.

  13. Extended exploding reflector concept for computing prestack traveltimes for waves of different type in the DSR framework

    KAUST Repository

    Duchkov, Anton A.

    2013-09-22

    The double-square-root (DSR) equation can be viewed as a Hamilton-Jacobi equation describing kinematics of downward data continuation in depth. It describes simultaneous propagation of source and receiver rays which allows computing reflection wave prestack traveltimes (for multiple sources) in a one run thus speeding up solution of the forward problem. Here we give and overview of different alternative forms of the DSR equation which allows stepping in two-way time and subsurface offset instead of depth. Different forms of the DSR equation are suitable for computing different types of waves including reflected, head and diving waves. We develop a WENO-RK numerical scheme for solving all mentioned forms of the DSR equation. Finally the extended exploding reflector concept can be used for computing prestack traveltimes while initiating the numerical solver as if a reflector was exploding in extended imaging space.

  14. Effects of furan derivatives on biohydrogen fermentation from wet steam-exploded cornstalk and its microbial community.

    Science.gov (United States)

    Liu, Zhidan; Zhang, Chong; Wang, Linjun; He, Jianwei; Li, Baoming; Zhang, Yuanhui; Xing, Xin-Hui

    2015-01-01

    Understanding the role of furan derivatives, furfural (FUR) and 5-hydroxymethyl furfural (HMF), is important for biofuel production from lignocellulosic biomass. In this study, the effects of furan derivatives on hydrogen fermentation from wet steam-exploded cornstalk were investigated. The control experiments with only seed sludge indicated that HMF addition of up to 1g/L stimulated hydrogen production. Similar results were obtained using steam-exploded cornstalk as the feedstock. Hydrogen productivity was increased by up to 40% with the addition of HMF. In addition, over 90% of furan derivatives with an initial concentration below 1g/L were degraded. Pyosequencing showed that the addition of HMF and FUR resulted in different microbial communities. HMF led to a higher proportion of the genera Clostridium and Ruminococcaceae, supporting the increased hydrogen production. This study suggested that hydrogen fermentation could be a detoxifying step for steam-exploded cornstalk, and HMF and FUR exhibited different functions for hydrogen fermentation. Copyright © 2014 Elsevier Ltd. All rights reserved.

  15. Enzymatic hydrolysis and succinic acid fermentation from steam-exploded corn stalk at high solid concentration by recombinant Escherichia coli.

    Science.gov (United States)

    Wu, Dexi; Li, Qiang; Wang, Dan; Dong, Yugang

    2013-08-01

    Steam-exploded corn stalk biomass was used as the substrate for succinic acid production via lignocellulose enzymatic hydrolysis and fermentation. Succinic acid fermentation was investigated in Escherichia coli strains overexpressing cyanobacterium Anabaena sp. 7120 ecaA gene encoding carbonic anhydrase (CA). For the washed steam-exploded corn stalk at 30 % substrate concentration, i.e., 30 % water-insoluble solids (WIS), enzymatic hydrolysis yielded 97.5 g/l glucose solution and a cellulose conversion of 73.6 %, thus a high succinic acid level up to 38.6 g/l. With the unwashed steam-exploded corn stalk, though a cellulose conversion of 71.2 % was obtained in hydrolysis at 30 % solid concentration (27.9 % WIS), its hydrolysate did not ferment at all, and the hydrolysate of 25 % solid loading containing 3.8 g/l acetic acid and 168.2 mg/l furfural exerted a strong inhibition on succinic acid production.

  16. Hydrodynamic Models of Line-Driven Accretion Disk Winds III: Local Ionization Equilibrium

    Science.gov (United States)

    Pereyra, Nicolas Antonio; Kallman, Timothy R.; White, Nicholas E. (Technical Monitor)

    2002-01-01

    We present time-dependent numerical hydrodynamic models of line-driven accretion disk winds in cataclysmic variable systems and calculate wind mass-loss rates and terminal velocities. The models are 2.5-dimensional, include an energy balance condition with radiative heating and cooling processes, and includes local ionization equilibrium introducing time dependence and spatial dependence on the line radiation force parameters. The radiation field is assumed to originate in an optically thick accretion disk. Wind ion populations are calculated under the assumption that local ionization equilibrium is determined by photoionization and radiative recombination, similar to a photoionized nebula. We find a steady wind flowing from the accretion disk. Radiative heating tends to maintain the temperature in the higher density wind regions near the disk surface, rather than cooling adiabatically. For a disk luminosity L (sub disk) = solar luminosity, white dwarf mass M(sub wd) = 0.6 solar mass, and white dwarf radii R(sub wd) = 0.01 solar radius, we obtain a wind mass-loss rate of M(sub wind) = 4 x 10(exp -12) solar mass yr(exp -1) and a terminal velocity of approximately 3000 km per second. These results confirm the general velocity and density structures found in our earlier constant ionization equilibrium adiabatic CV wind models. Further we establish here 2.5D numerical models that can be extended to QSO/AGN winds where the local ionization equilibrium will play a crucial role in the overall dynamics.

  17. CONSTRAINING TYPE Ia SUPERNOVA MODELS: SN 2011fe AS A TEST CASE

    Energy Technology Data Exchange (ETDEWEB)

    Roepke, F. K.; Seitenzahl, I. R. [Institut fuer Theoretische Physik und Astrophysik, Universitaet Wuerzburg, Am Hubland, D-97074 Wuerzburg (Germany); Kromer, M.; Taubenberger, S.; Ciaraldi-Schoolmann, F.; Hillebrandt, W.; Benitez-Herrera, S. [Max-Planck-Institut fuer Astrophysik, Karl-Schwarzschild-Str. 1, D-85741 Garching (Germany); Pakmor, R. [Heidelberger Institut fuer Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg (Germany); Sim, S. A. [Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611 (Australia); Aldering, G.; Childress, M.; Fakhouri, H. K. [Physics Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 (United States); Antilogus, P.; Bongard, S.; Canto, A.; Cellier-Holzem, F. [Laboratoire de Physique Nucleaire et des Hautes Energies, Universite Pierre et Marie Curie Paris 6, Universite Paris Diderot Paris 7, CNRS-IN2P3, 4 place Jussieu, 75252 Paris Cedex 05 (France); Baltay, C. [Department of Physics, Yale University, New Haven, CT 06250-8121 (United States); Buton, C. [Physikalisches Institut, Universitaet Bonn, Nussallee 12, 53115 Bonn (Germany); Chotard, N.; Copin, Y. [Universite de Lyon, F-69622, Lyon (France); Universite de Lyon 1, Villeurbanne (France); CNRS/IN2P3, Institut de Physique Nucleaire de Lyon (France); and others

    2012-05-01

    The nearby supernova SN 2011fe can be observed in unprecedented detail. Therefore, it is an important test case for Type Ia supernova (SN Ia) models, which may bring us closer to understanding the physical nature of these objects. Here, we explore how available and expected future observations of SN 2011fe can be used to constrain SN Ia explosion scenarios. We base our discussion on three-dimensional simulations of a delayed detonation in a Chandrasekhar-mass white dwarf and of a violent merger of two white dwarfs (WDs)-realizations of explosion models appropriate for two of the most widely discussed progenitor channels that may give rise to SNe Ia. Although both models have their shortcomings in reproducing details of the early and near-maximum spectra of SN 2011fe obtained by the Nearby Supernova Factory (SNfactory), the overall match with the observations is reasonable. The level of agreement is slightly better for the merger, in particular around maximum, but a clear preference for one model over the other is still not justified. Observations at late epochs, however, hold promise for discriminating the explosion scenarios in a straightforward way, as a nucleosynthesis effect leads to differences in the {sup 55}Co production. SN 2011fe is close enough to be followed sufficiently long to study this effect.

  18. Discovery of millisecond pulsar companions in the globular cluster 47 Tucanae .

    Science.gov (United States)

    Rivera-Sandoval, L. E.; van den Berg, M.; Heinke, C. O.; Cohn, H. N.; Lugger, P. M.; Freire, P.; Anderson, J.; Serenelli, A. M.; Althaus, L. G.; Cool, A. M.; Grindlay, J. E.; Edmonds, P. D.; Wijnands, R.; Ivanova, N.

    We present near-ultraviolet and optical Hubble Space Telescope observations of the likely white dwarf companions to radio millisecond pulsars 47 Tuc Q, S, T, Y, and the previously identified white dwarf counterpart to 47 Tuc U. All five near-ultraviolet companions show colors that agree with the ones resulting from He-core white dwarf (He WD) cooling models for white dwarf masses ˜0.16-0.3 Msun . We find that the counterparts to 47 Tuc U and 47 Tuc T show Hα absorption, as is characteristic for white dwarfs with a H envelope, such as He WDs.

  19. Three-dimensional Delayed-Detonation Model of Type Ia Supernovae

    Science.gov (United States)

    Gamezo, Vadim N.; Khokhlov, Alexei M.; Oran, Elaine S.

    2005-04-01

    We study a Type Ia supernova explosion using large-scale three-dimensional numerical simulations based on reactive fluid dynamics with a simplified mechanism for nuclear reactions and energy release. The initial deflagration stage of the explosion involves a subsonic turbulent thermonuclear flame propagating in the gravitational field of an expanding white dwarf. The deflagration produces an inhomogeneous mixture of unburned carbon and oxygen with intermediate-mass and iron-group elements in central parts of the star. During the subsequent detonation stage, a supersonic detonation wave propagates through the material unburned by the deflagration. The total energy released in this delayed-detonation process, (1.3-1.6)×1051 ergs, is consistent with a typical range of kinetic energies obtained from observations. In contrast to the deflagration model, which releases only about 0.6×1051 ergs, the delayed-detonation model does not leave carbon, oxygen, and intermediate-mass elements in central parts of a white dwarf. This removes the key disagreement between three-dimensional simulations and observations, and makes a delayed detonation the mostly likely mechanism for Type Ia supernova explosions.

  20. Comparison of saccharification and fermentation of steam exploded rice straw and rice husk.

    Science.gov (United States)

    Wood, Ian P; Cao, Huong-Giang; Tran, Long; Cook, Nicola; Ryden, Peter; Wilson, David R; Moates, Graham K; Collins, Samuel R A; Elliston, Adam; Waldron, Keith W

    2016-01-01

    Rice cultivation produces two waste streams, straw and husk, which could be exploited more effectively. Chemical pretreatment studies using rice residues have largely focussed on straw exploitation alone, and often at low substrate concentrations. Moreover, it is currently not known how rice husk, the more recalcitrant residue, responds to steam explosion without the addition of chemicals. The aim of this study has been to systematically compare the effects of steam explosion severity on the enzymatic saccharification and simultaneous saccharification and fermentation of rice straw and husk produced from a variety widely grown in Vietnam (Oryza sativa, cv. KhangDan18). Rice straw and husk were steam exploded (180-230 °C for 10 min) into hot water and washed to remove fermentation inhibitors. In both cases, pretreatment at 210 °C and above removed most of the noncellulosic sugars. Prolonged saccharification at high cellulase doses showed that rice straw could be saccharified most effectively after steam explosion at 210 °C for 10 min. In contrast, rice husk required more severe pretreatment conditions (220 °C for 10 min), and achieved a much lower yield (75 %), even at optimal conditions. Rice husk also required a higher cellulase dose for optimal saccharification (10 instead of 6 FPU/g DM). Hemicellulase addition failed to improve saccharification. Small pilot scale saccharification at 20 % (w/v) substrate loading in a 10 L high torque bioreactor resulted in similarly high glucose yields for straw (reaching 9 % w/v), but much less for husk. Simultaneous saccharification and fermentation under optimal pretreatment and saccharification conditions showed similar trends, but the ethanol yield from the rice husk was less than 40 % of the theoretical yield. Despite having similar carbohydrate compositions, pretreated rice husk is much less amenable to saccharification than pretreated rice straw. This is likely to attenuate its use as a biorefinery

  1. Anisotropic models for compact stars

    Energy Technology Data Exchange (ETDEWEB)

    Maurya, S.K.; Dayanandan, Baiju [University of Nizwa, Department of Mathematical and Physical Sciences, College of Arts and Science, Nizwa (Oman); Gupta, Y.K. [Jaypee Institute of Information Technology University, Department of Mathematics, Noida, Uttar Pradesh (India); Ray, Saibal [Government College of Engineering and Ceramic Technology, Department of Physics, Kolkata, West Bengal (India)

    2015-05-15

    In the present paper we obtain an anisotropic analog of the Durgapal and Fuloria (Gen Relativ Gravit 17:671, 1985) perfect fluid solution. The methodology consists of contraction of the anisotropic factor Δ with the help of both metric potentials e{sup ν} and e{sup λ}. Here we consider e{sup λ} the same as Durgapal and Fuloria (Gen Relativ Gravit 17:671, 1985) did, whereas e{sup ν} is as given by Lake (Phys Rev D 67:104015, 2003). The field equations are solved by the change of dependent variable method. The solutions set mathematically thus obtained are compared with the physical properties of some of the compact stars, strange star as well as white dwarf. It is observed that all the expected physical features are available related to the stellar fluid distribution, which clearly indicates the validity of the model. (orig.)

  2. Magnetospherically-trapped dust and a possible model for the unusual transits at WD 1145+017

    Science.gov (United States)

    Farihi, J.; von Hippel, T.; Pringle, J. E.

    2017-10-01

    The rapidly evolving dust and gas extinction observed towards WD 1145+017 has opened a real-time window onto the mechanisms for destruction-accretion of planetary bodies onto white dwarf stars, and has served to underline the importance of considering the dynamics of dust particles around such objects. Here it is argued that the interaction between (charged) dust grains and the stellar magnetic field is an important ingredient in understanding the physical distribution of infrared emitting particles in the vicinity of such white dwarfs. These ideas are used to suggest a possible model for WD 1145+017 in which the unusual transit shapes are caused by opaque clouds of dust trapped in the stellar magnetosphere. The model can account for the observed transit periodicities if the stellar rotation is near 4.5 h, as the clouds of trapped dust are then located near or within the co-rotation radius. The model requires the surface magnetic field to be at least around some tens of kG. In contrast to the eccentric orbits expected for large planetesimals undergoing tidal disintegration, the orbits of magnetospherically-trapped dust clouds are essentially circular, consistent with the observations.

  3. Simultaneous modelling of X-ray emission and optical polarization of intermediate polars: the case of V405 Aur

    Science.gov (United States)

    J. Lima, I.; Vilega Rodrigues, C.; Medeiros Gomes Silva, K.; Luna, G.; D Amico, F.; Goulart Coelho, J.

    2017-10-01

    Intermediate polars are compact binaries in which mass transfer occurs from a low-mass star onto a magnetic white dwarf. A shock structure is formed in the magnetic accretion column nearby the white-dwarf surface. High-energy emission is produced in the post-shock region and the main physical process envolved is bremsstrahlung and line emission. Some systems show optical polarization, which may be also originated in the post-shock region. Our main goal is to study the magnetic structure of intermediate polars by simultaneously modelling optical polarimetry and X-ray data using the CYCLOPS code. This code was developed by our group to peform multi-wavelength fitting of the accretion column flux. It considers cyclotron and free-free emission from a 3D post-shock region, which is non-homogeneous in terms of density, temperature, and magnetic field. In this study, we present our modelling of the optical polarization and X-ray emission of V405 Aurigae, the intermediate polar that has the highest magnetic field. Previous studies of this system were not successful in proposing a geometry that explains both the optical and X-ray emissions.

  4. Characterization of Turbulent Non-Uniform Exploding Wire Plasma Using High Resolution Interferometry, Schlieren and Shadowography Imaging

    Science.gov (United States)

    Caplinger, J.; Sarkisov, G.; Wallerstein, A. J.; Sotnikov, V.; Lundberg, J.; Reed, Z.

    2014-10-01

    High resolution interferogram, Schlieren, and shadowgraph imaging has been used to characterize an exploding wire plasma. Using an 80 kV high voltage pulse generator with a rise time of 5 ns, exploding wire plasmas are created in aluminum, gold, tin, stainless steel, platinum and silver wires. The plasma is probed over a period of 3-7 ns using a 532 nm frequency doubled Nd:YAG Q-switched laser. The resulting laser radiation is imaged as an interferogram using an air-wedge interferometer, a shadowgraph and as a Schlieren image using two CCDs. Calculations resulting from the interferograms reveal ionizations between 10-20% for Aluminum wires at atmospheric pressure. This is confirmed by the Schlieren images as the refraction caused by neutrals is dominant. Single wire, two parallel wires, and other two wire configurations are investigated. Additionally, influence of chamber pressure on plasma uniformity, shock wave propagation velocity and instabilities is presented. Work supported by the Air Force Research Laboratory.

  5. Processing temperature driven morphological evolution of ZnO nanostructures prepared by electro-exploding wire technique

    Science.gov (United States)

    Kumar, Lalit; Medwal, Rohit; Sen, P.; Annapoorni, S.

    2014-03-01

    This article presents an effective approach for the synthesis of ZnO nanoparticles with desired morphology via an environmentally benevolent electro-exploding wire (EEW) technique. In this process, ZnO nanoparticles evolve through the plasma generated from the parent Zn metal. Compared to other typical chemical methods, electro-exploding wire technique is a simple and economical technique that normally operates in water or organic liquids under ambient conditions. The effect of different processing temperatures in the range (5-80 °C), on the morphology of ZnO nanoparticles is clearly demonstrated. At 5 °C, nanoparticles with spherical morphology are observed. However, elliptical morphology is observed at room temperature and multipod nanorods at 50 °C and 80 °C. The evolution of ZnO phase is investigated with the help of time dependent UV-vis absorption and photoluminescence (PL) studies. The mechanism of formation and different morphologies of ZnO nanoparticles formed are also proposed.

  6. Detecting supersymmetric Q-balls with neutron stars

    OpenAIRE

    Madsen, Jes

    1998-01-01

    Supersymmetric Q-balls trapped in neutron stars or white dwarfs may cause the stars to explode. Trapping of Q-balls in neutron stars is shown to be less likely, but trapping in neutron star progenitors more likely than hitherto assumed, making neutron stars very sensitive Q-ball "detectors". White dwarfs only trap potentially dangerous Q-balls in a narrow parameter range.

  7. Conversion of C6 and C5 sugars in undetoxified wet exploded bagasse hydrolysates using Scheffersomyces (Pichia) stipitis CBS6054

    DEFF Research Database (Denmark)

    Biswas, Rajib; Uellendahl, Hinrich; Ahring, Birgitte Kiær

    2013-01-01

    Sugarcane bagasse is a potential feedstock for cellulosic ethanol production, rich in both glucan and xylan. This stresses the importance of utilizing both C6 and C5 sugars for conversion into ethanol in order to improve the process economics. During processing of the hydrolysate degradation...... products such as acetate, 5-hydroxymethylfurfural (HMF) and furfural are formed, which are known to inhibit microbial growth at higher concentrations. In the current study, conversion of both glucose and xylose sugars into ethanol in wet exploded bagasse hydrolysates was investigated without detoxification...... using Scheffersomyces (Pichia) stipitis CBS6054, a native xylose utilizing yeast strain. The sugar utilization ratio and ethanol yield (Yp/s) ranged from 88-100% and 0.33-0.41 ± 0.02 g/g, respectively, in all the hydrolysates tested. Hydrolysate after wet explosion at 185°C and 6 bar O2, composed...

  8. Kinetics of Moisture Absorption for Alkali Extracted Steam-Exploded Fiber Filled High-Density Polyethylene Composites

    Science.gov (United States)

    Taib, R. M.; Ramarad, S.; Ishak, Z. A. M.; Rozman, H. D.

    2010-03-01

    Acacia mangium wood fiber derived from steam-explosion and fiber fractionation treatment was used as fillers for high-density polyethylene (HDPE). The alkali extracted steam-exploded fibers (AEF) obtained were acetylated to produce acetylated fibers (AAEF) having three different weight percent gain (WPG). Composites of AEF or AAEF and HDPE were prepared via 2-roll mill, compression molded and cut into dumbbell specimens. All samples were immersed in water at room temperature for 30 days. The process of absorption of water by all composites followed the kinetics and mechanisms described by the Fick's theory. Diffusion coefficient (D) values increased with filler loading but decreased with increasing WPG of the AAEF fiber. Further decrease was observed when maleated polyethylene (MAPE) was added to the composite system. This was due to improved fiber-matrix adhesion that restricts movement of water molecules from further penetrate inside the composite structures.

  9. Steam-exploded biomass saccharification is predominately affected by lignocellulose porosity and largely enhanced by Tween-80 in Miscanthus.

    Science.gov (United States)

    Sun, Dan; Alam, Aftab; Tu, Yuanyuan; Zhou, Shiguang; Wang, Yanting; Xia, Tao; Huang, Jiangfeng; Li, Ying; Zahoor; Wei, Xiaoyang; Hao, Bo; Peng, Liangcai

    2017-09-01

    In this study, total ten Miscanthus accessions exhibited diverse cell wall compositions, leading to largely varied hexoses yields at 17%-40% (% cellulose) released from direct enzymatic hydrolysis of steam-exploded (SE) residues. Further supplied with 2% Tween-80 into the enzymatic digestion, the Mis7 accession showed the higher hexose yield by 14.8-fold than that of raw material, whereas the Mis10 had the highest hexoses yield at 77% among ten Miscanthus accessions. Significantly, this study identified four wall polymer features that negatively affect biomass saccharification as pbiomass enzymatic digestion. Hence, this study provides the potential strategy to enhance biomass saccharification using optimal biomass process technology and related genetic breeding in Miscanthus and beyond. Copyright © 2017 Elsevier Ltd. All rights reserved.

  10. Cu/Cu{sub 2}O/CuO nanoparticles: Novel synthesis by exploding wire technique and extensive characterization

    Energy Technology Data Exchange (ETDEWEB)

    Sahai, Anshuman [Department of Physics and Materials Science and Engineering, Jaypee Institute of Information Technology, A-10, Sector-62, Noida 201307 (India); Goswami, Navendu, E-mail: navendugoswami@gmail.com [Department of Physics and Materials Science and Engineering, Jaypee Institute of Information Technology, A-10, Sector-62, Noida 201307 (India); Kaushik, S.D. [UGC-DAE-Consortium for Scientific Research Mumbai Centre, R5 Shed, BARC, Mumbai 400085 (India); Tripathi, Shilpa [UGC-DAE Consortium for Scientific Research, Indore, M.P. (India)

    2016-12-30

    Highlights: The salient features of this research article are following: • Mixed phase synthesis of Cu/Cu{sub 2}O/CuO nanoparticles prepared by Exploding Wire Technique (EWT). • Predominant Cu/Cu{sub 2}O phases along with minor CuO phase revealed through XRD, TEM, Raman, FTIR, UV–Visible and PL analyses. • XPS analysis provided direct evidences of Cu{sup 2+} and Cu{sup +} along with O deficiency for prepared nanoparticles. • Room temperature weak ferromagnetic behaviour was demonstrated for Cu/Cu{sub 2}O/CuO nanoparticles. - Abstract: In this article, we explore potential of Exploding Wire Technique (EWT) to synthesize the copper nanoparticles using the copper metal in a plate and wire geometry. Rietveld refinement of X-ray diffraction (XRD) pattern of prepared material indicates presence of mixed phases of copper (Cu) and copper oxide (Cu{sub 2}O). Agglomerates of copper and copper oxide comprised of ∼20 nm average size nanoparticles observed through high resolution transmission electron microscope (HRTEM) and energy dispersive x-ray (EDX) spectroscopy. Micro-Raman (μR) and Fourier transform infrared (FTIR) spectroscopies of prepared nanoparticles reveal existence of additional minority CuO phase, not determined earlier through XRD and TEM analysis. μR investigations vividly reveal cubic Cu{sub 2}O and monoclinic CuO phases based on the difference of space group symmetries. In good agreement with μRaman analysis, FTIR stretching modes corresponding to Cu{sub 2}-O and Cu-O were also distinguished. Investigations of μR and FTIR vibrational modes are in accordance and affirm concurrence of CuO phases besides predominant Cu and Cu{sub 2}O phase. Quantum confinement effects along with increase of band gaps for direct and indirect optical transitions of Cu/Cu{sub 2}O/CuO nanoparticles are reflected through UV–vis (UV–vis) spectroscopy. Photoluminescence (PL) spectroscopy spots the electronic levels of each phase and optical transitions processes

  11. Simultaneous saccharification and fermentation of steam exploded duckweed: Improvement of the ethanol yield by increasing yeast titre.

    Science.gov (United States)

    Zhao, X; Moates, G K; Elliston, A; Wilson, D R; Coleman, M J; Waldron, K W

    2015-10-01

    This study investigated the conversion of Lemna minor biomass to bioethanol. The biomass was pre-treated by steam explosion (SE, 210°C, 10 min) and then subjected to simultaneous saccharification and fermentation (SSF) using Cellic® CTec 2 (20 U or 0.87 FPU g(-1) substrate) cellulase plus β-glucosidase (2 U g(-1) substrate) and a yeast inoculum of 10% (v/v or 8.0×10(7) cells mL(-1)). At a substrate concentration of 1% (w/v) an ethanol yield of 80% (w/w, theoretical) was achieved. However at a substrate concentration of 20% (w/v), the ethanol yield was lowered to 18.8% (w/w, theoretical). Yields were considerably improved by increasing the yeast titre in the inoculum or preconditioning the yeast on steam exploded liquor. These approaches enhanced the ethanol yield up to 70% (w/w, theoretical) at a substrate concentration of 20% (w/v) by metabolising fermentation inhibitors. Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.

  12. Simultaneous saccharification and co-fermentation for improving the xylose utilization of steam exploded corn stover at high solid loading.

    Science.gov (United States)

    Liu, Zhi-Hua; Chen, Hong-Zhang

    2016-02-01

    Simultaneous saccharification and co-fermentation (SSCF) of steam exploded corn stover (SECS) was investigated at 5-25% solid loadings compared with other conversion processes. SECS was washed with a 15-fold excess of deionized water to remove inhibitors of hydrolysis and fermentation. The concentration, yield, and productivity of ethanol was 34.3g/L, 90.0%, 2.61g/L/h in the co-fermentation of 60g/L glucose and 10g/L xylose by Saccharomyces cerevisiae IPE003. Ethanol concentration and productivity increased with increasing solid loading while ethanol yield decreased in all conversion processes of SECS. Glucan and xylan conversion was 82.0% and 82.1% in SSCF at 20% solid loading, respectively, while the concentration, yield and productivity of ethanol was 60.8g/L, 75.3% and 0.63g/L/h. The feeding strategy of SECS addition within 24h improved the SSCF performance. Therefore, SSCF increased ethanol productivity and was an effective conversion process for ethanol production at high solid loading. Copyright © 2015 Elsevier Ltd. All rights reserved.

  13. The Response of Model and Astrophysical Thermonuclear Flames to Curvature and Stretch

    Science.gov (United States)

    Dursi, L. J.; Zingale, M.; Calder, A. C.; Fryxell, B.; Timmes, F. X.; Vladimirova, N.; Rosner, R.; Caceres, A.; Lamb, D. Q.; Olson, K.; Ricker, P. M.; Riley, K.; Siegel, A.; Truran, J. W.

    2003-10-01

    Critically understanding the standard candle-like behavior of Type Ia supernovae requires understanding their explosion mechanism. One family of models for Type Ia supernovae begins with a deflagration in a carbon-oxygen white dwarf that greatly accelerates through wrinkling and flame instabilities. While the planar speed and behavior of astrophysically relevant flames is increasingly well understood, more complex behavior, such as the flame's response to stretch and curvature, has not been extensively explored in the astrophysical literature; this behavior can greatly enhance or suppress instabilities and local flame-wrinkling, which in turn can increase or decrease the bulk burning rate. In this paper, we explore the effects of curvature on both nuclear flames and simpler model flames to understand the effect of curvature on the flame structure and speed.

  14. The formation of glycine and other complex organic molecules in exploding ice mantles.

    Science.gov (United States)

    Rawlings, J M C; Williams, D A; Viti, S; Cecchi-Pestellini, C; Duley, W W

    2014-01-01

    of explaining the large abundances of all three isomers of C2H4O2 that are observationally inferred for star-forming regions. However, the model currently does not provide an obvious explanation for the predominance of methyl formate, suggesting that some refinement to our (very simplistic) chemistry is necessary. The model also predicts the production of glycine at a (lower) abundance level, that is consistent with its marginal detection in astrophysical sources.

  15. The Peculiar SN 2005hk: Do Some Type Ia Supernovae Explode As Deflagrations?

    Energy Technology Data Exchange (ETDEWEB)

    Phillips, M.M.; Li, W.; Frieman, J.A.; Blinnikov, S.I.; DePoy, D.; Prieto, J.L.; Milne, P.; Contreras, C.; Folatelli, Gaston; Morrell, N.; Hamuy, M.; Suntzeff, N.B.; Roth, M.; Gonzalez, S.; Krzeminski, W.; Filippenko, A.V.; Freedman, W.L.; Chornock, R.; Jha, S.; Madore, B.F.; Persson, S.E.; /Las Campanas Observ. /UC, Berkeley, Astron. Dept.

    2006-11-14

    We present extensive u{prime}g{prime}r{prime}i{prime} BV RIY JHK{sub s} photometry and optical spectroscopy of SN 2005hk. These data reveal that SN 2005hk was nearly identical in its observed properties to SN 2002cx, which has been called 'the most peculiar known type Ia supernova'. Both supernovae exhibited high ionization SN 1991T-like pre-maximum spectra, yet low peak luminosities like SN 1991bg. The spectra reveal that SN 2005hk, like SN 2002cx, exhibited expansion velocities that were roughly half those of typical type Ia supernovae. The R and I light curves of both supernovae were also peculiar in not displaying the secondary maximum observed for normal type Ia supernovae. Our Y JH photometry of SN 2005hk reveals the same peculiarity in the near-infrared. By combining our optical and near-infrared photometry of SN 2005hk with published ultraviolet light curves obtained with the Swift satellite, we are able to construct a bolometric light curve from {approx} 10 days before to {approx}60 days after B maximum. The shape and unusually low peak luminosity of this light curve, plus the low expansion velocities and absence of a secondary maximum at red and near-infrared wavelengths, are all in reasonable agreement with model calculations of a 3D deflagration which produces {approx} 0.25 M{sub {circle_dot}} of {sup 56}Ni.

  16. The exploding spark: workplace violence in an infectious disease hospital--a longitudinal study.

    Science.gov (United States)

    Magnavita, Nicola

    2013-01-01

    Workplace violence (WV) is an important occupational hazard for healthcare workers (HCWs). A longitudinal study was carried out on HCWs from an infectious disease hospital. Work-related stress, anxiety, and depression were measured at baseline in 2003, and they were reassessed in 2005, along with the assaults that occurred in the previous year. One-year prevalences of 6.2% and 13.9% were reported for physical and verbal aggressions, respectively. Perpetrators were mainly patients. The professional groups most frequently attacked were physicians, followed by nurses. Workers with job strain at baseline had a significant risk of being subject to aggression (OR 7.7; CI 95%, 3.3-17.9) in the following year. The relationship between job strain and subsequent WV remained significant even after correction for anxiety, depression, and other confounders. Conversely, experiencing WV was associated with a high risk of job strain and effort-reward imbalance in the following year. The final levels of anxiety and depression were predicted using regression models that included physical aggression among predictive variables. WV is the spark that sets off a problematic work situation. Effective prevention of WV can only be achieved within the framework of an overall improvement in the quality of work.

  17. The Exploding Spark: Workplace Violence in an Infectious Disease Hospital—A Longitudinal Study

    Directory of Open Access Journals (Sweden)

    Nicola Magnavita

    2013-01-01

    Full Text Available Objectives. Workplace violence (WV is an important occupational hazard for healthcare workers (HCWs. Methods. A longitudinal study was carried out on HCWs from an infectious disease hospital. Work-related stress, anxiety, and depression were measured at baseline in 2003, and they were reassessed in 2005, along with the assaults that occurred in the previous year. Results. One-year prevalences of 6.2% and 13.9% were reported for physical and verbal aggressions, respectively. Perpetrators were mainly patients. The professional groups most frequently attacked were physicians, followed by nurses. Workers with job strain at baseline had a significant risk of being subject to aggression (OR 7.7; CI 95%, 3.3–17.9 in the following year. The relationship between job strain and subsequent WV remained significant even after correction for anxiety, depression, and other confounders. Conversely, experiencing WV was associated with a high risk of job strain and effort-reward imbalance in the following year. The final levels of anxiety and depression were predicted using regression models that included physical aggression among predictive variables. Conclusions. WV is the spark that sets off a problematic work situation. Effective prevention of WV can only be achieved within the framework of an overall improvement in the quality of work.

  18. Cu/Cu2O/CuO nanoparticles: Novel synthesis by exploding wire technique and extensive characterization

    Science.gov (United States)

    Sahai, Anshuman; Goswami, Navendu; Kaushik, S. D.; Tripathi, Shilpa

    2016-12-01

    In this article, we explore potential of Exploding Wire Technique (EWT) to synthesize the copper nanoparticles using the copper metal in a plate and wire geometry. Rietveld refinement of X-ray diffraction (XRD) pattern of prepared material indicates presence of mixed phases of copper (Cu) and copper oxide (Cu2O). Agglomerates of copper and copper oxide comprised of ∼20 nm average size nanoparticles observed through high resolution transmission electron microscope (HRTEM) and energy dispersive x-ray (EDX) spectroscopy. Micro-Raman (μR) and Fourier transform infrared (FTIR) spectroscopies of prepared nanoparticles reveal existence of additional minority CuO phase, not determined earlier through XRD and TEM analysis. μR investigations vividly reveal cubic Cu2O and monoclinic CuO phases based on the difference of space group symmetries. In good agreement with μRaman analysis, FTIR stretching modes corresponding to Cu2-O and Cu-O were also distinguished. Investigations of μR and FTIR vibrational modes are in accordance and affirm concurrence of CuO phases besides predominant Cu and Cu2O phase. Quantum confinement effects along with increase of band gaps for direct and indirect optical transitions of Cu/Cu2O/CuO nanoparticles are reflected through UV-vis (UV-vis) spectroscopy. Photoluminescence (PL) spectroscopy spots the electronic levels of each phase and optical transitions processes occurring therein. Iterative X-ray photoelectron spectroscopy (XPS) fitting of core level spectra of Cu (2p3/2) and O (1s), divulges presence of Cu2+ and Cu+ in the lattice with an interesting evidence of O deficiency in the lattice structure and surface adsorption. Magnetic analysis illustrates that the prepared nanomaterial demonstrates ferromagnetic behaviour at room temperature.

  19. Periodic peristalsis increasing acetone-butanol-ethanol productivity during simultaneous saccharification and fermentation of steam-exploded corn straw.

    Science.gov (United States)

    Li, Jingwen; Wang, Lan; Chen, Hongzhang

    2016-11-01

    The acetone-butanol-ethanol (ABE) fermentation of lignocellulose at high solids content has recently attracted extensive attention. However, the productivity of high solids ABE fermentation of lignocellulose is typically low in traditional processes due to the lack of efficient intensifying methods. In the present study, periodic peristalsis, a novel intensifying method, was applied to improve ABE production by the simultaneous saccharification and fermentation (SSF) of steam-exploded corn straw using Clostridium acetobutylicum ATCC824. The ABE concentration and the ABE productivity of SSF at a solids content of 17.5% (w/w) with periodic peristalsis were 17.1 g/L and 0.20 g/(L h), respectively, which were higher than those obtained under static conditions (15.2 g/L and 0.14 g/(L h)). The initial sugar conversion rate over the first 12 h with periodic peristalsis was 4.67 g/(L h) at 10 FPU/g cellulase dosage and 15% (w/w) solids content, an increase of 49.7% compared with the static conditions. With periodic peristalsis, the period of batch fermentation was shortened from 108 h to 84 h. The optimal operating regime was a low frequency (6 h -1 ) of periodic peristalsis in the acid-production phase (0-48 h) of SSF. Therefore, periodic peristalsis should be an effective intensifying method to increase the productivity of ABE fermentation at high solids content. Copyright © 2016 The Society for Biotechnology, Japan. Published by Elsevier B.V. All rights reserved.

  20. The techniques of metallic foil electrically exploding driving hypervelocity flyer to more than 10 km/s for shock wave physics experiments.

    Science.gov (United States)

    Wang, Guiji; He, Jia; Zhao, Jianheng; Tan, Fuli; Sun, Chengwei; Mo, Jianjun; Xong, Xin; Wu, Gang

    2011-09-01

    Electrical explosion of metallic foil or wire is widely used to the fields of material science (preparation of nao-meter materials), dynamics of materials, and high energy density physics. In this paper, the techniques of gaining hypervelocity flyer driven by electrical explosion of metallic foil were researched, which are used to study dynamics of materials and hypervelocity impact modeling of space debris. Based on low inductance technologies of pulsed storage energy capacitor, detonator switch and parallel plate transmission lines with solid films insulation, two sets of experimental apparatuses with storage energy of 14.4 kJ and 40 kJ were developed for launching hypervelocity flyer. By means of the diagnostic technologies of velocity interferometer system for any reflectors and fibre-optic pins, the hypervelocity polyester (Mylar) flyers were gained. For the apparatus of 14.4 kJ, flyer of diameter φ6 ~ φ10 mm and thickness of 0.1 ~ 0.2 mm was accelerated to the hypervelocity of 10 ~ 14 km/s. And for the apparatus of 40 kJ, flyer of diameter φ20 ~ 30 mm and thickness of 0.2 mm was launched to the velocity of 5 ~ 8 km/s. The flatness of the flyer is not more than 34 ns for the flyer with diameter of 20 mm, and less than 22 ns for the flyer with diameter of 10 mm. Based on the Lagrange hydrodynamic code, one dimensional simulation was done by introducing database of equation of states, discharging circuit equation and Joule heat equation, and modifying energy equation. The simulation results are well agreed with the experimental results in accelerating processing. The simulation results can provide good advices in designing new experiments and developing new experimental devices. Finally, some experiments of materials dynamics and hypervelocity impact of space debris were done by using the apparatus above. The results show that the apparatus of metallic foil electrically exploding driving hypervelocity flyer is a good and versatile tool for shock dynamics.

  1. Neutrino and gravitational wave signal of a delayed-detonation model of type Ia supernovae

    Science.gov (United States)

    Seitenzahl, Ivo R.; Herzog, Matthias; Ruiter, Ashley J.; Marquardt, Kai; Ohlmann, Sebastian T.; Röpke, Friedrich K.

    2015-12-01

    The progenitor system(s) and the explosion mechanism(s) of type Ia supernovae (SNe Ia) are still under debate. Nonelectromagnetic observables, in particular, gravitational waves and neutrino emission, of thermoclear supernovae are a complementary window to light curves and spectra for studying these enigmatic objects. A leading model for SNe Ia is the thermonuclear incineration of a near-Chandrasekhar mass carbon-oxygen white dwarf star in a "delayed detonation." We calculate a three-dimensional hydrodynamic explosion for the N100 delayed-detonation model extensively discussed in the literature, taking the dynamical effects of neutrino emission from all important contributing source terms into account. Although neutrinos carry away 2 ×1049 erg of energy, we confirm the common view that neutrino energy losses are dynamically not very important, resulting in only a modest reduction of final kinetic energy by 2%. We then calculate the gravitational wave signal from the time evolution of the quadrupole moment. Our model radiates 7 ×1039 erg in gravitational waves and the spectrum has a pronounced peak around 0.4 Hz. Depending on viewing angle and polarization, we find that the future space-based gravitational wave missions DECIGO and BBO would be able to detect our source to a distance of ˜1.3 Mpc . We predict a clear signature of the deflagration-to-detonation transition in the neutrino and the gravitational wave signals. If observed, such a feature would be a strong indicator of the realization of delayed detonations in near-Chandrasekhar mass white dwarfs.

  2. Exploding Taylor Cones

    CERN Document Server

    Creasey, John C; Ristenpart, William D

    2010-01-01

    Application of a sufficiently strong electric field to an aqueous solution induces a phenomenon known as `electrohydraulic discharge'. The electric field causes the water to break down, generating either a corona (at lower field strengths) or a pulsed arc (at higher field strengths). The discharge typically results in a complex combination of physical processes (e.g., cavitation and light emission) and chemical reactions (e.g., generation of free radicals and nonthermal plasmas). The combination of physical and chemical processes tends to destroy any organic molecules present, and accordingly electrohydraulic discharges are currently being investigated as a potentially inexpensive and environmentally friendly means for purifying drinking water and removing contaminants from wastewater. Two types of electrode configurations have been the main focus of research to date: (i) a `one-phase' system with both electrodes immersed in water, or (ii) a `two-phase' system with one electrode in air and the other submerged...

  3. The Book Club Exploded

    Science.gov (United States)

    Hoffert, Barbara

    2006-01-01

    One leader, 12 readers, and a few well-thumbed copies of Jane Austen's "Pride and Prejudice." That is all a book club once required, but this is no longer the case. This article describes how the runaway popularity of book clubs has brought with it a whole new set of possibilities. Thematic discussion? A fiction/nonfiction mix? Videoconferencing?…

  4. Temperatures of exploding nuclei

    Energy Technology Data Exchange (ETDEWEB)

    Serfling, V.; Schwarz, C.; Begemann-Blaich, M.; Fritz, S.; Gross, C.; Kleinevoss, U.; Kunze, W.D; Lynen, U.; Mahi, M.; Mueller, W.F.J.; Odeh, T.; Schnittker, M.; Trautmann, W.; Woerner, A.; Xi, H. [Gesellschaft fuer Schwerionenforschung mbH, Darmstadt (Germany); Bassini, R.; Iori, I.; Moroni, A.; Petruzzelli, F. [Milan Univ. (Italy). Ist. di Scienze Fisiche]|[Istituto Nazionale di Fisica Nucleare, Milan (Italy); Gaff, S.J.; Kunde, G.J. [Michigan State Univ., East Lansing, MI (United States). Dept. of Physics and Astronomy]|[Michigan State Univ., East Lansing, MI (United States). National Superconducting Cyclotron Lab.; Imme, G.; Maddalena, V.; Nociforo, C.; Raciti, G.; Riccobene, G.; Romano, F.P.; Saija, A.; Sfienti, C.; Verde, G. [Catania Univ. (Italy). Dipt. di Fisica]|[Istituto Nazionale di Fisica Nucleare, Catania (Italy); Moehlenkamp, T.; Seidel, W. [Forschungszentrum Rossendorf e.V. (FZR), Dresden (Germany); Ocker, B.; Schuettauf, A. [Frankfurt Univ. (Germany). Inst. fuer Kernphysik; Pochodzalla, J. [Max-Planck-Institut fuer Kernphysik, Heidelberg (Germany); Trzcinski, A.; Zwieglinski, B. [Soltan Inst. for Nuclear Studies, Warsaw (Poland)

    1998-01-01

    Breakup temperatures in central collisions of {sup 197}Au+{sup 197}Au at bombarding energies E/A=50 to 200 MeV were determined with two methods. Isotope temperatures, deduced from double ratios of hydrogen, helium, and lithium isotopic yields, increase monotonically with bombarding energy from 5 MeV to 12 MeV, in qualitative agreement with a scenario of chemical freeze-out after adiabatic expansion. Excited-state temperatures, derived from yield ratios of states in {sup 4}He, {sup 5,6}Li, and {sup 8}Be, are about 5 MeV, independent of the projectile energy, and seem to reflect the internal temperature of fragments at their final separation from the system. (orig.)

  5. Exploding Water Drops

    Science.gov (United States)

    Reich, Gary

    2016-01-01

    Water has the unusual property that it expands on freezing, so that ice has a specific gravity of 0.92 compared to 1.0 for liquid water. The most familiar demonstration of this property is ice cubes floating in a glass of water. A more dramatic demonstration is the ice bomb shown in Fig. 1. Here a cast iron flask is filled with water and tightly…

  6. EG And: Providing the Missing Link Required for Modelling Red Giant Mass-loss

    Science.gov (United States)

    Espey, Brian

    2009-07-01

    For the majority of red giant stars the basic mass-loss processes at work are unknown. Indeed, for stars of spectral types between K0 III and M5-M6 III, much remains unknown about the regions above the visible photosphere and the transportation of the processed material outwards to the ISM. Eclipsing symbiotic binary systems, consisting of an evolved giant in orbit with a white dwarf, provide an opportunity to take advantage of the finite size of the hot component to probe different levels of the chromosphere and wind acceleration region in absorption. This provides spatially resolved thermal, ionisation and dynamic information on the wind which can then be compared against predictions of hydrodynamical stellar atmosphere codes. The symbiotic binary EG And can be considered as a rosetta stone for understanding the winds of these objects. The system is ideal on a number of counts for utilising the ultraviolet eclipse of the white dwarf {WD} component to probe, layer-by-layer, the thermal and dynamic conditions at the very base of the wind and chromosphere of the RG. This information is vital for constraining, testing and calibrating the new generation of cool giant wind+chromosphere models and is not possible to obtain for isolated RGs. This team has studied the UV eclipses of this system in depth and detail, however in order to definitively constrain the wind acceleration profile and identify the location of the temperature rise just above the photosphere we require 4 STIS E140M observations of EG And at specific orbital phases. We are also requesting a E230M observation of an isolated spectral standard, corresponding to the RG inthe binary, which will help place the EG And results into the context of the general RG population from analysis of the MgII wind diagnostic lines.

  7. Using of fiber-array diagnostic to measure the propagation of fast axial ionization wave during breakdown of electrically exploding tungsten wire in vacuum

    Science.gov (United States)

    Shi, Huantong; Zou, Xiaobing; Wang, Xinxin

    2017-12-01

    The physical process of electrical explosion of wires in vacuum is featured with the surface discharge along the wire, which generates the corona plasma layer and terminates the Joule heating of the wire core. In this paper, a fiber-array probe was designed to directly measure the radiation of surface arc with spatial and temporal resolution. The radiation of the exploding wire was casted to the section of an optical-fiber-array by a lens and transmitted to PIN diodes and finally collected with an oscilloscope. This probe enables direct diagnostics of the evolution of surface discharge with high temporal resolution and certain spatial resolution. The radiation of a tungsten wire driven by a positive current pulse was measured, and results showed that surface discharge initiates near the cathode and propagates toward the anode with a speed of 7.7 ± 1.6 mm/ns; further estimations showed that this process is responsible for the "conical" structure of the exploding wire.

  8. The Impact of Accurate Distances on UV Spectroscopy of White Dwarfs and Cataclysmic Variables

    Science.gov (United States)

    2009-01-01

    Institute, 3700 San Martin Drive, Baltimore, MD 21218 Abstract. J-MAPS and Gaia are two asterometry missions to be launched in 2012. The positions of...cataclysmic variables, accretion disks PACS: 97.10.Vm, 97.10.Ex, 97.20.Rp, 97.80.Gm INTRODUCTION Gaia Leveraging upon the highly successful Hipparcos...satellite, Gaia is an astrophysics driven mission, which utilizes the twin eld-of-view scanning concept, updated with modern technology, to achieve

  9. Black holes, white dwarfs, and neutron stars the physics of compact objects

    CERN Document Server

    Shapiro, Stuart Louis

    1983-01-01

    This self-contained textbook brings together many different branches of physics--e.g. nuclear physics, solid state physics, particle physics, hydrodynamics, relativity--to analyze compact objects. The latest astronomical data is assessed

  10. Radiative Transfer in the Refractive Atmospheres of Very Cool White Dwarfs

    OpenAIRE

    Piotr Kowalski; Saumon, D.; [Philo-Dicaeus] 

    2004-01-01

    We consider the problem of radiative transfer in stellar atmospheres where the index of refraction departs from unity and is a function of density and temperature. We present modified Feautrier and Lambda-iteration methods to solve the equation of radiative transfer with refraction in a plane parallel atmosphere. These methods are general and can be used in any problem with 1-D geometry where the index of refraction is a monotonically varying function of vertical optical depth. We present an ...

  11. A Hot White Dwarf Luminosity Function from the Sloan Digital Sky Survey

    Science.gov (United States)

    2009-01-01

    Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology , the Korean Scientist Group...Werner (Berlin, Heidelberg, New York: Springer-Verlag), Lecture Notes in Physics, 443, 41 Wood, P. R., & Faulkner, D. J. 1986, ApJ, 307, 65 York, D. G., Adelman, J., Anderson, J. E., Jr., et al. 2000, AJ, 120, 1579

  12. Astrophysical Model Selection in Gravitational Wave Astronomy

    Science.gov (United States)

    Adams, Matthew R.; Cornish, Neil J.; Littenberg, Tyson B.

    2012-01-01

    Theoretical studies in gravitational wave astronomy have mostly focused on the information that can be extracted from individual detections, such as the mass of a binary system and its location in space. Here we consider how the information from multiple detections can be used to constrain astrophysical population models. This seemingly simple problem is made challenging by the high dimensionality and high degree of correlation in the parameter spaces that describe the signals, and by the complexity of the astrophysical models, which can also depend on a large number of parameters, some of which might not be directly constrained by the observations. We present a method for constraining population models using a hierarchical Bayesian modeling approach which simultaneously infers the source parameters and population model and provides the joint probability distributions for both. We illustrate this approach by considering the constraints that can be placed on population models for galactic white dwarf binaries using a future space-based gravitational wave detector. We find that a mission that is able to resolve approximately 5000 of the shortest period binaries will be able to constrain the population model parameters, including the chirp mass distribution and a characteristic galaxy disk radius to within a few percent. This compares favorably to existing bounds, where electromagnetic observations of stars in the galaxy constrain disk radii to within 20%.

  13. Ethanol production from wet-exploded wheat straw hydrolysate by thermophilic anaerobic bacterium Thermoanaerobacter BG1L1 in a continuous immobilized reactor

    DEFF Research Database (Denmark)

    Georgieva, Tania I.; Mikkelsen, Marie Just; Ahring, Birgitte Kiær

    2008-01-01

    Thermophilic ethanol fermentation of wet-exploded wheat straw hydrolysate was investigated in a continuous immobilized reactor system. The experiments were carried out in a lab-scale fluidized bed reactor (FBR) at 70C. Undetoxified wheat straw hydrolysate was used (3-12% dry matter), corresponding...... was not detoxified, ethanol yield in a range of 0.39-0.42 g/g was obtained. Overall, sugar efficiency to ethanol was 68-76%. The reactor was operated continuously for approximately 143 days, and no contamination was seen without the use of any agent for preventing bacterial infections. The tested microorganism has...... considerable potential to be a novel candidate for lignocellulose bioconversion into ethanol. The work reported here also demonstrates that the use of FBR configuration might be a viable approach for thermophilic anaerobic ethanol fermentation....

  14. Comparison of SHF and SSF processes from steam-exploded wheat straw for ethanol production by xylose-fermenting and robust glucose-fermenting Saccharomyces cerevisiae strains

    DEFF Research Database (Denmark)

    Tomas Pejo, Elia; Oliva, Jose M.; Ballesteros, Mercedes

    2008-01-01

    , it showed an ethanol yield on consumed sugars of 0.43 g/g and a volumetric ethanol productivity of 0.7 g/Lh for the first 3 h. Ethanol concentrations obtained in SSF processes were in all cases higher than those from SHF at the same conditions. Furthermore, using the whole slurry, final ethanol......In this study, bioethanol production from steam-exploded wheat straw using different process configurations was evaluated using two Saccharomyces cerevisiae strains, F12 and Red Star. The strain F12 has been engineerically modified to allow xylose consumption as cereal straw contain considerable...... amounts of pentoses. Red Star is a robust hexose-fermenting strain used for industrial fuel ethanol fermentations and it was used for comparative purposes. The highest ethanol concentration, 23.7 g/L, was reached using the whole slurry (10%, w/v) and the recombinant strain (F12) in an SSF process...

  15. Development of detonations in degenerate stars

    Science.gov (United States)

    Blinnikov, S. I.; Khokhlov, A. M.

    1986-04-01

    It is now widely believed that thermal instability at the center of a carbon-oxygen white dwarf will produce a deflagration wave. In this paper, numerical analysis of the preoutburst conditions near the center of a C-O white dwarf as well as of the three stages of the thermonuclear explosion of the star (i.e., the stages of spontaneous burning front, shock formation, and postshock burning and shock amplification) is presented. It is demonstrated that when a degenerate C-O star explodes as a supernova, thermal instability may cause the thermonuclear burning front to develop into a detonation (rather than a deflagration) regime.

  16. Mild chemical pretreatments are sufficient for complete saccharification of steam-exploded residues and high ethanol production in desirable wheat accessions.

    Science.gov (United States)

    Zahoor; Tu, Yuanyuan; Wang, Lingqiang; Xia, Tao; Sun, Dan; Zhou, Shiguang; Wang, Yanting; Li, Ying; Zhang, Heping; Zhang, Tong; Madadi, Meysam; Peng, Liangcai

    2017-11-01

    In this study, a combined pretreatment was performed in four wheat accessions using steam explosion followed with different concentrations of H 2 SO 4 or NaOH, leading to increased hexoses yields by 3-6 folds from enzymatic hydrolysis. Further co-supplied with 1% Tween-80, Talq90 and Talq16 accessions exhibited an almost complete enzymatic saccharification of steam-exploded (SE) residues after 0.5% H 2 SO 4 or 1% NaOH pretreatment, with the highest bioethanol yields at 18.5%-19.4%, compared with previous reports about wheat bioethanol yields at 11%-17% obtained under relatively strong pretreatment conditions. Furthermore, chemical analysis indicated that much enhanced saccharification in Talq90 and Talq16 may be partially due to their relatively low cellulose CrI and DP values and high hemicellulose Ara and H-monomer levels in raw materials and SE residues. Hence, this study has not only demonstrated a mild pretreatment technology for a complete saccharification, but it has also obtained the high ethanol production in desirable wheat accessions. Copyright © 2017 Elsevier Ltd. All rights reserved.

  17. High resolution absorption spectroscopy of exploding wire plasmas using an x-pinch x-ray source and spherically bent crystal.

    Science.gov (United States)

    Knapp, P F; Pikuz, S A; Shelkovenko, T A; Hammer, D A; Hansen, S B

    2011-06-01

    We present here the use of absorption spectroscopy of the continuum radiation from x-pinch-produced point x-ray sources as a diagnostic to investigate the properties of aluminum plasmas created by pulsed power machines. This technique is being developed to determine the charge state, temperature, and density as a function of time and space under conditions that are inaccessible to x-ray emission spectroscopic diagnostics. The apparatus and its characterization are described, and the spectrometer dispersion, magnification, and resolution are calculated and compared with experimental results. Spectral resolution of about 5000 and spatial resolution of about 20 μm are demonstrated. This spectral resolution is the highest available to date in an absorption experiment. The beneficial properties of the x-pinch x-ray source as the backlighter for this diagnostic are the small source size (<5 μm), smooth continuum radiation, and short pulse duration (<0.1 ns). Results from a closely spaced (1 mm) exploding wire pair are shown and the general features are discussed. © 2011 American Institute of Physics

  18. Two-step size reduction and post-washing of steam exploded corn stover improving simultaneous saccharification and fermentation for ethanol production.

    Science.gov (United States)

    Liu, Zhi-Hua; Chen, Hong-Zhang

    2017-01-01

    The simultaneous saccharification and fermentation (SSF) of corn stover biomass for ethanol production was performed by integrating steam explosion (SE) pretreatment, hydrolysis and fermentation. Higher SE pretreatment severity and two-step size reduction increased the specific surface area, swollen volume and water holding capacity of steam exploded corn stover (SECS) and hence facilitated the efficiency of hydrolysis and fermentation. The ethanol production and yield in SSF increased with the decrease of particle size and post-washing of SECS prior to fermentation to remove the inhibitors. Under the SE conditions of 1.5MPa and 9min using 2.0cm particle size, glucan recovery and conversion to glucose by enzymes were 86.2% and 87.2%, respectively. The ethanol concentration and yield were 45.0g/L and 85.6%, respectively. With this two-step size reduction and post-washing strategy, the water utilization efficiency, sugar recovery and conversion, and ethanol concentration and yield by the SSF process were improved. Copyright © 2016 Elsevier Ltd. All rights reserved.

  19. Modeling The Most Luminous Supernova Associated with a Gamma-Ray Burst, SN 2011kl

    Science.gov (United States)

    Wang, Shan-Qin; Cano, Zach; Wang, Ling-Jun; Zheng, WeiKang; Dai, Zi-Gao; Filippenko, Alexei V.; Liu, Liang-Duan

    2017-12-01

    We study the most luminous known supernova (SN) associated with a gamma-ray burst (GRB), SN 2011kl. The photospheric velocity of SN 2011kl around peak brightness is 21,000 ± 7000 km s-1. Owing to different assumptions related to the light-curve (LC) evolution (broken or unbroken power-law function) of the optical afterglow of GRB 111209A, different techniques for the LC decomposition, and different methods (with or without a near-infrared contribution), three groups derived three different bolometric LCs for SN 2011kl. Previous studies have shown that the LCs without an early-time excess preferred a magnetar model, a magnetar+56Ni model, or a white dwarf tidal disruption event model rather than the radioactive heating model. On the other hand, the LC shows an early-time excess and dip that cannot be reproduced by the aforementioned models, and hence the blue-supergiant model was proposed to explain it. Here, we reinvestigate the energy sources powering SN 2011kl. We find that the two LCs without the early-time excess of SN 2011kl can be explained by the magnetar+56Ni model, and the LC showing the early excess can be explained by the magnetar+56Ni model taking into account the cooling emission from the shock-heated envelope of the SN progenitor, demonstrating that this SN might primarily be powered by a nascent magnetar.

  20. Dickens and the Exploding World

    DEFF Research Database (Denmark)

    Rainsford, Dominic

    2014-01-01

    ’s relationship to debates about the reach of social and ethical responsibility. Taking off from the concept of ‘telescopic philanthropy’ in Bleak House, and paying attention to recent applications of Dickens beyond Europe and North America, I will consider the relevance of his writing to the question of how...

  1. Physics beyond the Standard Model from hydrogen spectroscopy

    CERN Document Server

    Ubachs, Wim; Eikema, Kjeld S E; Salumbides, Edcel J

    2015-01-01

    Spectroscopy of hydrogen can be used for a search into physics beyond the Standard Model. Differences between the absorption spectra of H$_2$ as observed at high redshift and those measured in the laboratory can be interpreted in terms of possible variations of the proton-electron mass ratio. Investigation of some ten of such absorbers in the redshift range $z= 2.0-4.2$ yields a constraint of $|\\Delta\\mu/\\mu|< 5 \\times 10^{-6}$ at 3$\\sigma$. Observation of H$_2$ from the photospheres of white dwarf stars inside our Galaxy delivers a constraint of similar magnitude on a dependence of $\\mu$ on a gravitational potential $10^4$ times as strong as on the Earth's surface. Laser-based precision measurements of dissociation energies, vibrational splittings and rotational level energies in H$_2$ molecules and their deuterated isotopomers HD and D$_2$ produce values for the rovibrational binding energies fully consistent with quantum ab initio calculations including relativistic and quantum electrodynamical (QED) ef...

  2. Time and space resolved measurement of the electron temperature, mass density and ionization state in the ablation plasma between two exploding Al wires

    Energy Technology Data Exchange (ETDEWEB)

    Knapp, P. F.; Pikuz, S. A.; Shelkovenko, T. A.; Hammer, D. A. [Laboratory of Plasma Studies, Cornell University, 439 Rhodes Hall, Ithaca, New York 14853 (United States); Hansen, S. B. [Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States)

    2012-05-15

    We have determined the properties of plasma around and between two exploding wires using high-resolution x-ray absorption spectroscopy. Plasma densities and temperatures ranging from Greater-Than-Or-Equivalent-To 0.1g/cm{sup 3} and a few eV to less than 0.01 g/cm{sup 3} and 30 eV have been measured in experiments at Cornell University with two 40 {mu}m aluminum (Al) wires spaced 1 mm apart driven by {approx}150 kA peak current pulses with 100 ns rise time. The wire plasma was backlit by the 1.4-1.6 keV continuum radiation produced by a Mo wire X-pinch. The spectrometer employed two spherically bent quartz crystals to record the absorption and backlighter spectra simultaneously. The transition between the dense Al wire core and the coronal plasma is seen as a transition from cold K-edge absorption to Mg-, Na-, and finally Ne-like absorption at the boundary. In the plasma that accumulates between the wires, ionization states up to C-Like Al are observed. The spectrometer geometry and {approx}2{mu}m X-pinch source size provide 0.3 eV spectral resolution and 20 {mu}m spatial resolution enabling us to see 1s{yields} 2p satellite transitions as separate lines as well as O-, F-, and Ne-like 1s{yields} 3p transitions that have not been seen before. A step wedge was used to calibrate the transmission, enabling density to be measured within a factor of two and temperature to be measured within {+-}25%. A genetic algorithm was developed to fit synthetic spectra calculated using the collisional-radiative code SCRAM to the experimental spectra. In order to obtain agreement it was necessary to assume multiple plasma regions with variable thicknesses, thereby allowing the inferred plasma conditions to vary along the absorption path.

  3. V3885 Sagittarius: A Comparison With a Range of Standard Model Accretion Disks

    Science.gov (United States)

    Linnell, Albert P.; Godon, Patrick; Hubeny, Ivan; Sion, Edward M; Szkody, Paula; Barrett, Paul E.

    2009-01-01

    A chi-squared analysis of standard model accretion disk synthetic spectrum fits to combined Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph spectra of V3885 Sagittarius, on an absolute flux basis, selects a model that accurately represents the observed spectral energy distribution. Calculation of the synthetic spectrum requires the following system parameters. The cataclysmic variable secondary star period-mass relation calibrated by Knigge in 2006 and 2007 sets the secondary component mass. A mean white dwarf (WD) mass from the same study, which is consistent with an observationally determined mass ratio, sets the adopted WD mass of 0.7M(solar mass), and the WD radius follows from standard theoretical models. The adopted inclination, i = 65 deg, is a literature consensus, and is subsequently supported by chi-squared analysis. The mass transfer rate is the remaining parameter to set the accretion disk T(sub eff) profile, and the Hipparcos parallax constrains that parameter to mas transfer = (5.0 +/- 2.0) x 10(exp -9) M(solar mass)/yr by a comparison with observed spectra. The fit to the observed spectra adopts the contribution of a 57,000 +/- 5000 K WD. The model thus provides realistic constraints on mass transfer and T(sub eff) for a large mass transfer system above the period gap.

  4. Modelling the evolution of solar-mass stars with a range of metallicities using MESA

    Directory of Open Access Journals (Sweden)

    Jones E.F.

    2015-01-01

    Full Text Available The nuclides 1,2H, 3,4He, 7Li, 7Be, 8B, 12,13C, 13-15N, 14-18O, 17-19F, 18-22Ne, 22Mg, and 24Mg were used in the code package MESA (Modules for Experiments in Stellar Astrophysics[Paxton] to model a one-solar-mass star with a range of metallicities, z, from 0 to 0.1. On HR diagrams of each star model’s luminosity and effective temperature from before zero-age main sequence (pre-ZAMS to white dwarf, oscillations were noted in the horizontal branch at intervals from z = 0 to 0.0070. At z, = 0, the calculated stellar lifetime is 6.09x109 years. The calculated lifetime of the model stars increases to a maximum of 1.25x1010 years at z = 0.022 and then decreases to 2.59x109 years at z = 0.1. A piecewise fit of the model lifetimes vs. metallicity was obtained.

  5. Discovering Planetary Nebula Geometries: Explorations with a Hierarchy of Models

    Science.gov (United States)

    Huyser, Karen A.; Knuth, Kevin H.; Fischer, Bernd; Schumann, Johann; Granquist-Fraser, Domhnull; Hajian, Arsen R.

    2004-01-01

    Astronomical objects known as planetary nebulae (PNe) consist of a shell of gas expelled by an aging medium-sized star as it makes its transition from a red giant to a white dwarf. In many cases this gas shell can be approximately described as a prolate ellipsoid. Knowledge of the physics of ionization processes in this gaseous shell enables us to construct a model in three dimensions (3D) called the Ionization-Bounded Prolate Ellipsoidal Shell model (IBPES model). Using this model we can generate synthetic nebular images, which can be used in conjunction with Hubble Space Telescope (HST) images of actual PNe to perform Bayesian model estimation. Since the IBPES model is characterized by thirteen parameters, model estimation requires the search of a 13-dimensional parameter space. The 'curse of dimensionality,' compounded by a computationally intense forward problem, makes forward searches extremely time-consuming and frequently causes them to become trapped in local solutions. We find that both the speed and of the search can be improved by judiciously reducing the dimensionality of the search space. Our basic approach employs a hierarchy of models of increasing complexity that converges to the IBPES model. Earlier studies establish that a hierarchical sequence converges more quickly, and to a better solution, than a search relying only on the most complex model. Here we report results for a hierarchy of five models. The first three models treat the nebula as a 2D image, while the last two models explore its characteristics as a 3D object and enable us to characterize the physics of the nebula. This five-model hierarchy is applied to HST images of ellipsoidal PNe to estimate their geometric properties and gas density profiles.

  6. The Progenitors of Type Ia Supernovae. II. Are they Double-degenerate Binaries? The Symbiotic Channel

    Science.gov (United States)

    Di Stefano, R.

    2010-08-01

    In order for a white dwarf (WD) to achieve the Chandrasekhar mass, MC , and explode as a Type Ia supernova (SNIa), it must interact with another star, either accreting matter from or merging with it. The failure to identify the class o