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Sample records for white dwarf g191-b2b

  1. Interstellar medium and the highly ionized species observed in the spectrum of the nearby white dwarf G191-B2B

    International Nuclear Information System (INIS)

    High-resolution spectra of the neargy (48 pc) white dwarf G191-B2B obtained with the International Ultraviolet Explorer (IUE) 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, Fe II are also seen in the spectrum. Analysis of these features indicates an average neutral hydrogen number density, n/sub Htsi/ = 6.4 x 10-3, for this line of sight. In combination with the recent EUV and soft X-ray results, we interpret this to mean that the interstellar medium in the most immediate solar vicinity is of the ''normal'' density (nroughly-equal0.1 cm-3) 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 in its present form

  2. Stellar laboratories II. New Zn IV and Zn V oscillator strengths and their validation in the hot white dwarfs G191-B2B and RE0503-289

    CERN Document Server

    Rauch, T; Quinet, P; Kruk, J W

    2014-01-01

    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. 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. 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 RE0503-289. 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 an...

  3. 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 RE0503-289

    CERN Document Server

    Rauch, T; Quinet, P; Kruk, J W

    2015-01-01

    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. 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 RE0503-289 and to determine their photospheric Ga abundances. 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 RE0503-289. 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 RE0503-289. The identification of Ga IV and Ga VI lines is uncertain because th...

  4. The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. I. NLTE spectral analysis of the DA-type white dwarf G 191-B2B

    CERN Document Server

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

    2013-01-01

    H-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 observation. We establish a database of theoretical spectra of stellar flux standards that are easily accessible via a web interface. 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 G 191-B2B, we demonstrate that the model reproduces not only its overall continuum shape but also the numerous metal lines exhibited in its ultraviolet spectrum. TheoSSA is in operation and contains presently a variety of SEDs for DA white dwarfs. It will be extended in the near future and can host SEDs of all primary and ...

  5. 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.

  6. Deuterium Abundance Toward G191-B2B: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission

    Science.gov (United States)

    Lemoine, M.; Vidal-Madjar, A.; Hebrard, G.; Desert, J.-M.; Ferlet, R.; LecavelierdesEtangs, A.; Howk, J. C.; Andre, M.; Blair, W. P.; Friedman, S. D.; Oegerle, William R. (Technical Monitor)

    2002-01-01

    High-resolution spectra of the hot white dwarf G191-B2B covering the wavelength region 905-1187A were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). This data was used in conjunction with existing high-resolution Hubble Space Telescope STIS observations to evaluate the total H(sub I), D(sub I), O(sub I) and N(sub I) column densities along the line of sight. Previous determinations of N(D(sub I)) based upon GHRS (Goddard High Resolution Spectrograph) and STIS (Space Telescope Imaging Spectrograph) observations were controversial due to the saturated strength of the D(sub I) Lyman alpha line. In the present analysis the column density of D(sub I) has been measured using only the unsaturated Lyman beta and Lyman gamma lines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental character series has been performed. The column densities derived are: log N(D(sub I)) = 13.40+/-0.07, log N(O(sub I)) = 14.86+/-0.07, and log N(N(sub I)) = 13.87+/-0.07 quoted with 2sigma, uncertainties. The measurement of the H(sub I) column density by profile fitting of the Lyman alpha line has been found to be unsecure. If additional weak hot interstellar components are added to the three detected clouds along the line of sight, the H(sub I)) column density can be reduced quite significantly, even though the signal-to-noise ratio and spectral resolution at Lyman alpha are excellent. The new estimate of N(H(sub I)) toward G191-B2B reads: logN(H (sub I)) = 18.18+/-0.18 (2sigma uncertainty), so that the average (D/H) ratio on the line of sight is: (D/H)= 1.66(+0.9/-0.6) x 10(exp -5) (2sigma uncertainty).

  7. Deuterium abundance toward G191-B2B Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission

    CERN Document Server

    Lemoine, M; Hébrard, G; Désert, J M; Ferlet, R; Etangs, A L; Howk, J C; André, M; Blair, W P; Friedman, S D; Kruk, J W; Lacour, S; Moos, H W; Sembach, K R; Chayer, P; Jenkins, E B; Köster, D; Linsky, J L; Wood, B E; Oegerle, W R; Sonneborn, G; York, D G

    2001-01-01

    High-resolution spectra of the hot white dwarf G191-B2B, covering the wavelength region 905-1187A, were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). This data was used in conjunction with existing high-resolution Hubble Space Telescope STIS observations to evaluate the total HI, DI, OI and NI column densities along the line of sight. Previous determinations of N(DI) based upon GHRS and STIS observations were controversial due to the saturated strength of the DI Lyman-alpha line. In the present analysis the column density of DI has been measured using only the unsaturated Lyman-beta and Lyman-gamma lines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental characteristics has been performed. The column densities derived are: log N(DI) = 13.40 +/-0.07, log N(OI) = 14.86 +/-0.07, and log N(NI) = 13.87 +/-0.07 quoted with 2-sigma uncertainties. The measurement of the HI column ...

  8. The D/H Ratio in Interstellar Gas Towards G191-B2B

    CERN Document Server

    Sahu, M S; Bruhweiler, F C; Gull, T R; Bowers, C A; Lindler, D; Feggans, K; Barstow, M A; Hubeny, I; Holberg, J B

    1999-01-01

    We reinvestigate the question of spatial variation of the local D/H abundance, using both archival GHRS spectra, and new echelle spectra of G191-B2B obtained with the Space Telescope Imaging Spectrograph (STIS) aboard HST. Our analysis uses stratified line-blanketed non-LTE model atmosphere calculations to determine the shape of the intrinsic WD Lyman-alpha profile and estimate the WD photospheric contamination of the interstellar lines. Although three velocity components were reported previously towards G191-B2B, we detect only two velocity components. The first component is at V(hel) ~ 8.6 km/s and the second at V(hel) ~ 19.3 km/s, which we identify with the Local Interstellar Cloud (LIC). From the STIS data we derive D/H = 1.60(+0.39,-0.27)X10^-5 for the LIC component, and D/H > 1.26X10^-5 for the 8.6 km/s component (uncertainties denote 2-sigma or 95% confidence limits). The STIS data provide no evidence for local or component-to-component variation in the D/H ratio. Despite using two velocity components ...

  9. Limits on the dependence of the fine-structure constant on gravitational potential from white-dwarf spectra.

    Science.gov (United States)

    Berengut, J C; Flambaum, V V; Ong, A; Webb, J K; Barrow, John D; Barstow, M A; Preval, S P; Holberg, J B

    2013-07-01

    We propose a new probe of the dependence of the fine-structure constant ? on a strong gravitational field using metal lines in the spectra of white-dwarf stars. Comparison of laboratory spectra with far-UV astronomical spectra from the white-dwarf star G191-B2B recorded by the Hubble Space Telescope Imaging Spectrograph gives limits of ??/?=(4.2±1.6)×10(-5) and (-6.1±5.8)×10(-5) from FeV and NiV spectra, respectively, at a dimensionless gravitational potential relative to Earth of ???5×10(-5). With better determinations of the laboratory wavelengths of the lines employed these results could be improved by up to 2 orders of magnitude. PMID:23862989

  10. Uv spectra of nearby white dwarfs and the nature of the local interstellar medium

    International Nuclear Information System (INIS)

    We have investigated the local interstellar medium in the directions of four white dwarfs, G191-B2B, W1346, HD 149499B, and Sirius B. All the observational data were obtained at the high-resolution mode (lambda/?lambdaroughly-equal104) in the spectral range from about 1150 to 3200 A with the International Ultraviolet Explorer (IUE). Interstellar absorption lines of several elements in various stages of ionization are seen against the continuum of the white dwarfs. Low average hydrogen number densities (n-bar/sub HtsI/) are found. They range from n-bar/sub HtsI/ = 0.08 cm-3 for Sirius B, the nearest white dwarf (2.7 pc), to n-bar/sub HtsI/ = 0.006 cm-3 for G191-B2B, the most distant white dwarf (48 pc) studied. The results show, when combined with other recent ultraviolet, EUV, and diffuse X-ray observations, that: (a) the Sun is located inside a low-density (n-bar/sub HtsI/roughly-equal0.1 cm-3) cloud; (b) beyond 2--3 pc from the Sun, this cloud is surrounded, at least in most directions, by an extended region of hot (Troughly-equal10/sup 5en-dash6/ K) thin (nroughly-equal10-2 to 10-3 cm-3) interstellar plasma with no evidence for additional clouds in the lines of sight studied; (c) the elemental depletions of C, N, O, Si, Mg, and possibly Fe are low in the solar vicinity as previously found toward ? Vir, (d) the Sun is moving through this cloud at a relative velocity of about 20 km s-relative velocity of about 20 km s-1; and (e) the current results, which are quite consistent with previous ultraviolet, EUV, and diffuse X-ray observations, have significant bearings on the theoretical modeling of the interstellar medium. Subject headings: interstellar: abundances: interstellar: matter: stars: white dwarfs: ultraviolet: spectra

  11. Hot White Dwarfs

    OpenAIRE

    Sion, Edward M.

    2011-01-01

    The article covers the physical properties and evolution of single white dwarfs ranging in temperature from 20,000K to 200,000 and higher, the hottest know electron-degenerate stars. After discussing the classification of their spectra, the author reviews the known properties, parameters, evolutionary state, as well as persisting and new puzzles regarding all spectroscopic subclasses of hot white dwarfs: the hot DA white dwarfs, the DAO white dwarfs, the PG1159 degenerates, ...

  12. Accreting white dwarfs

    OpenAIRE

    Hernanz, Margarita; Jose?, Jordi

    2010-01-01

    Thermonuclear (type Ia) supernovae are explosions in accreting white dwarfs, but the exact scenario leading to these explosions is still unclear. An important step to clarify this point is to understand the behaviour of accreting white dwarfs in close binary systems. The characteristics of the white dwarf (mass, chemical composition, luminosity), the accreted material (chemical composition) and those related with the properties of the binary system (mass accretion rate), are...

  13. When White Dwarfs Collide

    Science.gov (United States)

    Hawley, Wendy Phyllis

    3D models of white dwarf collisions are used to assess the likelihood of double-degenerate mergers as progenitors for Type Ia supernovae (henceforth SNIa) and to identify observational signatures of double-degenerate collisions. Observations of individual SNIa, SNIa rates in different galaxy types, and double white dwarf binary systems suggest that mergers or collisions between two white dwarfs play a role in the overall SNIa population. Given the possibility of two progenitor systems (single-degenerate and double-degenerate), the sample of SNIa used in cosmological calcula- tions needs to be carefully examined. To improve calculations of cosmological parameters, the development of calibrated diagnostics for double-degenerate progenitor SNIa is essential. Head-on white dwarf collision simulations are used to provide an upper limit on the 56Ni production in white dwarf collisions. In chapter II, I explore zero impact parameter collisions of white dwarfs using the Eulerian grid code FLASH. The initial 1D white dwarf profiles are created assuming hydrostatic equilibrium and a uniform composition of 50% 12C and 50% 16O. The masses range from 0.64 to 0.81 solar masses and have an isothermal temperature of 107 K. I map these 1D models onto a 3D grid, where the dimensions of the grid are each eight times the white dwarf radius, and the dwarfs are initially placed four white dwarf radii apart (center to center). To provide insight into a larger range of physical possibilities, I also model non-zero impact parameter white dwarf collisions (Chapter III). Although head-on white dwarf collisions provide an upper limit on 56Ni production, non-zero impact parameter collisions provide insight into a wider range of physical scenarios. The initial conditions (box size, initial separation, composition, and initial temperature) are identical to those used for the head-on collisions (Chapter II) for the same range of masses. For each mass pair- ing, collision simulations are carried out at impact parameters b=1 and b=2 (grazing). Finally, I will address future work to be performed (Chapter IV).

  14. Progenitors of white dwarfs

    International Nuclear Information System (INIS)

    Direct observational evidence is presented which indicates that the immediate progenitors of white dwarfs are the central stars of planetary nebulae (approximately 70%), other post-AGB objects (approximately 30%), and post-HB objects not massive enough to climb the AGB (approximately 0.3%). The combined birth rate for these objects is in satisfactory agreement with the death rate of main-sequence stars and the birth rate of white dwarfs

  15. White dwarf pulsations

    International Nuclear Information System (INIS)

    The DA white dwarfs are those which show only the Stark-broadened lines of hydrogen in their spectra. They comprise about 80% of the total white dwarf population. A subset of the DA dwarfs, the ZZ Ceti stars, form a highly homogeneous class of nonradially pulsating variable stars. In this paper we shall review the observations from which both the physical properties of the stars and the characteristics of the pulsations have been derived. Data obtained since the last review of these variables (Robinson 1979) is stressed, as these data are forcing a somewhat revised understanding of the ZZ Ceti stars and their relationship to investigations of white dwarfs and to pulsating variable stars, in general. (orig.)

  16. White Dwarf Stars

    Science.gov (United States)

    Koester, Detlev

    White dwarfs are the final stage for more than 95% of all stars. Their population statistics and properties contain a wealth of information about the history of star formation in our galaxy, the ages of stellar systems, and the relation between original mass at birth and that of the final remnant. They are also interesting individually as laboratories for physical conditions not easily reached in terrestrial labs: macroscopic manifestation of the Pauli principle, high densities and pressures, and extremely high magnetic fields. After a brief introduction with some historical milestones the observational status is reviewed: spectroscopic classification, determination of stellar parameters from spectroscopic and photometric observations, effective temperatures, surface gravities, radii, and masses. The next sections deal with the physics of the interior and evolution of white dwarfs, leading to the mass-radius relation and cooling times. Going back closer to the observations again, the physical processes in the outer layers are discussed: gravitational separation, diffusion, radiative levitation, accretion, and convective mixing. This leads to a review of our current understanding of the origin of spectral types and their interrelation. A final section gives brief introductions to topics at the center of current research: white dwarfs in open and globular clusters, debris disks, the origin of accreted metals in the atmospheres, magnetic fields and their origin, variable white dwarfs, and white dwarfs in binaries. This chapter was finished in February 2010 and reflects the status of knowledge at that time.

  17. White Dwarf Stars

    Science.gov (United States)

    Kepler, S. O.

    2014-10-01

    White dwarfs are the evolutionary endpoint for nearly 95% of all stars born in our Galaxy, the final stages of evolution of all low- and intermediate mass stars, i.e., main sequence stars with masses below (8.5± 1.5) M_{odot}, depending on metallicity of the progenitor, mass loss and core overshoot. Massive white dwarfs are intrinsically rare objects, tand produce a gap in the determination of the initial vs. final mass relation at the high mass end (e.g. Weidemann 2000 A&A, 363, 647; Kalirai et al. 2008, ApJ, 676, 594; Williams, Bolte & Koester 2009, ApJ, 693, 355). Main sequences stars with higher masses will explode as SNII (Smartt S. 2009 ARA&A, 47, 63), but the limit does depend on the metallicity of the progenitor. Massive white dwarfs are probably SNIa progenitors through accretion or merger. They are rare, being the final product of massive stars (less common) and have smaller radius (less luminous). Kepler et al. 2007 (MNRAS, 375, 1315), Kleinman et al. 2013 (ApJS, 204, 5) estimate only 1-2% white dwarfs have masses above 1 M_{odot}. The final stages of evolution after helium burning are a race between core growth and loss of the H-rich envelope in a stellar wind. When the burning shell is exposed, the star rapidly cools and burning ceases, leaving a white dwarf. As they cool down, the magnetic field freezes in, ranging from a few kilogauss to a gigagauss. Peculiar type Ia SN 2006gz, SN 2007if, SN 2009dc, SN 2003fg suggest progenitors in the range 2.4-2.8 M_{odot}, and Das U. & Mukhopadhyay B. (2012, Phys. Rev. D, 86, 042001) estimate that the Chandrasekhar limit increases to 2.3-2.6 M_{odot} for extremely high magnetic field stars, but differential rotation induced by accretion could also increase it, according to Hachisu I. et al. 2012 (ApJ, 744, 69). García-Berro et al. 2012, ApJ, 749, 25, for example, proposes double degenerate mergers are the progenitors of high-field magnetic white dwarfs. We propose magnetic fields enhance the line broadening in WDs, causing an overestimated surface gravity, and ultimately determine if these magnetic fields are likely developed through the star's own surface convection zone, or inherited from massive Ap/Bp progenitors. We discovered around 20 000 spectroscopic white dwarfs with the Sloan Digital Sky Survey (SDSS), with a corresponding increase in relatively rare varieties of white dwarfs, including the massive ones (Kleinman et al. 2013, ApJS, 204, 5, Kepler et al. 2013, MNRAS, 439, 2934). The mass distributions of the hydrogen-rich (DA) measured from fitting the spectra with model atmospheres calculated using unidimensinal mixing lenght-theory (MLT) shows the average mass (as measured by the surface gravity) increases apparently below 13 000K for DAs (e.g. Bergeron et al. 1991, ApJ, 367, 253; Tremblay et al. 2011, ApJ, 730, 128; Kleinman et al. 2013). Only with the tridimensional (3D) convection calculations of Tremblay et al. 2011 (A&A, 531, L19) and 2013 (A&A, 552, 13; A&A, 557, 7; arXiv 1309.0886) the problem has finally been solved, but the effects of magnetic fields are not included yet in the mass determinations. Pulsating white dwarf stars are used to measure their interior and envelope properties through seismology, and together with the luminosity function of white dwarf stars in clusters and around the Sun are valuable tools for the study of high density physics, and the history of stellar formation.

  18. 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.

  19. Planetesimals Around White Dwarfs

    Science.gov (United States)

    Xu, Siyi

    Evidence is convincing that a big fraction of extrasolar planetesimals can survive the red giant stage of a star and persist into the white dwarf phase. We argue that eventually, some of them do get perturbed into the tidal radius of the white dwarf and disrupted, creating a dusk disk and polluting the star's pure hydrogen or helium atmosphere at the same time. I have been performing multi-wavelength observations to study these intriguing systems. With Spitzer/IRAC, I find a depletion of dust disk around cool white dwarfs relative to the warmer sample, possibly due to accretion from cometary objects. With the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope and the High- Resolution Echelle Spectrometer (HIRES) on Keck, I find that to zeroth order, the elemental compositions of extrasolar planetesimals very much resemble that of bulk Earth. A more detailed comparison with the solar system meteorites shows that post-nebular processing is common among extrasolar planetesimals. Overall, the bulk composition of Earth is normal compared to the current sample of extrasolar planetesimals.

  20. Axion cooling of white dwarfs

    CERN Document Server

    Isern, J; 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.

  1. ASTRO-H White Paper - White Dwarf

    OpenAIRE

    Mukai, K.; Yuasa, T.; Harayama, A.; Hayashi, T.; Ishida, M.; Long, K. S.; Terada, Y.; Tsujimoto, M.; Group, On Behalf Of The Astro-h Science Working

    2014-01-01

    Interacting binaries in which a white dwarf accretes material from a companion --- cataclysmic variables (CVs) in which the mass loss is via Roche-lobe overflow, and symbiotic stars in which the white dwarf captures the wind of 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 baffles observers and theorists alike. They are likely to be a significant contributor to the Gal...

  2. Equations of State for White Dwarfs

    OpenAIRE

    Heikkila?, Elena

    2009-01-01

    This thesis is about deriving a few equations of state for white dwarfs below the regime of neutron drip. White dwarfs - also called degenerate dwarfs, composed mostly of electron-degenerate matter - are luminous and the color of the light they are emitting is white, hence their name. Because of the relatively enormous density, the gravitational potential of a white dwarf causes a collapse. White dwarfs are classified as compact objects, meaning that their life begins wh...

  3. White dwarf-red dwarf binaries in the Galaxy

    OpenAIRE

    Besselaar, E. J. M. 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 intrinsically equally bright, these binaries stand out in any colour-colour diagram. These studies have mainly used the largest astronomical database available at present, the Sloan Digital Sky Su...

  4. Asteroseismology of white dwarf stars

    CERN Document Server

    Córsico, A H

    2014-01-01

    Most of low- and intermediate-mass stars that populate the Universe will end their lives as white dwarf stars. These ancient stellar remnants have encrypted inside a precious record of the evolutionary history of the progenitor stars, providing a wealth of information about the evolution of stars, star formation, and the age of a variety of stellar populations, such as our Galaxy and open and globular clusters. While some information like surface chemical composition, temperature and gravity of white dwarfs can be inferred from spectroscopy, the internal structure of these compact stars can be unveiled only by means of asteroseismology, an approach based on the comparison between the observed pulsation periods of variable stars and the periods of appropriate theoretical models. In this communication, we first briefly describe the physical properties of white dwarf stars and the various families of pulsating white dwarfs known up to the present day, and then we present two recent analysis carried out by the La...

  5. Gaia photometry for white dwarfs

    Science.gov (United States)

    Carrasco, J. M.; Catalán, S.; Jordi, C.; Tremblay, P.-E.; Napiwotzki, R.; Luri, X.; Robin, A. C.; Kowalski, P. M.

    2014-05-01

    Context. White dwarfs can be used to study the structure and evolution of the Galaxy by analysing their luminosity function and initial mass function. Among them, the very cool white dwarfs provide the information for the early ages of each population. Because white dwarfs are intrinsically faint only the nearby (~ 20 pc) sample is reasonably complete. The Gaia space mission will drastically increase the sample of known white dwarfs through its 5-6 years survey of the whole sky up to magnitude V = 20-25. Aims: We provide a characterisation of Gaia photometry for white dwarfs to better prepare for the analysis of the scientific output of the mission. Transformations between some of the most common photometric systems and Gaia passbands are derived. We also give estimates of the number of white dwarfs of the different galactic populations that will be observed. Methods: Using synthetic spectral energy distributions and the most recent Gaia transmission curves, we computed colours of three different types of white dwarfs (pure hydrogen, pure helium, and mixed composition with H/He = 0.1). With these colours we derived transformations to other common photometric systems (Johnson-Cousins, Sloan Digital Sky Survey, and 2MASS). We also present numbers of white dwarfs predicted to be observed by Gaia. Results: We provide relationships and colour-colour diagrams among different photometric systems to allow the prediction and/or study of the Gaia white dwarf colours. We also include estimates of the number of sources expected in every galactic population and with a maximum parallax error. Gaia will increase the sample of known white dwarfs tenfold to about 200 000. Gaia will be able to observe thousands of very cool white dwarfs for the first time, which will greatly improve our understanding of these stars and early phases of star formation in our Galaxy. Tables 6 and 7 are available in electronic form at http://www.aanda.orgFull Tables 3-5 are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/565/A11

  6. White Dwarf Planets from GAIA

    CERN Document Server

    Silvotti, Roberto; Lattanzi, Mario

    2010-01-01

    We investigate the potential of high-precision astrometry with GAIA for detection of giant planetary companions to nearby white dwarfs. If one considers that, to date, no confirmed planets around single white dwarfs are known, the results from GAIA will be crucial to study the late-stage evolution of planetary systems and to verify the possibility that 2nd-generation planets are formed.

  7. 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.

  8. Singing and dancing white dwarfs

    International Nuclear Information System (INIS)

    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.

  9. Gaia photometry for white dwarfs

    CERN Document Server

    Carrasco, J M; Jordi, C; Tremblay, P E; Napiwotzki, R; Luri, X; Robin, A C; Kowalski, P M

    2014-01-01

    Context. White dwarfs can be used to study the structure and evolution of the Galaxy by analysing their luminosity function and initial mass function. Among them, the very cool white dwarfs provide the information for the early ages of each population. Because white dwarfs are intrinsically faint only the nearby (about 20 pc) sample is reasonably complete. The Gaia space mission will drastically increase the sample of known white dwarfs through its 5-6 years survey of the whole sky up to magnitude V = 20-25. Aims. We provide a characterisation of Gaia photometry for white dwarfs to better prepare for the analysis of the scientific output of the mission. Transformations between some of the most common photometric systems and Gaia passbands are derived. We also give estimates of the number of white dwarfs of the different galactic populations that will be observed. Methods. Using synthetic spectral energy distributions and the most recent Gaia transmission curves, we computed colours of three different types of...

  10. Observations of Ultracool White Dwarfs

    OpenAIRE

    Oppenheimer, B. R.; Saumon, D.; Hodgkin, S. T.; Jameson, R. F.; Hambly, N. C.; Chabrier, G.; Filippenko, A. V.; Coil, A. L.; Brown, M. E.

    2001-01-01

    We present new spectroscopic and photometric measurements of the white dwarfs LHS 3250 and WD 0346+246. Along with F351-50, these white dwarfs are the coolest ones known, all with effective temperatures below 4000 K. Their membership in the Galactic halo population is discussed, and detailed comparisons of all three objects with new atmosphere models are presented. The new models consider the effects of mixed H/He atmospheres and indicate that WD 0346+246 and F351-50 have pr...

  11. Gravitational Radiation from Strongly Magnetized White Dwarfs

    OpenAIRE

    Heyl, Jeremy S.

    2000-01-01

    The magnetic fields of white dwarfs distort their shape generating an anisotropic moment of inertia. A magnetized white dwarf which rotates obliquely relative to the symmetry axis has a mass quadrupole moment which varies in time, so it will emit gravitational radiation. LISA may be able to detect the gravitational waves from two nearby, quickly rotating white dwarfs.

  12. The evolution of iron white dwarf stars

    Directory of Open Access Journals (Sweden)

    J. A. Panei

    2001-01-01

    Full Text Available Recent measurements by Hipparcos provide strong observational evidence supporting the existence of white dwarf stars with iron-rich core composition. Here we examine the evolution of iron-rich white dwarfs, for which the cooling is substancially accelerated as compared with the standard carbon-oxigen white dwarfs.

  13. White Dwarfs constrain Dark Forces

    CERN Document Server

    Dreiner, Herbert K; Isern, Jordi; Ubaldi, Lorenzo

    2013-01-01

    The white dwarf luminosity function, which provides information about their cooling, has been measured with high precision in the past few years. Simulations that include well known Standard Model physics give a good fit to the data. This leaves little room for new physics and makes these astrophysical objects a good laboratory for testing models beyond the Standard Model. It has already been suggested that white dwarfs might provide some evidence for the existence of axions. In this work we study the constraints that the white dwarf luminosity function puts on physics beyond the Standard Model involving new light particles (fermions or bosons) that can be pair-produced in a white dwarf and then escape to contribute to its cooling. We show, in particular, that we can severely constrain the parameter space of models with dark forces and light hidden sectors (lighter than a few tens of keV). The bounds we find are often more competitive than those from current lab searches and those expected from most future se...

  14. White dwarf research with Gaia

    CERN Document Server

    Jordan, Stefan

    2014-01-01

    The results of the Gaia mission will have tremendous influence on many topics in white dwarf research. In this paper the current status of the Gaia mission is described. At the end a short outlook on the release scenario and the expected accuracy of the Gaia data is provided.

  15. The Physics of White Dwarfs.

    Science.gov (United States)

    Van Horn, Hugh M.

    1979-01-01

    Describes the current understanding of the structure and evolution of the white dwarf stars that was gained as a result of the increasingly sensitive and detailed astronomical observations coupled with calculations of the properties of matter under extreme conditions. (Author/GA)

  16. The Population of Local White Dwarf Stars

    Science.gov (United States)

    Holberg, J. B.

    2000-10-01

    The most recent version of the Catalogue of Spectroscopically Identified White Dwarfs (Mc Cook & Sion 1999) lists 2249 white dwarf stars and much new photometric and astrometric data. Approximately 6 either trigonometric parallaxes or distance moduli which place them within 20 pc of the sun. Most of these nearby white dwarfs are isolated stars, but 25 systems, including double degenerate systems. The sample of local white dwarfs is largely complete out to a distance of 13 pc, allowing estimates of the local number density and mass density of white dwarfs stars. This work was supported in part by NASA Grant NAG5-9408

  17. White dwarfs - the once and future suns

    International Nuclear Information System (INIS)

    The history and properties of white dwarfs (Bessel's conclusion that Sirius and Procyon have invisible companions, Clark's discovery of Sirius B, Adams and Russell's study of white dwarf spectra, Chandrasekhar's explanation of white dwarf structure by equations incorporating quantum mechanics and relativity) are treated. Formation of white dwarfs, degeneracy, binary white dwarfs (and novae and supernovae) are explained. A mystery nearly 50 years old regarding the spectrum of the star Greenwich +70 degrees-8247 has been solved: it involves a stationary line phenomenon and a magnetic field of 300-500 million gauss. Processes being studied in white dwarfs and white dwarf models include gravitational settling, accretion, dredge-up, radiation pressure, and diffusive hydrogen burning

  18. DE CVn: A bright, eclipsing red dwarf - white dwarf binary

    OpenAIRE

    Besselaar, E. J. M. Den; Greimel, R.; Morales-rueda, L.; Nelemans, G.; Thorstensen, J. R.; Marsh, T. R.; Dhillon, V. S.; Robb, R. M.; Balam, D. D.; Guenther, E. W.; Kemp, J; Augusteijn, T.; Groot, P. J.

    2007-01-01

    Close white dwarf - red dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarf - red dwarf binary with a relatively short (~8.7 hours) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters which we discuss in the frame work of common-envelope evolution. Pho...

  19. White dwarfs in the WTS: Eclipsing binaries

    Directory of Open Access Journals (Sweden)

    Burleigh M.R.

    2013-04-01

    Full Text Available We have identified photometric white dwarf candidates in the WFCAM transit survey through a reduced proper motion versus colour approach. Box-fitting with parameters adjusted to detect the unique signature of a white dwarf + planet/brown dwarf transit/eclipse event was performed, as well as looking for variability due to the irradiation of the companions atmosphere by the white dwarf's high UV flux. We have also performed a simple sensitivity analysis in order to assess the ability of the survey to detect companions to white dwarfs via the transit method.

  20. Acoustic fluxes in white dwarfs

    International Nuclear Information System (INIS)

    Estimates of the acoustic flux generated in white dwarf convection zones are given. Wide ranges of mass, effective temperature, and convective efficiency have been considered. Homogeneous hydrogen- and helium-rich convective envelope models were computed as well as inhomogeneous models representing DA white dwarfs with thin layers of hydrogen surrounding helium-dominated regions. The calculations of the transition zone that separates the hydrogen- and helium-rich layers of these models is extended to take into account the fact that both hydrogen and helium are non-trace species. Only very thin layers of hydrogen are allowed if an underlyign helium convection zone is to produce acoustic noise in these inhomogeneous models. It is suggested that such layers are unstable against mixing. Using the input acoustic flux of the homogeneous envelopes, the soft X-ray luminosities of acoustically heated white dwarf coronae are estimated according to Hearn's coronal model. If such coronae exist, they could be detectable by present day techniques

  1. Double white dwarfs and LISA

    CERN Document Server

    Marsh, T R

    2011-01-01

    Close pairs of white dwarfs are potential progenitors of Type~Ia supernovae and they are common, with of order 100 -- 300 million in the Galaxy. As such they will be significant, probably dominant, sources of the gravitational waves detectable by LISA. In the context of LISA's goals for fundamental physics, double white dwarfs are a source of noise, but from an astrophysical perspective, they are of considerable interest in their own right. In this paper I discuss our current knowledge of double white dwarfs and their close relatives (and possible descendants) the AM~CVn stars. LISA will add to our knowledge of these systems by providing the following unique constraints: (i) an almost direct measurement of the Galactic merger rate of DWDs from the detection of short period systems and their period evolution, (ii) an accurate and precise normalisation of binary evolution models at the shortest periods, (iii) a determination of the evolutionary pathways to the formation of AM~CVn stars, (iv) measurements of the...

  2. Infrared photometry of cool white dwarfs

    International Nuclear Information System (INIS)

    The results are presented of a search for the effects of pressure induced H2 dipole opacity on the infrared JHK magnitudes of cool white dwarfs. LHS 1126 is found to be a very cool (Tsub(e) approximately 4250 K) DC white dwarf with a H rich atmospheric composition dominated by H2 dipole opacity in the infrared. JHK photometry also favours a H rich atmospheric composition for the DK white dwarfs LP 658-2 and W 489. The surprisingly high proportion of hydrogen rich white dwarfs in the sample appears to suggest that the mechanism which inhibits the accretion of hydrogen in the hotter helium stars becomes less effective at low (Tsub(e) approximately 3+ ion in cool hydrogen rich white dwarf atmospheres is pointed out and it is suggested that the opacity due to this ion may be responsible for the blanketing observed in the U and B magnitudes of some cool white dwarfs. (author)

  3. Merging White Dwarfs and Thermonuclear Supernovae

    OpenAIRE

    Kerkwijk, Marten H.

    2012-01-01

    Thermonuclear supernovae result when interaction with a companion reignites nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway, a catastrophic gain in pressure, and the disintegration of the whole white dwarf. It is usually thought that fusion is reignited in near-pycnonuclear conditions when the white dwarf approaches the Chandrasekhar mass. I briefly describe two long-standing problems faced by this scenario, and our suggestion that these supern...

  4. The space distribution of DA white dwarfs

    International Nuclear Information System (INIS)

    We present a determination of the scaleheight and luminosity function of DA white dwarfs based on a new sample of 41 faint (B0=275±50 pc. Assuming such a scaleheight, the local luminosity function derived for the AAT sample shows good agreement with that obtained previously for the Palomar Green DA white dwarf sample, particularly in the overall normalization of the luminosity function. We derive a value for the local space density of DA white dwarfs with Mv3. (author)

  5. 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).

  6. Decoding Convection with White Dwarf Light Curves

    Science.gov (United States)

    Provencal, J. L.; Montgomery, M.; WET Team

    2013-12-01

    Convection remains one of the largest sources of theoretical uncertainty in our understanding of stellar physics. Current studies of convective energy transport are based on the Mixing Length Theory (MLT). As an example for white dwarfs, Bergeron et al. (1995) show that basic parameters such as flux, line profiles, energy distribution, color indices, and equivalent widths are extremely sensitive to the assumed MLT parameterization. This is compelling, since we use our knowledge of white dwarf interiors to calibrate white dwarf cooling sequences, provide detailed estimates for the ages of individual white dwarfs, and calibrate the age of the Galactic disk. The Whole Earth Telescope (WET) is engaged in a long term project to empirically determine the physical properties of convection in the atmospheres of pulsating white dwarfs. Our technique uses information from nonlinear (non-sinusoidal) pulse shapes of the target star to empirically probe the physical properties of its convection zone. We present current results from WET targets in 2008 - 2012.

  7. 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 thank them. The white dwarf community has been steadily growing since the first white dwarf workshop, held in Kiel (Germany) in 1974. Some of the participants in the first colloquium have already effectively retired; others - although officially retired - continue to attend successive workshops, Professor Weidemann, one of the first organizers, being a leading example. We hope we will be able to continue counting on them for many years. A very graphical view of the evolution of the field can be found in the homepage of Professor Detlev Koester, who has collected pictures of almost all the previous workshops:. Additionally, several astronomers coming from related fields have joined our (not so) small community. Most importantly, several generations of young scientists gave their first talks in these workshops. In summary our community is an active one, and we have close, durable and solid ties of friendship. We are optimistic and we foresee that the spirit of the previous workshops will continue in future editions. We would like to express our deepest gratitude to our sponsors: The Universitat Politècnica de Catalunya (UPC), the Institut de Ciències de l'Espai (CSIC), the Institute for Space Studies of Catalonia (IEEC), the Spanish Ministry of Education and Science, the Generalitat de Catalunya, the Ajuntament de Barcelona, the School of Civil Engineering of Barcelona and UPCnet. Finally, the IEEC staff and our graduate students have enthusiastically supported the organization of the workshop in every single detail; without them we would have not succeeded. We thank them especially. Also, we acknowledge the task of the Scientific Organizing Committee, which gave their full support in all the scientific tasks. Enrique García-Berro, UPC Margarida Hernanz, ICE (CSIC) Jordi Isern, ICE (CSIC) Santiago Torres, UPC Editors Conference photograph

  8. SDSS DR7 WHITE DWARF CATALOG

    Energy Technology Data Exchange (ETDEWEB)

    Kleinman, S. J.; Nitta, A. [Gemini Observatory, 670 North A' ohoku Place, Hilo, HI 96720 (United States); Kepler, S. O.; Pelisoli, Ingrid; Pecanha, Viviane; Costa, J. E. S. [Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS (Brazil); Koester, D. [Institut fuer Theoretische Physik und Astrophysik, Universitaet Kiel, D-24098 Kiel (Germany); Krzesinski, J. [Mt. Suhora Observatory, Pedagogical University of Cracow, ul. Podchorazych 2, 30-084 Cracow (Poland); Dufour, P.; Lachapelle, F.-R.; Bergeron, P. [Departement de Physique, Universite de Montreal, C. P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada); Yip, Ching-Wa [Department of Physics and Astronomy, The Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218 (United States); Harris, Hugh C. [United States Naval Observatory, Flagstaff Station, 10391 West Naval Observatory Road, Flagstaff, AZ 86001-8521 (United States); Eisenstein, Daniel J. [Harvard Smithsonian Center for Astrophysics, 60 Garden Street, MS 20, Cambridge, MA 02138 (United States); Althaus, L.; Corsico, A., E-mail: hch@nofs.navy.mil [Facultad de Ciencias Astronomicas y Geofisicas, Paseo del Bosque S/N, (1900) La Plata (Argentina)

    2013-01-15

    We present a new catalog of spectroscopically confirmed white dwarf stars from the Sloan Digital Sky Survey (SDSS) Data Release 7 spectroscopic catalog. We find 20,407 white dwarf spectra, representing 19,712 stars, and provide atmospheric model fits to 14,120 DA and 1011 DB white dwarf spectra from 12,843 and 923 stars, respectively. These numbers represent more than a factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalogs based on DR4 data. Our distribution of subtypes varies from previous catalogs due to our more conservative, manual classifications of each star in our catalog, supplementing our automatic fits. In particular, we find a large number of magnetic white dwarf stars whose small Zeeman splittings mimic increased Stark broadening that would otherwise result in an overestimated log g if fit as a non-magnetic white dwarf. We calculate mean DA and DB masses for our clean, non-magnetic sample and find the DB mean mass is statistically larger than that for the DAs.

  9. An overview of white dwarf stars

    Directory of Open Access Journals (Sweden)

    Charpinet S.

    2013-03-01

    Full Text Available We present a brief summary of what is currently known about white dwarf stars, with an emphasis on their evolutionary and internal properties. As is well known, white dwarfs represent the end products of stellar evolution for the vast majority of stars and, as such, bear the signatures of past events (such as mass loss, mixing phases, loss and redistribution of angular momentum, and thermonuclear burning that are of essential importance in the evolution of stars in general. In addition, white dwarf stars represent ideal testbeds for our understanding of matter under extreme conditions, and work on their constitutive physics (neutrino production rates, conductive and radiative opacities, interior liquid/solid equations of state, partially ionized and partially degenerate envelope equations of state, diffusion coefficients, line broadening mechanisms is still being actively pursued. Given a set of constitutive physics, cooling white dwarfs can be used advantageously as cosmochronometers. Moreover, the field has been blessed by the existence of four distinct families of pulsating white dwarfs, each mapping a different evolutionary phase, and this allows the application of the asteroseismological method to probe and test their internal structure and evolutionary state. We set the stage for the reviews that follow on cooling white dwarfs as cosmochronometers and physics laboratories, as well as on the properties of pulsating white dwarfs and the asteroseismological results that can be inferred.

  10. Dyson Spheres around White Dwarfs

    CERN Document Server

    Semiz, ?brahim

    2015-01-01

    A Dyson Sphere is a hypothetical structure that an advanced civilization might build around a star to intercept all of the star's light for its energy needs. One usually thinks of it as a spherical shell about one astronomical unit (AU) in radius, and surrounding a more or less Sun-like star; and might be detectable as an infrared point source. We point out that Dyson Spheres could also be built around white dwarfs. This type would avoid the need for artificial gravity technology, in contrast to the AU-scale Dyson Spheres. In fact, we show that parameters can be found to build Dyson Spheres suitable --temperature- and gravity-wise-- for human habitation. This type would be much harder to detect.

  11. White Dwarfs population as seen by Gaia

    Science.gov (United States)

    Carrasco, J. M.; Catalán, S.; Jordi, C.; Tremblay, P.-E.; Napiwotzki, R.; Luri, X.; Robin, A. C.; Kowalski, P. M.; Reylé, C.

    2013-05-01

    The launch of Gaia satellite of ESA is approaching (scheduled in 2013) and the scientific community is preparing for the maximal scientific return. As white dwarfs are very faint (especially in the very cool regime, Teff ? 5,000 K), they are very hard to detect and only the closest ones have been observed until now. Gaia, through its 5--6 years survey of the whole sky up to magnitude 20-25 (depending on the colour of the source), will drastically increase the sample of known white dwarfs and provide a lot of new science in this field. Using synthetic spectral energy distribution libraries and the most recent Gaia transmission curves, we derive colours of three different kinds of white dwarfs (pure hydrogen, pure helium and mixed composition with H/He=0.1). With these colours we derive transformations to other common photometric systems (Johnson-Cousins, Sloan Digital Sky Survey and 2MASS). Different relationships have to be considered for different white dwarfs compositions. Pure-He white dwarfs show an unique behaviour valid at different temperatures, but pure-H white dwarfs need to be analysed in two different temperature regimes, as their behaviour changes around Teff =5,000 K. We also compare the estimations of number of white dwarfs as predicted by the Gaia Universe Model Snapshot and by a different model of white dwarfs population (Napiwotzky's simulations). Among all white dwarfs observed, the most interesting ones will be those in the very cool regime. According to our simulations, Gaia will be able to observe thousands of them for the first time.

  12. Spectral Evolution of White Dwarf Stars

    Science.gov (United States)

    Chen, Eugene Y.; Hansen, B. M. S.

    2007-12-01

    We made realistic calculations on the spectral evolution of white dwarf stars under effects of Interstellar Medium accretion. In particular, we exploited the H-He plasma mixing scheme given by Saumon, Chabrier and van Horn(1995) to properly account for the evolution of helium abundance at the white dwarf surface. We incorporate this change of composition into a self-consistent calculation of surface boundary conditions and convective mixing. We show that, under reasonable initial conditions, white dwarf stars could undergo complicated spectral evolution during the course of their cooling. We compare our results to the observed features such as the non-DA gap of Bergeron, Ruiz and Leggett(1997).

  13. Surface compositions of magnetic white dwarfs

    International Nuclear Information System (INIS)

    Magnetic white dwarfs have mixed spectra (of hydrogen and helium) much more frequently than nonmagnetic white dwarfs. Many authors have suggested that this is caused by the slowing down of gravitational settling by the magnetic field. This is shown not to be the case. The magnetic field can effectively stop diffusion only at very small optical depths (tau-3), not in the usual line forming region. Instead it is suggested that the magnetic field stops the selective wind of hydrogen postulated by Michaud and Fontaine to explain the near complete absence of hydrogen from nonmagnetic, non-DA white dwarfs

  14. Metal Abundances in Hot DO White Dwarfs

    CERN Document Server

    Werner, K; Ringat, E; Kruk, J W

    2012-01-01

    The relatively high abundance of carbon in the hot DO white dwarf RE0503-289 indicates that it is a descendant of a PG1159 star. This is corroborated by the recent detection of the extremely high abundances of trans-Fe elements which stem from s-process nucleosynthesis in the precursor AGB star, dredged up by a late He-shell flash and possibly amplified by radiative levitation. On the other hand, the hottest known DO white dwarf, KPD0005+5106, cannot have evolved from a PG1159 star but represents a distinct He-rich evolutionary sequence that possibly originates from a binary white dwarf merger.

  15. Merging white dwarfs and thermonuclear supernovae.

    Science.gov (United States)

    van Kerkwijk, M H

    2013-06-13

    Thermonuclear supernovae result when interaction with a companion reignites nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway, a catastrophic gain in pressure and the disintegration of the whole white dwarf. It is usually thought that fusion is reignited in near-pycnonuclear conditions when the white dwarf approaches the Chandrasekhar mass. I briefly describe two long-standing problems faced by this scenario, and the suggestion that these supernovae instead result from mergers of carbon-oxygen white dwarfs, including those that produce sub-Chandrasekhar-mass remnants. I then turn to possible observational tests, in particular, those that test the absence or presence of electron captures during the burning. PMID:23630372

  16. White Dwarfs, Neutron Stars and Black Holes

    Science.gov (United States)

    Szekeres, P.

    1977-01-01

    The three possible fates of burned-out stars: white dwarfs, neutron stars and black holes, are described in elementary terms. Characteristics of these celestial bodies, as provided by Einstein's work, are described. (CP)

  17. White Dwarf Model for PAMELA Positron Excess

    Science.gov (United States)

    Kashiyama, Kazumi; Ioka, Kunihito

    2010-10-01

    We suggest that white dwarf pulsars can compete with neutron star pulsars for producing the excesses of cosmic ray positrons and electrons (e+/-) observed by the PAMELA, ATIC/PPB-BETS, Fermi and HESS experiments. A double degenerate white dwarf binary mergers into a white dwarf pulsar with rotational energy (~1050 erg) comparable to a neutron star pulsar. The birth rate (~1/100 yr) is also similar, providing the right energy budget. Applying the neutron star theory, we show that the white dwarf pulsars can produce e+/-, up to ~10 TeV for high magnetic fields (>108 G). In contrast to the neutron star case, the adiabatic energy losses of e+/- are negligible since their injection continues after the nebula expansion. The long activity also enhances the nearby sources, potentially dominating the quickly cooled e+/- above TeV energy, detectable by the future CALET experiment.

  18. Accretion Flows in Magnetic White Dwarf Systems

    Science.gov (United States)

    Imamura, James N.

    2005-01-01

    We received Type A and B funding under the NASA Astrophysics Data Program for the analysis and interpretation of hard x-ray data obtained by the Rossi X-ray Timing Explorer and other NASA sponsored missions for Intermediate Polars (IPS) and Polars. For some targets, optical data was available. We reduced and analyzed the X-ray spectra and the X-ray and optical (obtained at the Cerro Tololo Inter-American Observatory) timing data using detailed shock models (which we constructed) to place constraints on the properties of the accreting white dwarfs, the high energy emission mechanisms of white dwarfs, and the large-scale accretion flows of Polars and IPS. IPS and Polars are white dwarf mass-transfer binaries, members of the larger class of cata,clysmic variables. They differ from the bulk of the cataclysmic variables in that they contain strongly magnetic white dwarfs; the white dwarfs in Polars have B, = 7 to 230 MG and those in IPS have B, less than 10 MG. The IPS and Polars are both examples of funneled accretion flows in strong magnetic field systems. The IPS are similar to x-ray pulsars in that accretion disks form in the systems which are disrupted by the strong stellar magnetic fields of the white dwarfs near the stellar surface from where the plasma is funneled to the surface of the white dwarf. The localized hot spots formed at the footpoints of the funnels coupled with the rotation of the white dwarf leads to coherent pulsed x-ray emission. The Polars offer an example of a different accretion topology; the magnetic field of the white dwarf controls the accretion flow from near the inner Lagrangian point of the system directly to the stellar surface. Accretion disks do not form. The strong magnetic coupling generally leads to synchronous orbital/rotational motion in the Polars. The physical system in this sense resembles the Io/Jupiter system. In both IPS and Polars, pulsed emission from the infrared to x-rays is produced as the funneled flows merge onto the white dwarfs through the formation of strong radiating shock waves. A comparative study of the IPS and Polars can elucidate the primary effects of the magnetic fields on the dynamics and thermodynamics in accreting white dwarf systems.

  19. Circumstellar Disks at White Dwarfs: Observations

    OpenAIRE

    Farihi, J.

    2011-01-01

    The current picture painted by the observations of circumstellar dust at white dwarfs, and the consequent atmospheric pollution, is of a surviving planetary system. This chapter recounts in detail both the discovery and empirical characterization of dust disks at single white dwarfs, including all observational data available up to early 2011. Observations of the disks themselves and the accreted heavy elements within the stellar photospheres are consistent with tidally disr...

  20. UBV photometry of hot DA white dwarfs

    International Nuclear Information System (INIS)

    Johnson UBV photometry has been obtained photoelectrically for a set of DA white dwarfs with effective temperatures greater than 20,000 K and for the AM Her type binary HO538 + 608. Most of the white dwarfs lie within existing Einstein IPC or EXOSAT LE soft X-ray fields, therefore they are of interest as potential serendipitous soft X-ray sources. In addition, high dispersion spectroscopy has been used to differentiate seven of these objects to be subdwarfs. 14 refs

  1. UBV photometry of hot DA white dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Kidder, K.M.; Holberg, J.B.; Mason, P.A. (Arizona Univ., Tucson (USA) Louisiana State Univ., Baton Rouge (USA))

    1991-02-01

    Johnson UBV photometry has been obtained photoelectrically for a set of DA white dwarfs with effective temperatures greater than 20,000 K and for the AM Her type binary HO538 + 608. Most of the white dwarfs lie within existing Einstein IPC or EXOSAT LE soft X-ray fields, therefore they are of interest as potential serendipitous soft X-ray sources. In addition, high dispersion spectroscopy has been used to differentiate seven of these objects to be subdwarfs. 14 refs.

  2. Pulsations in white dwarfs: Selected topics

    Directory of Open Access Journals (Sweden)

    Saio H.

    2013-03-01

    Full Text Available This paper presents a very brief overview of the observed properties of g-mode pulsations in variable white dwarfs. We then discuss a few selected topics: Excitation mechanisms (kappa- and convection- mechanisms, and briefly the effect of a strong magnetic field (? 1 MG on g-modes as recently found in a hot DQ (carbon-rich atmosphere white dwarf. In the discussion of excitation mechanisms, a simple interpretation for the convection mechanism is given.

  3. Comparison of theoretical white dwarf cooling timescales

    CERN Document Server

    Salaris, Maurizio; García-Berro, Enrique

    2013-01-01

    An accurate assessment of white dwarf cooling times is paramount to place white dwarf cosmochronology of Galactic populations on more solid grounds. This issue is particularly relevant in view of the enhanced observational capabilities provided by the next generation of Extremely Large Telescopes, that will offer more avenues to employ white dwarfs as probes of Galactic evolution and test-beds of fundamental physics. We estimate for the first time the consistency of results obtained from two independent and widely used evolutionary codes (BaSTI and LPCODE) for white dwarf models with fixed mass and chemical stratification, when the same input physics is employed in both codes. We considered 0.55Msun white dwarf models with both pure carbon and uniform carbon-oxygen (50/50 mass fractions) core. We have assessed for the first time the maximum possible accuracy in the current estimates of white dwarf cooling times, resulting only from the different implementations of the stellar evolution equations and homogeneo...

  4. General Relativistic Calculations for White Dwarf Stars

    CERN Document Server

    Mathew, Arun

    2014-01-01

    Properties of white dwarf stars composed of relativistic degenerate electron gas are studied in general relativity. Mass-Radius relation for white dwarf stars is investigated by solving the Tolman-Oppenheimer-Volkoff equation for hydrostatic equilibrium considering the electron gas as non-interacting. Chandrasekhar's limiting mass of $1.452 M_\\odot$ is slightly modified to $1.397 M_\\odot$ for $^4_2$He (and $^{12}_{\\ 6}$C) white dwarf stars. The critical mass for iron white dwarf stars is also calculated using general relativity, which is found to be $1.204 M_\\odot$. The critical densities for gravitational instability is compared with the neutronization threshold. It is found that $^4_2$He and $^{12}_{\\ 6}$C white dwarf stars are stable aganist neutronization with the critical value of $1.397 M_\\odot$ whereas the critical masses for stability are smaller due to neutronization for $^{16}_{\\ 8}$O, $^{20}_{10}$Ne, $^{24}_{12}$Mg, $^{28}_{14}$Si, $^{32}_{16}$S and $^{56}_{26}$Fe white dwarf stars. The maximum sta...

  5. Possible new class of dense white dwarfs

    International Nuclear Information System (INIS)

    If the strange quark matter hypothesis is true, then a new class of white dwarfs can exist whose nuclear material in their deep interiors can have a density as high as the neutron drip density, a few hundred times the density in maximum-mass white dwarfs and 4x104 the density in dwarfs of mass, M?0.6 Mcircle-dot. Their masses fall in the approximate range 10-4 to 1 Mcircle-dot. They are stable against acoustical modes of vibration. A strange quark core stabilizes these stars, which otherwise would have central densities that would place them in the unstable region of the sequence between white dwarfs and neutron stars. copyright 1995 American Institute of Physics

  6. Possible new class of dense white dwarfs

    International Nuclear Information System (INIS)

    If the strange matter hypothesis of Bodmer and Witten is true, then a new class of white dwarfs can exist whose nuclear material in their deep interiors can have a density as high as the neutron drip density, a few hundred times the density in maximum-mass white dwarfs and 4x104 the density in dwarfs of typical mass, M?0.6Mcircle-dot. Their masses fall in the approximate range 10-4--1Mcircle-dot. They are stable against acoustical modes of vibration. A strange quark core stabilizes these stars, which otherwise would have central densities that would place them in the unstable region of the sequence between white dwarfs and neutron stars

  7. Rare White dwarf stars with carbon atmospheres

    CERN Document Server

    Dufour, P; Fontaine, G; Behara, N

    2007-01-01

    White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 msun and 8-10 msun, where msun is the mass of the Sun (more massive stars end their life as either black holes or neutron stars). The theory of stellar evolution predicts that the majority of white dwarfs have a core made of carbon and oxygen, which itself is surrounded by a helium layer and, for ~80 per cent of known white dwarfs, by an additional hydrogen layer. All white dwarfs therefore have been traditionally found to belong to one of two categories: those with a hydrogen-rich atmosphere (the DA spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we report the discovery of several white dwarfs with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch evolution, althou...

  8. The Search for Brown Dwarfs around White Dwarfs

    OpenAIRE

    Farihi, J.; Becklin, E. E.; Zuckerman, B.

    2002-01-01

    The infrared search for substellar companions to nearby white dwarfs has been going for a little more than a decade. The most recent phase has been a wide field proper motion search carried out primarily at Steward Observatory, where we are complete down to J=18. Earlier phases included near field searches at the IRTF and Keck Observatory. In the last year we have discovered ten previously unrecognized faint proper motion companions. Of the recent discoveries, most are white...

  9. Measuring M dwarf Winds with DAZ White Dwarfs

    CERN Document Server

    Debes, J H

    2006-01-01

    Hydrogen atmosphere white dwarfs with metal lines, so-called DAZs, show evidence for ongoing accretion of material onto their surfaces. Some DAZs are known to have unresolved M dwarf companions, which could account for the observed accretion through a stellar wind. I combine observed Ca abundances of the DAZs with information on the orbital separation of their M dwarf companions to infer the mass loss rate of the M dwarfs. I find that for three of the six known DAZs with M dwarf companions, a stellar wind can plausibly explain the observed accretion on the white dwarfs assuming Bondi-Hoyle accretion of solar abundance stellar winds on the order of 10$^{-14}-10^{-16}\\Msun$ yr$^{-1}$. The rest of the sample have companions with orbits $\\gtorder$ 1~AU, and require companion mass loss rates of $> 10^{-11}\\Msun$ yr$^{-1}$. I conclude that there must be an alternative explanation for accretion of material onto DAZs with widely separated companions. The inferred winds for two of the close binaries are orders of magn...

  10. Brown dwarfs as close companions to white dwarfs

    International Nuclear Information System (INIS)

    The influence of the radiation flux emitted by a white dwarf primary on the evolution of a closely orbiting brown dwarf (BD) companion is investigated. Full stellar evolutionary calculations are presented for both isolated and thermal bath cases, including effects of large variations in the atmospheric grain opacities. High grain opacities significantly increase the radii of the BDs, but the thermal bath does not. The major influence of the thermal bath is to increase substantially the surface temperature and luminosity of the BD at a given age. These results are compared with the observational properties of the possible BD companion of the white dwarf G29-38. Inclusion of both physical effects, high grain opacities and thermal bath, increases the mass range (0.034-0.063 solar masses) of viable models significantly, yet the final determination of whether the object is indeed a BD requires improvements in the observations of the system's properties. 37 refs

  11. A Search for Close Red Dwarf-White Dwarf Binaries

    Science.gov (United States)

    Boyd, Mark R.; Henry, Todd J.; Subasavage, John P.

    2011-08-01

    We propose to observe 59 objects suspected to be red dwarf-white dwarf (RD-WD) binaries with separations BRI plate photometry and 2MASS JHK photometry in accordance with the positions of known RD-WD binaries. This effort will identify candidates for detailed observations as part of the RECONS astrometric program on the CTIO 0.9m to yield accurate parallaxes and photocentric orbits. The parallaxes will then be used to determine the ages of the systems from WD cooling curves, and the orbits will eventually be used to measure dynamical masses. Ultimately, we aim to increase significantly the number of dynamical masses for white dwarfs because currently only three have been determined to 5% accuracy. The first observational step outlined here will allow us to identify appropriate systems for long-term work. This 1.0m project is likely to become the undergraduate senior thesis work of the PI.

  12. Magnetic white dwarfs in binary systems

    International Nuclear Information System (INIS)

    The cataclysmic variables are close binary systems consisting of a late type star and a collapsed star, usually a white dwarf, undergoing mass exchange. According to the standard model, the late type star (the secondary) fills its Roche lobe and material escaping from the inner Lagrangian point is transferred to the primary by means of a mass transfer stream and an accretion disc. Recently a new class of cataclysmic variables has been discovered with distinctive optical properties that are inexplicable in terms of the standard model. These systems known as the AM Herculis type variables have provided the first direct evidence for the presence of strong magnetic fields in the white dwarfs of some cataclysmic variables. A review is presented of some of the important properties of these variables with emphasis on the unique system VV Puppis which has provided the first unequivocal evidence for high harmonic cyclotron radiation from white dwarfs

  13. HST spectrophotometry of accreting white dwarf pulsators

    Science.gov (United States)

    Mukadam, Anjum S.; Szkody, Paula; Gaensicke, Boris T.

    2015-01-01

    The discovery of non-radial pulsations in cataclysmic variables has opened a new venue of opportunity to probe the stellar parameters of accreting variable white dwarfs using asteroseismic techniques. A unique model fit to the observed periods of the variable white dwarf can reveal information about the stellar mass, core composition, age, rotation rate, magnetic field strength, and distance. Mode identification is an essential step in determining an unambiguous model fit, that could be achieved by determining optical and ultra-violet pulsation amplitudes. We will be presenting our results on ultra-violet HST observations acquired with contemporaneous ground based optical data for several cataclysmic variables. The HST spectrophotometry also yields the effective temperatures of the accreting white dwarfs, allowing us to improve our present determination of the instability strip for accreting pulsators. We thank NASA for the grant HST-GO12870 that has supported this research.

  14. 3D Model Atmospheres of White Dwarfs

    Science.gov (United States)

    Tremblay, Pier-Emmanuel; Ludwig, H.; Steffen, M.; Freytag, B.

    2014-01-01

    We present the first grid of 3D model atmospheres for hydrogen-atmosphere (DA) white dwarfs. These CO5BOLD radiation-hydrodynamics simulations, unlike the previous 1D calculations, do not rely on the mixing-length theory for the treatment of convection. The simulations have been employed to compute model spectra and we compared our improved Balmer line profiles to spectroscopic data from the Sloan Digital Sky Survey and the White Dwarf Catalog. The 3D surface gravities are found to be as much as 0.3 dex lower than the values derived from 1D models. The white dwarfs with a radiative and a convective atmosphere have derived mean masses that are the same within 0.01 Msun with our new models, in much better agreement with our understanding of stellar evolution.

  15. Recombination energy in double white dwarf formation

    CERN Document Server

    Nandez, Jose L A; Lombardi, James C

    2015-01-01

    In this Letter we investigate the role of recombination energy during a common envelope event. We confirm that taking this energy into account helps to avoid the formation of the circumbinary envelope commonly found in previous studies. For the first time, we can model a complete common envelope event, with a clean compact double white dwarf binary system formed at the end. The resulting binary orbit is almost perfectly circular. In addition to considering recombination energy, we also show that between 1/4 and 1/2 of the released orbital energy is taken away by the ejected material. We apply this new method to the case of the double-white dwarf system WD 1101+364, and we find that the progenitor system at the start of the common envelope event consisted of a $\\sim1.5M_\\odot$ red giant star in a $\\sim 30$ day orbit with a white dwarf companion.

  16. White dwarfs: connection with masses of the parent stars

    International Nuclear Information System (INIS)

    A relationship between the mass of a white dwarf and the mass of the parent star on the main sequence is established. The white dwarf birth-rate matches the birth-rate (death-rate) of main sequence stars

  17. 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

  18. Discovery of a peculiar DQ white dwarf

    OpenAIRE

    Carollo, D.; Hodgkin, S. T.; Spagna, A.; Smart, R. L.; Lattanzi, M. G.; Mclean, B. J.; Pinfield, D. J.

    2002-01-01

    We report the discovery of a new carbon rich white dwarf that was identified during a proper motion survey for cool white dwarfs based on photographic material used for the construction of the Guide Star Catalog II. Its large proper motion (0.48 arcsec/yr) and faint apparent magnitude (V = 18.7) suggest a nearby object of low luminosity. A low-resolution spectrum taken with the William Herschel Telescope clearly shows strong C2 Deslandres-d'Azambuja and Swan bands, which ide...

  19. 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.

  20. FIRST DIRECT EVIDENCE THAT BARIUM DWARFS HAVE WHITE DWARF COMPANIONS

    International Nuclear Information System (INIS)

    Barium II (Ba) stars are chemically peculiar F-, G-, and K-type objects that show enhanced abundances of s-process elements. Since s-process nucleosynthesis is unlikely to take place in stars prior to the advanced asymptotic giant branch (AGB) stage, the prevailing hypothesis is that each present Ba star was contaminated by an AGB companion which is now a white dwarf (WD). Unless the initial mass ratio of such a binary was fairly close to unity, the receiving star is thus at least as likely to be a dwarf as a giant. So although most known Ba stars appear to be giants, the hypothesis requires that Ba dwarfs be comparably plentiful and moreover that they should all have WD companions. However, despite dedicated searches with the IUE satellite, no WD companions have been directly detected to date among the classical Ba dwarfs, even though some 90% of those stars are spectroscopic binaries, so the contamination hypothesis is therefore presently in some jeopardy. In this paper, we analyze recent deep, near-UV and far-UV Galaxy Evolution Explorer (GALEX) exposures of four of the brightest of the class (HD 2454, 15360, 26367, and 221531), together with archived GALEX data for two newly recognized Ba dwarfs: HD 34654 and HD 114520 (which also prove to be spectroscopic binaries). The GALEX observations of the Ba dwarfs as a group show a significant far-UV excess compared to a control sample of normal F-type dwarfs. We suggest that this ensemble far-UV excess constitutes the fi ensemble far-UV excess constitutes the first direct evidence that Ba dwarfs have WD companions.

  1. Are there spots on magnetic white dwarfs

    International Nuclear Information System (INIS)

    The temporal variation of the circular polarization observed in the magnetic white dwarf G195--19 is interpreted in terms of an oblique rotator model. The peculiar behavior observed in red light strongly suggests the presence of a spot on the surface of this star. A model based on this assumption is found to be consistent with observations

  2. Implications of white dwarf galactic halos

    CERN Document Server

    Adams, F C; Adams, Fred C; Laughlin, Greg

    1996-01-01

    Motivated by recent measurements which suggest that roughly half the mass of the galactic halo may be in the form of white dwarfs, we study the implications of such a halo. We first use current limits on the infrared background light and the galactic metallicity to constrain the allowed initial mass function (IMF) of the stellar population that produced the white dwarfs. The IMF must be sharply peaked about a characteristic mass scale M_C \\approx 2.3 M_\\odot. Since only a fraction of the initial mass of a star is incorporated into the remnant white dwarf, we argue that the mass fraction of white dwarfs in the halo is likely to be 25\\% or less, and that 50\\% is an extreme upper limit. We use the IMF results to place corresponding constraints on the primordial initial conditions for star formation. The initial conditions must be much more homogeneous and skewed toward higher temperatures (T_{\\rm gas} \\sim 200 K) than the conditions which lead to the present day IMF. Next we determine the luminosity function of ...

  3. Electron-cyclotron maser emission from white-dwarf pairs and white-dwarf planetary systems

    OpenAIRE

    Willes, Andrew J.; Wu, Kinwah

    2003-01-01

    By analogy to Jovian radio emissions powered by the electromagnetic interaction between Jupiter and its moons, we propose that close magnetic-nonmagnetic white-dwarf pairs and white-dwarf planetary systems are strong radio sources. A simple model is developed to predict the flux densities of radio emission generated by a loss-cone-driven electron-cyclotron maser. The radio emission from these systems has high brightness temperatures, is highly polarized, and varies on a peri...

  4. The Prototype Dwarf Nova SS Cygni: The White Dwarf Exposed

    Science.gov (United States)

    Myszka, Janine; Sion, E.; Godon, P.

    2010-01-01

    We have carried out a combined Hubble Space Telescope and FUSE FUV spectroscopic analysis of the prototype dwarf nova SS Cygni during quiescence. The FUSE and HST spectra were obtained at comparable times after outburst and have matching flux levels where the two spectra overlap. In our synthetic spectral analysis, we have used SS Cygni's accurate HST FGS parallax giving d = 166 pc, a newly determined mass for the accreting white dwarf (Bitner, Robinson, & Behr, 2007, ApJ) of Mwd = 0.81 Msun (lower than the previous, widely used 1.2 Msun) and a widely adopted reddening of E(B-V) = 0.04. Our best-fit model solution to the combined HST + FUSE spectral energy distribution consists of a hot white dwarf with Teff = 45,000K, log g = 8.3 with a solar composition accreted atmosphere. Accretion disk models matching a distance of 166 pc fit badly in the FUSE range while, if the distance is a free parameter, the only accretion disk model which fits well is for a distance of 600-800 pc and the accretion rate (10-8 Msun/yr!) is unacceptably high for a dwarf nova in quiescence. We discuss the implications of the white dwarf temperature for the time-averaged accretion rate and long term compressional heating models. This work is supported by NSF grant AST08-07892 and NASA ADP grant NNX08AJ39G, both to Villanova University, the Villanova Undergraduate Research Award and partial support from the Delaware Space Grant Consortium.

  5. Pulsating white dwarfs in cataclysmic variables

    Science.gov (United States)

    Mukadam, A. S.; Szkody, P.; Gansicke, B. T.; Warner, B.; Woudt, P.; Sion, E.; Townsley, D.; Bildsten, L.; Solheim, J. E.; Fraser, O.; Haggard, D.

    2005-12-01

    Most cataclysmic variables do not eclipse and are thus difficult to study. With the recent discovery of pulsating white dwarfs in cataclysmic variables (see Warner & van Zyl 1998; van Zyl et al. 2000), we can use the technique of asteroseismology to study these objects. A unique model fit to the observed periods of the pulsating white dwarf contains information about the stellar mass, core composition, age, rotation rate, magnetic field strength, and distance (e.g. Winget et al. 1998). Characterizing such systems will help us grasp the effect of accretion on pulsations, important for all pulsating stars. Our preliminary observations using the 3.5m telescope at Apache Point Observatory suggest the discovery of two new pulsating white dwarfs among the cataclysmic variables of the Sloan Digital Sky Survey: WD0745+4538 and WD2143+1244. Precise measurements of periods requires a long time-base, but our present succinct light curve on WD0745+4538 indicates periods near 1010s and 647s with amplitudes of 6.6% and 5.7% respectively. Our hour-long light curve on WD2143+1244 shows a period at 616s with an amplitude of 0.8%. These periods are within the expected range for non-radial g-mode pulsations in white dwarfs. We have acquired ultra-violet (UV) time resolved spectroscopy of the previously discovered pulsating accreting white dwarfs WD0131-0901, WD1610-0102, and WD2205+1155 (Woudt & Warner 2004; Warner & Woudt 2004) using the Advanced Camera for Surveys mounted on the Hubble Space Telescope (HST). We intend to determine the effective temperature of these variables in order to better define the instability strip for accreting pulsating white dwarfs (Szkody et al. 2002). We also obtained multi-site optical observations of WD1610-0102 from the ground simultaneously with the HST campaign. We hope to utilize the UV and optical amplitudes to attempt a mode identification for the observed periodicities of WD1610-0102.

  6. LTE model atmospheres for accreting white dwarfs

    International Nuclear Information System (INIS)

    The results are presented of self-consistent atmosphere calculations appropriate for the optically thick regions of accretion flows on to white dwarfs, and which can be used to model the observed soft X-ray -EUV spectra of cataclysmic variables. The calculations take into account irradiation by hard X-rays from other parts of the accretion flow, and the pressure of the accretion flow itself. It is shown that radiation pressure limits the effective temperature Tsub(eff) of stable atmospheres to kTsub(eff) < or approx., 20-40 eV, depending on the white dwarf mass: the approximation that the atmosphere is fully ionized and that the Eddington limit applies is shown to be incorrect. (author)

  7. Watch This Space: Observing Merging White Dwarfs

    CERN Document Server

    Webbink, Ronald F

    2010-01-01

    The Laser Interferometer Space Antenna (LISA) will open the low-frequency (0.1-100 mHz) part of the gravitational wave spectrum to direct observation. Of order 3600 galactic close binary white dwarfs will be individually resolvable in its all-sky spectrum, of which a dozen systems are expected to be on the verge of merger, showing the effects of strong tidal heating and/or early onset of tidal mass transfer. Optical study of these systems would provide important insights into tidal dissipation mechanisms, and internal heating in merging white dwarfs that sets ignition conditions for potential type Ia supernovae. Theoretical modeling and instrumentation programs are needed now to enable a campaign for optical identifications to exploit this opportunity.

  8. General Relativistic and Newtonian White Dwarfs

    CERN Document Server

    Boshkayev, Kuantay; Ruffini, Remo; Siutsou, Ivan

    2015-01-01

    The properties of uniformly rotating white dwarfs (RWDs) are analyzed within the framework of Newton's gravity and general relativity. In both cases Hartle's formalism is applied to construct the internal and external solutions to the field equations. The white dwarf (WD) matter is described by the Chandrasekhar equation of state. The region of stability of RWDs is constructed taking into account the mass-shedding limit, inverse $\\beta$-decay instability, and the boundary established by the turning points of constant angular momentum $J$ sequences which separates stable from secularly unstable configurations. We found the minimum rotation period $\\sim0.28$ s in both cases and maximum rotating masses $\\sim1.534 M_{\\odot}$ and $\\sim1.516 M_{\\odot}$ for the Newtonian and general relativistic WDs, respectively. By using the turning point method we show that general relativistic WDs can indeed be axisymmetrically unstable whereas the Newtonian WDs are stable.

  9. General Relativistic and Newtonian White Dwarfs

    Science.gov (United States)

    Boshkayev, K.; Rueda, J. A.; Ruffini, R.; Siutsou, I.

    2015-01-01

    The properties of uniformly rotating white dwarfs (RWDs) are analyzed within the framework of Newton's gravity and general relativity. In both cases Hartle's formalism is applied to construct the internal and external solutions to the field equations. The white dwarf (WD) matter is described by the Chandrasekhar equation of state. The region of stability of RWDs is constructed taking into account the mass-shedding limit, inverse ?-decay instability, and the boundary established by the turning points of constant angular momentum J sequences which separates stable from secularly unstable configurations. We found the minimum rotation period ˜ 0.28 s in both cases and maximum rotating masses ˜ 1.534M? and ˜ 1.516M? for the Newtonian and general relativistic WDs, respectively. By using the turning point method we show that general relativistic WDs can indeed be axisymmetrically unstable whereas the Newtonian WDs are stable.

  10. 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.

  11. Search for Higgs shifts in white dwarfs

    OpenAIRE

    Onofrio, Roberto; Wegner, Gary A.

    2014-01-01

    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 ...

  12. White dwarfs in Be star binary systems

    Science.gov (United States)

    Apparao, K. M. V.

    1991-01-01

    An evaluation is made of possible reasons for the persistent inability to identify white dwarf stars in the Be binary systems. It is noted that many Be stars exhibiting large optical enhancements may be Be + WD and Be + He systems, and that observations of pulsations in the H-alpha emission, as well as observation of time delays between enhancements of optical line and continuum, can identify such systems.

  13. New mass limit of white dwarfs

    OpenAIRE

    Das, Upasana; Mukhopadhyay, Banibrata

    2013-01-01

    Is the Chandrasekhar mass limit for white dwarfs (WDs) set in stone? Not anymore -- recent observations of over-luminous, peculiar type Ia supernovae can be explained if significantly super-Chandrasekhar WDs exist as their progenitors, thus barring them to be used as cosmic distance indicators. However, there is no estimate of a mass limit for these super-Chandrasekhar WD candidates yet. Can they be arbitrarily large? In fact, the answer is no! We arrive at this revelation b...

  14. Cooling Models for Old White Dwarfs addendum

    CERN Document Server

    Hansen, B M S

    1999-01-01

    We present new white dwarf cooling models which incorporate an accurate outer boundary condition based on new opacity and detailed radiative transfer calculations. We find that helium atmosphere dwarfs cool considerably faster than has previously been claimed, while old hydrogen atmosphere dwarfs will deviate significantly from black body appearance. We use our new models to derive age limits for the Galactic disk. We find that the Liebert, Dahn & Monet (1988) luminosity function yields an age of only 6 Gyr if it is complete to stated limits. However, age estimates of individual dwarfs and the luminosity function of Oswalt et al (1995) are both consistent with disk ages as large as dark matter in Galactic halos. We find that previous attempts using inadequate cooling models were too severe and that direct detection limits allow a halo that is 11 Gyr old. If the halo is composed solely of helium atmosphere dwarfs, the lower age limit is only 7.5 Gyr. We also demonstrate the importance of studying the cooli...

  15. White Dwarf Convection Preceding Type Ia Supernovae

    Science.gov (United States)

    Zingale, Michael; Almgren, A. S.; Bell, J. B.; Malone, C. M.; Nonaka, A.; Woosley, S. E.

    2010-01-01

    In the single degenerate scenario for Type Ia supernovae, a Chandrasekhar mass white dwarf `simmers' for centuries preceding the ultimate explosion. During this period, reactions near the center drive convection throughout most of the interior of the white dwarf. The details of this convective flow determine how the first flames in the white dwarf ignite. Simulating this phase is difficult because the flows are highly subsonic. Using the low Mach number hydrodynamics code, MAESTRO, we present 3-d, full star models of the final hours of this convective phase, up to the point of ignition of a Type Ia supernova. We discuss the details of the convective velocity field and the locations of the initial hot spots. Finally, we show some preliminary results with rotation. Support for this work came from the DOE/Office of Nuclear Physics, grant No. DE-FG02-06ER41448 (Stony Brook), the SciDAC Program of the DOE Office of Mathematics, Information, and Computational Sciences under the DOE under contract No. DE-AC02-05CH11231 (LBNL), and the DOE SciDAC program, under grant No. DE-FC02-06ER41438 (UCSC). We made use of the jaguar machine via a DOE INCITE allocation at the Oak Ridge Leadership Computational Facility.

  16. Dark-matter admixed white dwarfs

    CERN Document Server

    Leung, S -C; Lin, L -M; Wong, K -W

    2013-01-01

    We study the equilibrium structures of white dwarfs with dark matter cores formed by non-self-annihilating dark matter DM particles with mass ranging from 1 GeV to 100 GeV, which are assumed to form an ideal degenerate Fermi gas inside the stars. For DM particles of mass 10 GeV and 100 GeV, we find that stable stellar models exist only if the mass of the DM core inside the star is less than O(10^-3) Msun and O(10^-6) Msun, respectively. The global properties of these stars, and in particular the corresponding Chandrasekhar mass limits, are essentially the same as those of traditional white dwarf models without DM. Nevertheless, in the 10 GeV case, the gravitational attraction of the DM core is strong enough to squeeze the normal matter in the core region to densities above neutron drip, far above those in traditional white dwarfs. For DM with particle mass 1 GeV, the DM core inside the star can be as massive as around 0.1 Msun and affects the global structure of the star significantly. In this case, the radiu...

  17. Magnetic Field Evolution in Accreting White Dwarfs

    CERN Document Server

    Cumming, A

    2002-01-01

    We discuss the evolution of the magnetic field of an accreting white dwarf. We first show that the timescale for ohmic decay in the liquid interior is 8 to 12 billion years for a dipole field, and 4 to 6 billion years for a quadrupole field. We then compare the timescales for ohmic diffusion and accretion at different depths in the star, and for a simplified field structure and spherical accretion, calculate the time-dependent evolution of the global magnetic field at different accretion rates. In this paper, we neglect mass loss by classical nova explosions and assume the white dwarf mass increases with time. In this case, the field structure in the outer layers of the white dwarf is significantly modified for accretion rates above the critical rate (1-5) x 10^(-10) solar masses per year. We consider the implications of our results for observed systems. We propose that accretion-induced magnetic field changes are the missing evolutionary link between AM Her systems and intermediate polars. The shorter ohmic ...

  18. Open Science Project in White Dwarf Research

    Science.gov (United States)

    Vornanen, T.

    2013-01-01

    I will propose a new way of advancing white dwarf research. Open science is a method of doing research that lets everyone who has something to say about the subject take part in the problem solving process. Already now, the amount of information we gather from observations, theory and modeling is too vast for any one individual to comprehend and turn into knowledge. And the amount of information just keeps growing in the future. A platform that promotes sharing of thoughts and ideas allows us to pool our collective knowledge of white dwarfs and get a clear picture of our research field. It will also make it possible for researchers in fields closely related to ours (AGB stars, planetary nebulae etc.) to join the scientific discourse. In the first stage this project would allow us to summarize what we know and what we don't, and what we should search for next. Later, it could grow into a large collaboration that would have the impact to, for example, suggest instrument requirements for future telescopes to satisfy the needs of the white dwarf community, or propose large surveys. A simple implementation would be a wiki page for collecting knowledge combined with a forum for more extensive discussions. These would be simple and cheap to maintain. A large community effort on the whole would be needed for the project to succeed, but individual workload should stay at a low level.

  19. The white dwarf luminosity function. I. Statistical errors and alternatives

    CERN Document Server

    Geijo, E M; Isern, J; García-Berro, E

    2006-01-01

    Over the years, several methods have been proposed to compute galaxy luminosity functions, from the most simple ones -counting sample objects inside a given volume- to very sophisticated ones -like the C- method, the STY method or the Choloniewski method, among others. However, only the V/Vmax method is usually employed in computing the white dwarf luminosity function and other methods have not been applied so far to the observational sample of spectroscopically identified white dwarfs. Moreover, the statistical significance of the white dwarf luminosity function has also received little attention and a thorough study still remains to be done. In this paper we study, using a controlled synthetic sample of white dwarfs generated using a Monte Carlo simulator, which is the statistical significance of the white dwarf luminosity function and which are the expected biases. We also present a comparison between different estimators for computing the white dwarf luminosity function. We find that for sample sizes larg...

  20. The WFCAM transit survey and cool white dwarfs

    Directory of Open Access Journals (Sweden)

    Pinfield D.

    2013-04-01

    Full Text Available We present results from our search for cool white dwarfs in the WTS (WFCAM Transit Survey. Repeat observations starting in 2007 allowed to produce deep stacked images in J and measure proper motions. We combine this with deep optical imaging to select cool white dwarf candidates (Teff < 5000?K. About 27 cool white dwarf candidates with proper motions above 0.10 arcsec/yr were identified in one of the fields representing 1/8th of the survey area. Follow-up spectroscopy with the 10.2?m GTC telescope at La Palma confirmed the white dwarf status for all observed candidates. On-going work is being carried out to increase the sample of cool white dwarfs that will allow a more comprehensive study of the thick disk/halo white dwarf population.

  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. Simulation of the white dwarf-white dwarf galactic background in the LISA data

    International Nuclear Information System (INIS)

    Laser Interferometer Space Antenna (LISA) is a proposed mission to detect and study gravitational radiation in the frequency range from 10-4 to 10-1 Hz. In the low part of its frequency band, the LISA data will contain a stochastic signal consisting of an incoherent superposition of hundreds of millions of gravitational wave signals radiated by inspiraling white-dwarf binaries present in our own galaxy. In order to estimate the LISA response to this background, we have simulated a population of white-dwarf binaries recently synthesized by one of us. Our approach relies on an analytic expressions of the LISA Time-Delay Interferometric responses to the gravitational radiation emitted by such systems, and it allows us to implement a computationally efficient and accurate simulation of the background in the LISA data. We find the amplitude of the galactic white-dwarf binary background in the LISA data to be modulated in time with a period of 1 year, reaching a minimum equal to about twice that of the LISA noise for a period of about 2 months around the time when the Sun-LISA direction is roughly oriented towards the Autumn equinox. This modulation means that the galactic white-dwarf background that will be observable by LISA is a cyclostationary random process with a period of 1 year. We summarize the theory of cyclostationary random processes and present the corresponding generalized spectral method needed to characterize such a process in the LISA o characterize such a process in the LISA data. We find that, by measuring the generalized spectral components of the white-dwarf background, LISA will be able to infer properties of the distribution of the white-dwarf binary systems present in our galaxy

  3. The Torino Observatory Parallax Program: White Dwarf Candidates

    OpenAIRE

    Smart, R. L.; Lattanzi, M. G.; Bucciarelli, B.; Massone, G.; Casalegno, R.; Chiumiento, G.; Drimmel, R.; Lanteri, L.; Marocco, F.; Spagna, A.

    2003-01-01

    We present parallax determinations for six white dwarf candidates in the Torino Observatory Parallax Program. The absolute parallaxes are found with precisions at the 2-3 milliarcsecond level. For WD 1126+185 we find a distance incompatible with being a white dwarf, implying an incorrect classification. For WD 2216+484 we find our distance is consistent with a simple DA white dwarf rather than a composite system as previously proposed in the literature. In general it is foun...

  4. Understanding the Cool DA White Dwarf, G29-38

    OpenAIRE

    Kleinman, S. J.; Nather, R. E.; Winget, D. E.; Clemens, J. C.; Bradley, P. A.; Kanaan, A.; Provencal, J. L.; Claver, C. F.; Watson, T. K.; Yanagida, K.; Nitta, A.; Dixson, J. S.; Wood, M. A.; Grauer, A. D.; Hine, B. P.

    1997-01-01

    The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post main sequence evolution, along with their cooling rates, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously b...

  5. GRMHD formulation of highly super-Chandrasekhar magnetized white dwarfs: Stable configurations of non-spherical white dwarfs

    OpenAIRE

    Das, Upasana; Mukhopadhyay, Banibrata

    2014-01-01

    The topic of magnetized super-Chandrasekhar white dwarfs is in the limelight, particularly in the last few years, since our proposal of their existence. By full-scale general relativistic magnetohydrodynamic (GRMHD) numerical analysis, we confirm in this work the existence of stable, highly magnetized, significantly super-Chandrasekhar white dwarfs with mass more than 3 solar mass. While a poloidal field geometry renders the white dwarfs oblate, a toroidal field makes them p...

  6. GRMHD formulation of highly super-Chandrasekhar magnetized white dwarfs: Stable configurations of non-spherical white dwarfs

    CERN Document Server

    Das, Upasana

    2014-01-01

    The topic of magnetized super-Chandrasekhar white dwarfs is in the limelight, particularly in the last few years, since our proposal of their existence. By full-scale general relativistic magnetohydrodynamic (GRMHD) numerical analysis, we confirm in this work the existence of stable, highly magnetized, significantly super-Chandrasekhar white dwarfs with mass more than 3 solar mass. While a poloidal field geometry renders the white dwarfs oblate, a toroidal field makes them prolate retaining an overall quasi-spherical shape, as speculated in our earlier work. These white dwarfs are expected to serve as the progenitors of over-luminous type Ia supernovae.

  7. Pulsations in carbon-atmosphere white dwarfs: A new chapter in white dwarf asteroseismology

    International Nuclear Information System (INIS)

    We present some of the results of a survey aimed at exploring the asteroseismological potential of the newly-discovered carbon-atmosphere white dwarfs. We show that, in certains regions of parameter space, carbon-atmosphere white dwarfs may drive low-order gravity modes. We demonstrate that our theoretical results are consistent with the recent exciting discovery of luminosity variations in SDSS J1426+5752 and some null results obtained by a team of scientists at McDonald Observatory. We also present follow-up photometric observations carried out by ourselves at the Mount Bigelow 1.6-m telescope using the new Mont4K camera. The results of follow-up spectroscopic observations at the MMT are also briefly reported, including the surprising discovery that SDSS J1426+5752 is not only a pulsating star but that it is also a magnetic white dwarf with a surface field near 1.2 MG. The discovery of g-mode pulsations in SDSS J1426+5752 is quite significant in itself as it opens a fourth asteroseismological 'window', after the GW Vir, V777 Her, and ZZ Ceti families, through which one may study white dwarfs.

  8. Search for Higgs Shifts in White Dwarfs

    Science.gov (United States)

    Onofrio, Roberto; Wegner, Gary A.

    2014-08-01

    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. Search for Higgs shifts in white dwarfs

    CERN Document Server

    Onofrio, Roberto

    2014-01-01

    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.

  10. Spectropolarimetric observations of cool DQ white dwarfs

    CERN Document Server

    Vornanen, Tommi; Berdyugin, Andrei

    2013-01-01

    Following our recent discovery of a new magnetic DQ white dwarf (WD) with CH molecular features, we report the results for the rest of the DQ WDs from our survey. We use high signal-to-noise spectropolarimetric data to search for magnetic fields in a sample of 11 objects. One object in our sample, WD1235+422, shows the signs of continuum circular polarization that is similar to some peculiar DQs with unidentified molecular absorption bands, but the low S/N and spectral resolution of these data make more observations necessary to reveal the true nature of this object.

  11. White-dwarf-white-dwarf galactic background in the LISA data

    International Nuclear Information System (INIS)

    LISA (Laser Interferometer Space Antenna) is a proposed space mission, which will use coherent laser beams exchanged between three remote spacecraft to detect and study low-frequency cosmic gravitational radiation. In the low part of its frequency band, the LISA strain sensitivity will be dominated by the incoherent superposition of hundreds of millions of gravitational wave signals radiated by inspiraling white-dwarf binaries present in our own Galaxy. In order to estimate the magnitude of the LISA response to this background, we have simulated a synthesized population that recently appeared in the literature. Our approach relies on entirely analytic expressions of the LISA time-delay interferometric responses to the gravitational radiation emitted by such systems, which allows us to implement a computationally efficient and accurate simulation of the background in the LISA data. We find the amplitude of the galactic white-dwarf binary background in the LISA data to be modulated in time, reaching a minimum equal to about twice that of the LISA noise for a period of about two months around the time when the Sun-LISA direction is roughly oriented towards the Autumn equinox. This suggests that, during this time period, LISA could search for other gravitational wave signals incoming from directions that are away from the galactic plane. Since the galactic white-dwarf background will be observed by LISA not as a stationary but rather as a cyclostationary random process wiher as a cyclostationary random process with a period of 1 yr, we summarize the theory of cyclostationary random processes, present the corresponding generalized spectral method needed to characterize such process, and make a comparison between our analytic results and those obtained by applying our method to the simulated data. We find that, by measuring the generalized spectral components of the white-dwarf background, LISA will be able to infer properties of the distribution of the white-dwarf binary systems present in our Galaxy

  12. The detection of photospheric calcium in a DBA white dwarf

    Science.gov (United States)

    Kenyon, Scott J.; Shipman, Harry L.; Sion, Edward M.; Aannestad, Per A.

    1988-05-01

    The authors report the detection of photospheric calcium absorption lines in the white dwarf star G200-39 (DBAZ4). The abundance of calcium relative to that of hydrogen is approximately solar, a result which lends support to the hypothesis that accretion of interstellar matter is responsible for hybrid composition white dwarfs.

  13. Detection of photospheric calcium in a DBA white dwarf

    International Nuclear Information System (INIS)

    The detection of photospheric calcium absorption lines in the white dwarf star G200-39 (DBAZ4) is reported. The abundance of calcium relative to that of hydrogen is approximately solar, a result which lends support to the hypothesis that accretion of interstellar matter is responsible for hybrid composition white dwarfs. 21 references

  14. Freak waves in white dwarfs and magnetars

    International Nuclear Information System (INIS)

    We report properties of ion acoustic freak waves that propagate in a plasma composed of warm ions and ultrarelativistic electrons and positrons. The dynamics of the nonlinear freak waves is governed by the nonlinear Schrödinger equation. The possible region for the freak waves to exist is defined precisely for typical parameters of white dwarfs and magnetars corona. It is found that for low wave number, the nonlinear ion-acoustic wave packets are structurally stable in magnetars corona than in white dwarfs. However, for large wave numbers the situation is opposite. The critical wave number threshold (kc), which indicates where the modulational instability sets in, is defined for both applications. It is seen that near to kc the freak wave amplitude becomes high, but it decreases whenever we stepped away from kc. For the wave numbers close to kc, the increase of the unperturbed density ratio of positrons-to-electrons (?) would lead to increase the freak wave amplitude, but for larger wave numbers the amplitude decreases with the increase of ?.

  15. 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.

  16. Open Science Project in White Dwarf Research

    CERN Document Server

    Vornanen, Tommi

    2012-01-01

    I will propose a new way of advancing white dwarf research. Open science is a method of doing research that lets everyone who has something to say about the subject take part in the problem solving process. Already now, the amount of information we gather from observations, theory and modelling is too vast for any one individual to comprehend and turn into knowledge. And the amount of information just keeps growing in the future. A platform that promotes sharing of thoughts and ideas allows us to pool our collective knowledge of white dwarfs and get a clear picture of our research field. It will also make it possible for researchers in fields closely related to ours (AGB stars, planetary nebulae etc.) to join the scientific discourse. In the first stage this project would allow us to summarize what we know and what we don't, and what we should search for next. Later, it could grow into a large collaboration that would have the impact to, for example, suggest instrument requirements for future telescopes to sa...

  17. Evolutionary and pulsational properties of white dwarf stars

    CERN Document Server

    Althaus, Leandro G; Isern, Jordi; a-Berro, Enrique Garcí

    2010-01-01

    Abridged. White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. The study of white dwarfs has potential applications to different fields of astrophysics. In particular, they can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, like our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow to use these stars as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. They can be used to constrain fundamental properties of elementary particles such as axions and neutrinos, and to study problems related to the variation of fundamental constants. In this work, we review the essentials of the physics of white dwarf stars. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs ...

  18. The incidence of magnetic fields in cool DZ white dwarfs

    CERN Document Server

    Hollands, Mark; Koester, Detlev

    2015-01-01

    Little is known about the incidence of magnetic fields among the coolest white dwarfs. Their spectra usually do not exhibit any absorption lines as the bound-bound opacities of hydrogen and helium are vanishingly small. Probing these stars for the presence of magnetic fields is therefore extremely challenging. However, external pollution of a cool white dwarf by, e.g., planetary debris, leads to the appearance of metal lines in its spectral energy distribution. These lines provide a unique tool to identify and measure magnetism in the coolest and oldest white dwarfs in the Galaxy. We report the identification of 7 strongly metal polluted, cool (T_eff < 8000 K) white dwarfs with magnetic field strengths ranging from 1.9 to 9.6 MG. An analysis of our larger magnitude-limited sample of cool DZ yields a lower limit on the magnetic incidence of 13+/-4 percent, noticeably much higher than among hot DA white dwarfs.

  19. A magnetic white dwarf in a detached eclipsing binary

    Science.gov (United States)

    Parsons, S. G.; Marsh, T. R.; Gänsicke, B. T.; Schreiber, M. R.; Bours, M. C. P.; Dhillon, V. S.; Littlefair, S. P.

    2013-11-01

    SDSS J030308.35+005444.1 is a close, detached, eclipsing white dwarf plus M dwarf binary which shows a large infrared excess which has been interpreted in terms of a circumbinary dust disc. In this paper, we present optical and near-infrared photometric and spectroscopic data for this system. At optical wavelengths, we observe heated pole caps from the white dwarf caused by accretion of wind material from the main-sequence star on to the white dwarf. At near-infrared wavelengths, we see the eclipse of two poles on the surface of the white dwarf by the main-sequence star indicating that the white dwarf is magnetic. Our spectroscopic observations reveal Zeeman-split emission lines in the hydrogen Balmer series, which we use to measure the magnetic field strength as 8 MG. This measurement indicates that the cyclotron lines are located in the infrared, naturally explaining the infrared excess without the need for a circumbinary dust disc. We also detect magnetically confined material located roughly midway between the two stars. Using measurements of the radial velocity amplitude and rotational broadening of the M star, we constrain the physical parameters of the system, a first for a magnetic white dwarf, and the location of the poles on the surface of the white dwarf. SDSS J030308.35+005444.1 is a pre-cataclysmic variable that will likely evolve into an intermediate polar in ˜1 Gyr.

  20. An unsuccessful search for brown dwarf companions to white dwarf stars

    International Nuclear Information System (INIS)

    A search was carried out for brown dwarf stars in the solar neighbourhood, using the IRAS data base, in connection with the 'missing mass' theory. The paper was presented at the workshop on 'Astrophysics of brown dwarfs', Virginia, USA, 1985. No brown dwarf companions to white dwarf stars were discovered. However it is suggested that a similar search with a more sensitive survey instrument may yield some detections. (U.K.)

  1. A SEARCH FOR ASTEROIDS, MOONS, AND RINGS ORBITING WHITE DWARFS

    International Nuclear Information System (INIS)

    Do white dwarfs host asteroid systems? Although several lines of argument suggest that white dwarfs may be orbited by large populations of asteroids, transits would provide the most direct evidence. We demonstrate that the Kepler mission has the capability to detect transits of white dwarfs by asteroids. Because white-dwarf asteroid systems, if they exist, are likely to contain many asteroids orbiting in a spatially extended distribution, discoveries of asteroid transits can be made by monitoring only a small number of white dwarfs, compatible with Kepler's primary mission, which is to monitor stars with potentially habitable planets. Possible future missions that survey 10 times as many stars with similar sensitivity and minute-cadence monitoring can establish the characteristics of asteroid systems around white dwarfs, such as the distribution of asteroid sizes and semimajor axes. Transits by planets would be more dramatic, but the probability that they will occur is lower. Ensembles of planetary moons and/or the presence of rings around planets can also produce transits detectable by Kepler. The presence of moons and rings can significantly increase the probability that Kepler will discover planets orbiting white dwarfs, even while monitoring only a small number of them.

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

    CERN Document Server

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

    2014-01-01

    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 w...

  3. Search for the coolest white dwarfs in the Galaxy

    CERN Document Server

    Catalan, S; Hodgkin, S; Pinfield, D; Hornillos, D Cristobal

    2010-01-01

    A number of so-called ultra-cool white dwarfs have been detected in different surveys so far. However, based on anecdotal evidence it is believed that most or all of these ultra-cool white dwarfs are low-mass products of binary evolution and thus not representative for the oldest white dwarfs. Their low mass causes relatively high luminosity making them the first cool white dwarfs detected in relatively shallow surveys. Deeper observations are needed for the oldest, high mass white dwarfs with the longest cooling times. We report results of an ongoing project that combines deep IR and optical data. This combination plus proper motion information will allow an unambiguous identification of very cool white dwarfs, since the spectral energy distributions are very different from other types of stellar objects. The atmospheric parameters that can be derived from the spectral energy distributions together with the proper motions inferred from the IR data can be used to construct the white dwarf luminosity functions...

  4. Maximum mass of magnetic white dwarfs

    CERN Document Server

    Paret, D Manreza; Horvath, J E

    2015-01-01

    We revisit in this work the problem of the maximum masses of magnetized White Dwarfs (WD). The impact of a strong magnetic field onto the structure equations is addressed. The pressures become anisotropic due to the presence of the magnetic field and split into a parallel and perpendicular components. We first construct stable solutions of TOV equations for the parallel pressures, and found that physical solutions vanish for the perpendicular pressure when $B \\gtrsim 10^{13}$ G. This fact establishes an upper bound for a magnetic field and the stability of the configurations in the (quasi) spherical approximation. Our findings also indicate that it is not possible to obtain stable magnetized WD with super Chandrasekhar masses because the values of the magnetic field needed for them are higher than this bound. To proceed into the anisotropic regime, we derived structure equations appropriated for a cylindrical metric with anisotropic pressures. From the solutions of the structure equations in cylindrical symme...

  5. Complexity and white-dwarf structure

    International Nuclear Information System (INIS)

    From the low-mass non-relativistic case to the extreme relativistic limit, the density profile of a white dwarf is used to evaluate the CLMC complexity measure [R. Lopez-Ruiz, H.L. Mancini, X. Calbet, Phys. Lett. A 209 (1995) 321]. Similarly to the recently reported atomic case where, by averaging shell effects, complexity grows with the atomic number [C.P. Panos, K.Ch. Chatzisavvas, Ch.C. Moustakidis, E.G. Kyrkou, Phys. Lett. A 363 (2007) 78; A. Borgoo, F. De Proft, P. Geerlings, K.D. Sen, Chem. Phys. Lett. 444 (2007) 186; J. Sanudo, R. Lopez-Ruiz, Int. Rev. Phys. 2 (2008) 223], here complexity grows as a function of the star mass reaching a maximum finite value in the Chandrasekhar limit

  6. New mass limit of white dwarfs

    CERN Document Server

    Das, Upasana

    2013-01-01

    Is the Chandrasekhar mass limit for white dwarfs (WDs) set in stone? Not anymore -- recent observations of over-luminous, peculiar type Ia supernovae can be explained if significantly super-Chandrasekhar WDs exist as their progenitors, thus barring them to be used as cosmic distance indicators. However, there is no estimate of a mass limit for these super-Chandrasekhar WD candidates yet. Can they be arbitrarily large? In fact, the answer is no! We arrive at this revelation by exploiting the flux freezing theorem in observed, accreting, magnetized WDs, which brings in Landau quantization of the underlying electron degenerate gas. This essay presents the calculations which pave the way for the ultimate (significantly super-Chandrasekhar) mass limit of WDs, heralding a paradigm shift 80 years after Chandrasekhar's discovery.

  7. HST Spectroscopy of the Hottest White Dwarfs

    CERN Document Server

    Rauch, Thomas

    2007-01-01

    Spectral analysis needs the observation of lines of successive ionization stages in order to evaluate the ionization equilibrium (of a particular species) which is a sensitive indicator for the effective temperature. Since stars with effective temperatures as high as 100000 K have their flux maximum in the extreme ultraviolet (EUV) wavelength range and due to the high degree of ionization, most of the metal lines are found in the ultraviolet (UV) range. Thus, high-S/N and high-resolution UV spectra are a pre-requisite for a precise analysis. Consequently, we employed the Faint Object Spectrograph (FOS), the Goddard High Resolution Spectrograph (GHRS), and the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST) in order to obtain suitable data. We present state-of-the-art analyses of the hottest (pre-) white dwarfs by means of NLTE model atmospheres which include the metal-line blanketing of all elements from hydrogen to nickel.

  8. TIDAL NOVAE IN COMPACT BINARY WHITE DWARFS

    International Nuclear Information System (INIS)

    Compact binary white dwarfs (WDs) undergoing orbital decay due to gravitational radiation can experience significant tidal heating prior to merger. In these WDs, the dominant tidal effect involves the excitation of outgoing gravity waves in the inner stellar envelope and the dissipation of these waves in the outer envelope. As the binary orbit decays, the WDs are synchronized from outside in (with the envelope synchronized first, followed by the core). We examine the deposition of tidal heat in the envelope of a carbon-oxygen WD and study how such tidal heating affects the structure and evolution of the WD. We show that significant tidal heating can occur in the star's degenerate hydrogen layer. This layer heats up faster than it cools, triggering runaway nuclear fusion. Such 'tidal novae' may occur in all WD binaries containing a CO WD, at orbital periods between 5 minutes and 20 minutes, and precede the final merger by 105-106 years.

  9. REMNANTS OF BINARY WHITE DWARF MERGERS

    Energy Technology Data Exchange (ETDEWEB)

    Raskin, Cody; Scannapieco, Evan; Timmes, F. X. [School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404 (United States); Fryer, Chris; Rockefeller, Gabriel [Los Alamos National Laboratories, Los Alamos, NM 87545 (United States)

    2012-02-10

    We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

  10. Remnants of Binary White Dwarf Mergers

    CERN Document Server

    Raskin, Cody; Fryer, Chris; Rockefeller, Gabriel; Timmes, F X

    2011-01-01

    We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

  11. Uniform period spacings in white dwarf models

    International Nuclear Information System (INIS)

    Asymptotic analysis of the equations of nonradial adiabatic oscillation shows that there is a uniform period spacing for g-modes with the same degree (l) and consecutive values of the radial wave number (n). If modes with the same l but different n are present in a pulsating star, then comparison of the period spacings in appropriate stellar models with period differences between the observed pulsation periods can provide mode identification, and thereby constrain other physical properties of the star. Since the pulsating white dwarfs are g-mode pulsators, those with rich power spectra may have period spacings that are integer multiples of some uniform period interval. The periods of PG1159 show spacings that correspond very closely to theoretical models for modes with l=1 and/or l=3. Since the period spacing of g-modes in DOV stars is sensitive to total stellar mass alone, this property of the period spectrum of PG1159 also strictly constrains its mass to be 0.60+-0.02 Msolar. New photometric observations of the other DOV stars are being undertaken, and progress continues in unravelling the complex lightcurve of the pulsating nucleus of the planetary nebula K1-16. Most compact pulsators are cooler than the DOV stars: the DBV and ZZ Ceti, or DAV, stars. In this paper, the authors calculate the period spacings for some representative DBV and DAV white dwarf models. They then briefly examine the applicability of the analysis of period spacings to the DBV and nalysis of period spacings to the DBV and DAV stars

  12. The Torino Observatory Parallax Program: White Dwarf Candidates

    CERN Document Server

    Smart, R L; Bucciarelli, B; Massone, G; Casalegno, R; Chiumiento, G; Drimmel, R; Lanteri, L; Marocco, F; Spagna, A

    2003-01-01

    We present parallax determinations for six white dwarf candidates in the Torino Observatory Parallax Program. The absolute parallaxes are found with precisions at the 2-3 milliarcsecond level. For WD 1126+185 we find a distance incompatible with being a white dwarf, implying an incorrect classification. For WD 2216+484 we find our distance is consistent with a simple DA white dwarf rather than a composite system as previously proposed in the literature. In general it is found that the published photometric distance is an overestimate of the distance found here.

  13. Comparing Two Mode Identification Techniques for a DB White Dwarf

    Science.gov (United States)

    Nitta, A.; Koester, D.; Chu, D.; Thompson, S. E.; Kepler, S. O.; Kleinman, S. J.; Winget, D. E.; Provencal, J. L.; Castanheira, B. G.

    2013-12-01

    Mode identification is one of the most important steps in the asteroseismological analysis of pulsating stars. For pulsating white dwarf stars, the method using pulsation periods has proven to be quite powerful. But there is another method, using the amplitude of the pulsation modes. The advantage of this method is that it can work on pulsators with small numbers of pulsation modes. Since the majority of white dwarf pulsators show a small number of pulsation modes, they are difficult to analyze using their pulsation periods alone. We have applied the mode identification method using pulsation amplitudes to GD 358, a pulsating white dwarf star previously studied successfully using pulsation periods. Both methods agree.

  14. Kinematics and Velocity Ellipsoid of the Solar Neighborhood White Dwarfs

    CERN Document Server

    Elsanhoury, W H; Abdel-Rahman, H I

    2015-01-01

    To determine the velocity ellipsoid of the solar neighborhood white dwarfs, we use the space velocity components of stars. Two samples of white dwarfs are used, 20 pc and 25 pc samples. Beside the two main samples, the solar velocity and velocity dispersions are calculated for the four subsamples, namely DA, non - DA, hot and cool white dwarfs. Comparison between the results of 20 pc sample and those of 25 pc sample gives good agreement, while the comparison between the other subsamples gives bad agreement. Dependence of the velocity dispersions and solar velocity on the chemical composition and effective temperatures are discussed.

  15. RE 0044+09: A new K dwarf rapid rotator with a white dwarf companion

    Science.gov (United States)

    Kellett, Barry J.; Bromage, Gordon E.; Brown, Alexander; Jeffries, Robin D.; James, David J.; Kilkenny, David; Robb, Russell M.; Wonnacott, David; Lloyd, Christopher; Clayton, C.

    1995-01-01

    We report the discovery of a new K dwarf rapid rotator with a potential white dwarf companion. The white dwarf accounts for over 90% of the observed extreme ultraviolet flux detected from this system. Analysis of ROSAT Wide Field Camera (WFC) and IUE data both suggest a white dwarf temperature of approximately 28,700 K. Optical photometry and the IUE long wavelength prime (LWP) spectrum (with the white dwarf contribution removed) imply that the late-type star has a spectral type of K1-3 V, and a distance of 55 +/- 5 pc. Using this distance, the observed IUE SWP flux, and the best-fit temperature results in a white dwarf radius of 0.0088 solar radius. The estimated white dwarf mass is then approximately 0.91 solar mass; somewhat over-massive compared to field white dwarfs. Optical photometry of the K star reveals a 'spot' modulation period of approximately 10 hr (now observed over 3 yr). However, radial velocity observations have revealed no significant variations. Spectroscopic observations place a low limit on the lithium abundance, but do show rapid rotation with a v sin i of 90 +/- 10 km/s. The K star was detected as a radio source at 3.6 cm (on two occasions) and 6 cm by the Very Large Array (VLA). The most likely evolutionary scenario is that the K star and hot white dwarf from either a wide binary or common proper motion pair with an age of 0.1-0.1 Gyr-consistent with the evolutionary timescale of the white dwarf and the rapid rotation of the K star. However, from the proper motion of the K star, this system does not seem to be associated with any of the known young stellar groups.

  16. RE 0044+09: A New K Dwarf Rapid Rotator with a White Dwarf Companion?

    Science.gov (United States)

    Kellett, Barry J.; Bromage, Gordon E.; Brown, Alexander; Jeffries, Robin D.; James, David J.; Kilkenny, David; Robb, Russell M.; Wonnacott, David; Lloyd, Christopher; Clayton, C.

    1995-01-01

    We report the discovery of a new K dwarf rapid rotator with a potential white dwarf companion. The white dwarf accounts for over 90 percent of the observed extreme ultraviolet flux detected from this system. Analysis of ROSAT Wide Field Camera and IUE data both suggest a white dwarf temperature of approx. 28,700 K. Optical photometry and the IUE LWP spectrum (with the white dwarf contribution removed) imply that the late-type star has a spectral type of K1-3 V, and a distance of 55 +/- 5 pc. Using this distance, the observed IUE SWP flux, and the best-fit temperature results in a white dwarf radius of 0.0088 solar radius. The estimated white dwarf mass is then approx. 0.91 solar mass; somewhat over-massive compared to field white dwarfs. Optical photometry of the K star reveals a 'spot' modulation period of approx. 10 hr (now observed over 3 yr). However, radial velocity observations have revealed no significant variations. Spectroscopic observations place a low limit on the lithium abundance, but do show rapid rotation with a v sin i of 90 +/- 10 km/s. The K star was detected as a radio source at 3.6 cm (on two occasions) and 6 cm by the Very Large Array. The most likely evolutionary scenario is that the K star and hot white dwarf form either a wide binary or common proper motion pair with an age of 0.1-0.2 Gyr - consistent with the evolutionary timescale of the white dwarf and the rapid rotation of the K star. However, from the proper motion of the K star, this system does not seem to be associated with any of the known young stellar groups.

  17. RE 0044+09: A new K dwarf rapid rotator with a white dwarf companion?

    Science.gov (United States)

    Kellett, Barry J.; Bromage, Gordon E.; Brown, Alexander; Jeffries, Robin D.; James, David J.; Kilkenny, David; Robb, Russell M.; Wonnacott, David; Lloyd, Christopher; Clayton, C.

    1995-01-01

    We report the discovery of a new K dwarf rapid rotator with a potential white dwarf companion. The white dwarf accounts for over 90% of the observed extreme ultraviolet flux detected from this system. Analysis of ROSAT Wide Field Camera (WFC) and IUE data both suggest a white dwarf temperature of approximately 28,700 K. Optical photometry and the IUE long wavelength prime (LWP) spectrum (with the white dwarf contribution removed) imply that the late-type star has a spectral type of K1-3 V, and a distance of 55 +/- 5 pc. Using this distance, the observed IUE SWP flux, and the best-fit temperature results in a white dwarf radius of 0.0088 solar radius. The estimated white dwarf mass is then approximately 0.91 solar mass; somewhat over-massive compared to field white dwarfs. Optical photometry of the K star reveals a 'spot' modulation period of approximately 10 hr (now observed over 3 yr). However, radial velocity observations have revealed no significant variations. Spectroscopic observations place a low limit on the lithium abundance, but do show rapid rotation with a v sin i of 90 +/- 10 km/s. The K star was detected as a radio source at 3.6 cm (on two occasions) and 6 cm by the Very Large Array (VLA). The most likely evolutionary scenario is that the K star and hot white dwarf from either a wide binary or common proper motion pair with an age of 0.1-0.1 Gyr-consistent with the evolutionary timescale of the white dwarf and the rapid rotation of the K star. However, from the proper motion of the K star, this system does not seem to be associated with any of the known young stellar groups.

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

    CERN Document Server

    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 context of SNIa progenitors. White dwarf/M dwarf pairs may be sufficiently plentiful to provide an adequate rate of explosions. 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...

  19. THREE NEW ECLIPSING WHITE-DWARF-M-DWARF BINARIES DISCOVERED IN A SEARCH FOR TRANSITING PLANETS AROUND M-DWARFS

    International Nuclear Information System (INIS)

    We present three new eclipsing white-dwarf/M-dwarf binary systems discovered during a search for transiting planets around M-dwarfs. Unlike most known eclipsing systems of this type, the optical and infrared emission is dominated by the M-dwarf components, and the systems have optical colors and discovery light curves consistent with being Jupiter-radius transiting planets around early M-dwarfs. We detail the PTF/M-dwarf transiting planet survey, part of the Palomar Transient Factory (PTF). We present a graphics processing unit (GPU)-based box-least-squares search for transits that runs approximately 8 × faster than similar algorithms implemented on general purpose systems. For the discovered systems, we decompose low-resolution spectra of the systems into white-dwarf and M-dwarf components, and use radial velocity measurements and cooling models to estimate masses and radii for the white dwarfs. The systems are compact, with periods between 0.35 and 0.45 days and semimajor axes of approximately 2 R? (0.01 AU). The M-dwarfs have masses of approximately 0.35 M?, and the white dwarfs have hydrogen-rich atmospheres with temperatures of around 8000 K and have masses of approximately 0.5 M?. We use the Robo-AO laser guide star adaptive optics system to tentatively identify one of the objects as a triple system. We also use high-cadence photometry to put an upper limit on the white-dwarf radius of 0.025 R? (95% confidence025 R? (95% confidence) in one of the systems. Accounting for our detection efficiency and geometric factors, we estimate that 0.08%-0.05%+0.10% (90% confidence) of M-dwarfs are in these short-period, post-common-envelope white-dwarf/M-dwarf binaries where the optical light is dominated by the M-dwarf. The lack of detections at shorter periods, despite near-100% detection efficiency for such systems, suggests that binaries including these relatively low-temperature white dwarfs are preferentially found at relatively large orbital radii. Similar eclipsing binary systems can have arbitrarily small eclipse depths in red bands and generate plausible small-planet-transit light curves. As such, these systems are a source of false positives for M-dwarf transiting planet searches. We present several ways to rapidly distinguish these binaries from transiting planet systems.

  20. Halo White Dwarfs, Thick Disks and a Sanity Check

    CERN Document Server

    Hansen, B M S

    2001-01-01

    The discovery of a population of high proper motion white dwarfs by Oppenheimer et al (2001) has caused a lot of speculation as to the origin of these stars. I show that the age distribution of the white dwarfs offers a kind of sanity check in these discussions. In particular, this population appears to have a similar age distribution to those in the standard, thin disk white dwarf population. This is not what is expected for either the halo or thick disk, which are thought to be old populations. It may indicate a different dynamical origin entirely, or it may be just be the tail of a larger distribution, implying a significantly larger total density in even older white dwarfs.

  1. General Relativistic White Dwarfs and Their Astrophysical Implications

    CERN Document Server

    Boshkayev, Kuantay; Ruffini, Remo; Siutsou, Ivan

    2014-01-01

    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.

  2. Ages of white dwarf-red subdwarf systems

    Directory of Open Access Journals (Sweden)

    Hektor Monteiro

    2006-01-01

    Full Text Available We provide the first age estimates for two recently discovered white dwarf-red subdwarf systems, LHS 193AB and LHS 300AB. These systems provide a new opportunity for linking the reliable age estimates for the white dwarfs to the (measurable metallicities of the red subdwarfs. We have obtained precise photometry in the VJRKCIKCJH bands and spectroscopy covering from 6,000°A to 9,000°A (our spectral coveragefor the two new systems, as well as for a comparison white dwarfmain sequence red dwarf system, GJ 283 AB. Using model grids, we estimate the cooling age as well as temperature, surface gravity, mass, progenitor mass and total lifetimes of the white dwarfs. The results indicate that the two new systems are probably ancient thick disk objects with ages of at least 6-9 gigayears (Gyr.

  3. The Local Population of White Dwarfs within 25 pc

    Science.gov (United States)

    Holberg, Jay B.; Oswalt, Terry D.; Sion, Edward M.

    2015-01-01

    We have extended the detailed survey of the local white dwarf population from 20 pc to 25 pc, effectively doubling the sample volume to now include 231 stars. The present 25 pc has an estimated completeness of 70% (the corresponding 20 pc sample is now 85% complete). The space density of white dwarfs remains at 4.8 ± 0.5 x 10-3 pc-3. There exists a curious excess of single stars in the sample 70% vs 30% in systems with one or more companions. A pronounced apparent deficiency remains between the eleven known Sirius-like systems present in the 20 pc sample and only a single such system presently known in the extended 25 pc sample. Also demonstrated, using explicit individual white dwarf cooling ages, is the feasibility of estimating the white dwarf birth rates over the last ~ 5 Gyr.This work is supported by NSF grant AST-1413537

  4. Direct imaging searches for planets around white dwarf stars

    OpenAIRE

    Burleigh, M.; Hogan, E.; Clarke, F.

    2005-01-01

    White dwarfs are excellent targets for direct imaging searches for extra-solar planets, since they are up to 104 times fainter than their main sequence progenitors, providing a huge gain in the contrast problem. In addition, the orbits of planetary companions that lie beyond the maximum extent of the Red Giant envelope are expected to widen considerably, improving resolution and further encouraging direct detection. We discuss current searches for planetary companions to white dwarfs, includi...

  5. A COMPREHENSIVE SPECTROSCOPIC ANALYSIS OF DB WHITE DWARFS

    International Nuclear Information System (INIS)

    We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 Msun for our sample, with a dispersion of only 0.09 Msun. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M sun) DB white dwarfs. We derive a luminosity function based on a subset of DB white dwarfs identified in the Palomar-Green Survey. We show that 20% of all white dwarfs in the temperature range of interest are DB stars, although the fraction drops to half this value above Teff ? 20,000 K. We also show that the persistence of DB stars with no hydrogen features at low temperatures is difficult to reconcile with a scenario involving accretion from the interstellar medium, often invoked to account for the observed hydrogen abundances in DBA stars. We present evidence for the existence of two different evolutionary channels that produce DB white dwarfs: the standard model where DA stars are transformed into DB stars through the convective dilution of a thin hydrogen layer and a second channel where DB stars retain a helium atmosphere throughout their evolution. We finally demonstrate that the instability strip of pulsating V777 Her white dwarfs contains no non-g V777 Her white dwarfs contains no non-variables, if the hydrogen content of these stars is properly accounted for.

  6. QUIESCENT NUCLEAR BURNING IN LOW-METALLICITY WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Miller Bertolami, Marcelo M.; 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, E-08860 Castelldefels (Spain)

    2013-09-20

    We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich (DA) white dwarfs with very low metallicity progenitors (Z = 0.0001). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main-sequence masses, covering the most usual interval of white dwarf masses—from 0.53 M {sub ?} to 0.83 M {sub ?}. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase in which nuclear reactions are still important and find that nuclear energy sources play a key role during long periods of time, considerably increasing the cooling times from those predicted by standard white dwarf models. In particular, we find that for this metallicity and for white dwarf masses smaller than about 0.6 M {sub ?}, nuclear reactions are the main contributor to the stellar luminosity for luminosities as low as log (L/L {sub ?}) ? –3.2. This, in turn, should have a noticeable impact in the white dwarf luminosity function of low-metallicity stellar populations.

  7. QUIESCENT NUCLEAR BURNING IN LOW-METALLICITY WHITE DWARFS

    International Nuclear Information System (INIS)

    We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich (DA) white dwarfs with very low metallicity progenitors (Z = 0.0001). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main-sequence masses, covering the most usual interval of white dwarf masses—from 0.53 M ? to 0.83 M ?. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase in which nuclear reactions are still important and find that nuclear energy sources play a key role during long periods of time, considerably increasing the cooling times from those predicted by standard white dwarf models. In particular, we find that for this metallicity and for white dwarf masses smaller than about 0.6 M ?, nuclear reactions are the main contributor to the stellar luminosity for luminosities as low as log (L/L ?) ? –3.2. This, in turn, should have a noticeable impact in the white dwarf luminosity function of low-metallicity stellar populations

  8. Evaporation and accretion of extrasolar comets following white dwarf kicks

    Science.gov (United States)

    Stone, Nicholas; Metzger, Brian D.; Loeb, Abraham

    2015-03-01

    Several lines of observational evidence suggest that white dwarfs receive small birth kicks due to anisotropic mass-loss. If other stars possess extrasolar analogues to the Solar Oort cloud, the orbits of comets in such clouds will be scrambled by white dwarf natal kicks. Although most comets will be unbound, some will be placed on low angular momentum orbits vulnerable to sublimation or tidal disruption. The dusty debris from these comets will manifest itself as an IR excess temporarily visible around newborn white dwarfs; examples of such discs may already have been seen in the Helix Nebula, and around several other young white dwarfs. Future observations with the James Webb Space Telescope may distinguish this hypothesis from alternatives such as a dynamically excited Kuiper Belt analogue. Although competing hypotheses exist, the observation that ?15 per cent of young white dwarfs possess such discs, if interpreted as indeed being cometary in origin, provides indirect evidence that low-mass gas giants (thought necessary to produce an Oort cloud) are common in the outer regions of extrasolar planetary systems. Hydrogen abundances in the atmospheres of older white dwarfs can, if sufficiently low, also be used to place constraints on the joint parameter space of natal kicks and exo-Oort cloud models.

  9. Presupernova evolution of accreting white dwarfs with rotation

    CERN Document Server

    Yoon, S C

    2004-01-01

    We discuss the effects of rotation on the evolution of accreting carbon-oxygen white dwarfs, with the emphasis on possible consequences in Type Ia supernova (SN Ia) progenitors. Starting with a slowly rotating white dwarf, we simulate the accretion of matter and angular momentum from a quasi-Keplerian accretion disk. The role of the various rotationally induced hydrodynamic instabilities for the transport of angular momentum inside the white dwarf is investigated. We find that the dynamical shear instability is the most important one in the highly degenerate core. Our results imply that accreting white dwarfs rotate differentially throughout,with a shear rate close to the threshold value for the onset of the dynamical shear instability. As the latter depends on the temperature of the white dwarf, the thermal evolution of the white dwarf core is found to be relevant for the angular momentum redistribution. As found previously, significant rotation is shown to lead to carbon ignition masses well above 1.4 Msun....

  10. DISCOVERY OF AN ULTRAMASSIVE PULSATING WHITE DWARF

    International Nuclear Information System (INIS)

    We announce the discovery of the most massive pulsating hydrogen-atmosphere white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12, 030 ± 210 K WD with a log g =9.08 ± 0.06, which corresponds to a mass of 1.20 ± 0.03 M?. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 to 595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains Type Ia supernovae progenitor systems.

  11. White dwarf cooling and large extra dimensions

    CERN Document Server

    Biesiada, M; Biesiada, Marek; Malec, Beata

    2002-01-01

    Theories of fundamental interactions with large extra dimensions have recently become very popular. Astrophysical bounds from the Sun, red-giants and SN1987a have already been derived by other authors for the theory proposed by Arkani-Hamed, Dimopoulos and Dvali. In this paper we consider G117-B15A pulsating white dwarf (ZZ Ceti star) for which the secular rate at which the period of its fundamental mode increases has been accurately measured and claimed that this mode of G117-B15A is perhaps the most stable oscillation ever recorded in the optical band. 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 theories with large extra dimensions. Within the framework of the theory with large extra dimensions proposed by Arkani-Hamed, Dimopoulos and Dvali we find the lower bound on string comapctification scale which is more stringent than solar or red-giant bounds.

  12. White dwarf cooling and large extra dimensions

    Science.gov (United States)

    Biesiada, Marek; Malec, Beata

    2002-02-01

    Theories of fundamental interactions with large extra dimensions have recently become very popular. Astrophysical bounds from the Sun, red giants, and SN 1987a have already been derived by other authors for the theory proposed by Arkani-Hamed, Dimopoulos, and Dvali. In this paper we consider the G117-B15A pulsating white dwarf (ZZ Ceti star) for which the secular rate at which the period of its fundamental mode increases has been accurately measured and claimed that this mode of G117-B15A is perhaps the most stable oscillation ever recorded in the optical band. 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 theories with large extra dimensions. Within the framework of the theory with large extra dimensions proposed by Arkani-Hamed, Dimopoulos, and Dvali we find the lower bound on string compactification scale Ms>14.3 TeV/c2 which is more stringent than solar or red-giant bounds.

  13. Discovery of an ultramassive pulsating white dwarf

    CERN Document Server

    Hermes, J J; Castanheira, Barbara G; Gianninas, A; Winget, D E; Montgomery, M H; Brown, Warren R; Harrold, Samuel T

    2013-01-01

    We announce the discovery of the most massive pulsating hydrogen-atmosphere (DA) white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log(g) = 9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03 Msun. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425-595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core ...

  14. ORBITAL EVOLUTION OF COMPACT WHITE DWARF BINARIES

    International Nuclear Information System (INIS)

    The newfound prevalence of extremely low mass (ELM, MHe ?) helium white dwarfs (WDs) in tight binaries with more massive WDs has raised our interest in understanding the nature of their mass transfer. Possessing small (Menv ? 10–3 M?) but thick hydrogen envelopes, these objects have larger radii than cold WDs and so initiate mass transfer of H-rich material at orbital periods of 6-10 minutes. Building on the original work of D'Antona et al., we confirm the 106 yr period of continued inspiral with mass transfer of H-rich matter and highlight the fact that the inspiraling direct-impact double WD binary HM Cancri likely has an ELM WD donor. The ELM WDs have less of a radius expansion under mass loss, thus enabling a larger range of donor masses that can stably transfer matter and become a He mass transferring AM CVn binary. Even once in the long-lived AM CVn mass transferring stage, these He WDs have larger radii due to their higher entropy from the prolonged H-burning stage.

  15. Magnetically powered outbursts from white dwarf mergers

    CERN Document Server

    Beloborodov, Andrei M

    2013-01-01

    Merger of a white dwarf binary creates a differentially rotating object which is expected to generate strong magnetic fields. Kinetic energy stored in differential rotation is partially dissipated in the magnetically dominated corona, which forms a hot variable outflow with ejection velocity comparable to $10^9$ cm s$^{-1}$. The outflow should carry significant mass and energy for hours to days, creating an expanding fireball with the following features. (i) The fireball is initially opaque and its internal energy is dominated by the trapped thermal radiation. The stored heat is partially converted to kinetic energy of the flow (through adiabatic cooling) and partially radiated away. (ii) Internal shocks develop in the fireball and increase its radiative output. (iii) A significant fraction of the emitted energy is in the optical band. As a result, a bright optical transient with luminosity $L\\sim 10^{41}-10^{42}$ erg s$^{-1}$ and a characteristic peak duration comparable to 1 day may be expected from the mer...

  16. On general relativistic uniformly rotating white dwarfs

    CERN Document Server

    Boshkayev, Kuantay; Ruffini, Remo; Siutsou, Ivan

    2012-01-01

    Uniformly rotating white dwarfs (RWDs) are analyzed within the framework of general relativity. The Hartle's formalism is applied to construct self-consistently the internal and external solutions to the Einstein equations. The relativistic Feynman-Metropolis-Teller EoS that generalizes the Salpeter's one taking fully into account the finite size of nuclei, the Coulomb interactions as well as electroweak equilibrium in a self-consistent relativistic fashion is used to describe the WD matter. The mass, radius, angular momentum, eccentricity and quadrupole moment of RWDs are calculated as a function of the central density and rotation angular velocity. We construct the region of stability of RWDs taking into account the mass-shedding limit, inverse beta-decay instability, and the boundary established by the turning points of constant angular momentum sequences that separates stable from secularly unstable configurations. We found the minimum rotation periods 0.3, 0.5, 0.7 and 2.2 seconds and maximum masses 1.50...

  17. White dwarf masses in cataclysmic variables

    CERN Document Server

    Wijnen, T P G; Schreiber, M R

    2015-01-01

    The white dwarf (WD) mass distribution of cataclysmic variables (CVs) has recently been found to dramatically disagree with the predictions of the standard CV formation model. The high mean WD mass among CVs is not imprinted in the currently observed sample of CV progenitors and cannot be attributed to selection effects. Two possibilities have been put forward: either the WD grows in mass during CV evolution, or in a significant fraction of cases, CV formation is preceded by a (short) phase of thermal time-scale mass transfer (TTMT) in which the WD gains a sufficient amount of mass. We investigate if either of these two scenarios can bring theoretical predictions and observations into agreement. We employed binary population synthesis models to simulate the present intrinsic CV population. We incorporated aspects specific to CV evolution such as an appropriate mass-radius relation of the donor star and a more detailed prescription for the critical mass ratio for dynamically unstable mass transfer. We also imp...

  18. The Binary White Dwarf LHS 3236

    Science.gov (United States)

    Harris, Hugh C.; Dahn, Conard C.; Dupuy, Trent J.; Canzian, Blaise; Guetter, Harry H.; Hartkopf, William I.; Ireland, Michael J.; Leggett, S. K.; Levine, Stephen E.; Liu, Michael C.; Luginbuhl, Christian B.; Monet, Alice K. B.; Stone, Ronald C.; Subasavage, John P.; Tilleman, Trudy; Walker, Richard L.

    2013-12-01

    The white dwarf LHS 3236 (WD1639+153) is shown to be a double-degenerate binary, with each component having a high mass. Astrometry at the U.S. Naval Observatory gives a parallax and distance of 30.86 ± 0.25 pc and a tangential velocity of 98 km s-1, and reveals binary orbital motion. The orbital parameters are determined from astrometry of the photocenter over more than three orbits of the 4.0 yr period. High-resolution imaging at the Keck Observatory resolves the pair with a separation of 31 and 124 mas at two epochs. Optical and near-IR photometry give a set of possible binary components. Consistency of all data indicates that the binary is a pair of DA stars with temperatures near 8000 and 7400 K and with masses of 0.93 and 0.91 M ? also possible is a DA primary and a helium DC secondary with temperatures near 8800 and 6000 K and with masses of 0.98 and 0.69 M ?. In either case, the cooling ages of the stars are ~3 Gyr and the total ages are Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

  19. Cool DZ white dwarfs in the SDSS

    CERN Document Server

    Koester, D; Gänsicke, B T; Dufour, P

    2011-01-01

    We report the identification of 26 cool DZ white dwarfs that lie across and below the main sequence in the Sloan Digital Sky Survey (SDSS) u-g vs. g-r two-color diagram; 21 of these stars are new discoveries. The sample was identified by visual inspection of all spectra of objects that fall below the main sequence in the two-color diagram, as well as by an automated search for characteristic spectral features over a large area in color space that included the main sequence. The spectra and photometry provided by the SDSS project are interpreted with model atmospheres, including all relevant metals. Effective temperatures and element abundances are determined, while the surface gravity has to be assumed and was fixed at the canonical value of log g = 8. These stars represent the extension of the well-known DZ sequence towards cooler temperatures and fill the gap around Teff = 6500 K present in a previous study. The metal abundances are similar to those in the hotter DZ, but the lowest abundances are missing, p...

  20. Spin and Magnetism of White Dwarfs

    CERN Document Server

    Kissin, Yevgeni

    2015-01-01

    The magnetism and rotation of white dwarf (WD) stars are investigated in relation to a hydromagnetic dynamo operating in the progenitor during shell burning phases. We find that the downward pumping of angular momentum in the convective envelope can, by itself, trigger dynamo action near the core-envelope boundary in an isolated intermediate-mass star. A solar-mass star must receive additional angular momentum following its rotational braking on the main sequence, either by a merger with a planet, or by tidal interaction in a stellar binary. Several arguments point to the outer core as the source for a magnetic field in the WD remnant: i) the outer third of a ~0.55$M_\\odot$ WD is processed during the shell burning phases of the progenitor; ii) escape of magnetic helicity through the envelope mediates the growth of (compensating) helicity in the core, as is needed to maintain a stable magnetic field in the remnant; and iii) intense radiation flux at the core boundary facilitates magnetic buoyancy within a rela...

  1. DISCOVERY OF AN ULTRAMASSIVE PULSATING WHITE DWARF

    Energy Technology Data Exchange (ETDEWEB)

    Hermes, J. J.; Castanheira, Barbara G.; Winget, D. E.; Montgomery, M. H.; Harrold, Samuel T. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Kepler, S. O. [Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS (Brazil); Gianninas, A. [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, OK 73019 (United States); Brown, Warren R., E-mail: jjhermes@astro.as.utexas.edu [Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 (United States)

    2013-07-01

    We announce the discovery of the most massive pulsating hydrogen-atmosphere white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12, 030 {+-} 210 K WD with a log g =9.08 {+-} 0.06, which corresponds to a mass of 1.20 {+-} 0.03 M{sub Sun }. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 to 595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains Type Ia supernovae progenitor systems.

  2. TWO NEW TIDALLY DISTORTED WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Hermes, J. J.; Montgomery, M. H.; Winget, D. E. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Kilic, Mukremin [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019 (United States); Brown, Warren R., E-mail: jjhermes@astro.as.utexas.edu [Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 (United States)

    2012-04-10

    We identify two new tidally distorted white dwarfs (WDs), SDSS J174140.49+652638.7 and J211921.96-001825.8 (hereafter J1741 and J2119). Both stars are extremely low mass (ELM, {<=} 0.2 M{sub Sun }) WDs in short-period, detached binary systems. High-speed photometric observations obtained at the McDonald Observatory reveal ellipsoidal variations and Doppler beaming in both systems; J1741, with a minimum companion mass of 1.1 M{sub Sun }, has one of the strongest Doppler beaming signals ever observed in a binary system (0.59% {+-} 0.06% amplitude). We use the observed ellipsoidal variations to constrain the radius of each WD. For J1741, the star's radius must exceed 0.074 R{sub Sun }. For J2119, the radius exceeds 0.10 R{sub Sun }. These indirect radius measurements are comparable to the radius measurements for the bloated WD companions to A-stars found by the Kepler spacecraft, and they constitute some of the largest radii inferred for any WD. Surprisingly, J1741 also appears to show a 0.23% {+-} 0.06% reflection effect, and we discuss possible sources for this excess heating. Both J1741 and J2119 are strong gravitational wave sources, and the time-of-minimum of the ellipsoidal variations can be used to detect the orbital period decay. This may be possible on a timescale of a decade or less.

  3. The Binary White Dwarf LHS 3236

    CERN Document Server

    Harris, Hugh; Dupuy, Trent; Canzian, Blaise; Guetter, Harry; Hartkopf, William; Ireland, Michael; Leggett, Sandy; Levine, Stephen; Liu, Michael; Luginbuhl, Christian; Monet, Alice; Stone, Ronald; Subasavage, John; Tilleman, Trudy; Walker, Richard

    2013-01-01

    The white dwarf LHS 3236 (WD1639+153) is shown to be a double-degenerate binary, with each component having a high mass. Astrometry at the U.S. Naval Observatory gives a parallax and distance of 30.86 +/- 0.25 pc and a tangential velocity of 98 km/s, and reveals binary orbital motion. The orbital parameters are determined from astrometry of the photocenter over more than three orbits of the 4.0-year period. High-resolution imaging at the Keck Observatory resolves the pair with a separation of 31 and 124 mas at two epochs. Optical and near-IR photometry give a set of possible binary components. Consistency of all data indicates that the binary is a pair of DA stars with temperatures near 8000 and 7400 K and with masses of 0.93 and 0.91 M_solar; also possible, is a DA primary and a helium DC secondary with temperatures near 8800 and 6000 K and with masses of 0.98 and 0.69 M_solar. In either case, the cooling ages of the stars are ~3 Gyr and the total ages are <4 Gyr. The combined mass of the binary (1.66--1....

  4. Orbital Evolution of Compact White Dwarf Binaries

    CERN Document Server

    Kaplan, David L; Steinfadt, Justin D R

    2012-01-01

    The new-found prevalence of extremely low mass (ELM, Mhe<0.2 Msun) helium white dwarfs (WDs) in tight binaries with more massive WDs has raised our interest in understanding the nature of their mass transfer. Possessing small (Menv~1e-3 Msun) but thick hydrogen envelopes, these objects have larger radii than cold WDs and so initiate mass transfer of H-rich material at orbital periods of 6-10 minutes. Building on the original work of D'Antona et al., we confirm the 1e6 yr period of continued inspiral with mass transfer of H-rich matter and highlight that the inspiraling direct-impact double WD binary HM Cancri likely has an ELM WD donor. The ELM WDs have less of a radius expansion under mass loss, thus enabling a larger range of donor masses that can stably transfer matter and become a He mass transferring AM CVn binary. Even once in the long-lived AM CVn mass transferring stage, these He WDs have larger radii due to their higher entropy from the prolonged H burning stage.

  5. A magnetic white dwarf in a detached eclipsing binary

    CERN Document Server

    Parsons, S G; Gänsicke, B T; Schreiber, M R; Bours, M C P; Dhillon, V S; Littlefair, S P

    2013-01-01

    SDSS J030308.35+005444.1 is a close, detached, eclipsing white dwarf plus M dwarf binary which shows a large infrared excess which has been interpreted in terms of a circumbinary dust disk. In this paper we present optical and near-infrared photometric and spectroscopic data for this system. At optical wavelengths we observe heated pole caps from the white dwarf caused by accretion of wind material from the main-sequence star on to the white dwarf. At near-infrared wavelengths we see the eclipse of two poles on the surface of the white dwarf by the main-sequence star, indicating that the white dwarf is magnetic. Our spectroscopic observations reveal Zeeman split emission lines in the hydrogen Balmer series, which we use to measure the magnetic field strength as 8MG. This measurement indicates that the cyclotron lines are located in the infrared, naturally explaining the infrared excess without the need for a circumbinary dust disk. We also detect magnetically-confined material located roughly midway between t...

  6. A precision study of two eclipsing white dwarf plus M dwarf binaries

    CERN Document Server

    Parsons, S G; Gänsicke, B T; Rebassa-Mansergas, A; Dhillon, V S; Littlefair, S P; Copperwheat, C M; Hickman, R D G; Burleigh, M R; Kerry, P; Koester, D; Gómez-Morán, A Nebot; Pyrzas, S; Savoury, C D J; Schreiber, M R; Schmidtobreick, L; Schwope, A D; Steele, P R; Tappert, C

    2011-01-01

    We use a combination of X-shooter spectroscopy, ULTRACAM high-speed photometry and SOFI near-infrared photometry to measure the masses and radii of both components of the eclipsing post common envelope binaries SDSS J1212-0123 and GK Vir. For both systems we measure the gravitational redshift of the white dwarf and combine it with light curve model fits to determine the inclinations, masses and radii. For SDSS J1212-0123 we find a white dwarf mass and radius of 0.439 +/- 0.002 Msun and 0.0168 +/- 0.0003 Rsun, and a secondary star mass and radius of 0.273 +/- 0.002 Msun and 0.306 +/- 0.007 Rsun. For GK Vir we find a white dwarf mass and radius of 0.564 +/- 0.014 Msun and 0.0170 +/- 0.0004 Rsun, and a secondary star mass and radius of 0.116 +/- 0.003 Msun and 0.155 +/- 0.003 Rsun. The mass and radius of the white dwarf in GK Vir are consistent with evolutionary models for a 50,000K carbon-oxygen core white dwarf. Although the mass and radius of the white dwarf in SDSS J1212-0123 are consistent with carbon-oxyge...

  7. 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-center ignition of the underlying WD in the double detonation scenario for Type Ia supernovae.

  8. A Proper Motion Survey For White Dwarfs With WFPC2

    CERN Document Server

    Nelson, C A; Axelrod, T S; Mould, J R; Alcock, C B; Freeman, K C; Nelson, Cailin A.; Cook, Kem H.; Axelrod, Tim S.; Mould, Jeremy R.; Alcock, Charles; Freeman, Ken C.

    2001-01-01

    We have performed a search for halo white dwarfs as high proper motion objects in a second epoch WFPC2 image of the Groth-Westphal strip. The survey covers 74.8 square arcmin, and is complete to V~26.5. We identify 24 high proper motion objects with mu > 0.014''/y. Five of these high proper motion objects are identified as strong white dwarf candidates on the basis of their color, (V-I) < 1.4. We also identify two marginal candidates whose photometric errors are within ~1 sigma of our color cutoff. We create a model of the Milky Way thin disk, thick disk and stellar halo and find that this sample of white dwarfs is clearly an excess above the ~1 detections expected from these known stellar populations. The origin of the excess signal is less clear. Possibly, the excess cannot be explained without invoking a fourth galactic component: a white dwarf dark halo. Previous work of this nature has separated white dwarf samples into various galactic components based on kinematics; distances, and thus velocities, a...

  9. The ELM Survey: Finding the Shortest Period Binary White Dwarfs

    Science.gov (United States)

    Canton, Paul; Gianninas, Alexandros; Kilic, Mukremin; Brown, Warren; Kenyon, Scott

    2014-08-01

    A new discovery space for short period binary white dwarfs has opened up with the availability of 14,600 deg^2 of SDSS Data Release 9 photometry. The Extremely Low-Mass (ELM) Survey takes advantage of this photometry and SDSS spectroscopy to identify compact systems with 1 hour or shorter orbital periods. To significantly increase the number of merging white dwarf systems known, we have proposed to obtain follow- up spectroscopic observations of all candidates with g ? 19 mag and photometric colors consistent with extremely low-mass (? 0.3 M_?) white dwarfs. Most of our 2012A Hale and 2012B KP 4m observing runs were lost to weather, yet we managed to identify at least one new short period binary. Our 2013A run on the KP 4m was successful in identifying many new ELM white dwarfs, and in our 2013B follow-up run we observed two new 3 hour binaries while also obtaining further data on a number of other merging systems. Here we propose to continue our program by observing our fall targets on the KP 4m telescope to constrain their binary orbital periods. Our two major science goals are to discover detached gravitational wave sources for fundamental tests of general relativity, and to constrain the formation rate and space density of merging white dwarfs. The latter is important for constraining the contribution of double degenerates to Type Ia and underluminous supernovae.

  10. A Multi-Survey Approach to White Dwarf Discovery

    CERN Document Server

    Sayres, Conor; Bergeron, P; Dufour, P; Davenport, James R A; AlSayyad, Yusra; Tofflemire, Benjamin M

    2012-01-01

    By selecting astrometric and photometric data from the Sloan Digital Sky Survey (SDSS), the L{\\'e}pine & Shara Proper Motion North Catalog (LSPM-North), the Two Micron All Sky Survey (2MASS), and the USNO-B1.0 catalog, we use a succession of methods to isolate white dwarf candidates for follow-up spectroscopy. Our methods include: reduced proper motion diagram cuts, color cuts, and atmospheric model adherence. We present spectroscopy of 26 white dwarfs obtained from the CTIO 4m and APO 3.5m telescopes. Additionally, we confirm 28 white dwarfs with spectra available in the SDSS DR7 database but unpublished elsewhere, presenting a total of 54 WDs. We label one of these as a recovered WD while the remaining 53 are new discoveries. We determine physical parameters and estimate distances based on atmospheric model analyses. Three new white dwarfs are modeled to lie within 25 pc. Two additional white dwarfs are confirmed to be metal-polluted (DAZ). Follow-up time series photometry confirms another object to be ...

  11. The First Mid-Infrared Spectra of Cool White Dwarfs

    CERN Document Server

    Kilic, Mukremin; Mullally, Fergal; Reach, William T; von Hippel, Ted

    2007-01-01

    We present the first mid-infrared spectra of two cool white dwarfs obtained with the Spitzer Space Telescope. We also present 3.5-8 micron photometry for 19 cool white dwarfs with 5000K < Teff < 9000K. We perform a detailed model atmosphere analysis of these white dwarfs by fitting their UBVRIJHK and Spitzer photometry with state-of-the-art model atmospheres, and demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our mid-IR photometry and 7.5-14.5 micron spectrum of WD0018-267 are consistent with a Teff = 5720K, pure hydrogen white dwarf model atmosphere. On the other hand, LHS 1126 remains peculiar with significant mid-IR flux deficits in all IRAC bands and a featureless spectrum in the 5.2-7.5 micron range. Even though this deficit is attributed to collision induced absorption (CIA) due to molecular hydrogen, the shape of the deficit cannot be explained with current CIA opacity calculations. The infrared portion of th...

  12. The formation of massive white dwarfs in cataclysmic binaries

    International Nuclear Information System (INIS)

    In contrast to the mass spectrum of single white dwarfs which has a single narrow peak at approximately 0.6 solar mass, the observed mass spectrum of white dwarfs of cataclysmic binaries (CB's) shows a rather uniform distribution of the masses in the range approximately 0.3 solar mass to approximately 1.3 solar mass. The formation of CB's with white dwarfs of less than about 0.8 solar mass can be understood as the result of a binary evolution according to low mass Case B or Case C with a subsequent spiraling-in in a common envelope. On the other hand the formation of massive white dwarfs of M approximately > 1 solar mass can be explained as the result of a massive Case B mass transfer yielding a helium star which subsequently undergoes a second Case B mass transfer (so called Case BB evolution). The ultimate product of such an evolution is either a CO-white dwarf with a mass up to the Chandrasekhar limit or a neutron star. The formation of CB's via Case BB evolution requires the binary to undergo at least one, most probably two separate phases of spiraling-in in a common envelope. (Auth.)

  13. A deeply eclipsing detached double helium white dwarf binary

    CERN Document Server

    Parsons, S G; Gänsicke, B T; Drake, A J; Koester, D

    2011-01-01

    Using Liverpool Telescope+RISE photometry we identify the 2.78 hour period binary star CSS 41177 as a detached eclipsing double white dwarf binary with a 21,100K primary star and a 10,500K secondary star. This makes CSS 41177 only the second known eclipsing double white dwarf binary after NLTT 11748. The 2 minute long primary eclipse is 40% deep and the secondary eclipse 10% deep. From Gemini+GMOS spectroscopy we measure the radial velocities of both components of the binary from the H{\\alpha} absorption line cores. These measurements, combined with the light curve information, yield white dwarf masses of M1 = 0.283\\pm0.064Msun and M2 = 0.274\\pm0.034Msun, making them both helium core white dwarfs. As an eclipsing, double-lined spectroscopic binary CSS 41177 is ideally suited to measuring precise, model-independent masses and radii. The two white dwarfs will merge in roughly 1.1 Gyr to form a single sdB star.

  14. A Comprehensive Spectroscopic Analysis of DB White Dwarfs

    CERN Document Server

    Bergeron, P; Dufour, Pierre; Beauchamp, A; Hunter, C; Saffer, Rex A; Gianninas, A; Ruiz, M T; Limoges, M -M; Dufour, Patrick; Fontaine, G; Liebert, James

    2011-01-01

    We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 Mo for our sample, with a dispersion of only 0.09 Mo. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M < 0.5 Mo) DB white dwarfs. We derive a luminosity function based on a subset of DB white dwarfs identified in the Palomar-Green survey. We show that 20% of all white dwarfs in the temperature range of interest are DB stars, although the fraction drops to half this value above Teff ~ 20,000 K. We also show that the persistence of DB stars with no hydrogen features at low temperatures is difficult to reconcile with a scenario involving accretion from the interstellar medium, often invoked to account for the observed hydrogen abundances in DBA stars. W...

  15. Evolutionary calculations of phase separation in crystallizing white dwarf stars

    CERN Document Server

    Montgomery, M H; Winget, D E; Wood, M A

    1999-01-01

    We present an exploration of the significance of Carbon/Oxygen phase separation in white dwarf stars in the context of self-consistent evolutionary calculations. Because phase separation can potentially increase the calculated ages of the oldest white dwarfs, it can affect the age of the Galactic disk as derived from the downturn in the white dwarf luminosity function. We find that the largest possible increase in ages due to phase separation is 1.5 Gyr, with a most likely value of approximately 0.6 Gyr, depending on the parameters of our white dwarf models. The most important factors influencing the size of this delay are the total stellar mass, the initial composition profile, and the phase diagram assumed for crystallization. We find a maximum age delay in models with masses of 0.6 solar masses, which is near the peak in the observed white dwarf mass distribution. We find that varying the opacities (via the metallicity) has little effect on the calculated age delays. In the context of Galactic evolution, a...

  16. Chandra grating spectroscopy of three hot white dwarfs

    CERN Document Server

    Adamczak, J; Rauch, T; Schuh, S; Drake, J J; Kruk, J W

    2012-01-01

    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 (LB 1919) and the other is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD 246). 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. LB 1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD 246 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. (abridged)

  17. The Spectral Evolution of Convective Mixing White Dwarfs, the non-DA Gap, and White Dwarf Cosmochronology

    CERN Document Server

    Chen, Eugene Y

    2012-01-01

    The spectral distribution of field white dwarfs shows a feature called the "non-DA gap". As defined by Bergeron et al., this is a temperature range (5100K--6100K) where relatively few non-DA stars are found, even though such stars are abundant on either side of the gap. It is usually viewed as an indication that a significant fraction of white dwarfs switch their atmospheric compositions back and forth between hydrogen-rich and helium-rich as they cool. In this paper, we present a Monte Carlo model of the Galactic disk white dwarf population, based on the spectral evolution model of Chen and Hansen. We find that the non-DA gap emerges naturally, even though our model only allows white dwarf atmospheres to evolve monotonically from hydrogen-rich to helium-rich through convective mixing. We conclude by discussing the effects of convective mixing on the white dwarf luminosity function and the use thereof for Cosmochronology.

  18. On high proper motion white dwarfs from photographic surveys

    CERN Document Server

    Reylé, C; Creze, M; Reyle, Celine; Robin, Annie C.; Creze, Michel

    2001-01-01

    The interpretation of high proper motion white dwarfs detected by Oppenheimer et al (2001) was the start of a tough controversy. While the discoverers identify a large fraction of their findings as dark halo members, others interpret the same sample as essentially made of disc and/or thick disc stars. We use the comprehensive description of galactic stellar populations provided by the "Besancon" model to produce a realistic simulation of Oppenheimer et al. data, including all observational selections and calibration biases. The conclusion is unambiguous: Thick disc white dwarfs resulting from ordinary hypotheses on the local density and kinematics are sufficient to explain the observed objects, there is no need for halo white dwarfs. This conclusion is robust to reasonable changes in model ingredients. The main cause of the misinterpretation seems to be that the velocity distribution of a proper motion selected star sample is severely biased in favour of high velocities. This has been neglected in previous an...

  19. Atypical Thermonuclear Supernovae from Tidally Crushed White Dwarfs

    International Nuclear Information System (INIS)

    Suggestive evidence has accumulated that intermediate mass black holes (IMBHs) exist in some globular clusters. Some stars will inevitably wander sufficiently close to the hole to suffer a tidal disruption. IMBHs can disrupt not only solar-type stars but also compact white dwarf stars. We investigate the fate of white dwarfs that approach the hole close enough to be disrupted and compressed to such an extent that explosive nuclear burning is triggered. Based on a precise modeling of the gas dynamics together with the nuclear reactions, it is argued that thermonuclear ignition is a natural outcome for white dwarfs of all masses passing well within the tidal radius. A good fraction of the star is accreted, yielding high luminosities that persist for up to a year. A peculiar, underluminous thermonuclear explosion accompanied by a soft X-ray transient signal would, if detected, be a compelling testimony for the presence of an IMBH

  20. Evaporation and Accretion of Extrasolar Comets Following White Dwarf Kicks

    CERN Document Server

    Stone, Nicholas; Loeb, Abraham

    2014-01-01

    Several lines of observational evidence suggest that white dwarfs receive small birth kicks due to anisotropic mass loss. If other stars possess extrasolar analogues to the Solar Oort cloud, the orbits of comets in such clouds will be scrambled by white dwarf natal kicks. Although most comets will be unbound, some will be placed on low angular momentum orbits vulnerable to sublimation or tidal disruption. The dusty debris from these comets will manifest itself as a debris disk temporarily visible around newborn white dwarfs; examples of such disks may already have been seen in the Helix Nebula, and around several other young WDs. Future observations with the James Webb Space Telescope will distinguish this hypothesis from alternatives such as a dynamically excited Kuiper Belt analogue. If interpreted as indeed being cometary in origin, the observation that >15% of young WDs possess such disks provides indirect evidence that low mass gas giants (thought necessary to produce an Oort cloud) are common in the out...

  1. White dwarf post common envelope binaries from the SDSS

    International Nuclear Information System (INIS)

    Post common envelope binaries (PCEBs) consisting of a white dwarf and a main sequence star are ideal systems to calibrate current theories of angular momentum loss in close compact binary stars. The potential held by PCEBs for further development of close binary evolution could so far not be exploited due to the small number of known systems and the inhomogeneity of the sample. The Sloan Digital Sky Survey is changing this scene dramatically, as it is very efficient in identifying white dwarf/main sequence (WDMS) binaries, including both wide systems whose stellar components evolve like single stars and - more interesting in the context of close binary evolution - PCEBs. We pursue a large-scale follow-up survey to identify and characterise the PCEBs among the WDMS binaries that have been found with SDSS. We here present preliminary results of our survey and derive first constraints on current models of white dwarf binary evolution.

  2. The isolated massive DA white dwarf GD 50

    Energy Technology Data Exchange (ETDEWEB)

    Bergeron, P.; Liebert, J.; Kidder, K.M.; Holberg, J.B.; Wesemael, F. (Steward Observatory, Tucson, AZ (USA) Arizona, University, Tucson Montreal, Universite, Montreal (Canada))

    1991-05-01

    Analysis of accurate hydrogen line profiles from optical and ultraviolet spectrophotometry shows that the hot DA white dwarf GD 50 (WD 0346-011) has an exceptionally high surface gravity of log g = 9.0 + or - 0.15; the derived parameters imply a mass of 1.2 solar mass and a radius of 0.0057 solar mass, if an interior composed of carbon and oxygen is assumed. As such, it is the first well-studied, isolated DA white dwarf with a likely mass larger than that of Sirius B. Moreover, the derived mass is large enough to consider the possibility that the interior is composed of oxygen, neon, and magnesium. If GD 50 has evolved as a single object, it should be quite young. Alternatively, the star could have formed as the result of a merger of a close pair of white dwarfs. 48 refs.

  3. The isolated massive DA white dwarf GD 50

    International Nuclear Information System (INIS)

    Analysis of accurate hydrogen line profiles from optical and ultraviolet spectrophotometry shows that the hot DA white dwarf GD 50 (WD 0346-011) has an exceptionally high surface gravity of log g = 9.0 + or - 0.15; the derived parameters imply a mass of 1.2 solar mass and a radius of 0.0057 solar mass, if an interior composed of carbon and oxygen is assumed. As such, it is the first well-studied, isolated DA white dwarf with a likely mass larger than that of Sirius B. Moreover, the derived mass is large enough to consider the possibility that the interior is composed of oxygen, neon, and magnesium. If GD 50 has evolved as a single object, it should be quite young. Alternatively, the star could have formed as the result of a merger of a close pair of white dwarfs. 48 refs

  4. Asteroseismology of pulsating DA white dwarfs with fully evolutionary models

    Directory of Open Access Journals (Sweden)

    Althaus L.G.

    2013-03-01

    Full Text Available We present a new approach for asteroseismology of DA white dwarfs that consists in the employment of a large set of non-static, physically sound, fully evolutionary models representative of these stars. We already have applied this approach with success to pulsating PG1159 stars (GW Vir variables. Our white dwarf models, which cover a wide range of stellar masses, effective temperatures, and envelope thicknesses, are the result of fully evolutionary computations that take into account the complete history of the progenitor stars from the ZAMS. In particular, the models are characterized by self-consistent chemical structures from the centre to the surface, a crucial aspect of white dwarf asteroseismology. We apply this approach to an ensemble of 44 bright DAV (ZZ Ceti stars.

  5. Neutrino Emission from Helium White Dwarfs with Condensed Cores

    CERN Document Server

    Bedaque, Paulo F; Cherman, Aleksey

    2012-01-01

    The possibility that ions in a helium white dwarf star are in a Bose-Einstein condensed state has been explored recently. In particular, it has been argued that the resulting novel quantum liquid has a new kind of quasiparticle excitation with a phonon-like dispersion relation. We investigate the neutrino emission rate due to this gapless state and the resulting impact on the total luminosity of helium white dwarf stars, as a possible observable way of detecting this exotic phase. If the condensation temperature for the quantum liquid state, which is currently not known very precisely, turns out to be high enough, our calculations indicate that neutrino emission due to the gapless mode would make a large contribution to the total luminosity of the helium white dwarf stars.

  6. Understanding the Cool DA White Dwarf, G29-38

    CERN Document Server

    Kleinman, S J; Winget, D E; Clemens, J C; Bradley, P A; Kanaan, A; Provencal, J L; Claver, C F; Watson, T K; Yanagida, K; Nitta, A; Dixson, J S; Wood, M A; Grauer, A D; Hine, B P; Fontaine, G; Liebert, J; Sullivan, D J; Wickramasinghe, D T; Marar, K; Seetha, S; Ashoka, B N; Meistas, E; Leibowitz, E M; Moskalik, P; Krzesínski, J; Solheim, J E; Bruvold, A; Kurtz, D W; Warner, B; Martínez, P; Vauclair, G; Dolez, N; Chevreton, M; Barstow, M A; Kepler, S O; Giovannini, O; Augusteijn, T; Hansen, C J; Kawaler, S D; Liebert, James; Martinez, Peter

    1997-01-01

    The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post main sequence evolution, along with their cooling rates, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously been explored through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning ten years, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modelling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the fir...

  7. Hot white dwarfs, OB subdwarfs and dark clouds

    International Nuclear Information System (INIS)

    A method is described which uses UBV photographic photometry to survey obscured regions in the Galactic plane for hot white dwarfs and OB subdwarfs. From an initial survey of two regions, the first towards the Coalsack and the second covering part of the Scorpius-Centaurus cloud, at least six white dwarfs, one probable interacting binary, five B subdwarfs, and two O subdwarfs have been found. The space densities computed from these, admittedly sparse, statistics are similar to those deduced from Green's (1980) analysis of the Palomar-Green survey and thus a factor of 3 higher than Fleming et al.'s (1986) reanalysis of those data. A space density of about 0.02 stars/cu pc for white dwarfs with M(V) less than or equal to 13.0 is found. 42 references

  8. Hot white dwarfs, OB subdwarfs and dark clouds

    Science.gov (United States)

    Reid, Neill; Wegner, Gary; Wickramasinghe, D. T.; Bessell, M. S.

    1988-07-01

    The authors describe a method of using UBV photographic photometry to survey obscured regions in the Galactic plane for hot white dwarfs and OB subdwarfs. From an initial survey of two regions, the first towards the Coalsack, and the second covering part of the Scorpius-Centaurus cloud, the authors have found at least six white dwarfs, one probable interacting binary, five B subdwarfs, and two O subdwarfs. The space densities computed from these, admittedly sparse, statistics are similar to those deduced from Green's analysis of the Palomar-Green survey and thus a factor of 3 higher than Fleming et al's recent reanalysis of those data. The authors find a space density of ?2.0×10-2stars pc-3 for white dwarfs with MV ? 13.0.

  9. Globular cluster interstellar media: ionized and ejected by white dwarfs

    Science.gov (United States)

    McDonald, I.; Zijlstra, A. A.

    2015-01-01

    Ultraviolet radiation from white dwarfs can efficiently clear Galactic globular clusters (GCs) of their intracluster medium (ICM). This solves the problem of the missing ICM in clusters, which is otherwise expected to build up to easily observable quantities. To show this, we recreate the ionizing flux in 47 Tuc, following randomly generated stars through their asymptotic giant branch (AGB), post-AGB and white dwarf evolution. Each white dwarf can ionize all the material injected into the cluster by stellar winds for ˜3 Myr of its evolution: ˜40 such white dwarfs exist at any point. Every GC's ICM should be ionized. The neutral cloud in M15 should be caused by a temporary overdensity. A pressure-supported ICM will expand over the cluster's tidal radius, where it will be truncated, allowing Jeans escape. The modelled Jeans mass-loss rate approximates the total stellar mass-loss rate, allowing efficient clearing of ICM. Any cluster's ICM mass should equal the mass injected by its stars over the sound travel time between the cluster core and tidal radius. We predict ˜11.3 M? of ICM within 47 Tuc, cleared over ˜4 Myr, compared to a dynamical time-scale of 4.3 Myr. We present a new mass hierarchy, discussing the transition between GCs dwarf galaxies.

  10. Searching for benchmark systems containing ultra-cool dwarfs and white dwarfs

    Directory of Open Access Journals (Sweden)

    Pinfield D.J.

    2013-04-01

    Full Text Available We have used the 2MASS all-sky survey and WISE to look for ultracool dwarfs that are part of multiple systems containing main sequence stars. We cross-matched L dwarf candidates from the surveys with Hipparcos and Gliese stars, finding two new systems. We consider the binary fraction for L dwarfs and main sequence stars, and further assess possible unresolved multiplicity within the full companion sample. This analysis shows that some of the L dwarfs in this sample might actually be unresolved binaries themselves. We have also identified a sample of common proper motion systems in which a main sequence star has a white dwarf as wide companion. These systems can help explore key issues in star evolution theory, as the initial-final mass relationship of white dwarfs, or the chromospheric activity-age relationship for stars still in the main sequence. Spectroscopy for 50 white dwarf candidates, selected from the SuperCOSMOS Science Archive, was obtained. We have also observed 6 of the main sequence star companions, and have estimated their effective temperatures, rotational and microturbulent velocities and metallicities.

  11. Cool White Dwarfs in the Sloan Digital Sky Survey

    OpenAIRE

    Kilic, Mukremin; Munn, Jeffrey A.; Harris, Hugh C.; Liebert, James; Von Hippel, Ted; Williams, Kurtis A.; Metcalfe, Travis S.; Winget, D. E.; Levine, Stephen E.

    2005-01-01

    A reduced proper motion diagram utilizing Sloan Digital Sky Survey (SDSS) photometry and astrometry and USNO-B plate astrometry is used to separate cool white dwarf candidates from metal-weak, high-velocity main sequence Population II stars (subdwarfs) in the SDSS Data Release 2 imaging area. Follow-up spectroscopy using the Hobby-Eberly Telescope, the MMT, and the McDonald 2.7m Telescope is used to demonstrate that the white dwarf and subdwarf loci separate cleanly in the r...

  12. The brightest pure-H ultracool white dwarf

    OpenAIRE

    Catalan, S.; Tremblay, P. -e; Pinfield, D. J.; Smith, L. C.; Zhang, Z. H.; Napiwotzki, R.; Marocco, F.; Day-jones, A. C.; Gomes, J.; Forde, K. P.; Lucas, P. W.; Jones, H. R. A.

    2012-01-01

    We report the identification of LSR J0745+2627 in the United Kingdom InfraRed Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool white dwarf with kinematics and age compatible with the thick-disk/halo population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high reduced proper motion value in the J band (H_J=21.87). We show how the infrared-reduced proper motion diagram is useful for selecting a sample of cool white dwarfs with low co...

  13. A Comprehensive Spectroscopic Analysis of DB White Dwarfs

    OpenAIRE

    Bergeron, P.; Wesemael, F.; Dufour, Pierre; Beauchamp, A.; Hunter, C.; Saffer, Rex A.; Gianninas, A.; Ruiz, M. T.; Limoges, M. -m; Dufour, Patrick; Fontaine, G.; Liebert, James

    2011-01-01

    We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 Mo for our sample, with a dispersion of only 0.09 Mo. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M < 0.5 Mo) DB ...

  14. Testing energy non-additivity in white dwarfs

    CERN Document Server

    Carmona, J M; Gracia-Ruiz, R; Loret, N

    2013-01-01

    We consider a particular effect which can be expected in scenarios of deviations from Special Relativity induced by Planckian physics: the loss of additivity in the total energy of a system of particles. We argue about the necessity to introduce a length scale to control the effects of non-additivity for macroscopic objects and consider white dwarfs as an appropriate laboratory to test this kind of new physics. We study the sensitivity of the mass-radius relation of the Chandrasekhar model to these corrections by comparing the output of a simple phenomenological model to observational data of white dwarfs.

  15. The formation of massive white dwarfs in cataclysmic binaries

    International Nuclear Information System (INIS)

    White dwarfs (WD's) in cataclysmic binaries (CB's) contrast with single white dwarfs in that, while the masses of most single WD's fall into a narrow range of --0.12 M/sub sub solar/, the masses of WD's in CB's are distributed quite uniformly over almost the entire permitted mass range. This makes the discussion of the formation of higher mass (?1 M/sub sub solar/) WD's of particular interest, and may shed some light on the evolution of CB's in general

  16. INFRARED SIGNATURES OF DISRUPTED MINOR PLANETS AT WHITE DWARFS

    International Nuclear Information System (INIS)

    Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T eff ? 3 x 108 g s-1 display a warm infrared excess from orbiting dust; the likely result of a tidally destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 106 g s-1 in the solar system zodiacal cloud and 1010 g s-1 often inferred for debris disks at main-sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

  17. Revisiting some physics issues related to the new mass limit for magnetized white dwarfs

    CERN Document Server

    Das, Upasana

    2013-01-01

    We clarify important physics issues related to the recently established new mass limit for magnetized white dwarfs which is significantly super-Chandrasekhar. The issues include, justification of high magnetic field and the corresponding formation of stable white dwarfs, shape of the white dwarfs under consideration, flux freezing, variation of magnetic field and related currents therein. We also attempt to address the observational connection of such highly magnetized white dwarfs.

  18. Hubble Space Telescope high resolution spectroscoy of the exposed white dwarf in the dwarf nova VW Hydri in quiescence: A rapidly rotating white dwarf

    Science.gov (United States)

    Sion, Edward M.; Huang, Min; Szkody, Paula; Cheng, Fu-Hua

    1995-01-01

    We obtained a far-ultraviolet spectrum of the dwarf nova VW Hyi in quiescence, with the Hubble Space Telescope Goddard High Resolution Spectrograph covering the region of the Si iv lambda(lambda)1393, 1402 resonance doublet. The broad, shallow Si iv doublet feature is fully resolved, has a total equivalent width of 2.8 A, and is the first metal absorption feature to be clearly detected in the exposed white dwarf. Our synthetic spectral analysis, using a model grid constructed with the code TLUSTY, resulted in a reasonable fit to a white dwarf photosphere with T(sub eff) = 22,000 +/- 2000 K, log g = 8.0 +/- 0.3, an approximately solar Si/H abundance, and a rotational velocity, v sin i approximately equal to 600 km/s. This rotation rate, while not definitive because it is based upon just one line transition, is 20% of the Keplerian (breakup) velocity of the white dwarf and hence does not account for the unexpectedly low boundary-layer luminosity inferred from the soft-X-ray/extreme ultra-violet bands where most of the boundary-layer luminosity should be radiated. The predicted boundary-layer luminosity for a 0.6 solar mass white dwarf accreting at the rate 10(exp -10) solar mass/yr and rotating at 600 km/s, corresponding to VW Hyi in quiescence, is 2 x 10(exp 32) ergs/s when proper account is taken of the rotational kinetic energy going into spinning up the white dwarf. If the boundary-layer area is equal to that of the white dwarf, then T(sub bl) = 24,000 K. This is essentially identical to the photspheric luminosity and temperature determined in far-ultraviolet photospheric analyses. If the boundary-layer area is 10(exp -3) of the white dwarf surface area, then T(sub bl) = 136,000 K.

  19. Interferometric Astrometry of the Detached White Dwarf - M Dwarf Binary Feige 24 Using Hubble Space Telescope Fine Guidance Sensor 3: White Dwarf Radius and Component Mass Estimates

    OpenAIRE

    Benedict, G. F.; Mcarthur, B. E.; Franz, O. G.; Wasserman, L. H.; Nelan, E.; Lee, J.; Fredrick, L. W.; Jefferys, W. H.; Altena, W.; Robinson, E. L.; Spiesman, W. J.; Shelus, P. J.; Hemenway, P. D.; Duncombe, R. L.; Story, D.

    2000-01-01

    With HST FGS 3 we have determined a parallax for the white dwarf - M dwarf interacting binary, Feige 24. The white dwarf (DA) component has an effective temperature, T_eff of approximately 56,000 K degrees. A weighted average with past parallax determinations (pi_abs = 14.6 +- 0.4 milliseconds of arc) narrows the range of possible radius values, compared to past estimates. We obtain R_DA = 0.0185 +- 0.0008 R_solar with uncertainty in the temperature and bolometric correction...

  20. Companion of the white dwarf G29-38 as a brown dwarf

    International Nuclear Information System (INIS)

    The white dwarf G29-38 is studied in order to determine whether it has a brown dwarf companion. A variety of spectra of the white dwarf shows it to be an essentially normal DA4V, without spectral peculiarities, that might have arisen from accretion or interaction with a close companion. The energy distribution of the infrared object G29-38B is obtained. The luminosity of the latter is roughly 4 x 10 to the -5th solar, its temperature between 1100 and 1500 K, with a strong preference for the lower half of that range. The radius corresponding to those temperatures, 0.18-0.10 solar, is larger than expected, but interpretation as a brown dwarf still appears most plausible. 22 references

  1. The White Dwarf Population in NGC 1039 (M34) and the White Dwarf Initial-Final Mass Relation

    OpenAIRE

    Rubin, Kate H. R.; Williams, Kurtis A.; Bolte, M.; Koester, Detlev

    2008-01-01

    We present the first detailed photometric and spectroscopic study of the white dwarfs (WDs) in the field of the ~225 Myr old (log tau_cl = 8.35) open cluster NGC 1039 (M34) as part of the ongoing Lick-Arizona White Dwarf Survey. Using wide-field UBV imaging, we photometrically select 44 WD candidates in this field. We spectroscopically identify 19 of these objects as WDs; 17 are hydrogen-atmosphere DA WDs, one is a helium-atmosphere DB WD, and one is a cool DC WD that exhibi...

  2. ULTRACAM observations of two accreting white dwarf pulsators

    CERN Document Server

    Copperwheat, C M; Dhillon, V S; Littlefair, S P; Woudt, P A; Warner, B; Steeghs, D; Gänsicke, B T; Southworth, J

    2008-01-01

    In this paper we present high time-resolution observations of GW Librae and SDSS J161033.64-010223.3 -- two cataclysmic variables which have shown periodic variations attributed to non-radial pulsations of the white dwarf. We observed both these systems in their quiescent states and detect the strong pulsations modes reported by previous authors. The identification of further periodicities in GW Lib is limited by the accretion-driven flickering of the source, but in the case of SDSS 1610 we identify several additional low-amplitude periodicities. In the case of SDSS 1610, there is evidence to suggest that the two primary signals have a different colour dependence, suggesting that they may be different spherical harmonic modes. We additionally observed GW Lib during several epochs following its 2007 dwarf nova outburst: the first time a dwarf nova containing a pulsating white dwarf has been observed in such a state. We do not observe any periodicities, suggesting that the heating of the white dwarf had either ...

  3. 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 ex...

  4. Central stars of planetary nebulae: The white dwarf connection

    CERN Document Server

    Werner, K

    2011-01-01

    This paper is focused on the transition phase between central stars and white dwarfs, i.e. objects in the effective temperature range 100,000 - 200,000 K. We confine our review to hydrogen-deficient stars because the common H-rich objects are subject of the paper by Ziegler et al. in these proceedings. We address the claimed iron-deficiency in PG1159 stars and [WC] central stars. The discovery of new Ne VII and Ne VIII lines in PG1159 stars suggests that the identification of O VII and O VIII lines that are used for spectral classification of [WCE] stars is wrong. We then present evidence for two distinct post-AGB evolutionary sequences for H-deficient stars based on abundance analyses of the He-dominated O(He) stars and the hot DO white dwarf KPD0005+5106. Finally, we report on evidence for an H-deficient post-super AGB evolution sequence represented by the hottest known, carbon/oxygen-atmosphere white dwarf H1504+65 and the recently discovered carbon-atmosphere "hot DQ" white dwarfs.

  5. Binary Star Origin of High Field Magnetic White Dwarfs

    CERN Document Server

    Tout, C A; Liebert, J; Ferrario, L; Pringle, J E

    2008-01-01

    White dwarfs with surface magnetic fields in excess of $1 $MG are found as isolated single stars and relatively more often in magnetic cataclysmic variables. Some 1,253 white dwarfs with a detached low-mass main-sequence companion are identified in the Sloan Digital Sky Survey but none of these is observed to show evidence for Zeeman splitting of hydrogen lines associated with a magnetic field in excess of 1MG. If such high magnetic fields on white dwarfs result from the isolated evolution of a single star then there should be the same fraction of high field white dwarfs among this SDSS binary sample as among single stars. Thus we deduce that the origin of such high magnetic fields must be intimately tied to the formation of cataclysmic variables. CVs emerge from common envelope evolution as very close but detached binary stars that are then brought together by magnetic braking or gravitational radiation. We propose that the smaller the orbital separation at the end of the common envelope phase, the stronger ...

  6. The frequency of planetary debris around young white dwarfs

    CERN Document Server

    Koester, Detlev; Farihi, Jay

    2014-01-01

    (Abridged) We present the results of the first unbiased survey for metal pollution among H-atmosphere (DA) white dwarfs with cooling ages of 20-200 Myr and 17000K 0.8 Msun is found to be currently accreting, which suggests a large fraction are double-degenerate mergers, and the merger discs do not commonly reform large planetesimals or otherwise pollute the remnant. We reconfirm our previous finding that two white dwarf Hyads are currently accreting rocky debris. At least 27%, and possibly up to ~50%, of all white dwarfs with cooling ages 20-200 Myr are accreting planetary debris. At Teff > 23000K, the luminosity of white dwarfs is likely sufficient to vaporize circumstellar dust, and hence no stars with strong metal-pollution are found. However, planetesimal disruption events should occur in this cooling age and Teff range as well, and likely result in short phases of high mass transfer rates. It appears that the formation of rocky planetary material is common around 2-3 Msun late B- and A-type stars.

  7. Rapidly accreting white dwarfs as supernova type Ia progenitors

    CERN Document Server

    Lepo, Kelly

    2013-01-01

    The nature of the progenitors of type Ia supernovae is still a mystery. While plausible candidates are known for both the single degenerate and double degenerate models, the observed numbers fall significantly short of what is required to reproduce the type Ia supernovae rate. Some of the most promising single-degenerate type Ia progenitors are recurrent novae and super-soft sources (SSS). White dwarfs with higher mass transfer rates can also be type Ia supernova progenitors. For these rapidly accreting white dwarfs (RAWD), more material than is needed for steady burning accretes on the white dwarf, and extends the white dwarf's photosphere. Unlike super-soft sources, such objects will likely not be detectable at soft X-ray energies, but will be bright at longer wavelengths, such as the far ultraviolet (UV). Possible examples include LMC N66 and the V Sagittae stars. We present a survey using multi-object spectrographs looking for RAWD in the central core of the SMC, from objects selected to be bright in the ...

  8. The Chandrasekhar's Equation for Two-Dimensional Hypothetical White Dwarfs

    CERN Document Server

    De, Sanchari

    2014-01-01

    In this article we have extended the original work of Chandrasekhar on the structure of white dwarfs to the two-dimensional case. Although such two-dimensional stellar objects are hypothetical in nature, we strongly believe that the work presented in this article may be prescribed as Master of Science level class problem for the students in physics.

  9. White dwarfs constraints on dark sector models with light particles

    International Nuclear Information System (INIS)

    The white dwarf luminosity function is well understood in terms of standard model physics and leaves little room for exotic cooling mechanisms related to the possible existence of new weakly interacting light particles. This puts significant constraints on the parameter space of models that contain a massive dark photon and light dark sector particles

  10. The brightest pure-H ultracool white dwarf

    CERN Document Server

    Catalan, S; Pinfield, D J; Smith, L C; Zhang, Z H; Napiwotzki, R; Marocco, F; Day-Jones, A C; Gomes, J; Forde, K P; Lucas, P W; Jones, H R A

    2012-01-01

    We report the identification of LSR J0745+2627 in the United Kingdom InfraRed Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool white dwarf with kinematics and age compatible with the thick-disk/halo population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high reduced proper motion value in the J band (H_J=21.87). We show how the infrared-reduced proper motion diagram is useful for selecting a sample of cool white dwarfs with low contamination. LSR J0745+2627 is also detected in the Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer (WISE). We have spectroscopically confirmed this object as a cool white dwarf using X-Shooter on the Very Large Telescope. A detailed analysis of its spectral energy distribution reveals that its atmosphere is compatible with a pure-H composition model with an effective temperature of 3880+-90 K. This object is the brightest pure-H ultracool white dwarf (Teff<4000 K) ever identified. We have constrained the dis...

  11. Bose-Einstein condensation in helium white dwarf stars. I

    Energy Technology Data Exchange (ETDEWEB)

    Mosquera, M.E. [Faculty of Astronomy and Geophysics, University of La Plata, Paseo del Bosque s.n., La Plata (Argentina); Department of Physics, University of La Plata, c.c. 67 1900, La Plata (Argentina); Civitarese, O., E-mail: osvaldo.civitarese@fisica.unlp.edu.a [Department of Physics, University of La Plata, c.c. 67 1900, La Plata (Argentina); Benvenuto, O.G.; De Vito, M.A. [Faculty of Astronomy and Geophysics, University of La Plata, Paseo del Bosque s.n., La Plata (Argentina); Instituto de Astrofisica La Plata, CCT (Argentina)

    2010-01-18

    The formation of a Bose-Einstein condensate in the interior of helium white dwarfs stars is discussed. Following the proposal made by Gabadadze and Rosen, we have explored the consequences of such a mechanism by calculating the cooling time of the stars. We have found that it is shorter than the value predicted by the standard model.

  12. Bose-Einstein condensation in helium white dwarf stars. I

    International Nuclear Information System (INIS)

    The formation of a Bose-Einstein condensate in the interior of helium white dwarfs stars is discussed. Following the proposal made by Gabadadze and Rosen, we have explored the consequences of such a mechanism by calculating the cooling time of the stars. We have found that it is shorter than the value predicted by the standard model.

  13. White dwarfs constraints on dark sector models with light particles

    Energy Technology Data Exchange (ETDEWEB)

    Ubaldi, Lorenzo [Physikalisches Institut der Universität Bonn, Nussallee 12, D-53115 Bonn (Germany)

    2014-06-24

    The white dwarf luminosity function is well understood in terms of standard model physics and leaves little room for exotic cooling mechanisms related to the possible existence of new weakly interacting light particles. This puts significant constraints on the parameter space of models that contain a massive dark photon and light dark sector particles.

  14. Hot accreting white dwarfs in the quasi-static approximation

    International Nuclear Information System (INIS)

    Properties of white dwarfs which are accreting hydrogen-rich matter at rates in the range 1.5 x 10-9 to 2.5 x 10-7 M/sub sun/ yr-1 are investigated in several approximations. Steady-burning models, in which matter is processed through nuclear-burning shells as rapidly as it is accreted, provide a framework for understanding the properties of models in which thermal pulses induced by hydrogen burning and helium burning are allowed to occur. In these latter models, the underlying carbon-oxygen core is chosen to be in a cycle-averaged steady state with regard to compressional heating and neutrino losses. Several of these models are evolved in the quasi-static approximation. Combining results obtained in the steady-burning approximation with those obtained in the quasi-static approximation, expressions are obtained for estimating, as functions of accretion rate and white dwarf mass, the thermal pulse recurrence period and the duration of hydrogen-burning phases. The time spent by an accreting model burning hydrogen as a large star of giant dimensions versus time spent burning hydrogen as a hot dwarf is also estimated as a function of model mass and accretion rate. Finally, suggestions for detecting observational counterparts of the theoretical models and suggestions for further theoretical investigations are offered. Subject headings: stars: accretion: stars: interiors: stars: novae: stars: symbiotic: stars: white dwarfsic: stars: white dwarfs

  15. Diffusion and hydrogen shell burning on slowly accreting white dwarfs

    International Nuclear Information System (INIS)

    The stability of hydrogen shell burning in accreted envelopes on white dwarfs is examined for the domain of low accretion rates. Account is taken of the effects of diffusion of CNO nuclei attributable to sedimentation associated with gravity and temperature gradients. Due to the direct competition with accretion, the effects of such sedimentation of CNO elements become important only for accretion rates smaller than M = 10-11 M/sub sun/ yr-1, for hydrogen shell burning in steady state approximation. Such a steady state is stable, for white dwarfs of mass -12 M/sub sun/ yr-1; this results from the weak temperature dependences of the proton-proton cycles and the relatively weak degeneracy at the burning shell. Reduction of CNO abundance levels due to sedimentation acts to enlarge this accretion rate range by approximately a factor of 2. The stable steady state is realized only when gas is accreted onto a hot white dwarf. For accretion onto cool white dwarfs, the stable regime is bypassed by recurrent shell flashes. In this latter case, the concentration of CNO elements will not be greatly affected by sedimentation because of its low efficiency at the low temperatures characterizing the accretion phase

  16. Lithium production in the merging of white dwarf stars

    CERN Document Server

    Longland, Richard; José, Jordi; García-Berro, Enrique; Althaus, Leandro G

    2012-01-01

    The origin of R Coronae Borealis stars has been elusive for over 200 years. Currently, two theories for their formation have been presented. These are the Final Flash scenario, in which a dying asymptotic giant branch (AGB) star throws off its atmosphere to reveal the hydrogen poor, heavily processed material underneath, and the double degenerate scenario, in which two white dwarfs merge to produce a new star with renewed vigour. Some theories predict that the temperatures reached during the latter scenario would destroy any lithium originally present in the white dwarfs. The observed lithium content of some R Coronae Borealis stars, therefore, is often interpreted as an indication that the Final Flash scenario best describes their formation. In this paper, it is shown that lithium production can, indeed, occur in the merging of a helium white dwarf with a carbon-oxygen white dwarf if their chemical composition, particularly that of 3He, is fully considered. The production mechanism is described in detail, an...

  17. Population synthesis for symbiotic stars with white dwarf accretors

    CERN Document Server

    Lu, G; Han, Z; Lu, Guoliang; Yungelson, Lev; Han, Zhanwen

    2006-01-01

    We have carried out a detailed study of symbiotic stars with white dwarf accretors by means of a population synthesis code. We estimate the total number of symbiotic stars with white dwarf accretors in the Galaxy as 1,200 - 15,000. This range is compatible with observational estimates. Two crucial physical parameters that define the birthrate and number of symbiotic stars are the efficiency of accretion by white dwarfs (which greatly depends on the separation of components after common envelope stage and stellar wind velocity) and the mass of the hydrogen layer which the white dwarf can accumulate prior to the hydrogen ignition. The theoretical estimate of the Galactic occurrence rate of symbiotic novae ranges from about 1.3 to about 13.5 per year out of which weak symbiotic novae comprise about 0.5 to 6.0 per year, depending on the model assumptions. We simulate the distributions of symbiotic stars over orbital periods, masses of components, mass-loss rates of cool components, mass-accretion rates of hot com...

  18. FIRST DETECTION OF KRYPTON AND XENON IN A WHITE DWARF

    International Nuclear Information System (INIS)

    We report on the first detection of the noble gases krypton (Z = 36) and xenon (54) in a white dwarf. About 20 Kr VI- 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 ± 0.5 and log Xe = –4.2 ± 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 that the observed metal abundances constrain the evolutionary history of the star. Its hydrogen deficiency may be the consequence of a late helium-shell flash or a binary white dwarf merger.

  19. FIRST DETECTION OF KRYPTON AND XENON IN A WHITE DWARF

    Energy Technology Data Exchange (ETDEWEB)

    Werner, Klaus; Rauch, Thomas; Ringat, Ellen [Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University Tuebingen, Sand 1, 72076 Tuebingen (Germany); Kruk, Jeffrey W. [NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)

    2012-07-01

    We report on the first detection of the noble gases krypton (Z = 36) and xenon (54) in a white dwarf. About 20 Kr VI- 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 {+-} 0.5 and log Xe = -4.2 {+-} 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 that the observed metal abundances constrain the evolutionary history of the star. Its hydrogen deficiency may be the consequence of a late helium-shell flash or a binary white dwarf merger.

  20. 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.

  1. 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. The result is in accordance with predictions from nonadiabatic stellar pulsation models. Such models are therefore reliable tools to investigate the interior structure of GW Vir variables. Conclusions. Our soft X-ray study reveals that the understanding of metal abundances in hot DA white dwarf atmospheres is still incomplete. On the other hand, model atmospheres of hydrogen-deficient PG1159-type stars are reliable and reproduce well the observed spectra from soft X-ray to optical wavelengths.

  2. The White Dwarf Luminosity Function: The Shape of Things to Come

    CERN Document Server

    Von Hippel, T; Munn, J A; Harris, H; Williams, K; Liebert, J; Winget, D E; Metcalfe, T S; Shipman, H; Wood, M A; Oswalt, T; Kleinman, S; Kleinman, A N

    2004-01-01

    We describe a new survey for cool white dwarfs that supplements Sloan Digital Sky Survey photometry with USNO proper motions and follow-up spectroscopy. To date we have discovered and spectroscopically confirmed 80 new moderate temperature and cool white dwarfs. We have also found a handful of high-velocity white dwarfs and we expect a sizable fraction of these to be thick disk or possibly halo objects. Our survey is designed to find ~10,000 new white dwarfs, although only ~60 will be among the faintest white dwarfs (M(V) >= 16), where most of the age-sensitivity resides. We discuss an extension of our survey to V ~ 22.

  3. Nova explosion of mass-accreting white dwarfs

    International Nuclear Information System (INIS)

    The evolution of mass-accreting white dwarfs has been computed from the onset of accretion through nova explosion. Two cases have been considered: a carbon-oxygen white dwarf of 1.3 Msub(solar mass) with the accretion rate of 1.0 x 10-10 Msub(solar mass) yr-1 and a helium white dwarf of 0.4 Msub(solar mass) with 1.0 x 10-8 Msub(solar mass) yr-1. Because the thermal evolution during the accretion phase has been fully taken into consideration in computation, the mass of the accreted hydrogen-rich envelope and the corresponding temperature distribution in the envelope have been determined. When a certain amount of the hydrogen-rich envelope has been formed, a hydrogen shell flash commences. The flashing shell lies in the midway between the bottom of the envelope and the stellar surface. In the case of the 1.3 Msub(solar mass) white dwarf, the flash has been found to grow strong enough to lead to a nova-like explosion, even for the normal abundance of CNO elements. The main driving mechanism of this explosion was a rapid transition of the envelope from white dwarf to supergiant characteristics. In order to compute such non-linear effects accurately we need very fine mesh points for the shells just overlying the burning shell in particular. Its importance is shown both by analyzing the stellar structure equation and by comparing our results with those obtained by other authors. (author)

  4. Brown Dwarfs, White Knights and Demons

    OpenAIRE

    Gyuk, G.; Evans, N. W.; Gates, E. I.

    1998-01-01

    This paper investigates the hypothesis that the lensing objects towards the Large Magellanic Cloud (LMC) are brown dwarfs by analysing the effects of velocity anisotropy on the inferred microlensing masses. To reduce the masses, the transverse velocity of the lenses with respect to the microlensing tube must be minimised. In the outer halo, radial anisotropy is best for doing this; closer to the solar circle, azimuthal anisotropy is best. By using a constraint on the total k...

  5. The frequency of planetary debris around young white dwarfs

    Science.gov (United States)

    Koester, D.; Gänsicke, B. T.; Farihi, J.

    2014-06-01

    Context. Heavy metals in the atmospheres of white dwarfs are thought in many cases to be accreted from a circumstellar debris disk, which was formed by the tidal disruption of a rocky planetary body within the Roche radius of the star. The abundance analysis of photospheric elements and conclusions about the chemical composition of the accreted matter are a new and promising method of studying the composition of extrasolar planetary systems. However, ground-based searches for metal-polluted white dwarfs that rely primarily on the detection of the Ca ii K line become insensitive at Teff > 15 000 K because this ionization state depopulates. Aims: We present the results of the first unbiased survey for metal pollution among hydrogen-atmosphere (DA type) white dwarfs with cooling ages in the range 20-200 Myr and 17 000 K board the Hubble Space Telescope in the far ultraviolet range between 1130 and 1435 Å. The atmospheric parameters were obtained using these spectra and optical observations from the literature. Element abundances were determined using theoretical models, which include the effects of element stratification due to gravitational settling and radiative levitation. Results: We find 48 of the 85 DA white dwarfs studied, or 56% show traces of heavy elements. In 25 stars (showing only Si and occasionally C), the elements can be explained by radiative levitation alone, although we argue that accretion has very likely occurred recently. The remaining 23 white dwarfs (27%), however, must be currently accreting. Together with previous studies from the ground and adopting bulk Earth abundances for the debris, accretion rates range from a few 105 g s-1 to a few 108 g s-1, with no evident trend in cooling age from ?40 Myr to ?2 Gyr. Only a single, modest case of metal pollution (? 23 000 K, in excellent agreement with the absence of infrared excess from dust around these warmer stars. The median, main sequence progenitor of our sample corresponds to an A-type star of ?2 M?, and we find 13 of 23 white dwarfs descending from main sequence 2-3 M?, late B- and A-type stars to be currently accreting. Only one of 14 targets with Mwd > 0.8 M? is found to be currently accreting, which suggests a large fraction of these stars result from double-degenerate mergers, and the merger disks do not commonly reform large planetesimals or otherwise pollute the remnant. We reconfirm our previous finding that two 625 Myr Hyades white dwarfs are currently accreting rocky planetary debris. Conclusions: At least 27% of all white dwarfs with cooling ages 20-200 Myr are accreting planetary debris, but that fraction could be as high as ?50%. At Teff > 23 000 K, the luminosity of white dwarfs is probably sufficient to vaporize circumstellar dust grains, so no stars with strong metal-pollution are found. Planetesimal disruption events should occur in this cooling age and temperature range as well, and they are likely to result in short phases of high mass-transfer rates. It appears that the formation of rocky planetary material is common around 2-3 M? late B- and A-type stars. Table 1 is available in electronic form at http://www.aanda.org

  6. Spitzer IRAC Observations of White Dwarfs. II. Massive Planetary and Cold Brown Dwarf Companions to Young and Old Degenerates

    CERN Document Server

    Farihi, J; Zuckerman, B

    2008-01-01

    This paper presents a sensitive and comprehensive IRAC 3-8 $\\mu$m photometric survey of white dwarfs for companions in the planetary mass regime with temperatures cooler than the known T dwarfs. The search focuses on descendants of intermediate mass stars with $M\\ga3$ $M_{\\odot}$ whose inner, few hundred AU regions cannot be probed effectively for massive planets and brown dwarfs by any alternative existing method. Furthermore, examination for mid-infrared excess explores an extensive range of orbital semimajor axes, including the intermediate 5-50 AU range poorly covered and incompletely accessible by other techniques at main sequence or evolved stars. Three samples of white dwarfs are chosen which together represent relatively young as well as older populations of stars: 9 open cluster white dwarfs, 22 high mass field white dwarfs, and 17 metal-rich field white dwarfs. In particular, these targets include: 7 Hyads and 4 field white dwarfs of similar age; 1 Pleiad and 19 field white dwarfs of similar age; va...

  7. A stellar prominence in the white dwarf/red dwarf binary QS Vir: evidence for a detached system

    OpenAIRE

    Parsons, S. G.; Marsh, T. R.; Ga?nsicke, B. T.; Tappert, C.

    2010-01-01

    Using high resolution UVES spectra of the eclipsing Post Common Envelope Binary QS Vir we detect material along the line of sight to the white dwarf at orbital phase $\\phi=0.16$. We ascribe this to a stellar prominence originating from the M dwarf secondary star which passes in front of the white dwarf at this phase. This creates sharp absorption features in the hydrogen Balmer series and Ca II H and K lines. The small size of the white dwarf allows us to place tight constra...

  8. Thermogalvanomagnetic phenomena in neutron stars and white dwarf stars

    International Nuclear Information System (INIS)

    The electron thermal and electrical conductivities and thermoelectric coefficient are found and the induction and heat transfer equations are presented for the degenerate cores of white dwarfs and degenerate layers of the envelopes of neutron stars with the density rho11 g/cm3 in the presence of nonquantizing magnetic field. The scattering of electrons on ions and on phonons and impurities is taken into account. The thermogalvanomagnetic phenomena which can take place im neutron stars and white dwarfs are discussed: the drift of the magnetic field towards the surface or into the interior by the heat flow emerging from the star, various mechanisms of nonuniform heating connected with the magnetic field. It is pointed out that in degenerate stars some mechanisms of inhomogeneous heat release cause by magnetic field can take place

  9. The low mass white dwarf companion to Beta Crateris

    CERN Document Server

    Burleigh, M R; Schenker, K J; Sills, A I; Wynn, G A; Dobbie, P D; Good, S A

    2001-01-01

    We present FUSE H Lyman series spectroscopy of the hot white dwarf companion to the 4th magnitude A1III star Beta Crateris, which shows that is has an unusually low mass, 0.43Msun, and has almost certainly evolved through binary interaction. This system could be a long-sought remnant of Algol-type evolution, although radial velocity measurements appear to show that the pair are not close. Instead, micro-variations in Beta Crt's proper motion as measured by Hipparcos suggests that the period could be as high as 10 years. However, a low mass white dwarf in a system with a period >3 years is difficult to explain by conventional models for binary evolution. We speculate on alternative models for the evolution of this system which involve an eccentric binary or multiple components.

  10. Testing common envelopes on double white dwarf binaries

    CERN Document Server

    Nandez, Jose L A; Lombardi, James C

    2015-01-01

    The formation of a double white dwarf binary likely involves a common envelope (CE) event between a red giant and a white dwarf (WD) during the most recent episode of Roche lobe overflow mass transfer. We study the role of recombination energy with hydrodynamic simulations of such stellar interactions. We find that the recombination energy helps to expel the common envelope entirely, while if recombination energy is not taken into account, a significant fraction of the common envelope remains bound. We apply our numerical methods to constrain the progenitor system for WD 1101+364 -- a double WD binary that has well-measured mass ratio of $q=0.87\\pm0.03$ and an orbital period of 0.145 days. Our best-fit progenitor for the pre-common envelope donor is a 1.5 $M_\\odot$ red giant.

  11. The metal content of hot DA white dwarf spectra

    CERN Document Server

    Dickinson, Nathan

    2012-01-01

    A study of high ion metal absorption features present in the spectra of hot DA white dwarfs is presented. An analysis of three DAs is performed, where previous studies came to conflicting conclusions as to the stars' nitrogen configurations. The nitrogen abundances were found to be in keeping with DAs of higher Teff, with a homogeneous distribution. A search for circumstellar gas discs was performed on eight stars, where circumstellar pollution may explain the differences between predicted and observed metal abundances. No positive detections were made. Already the subject of previous studies, the circumstellar absorption features seen at many hot DAs were again analysed, using a more advanced technique than those implemented in previous studies. This allowed, for the first time, column density measurements for all non-photospheric absorbing material. The derived column density measurements are consistent with those predicted to exist in white dwarf Stromgren Spheres, and the velocities of the absorbing mater...

  12. DA WHITE DWARFS OBSERVED IN THE LAMOST PILOT SURVEY

    International Nuclear Information System (INIS)

    A total of ?640, 000 objects from the LAMOST pilot survey have been publicly released. In this work, we present a catalog of DA white dwarfs (DAWDs) from the entire pilot survey. We outline a new algorithm for the selection of white dwarfs (WDs) by fitting Sérsic profiles to the Balmer H?, H?, and H? lines of the spectra, and calculating the equivalent width of the Ca II K line. Two thousand nine hundred sixty-four candidates are selected by constraining the fitting parameters and the equivalent width of the Ca II K line. All the spectra of candidates are visually inspected. We identify 230 DAWDs (59 of which are already included in the Villanova and SDSS WD catalogs), 20 of which are DAWDs with non-degenerate companions. In addition, 128 candidates are classified as DAWDs/subdwarfs, which means the classifications are ambiguous. The result is consistent with the expected DAWD number estimated based on the LEGUE target selection algorithm

  13. Quantum gases and white dwarfs with quantum gravity

    International Nuclear Information System (INIS)

    This paper addresses the effect of a generalized uncertainty principle produced by different approaches of quantum gravity within the Planck scale on statistical and thermodynamical properties of ideal fermion and boson gases. The partition function and some thermodynamical properties are investigated. The Bose–Einstein condensation and the ground state properties of fermion gases are also considered. The target approach is extended to a white dwarf as an application. The modified mass-radius relation is calculated. A decrease in the pressure of degenerate fermions due to the presence of quantum gravity leads to a contraction in the star radius. It is also found that the gravity background does not result in any change in white dwarf stability. (paper)

  14. Infrared Signatures of Disrupted Minor Planets at White Dwarfs

    CERN Document Server

    Farihi, J; Zuckerman, B

    2009-01-01

    Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T 3e8 g/s display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 1e6 g/s in the solar system zodiacal cloud and 1e10 g/s often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

  15. Evidence for helium in the magnetic white dwarf GD229

    CERN Document Server

    Jordan, S; Becken, W; Schweizer, W

    1998-01-01

    The nature of the strong absorption features in the white dwarf GD229 has been a real mystery ever since it was found to be magnetic in 1974. All attempts to explain the spectrum by line components of hydrogen failed. With the first sets of newly calculated line data for HeI in a strong magnetic field we could identify most of the absorption structures in the spectrum with stationary line components in a range of magnetic fields between 300 and 700 MG. This is much lower than previously speculated and is comparable to the highest fields found in hydrogen rich magnetic white dwarfs. The reason for the large number of spectral features in GD229 is the extreme accumulation of stationary components of transitions in He I in a narrow interval of field strengths.

  16. Weak magnetic fields in white dwarfs and their direct progenitors?

    CERN Document Server

    Jordan, S; Landstreet, J; Fossati, L; Valyanin, G G; Monin, D; Wade, G A; Werner, K; O'Toole, S J

    2012-01-01

    We have carried out a re-analysis of polarimetric data of central stars of planetary nebulae, hot subdwarfs, and white dwarfs taken with FORS1 (FOcal Reducer and low dispersion Spectrograph) on the VLT (Very Large Telescope), and added a large number of new observations in order to increase the sample. A careful analysis of the observations using only one wavelength calibration for the polarimetrically analysed spectra and for all positions of the retarder plate of the spectrograph is crucial in order to avoid spurious signals. We find that the previous detections of magnetic fields in subdwarfs and central stars could not be confirmed while about 10% of the observed white dwarfs have magnetic fields at the kilogauss level.

  17. Merging Binary Stars and the magnetic white dwarfs

    CERN Document Server

    Briggs, Gordon P; Tout, Christopher A; Wickramasinghe, Dayal T; Hurley, Jarrod R

    2014-01-01

    A magnetic dynamo driven by differential rotation generated when stars merge can explain strong fields in certain classes of magnetic stars, including the high field magnetic white dwarfs (HFMWDs). In their case the site of the differential rotation has been variously proposed to be within a common envelope, the massive hot outer regions of a merged degenerate core or an accretion disc formed by a tidally disrupted companion that is subsequently incorporated into a degenerate core. We synthesize a population of binary systems to investigate the stellar merging hypothesis for observed single HFMWDs. Our calculations provide mass distribution and the fractions of white dwarfs that merge during a common envelope phase or as double degenerate systems in a post common envelope phase. We vary the common envelope efficiency parameter alpha and compare with observations. We find that this hypothesis can explain both the observed incidence of magnetism and the mass distribution of HFMWDs for a wide range of alpha. In ...

  18. Spitzer Surveys of IR Excesses of White Dwarfs

    CERN Document Server

    Chu, Y -H; Bilíkovà, J; Riddle, A; Su, K Y -L

    2010-01-01

    IR excesses of white dwarfs (WDs) can be used to diagnose the presence of low-mass companions, planets, and circumstellar dust. Using different combinations of wavelengths and WD temperatures, circumstellar dust at different radial distances can be surveyed. The Spitzer Space Telescope has been used to search for IR excesses of white dwarfs. Two types of circumstellar dust disks have been found: (1) small disks around cool WDs with Teff 100,000 K. The small dust disks are within the Roche limit, and are commonly accepted to have originated from tidally crushed asteroids. The large dust disks, at tens of AU from the central WDs, have been suggested to be produced by increased collisions among Kuiper Belt-like objects. In this paper, we discuss Spitzer IRAC surveys of small dust disks around cool WDs, a MIPS survey of large dust disks around hot WDs, and an archival Spitzer survey of IR excesses of WDs.

  19. The white dwarf cooling sequence of 47 Tucanae

    CERN Document Server

    García-Berro, Enrique; Althaus, Leandro G; Bertolami, Marcelo M Miller

    2014-01-01

    47 Tucanae is one of the most interesting and well observed and theoretically studied globular clusters. This allows us to study the reliability of our understanding of white dwarf cooling sequences, to confront different methods to determine its age, and to assess other important characteristics, like its star formation history. Here we present a population synthesis study of the cooling sequence of the globular cluster 47 Tucanae. In particular, we study the distribution of effective temperatures, the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. We do so using an up-to-date population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. We find a good agreement between our theoretical models and the observed data. Thus, our study, rules out previous claims that there are still missing physics in the white dwarf cooling models at moderately high e...

  20. Low heat conduction in white dwarf boundary layers?

    CERN Document Server

    Liu, F K; Meyer-Hofmeister, E; Burwitz, V

    2008-01-01

    X-ray spectra of dwarf novae in quiescence observed by Chandra and XMM-Newton provide new information on the boundary layers of their accreting white dwarfs. Comparison of observations and models allows us to extract estimates for the thermal conductivity in the accretion layer and reach conclusions on the relevant physical processes. We calculate the structure of the dense thermal boundary layer that forms under gravity and cooling at the white dwarf surface on accretion of gas from a hot tenuous ADAF-type coronal inflow. The distribution of density and temperature obtained allows us to calculate the strength and spectrum of the emitted X-ray radiation. They depend strongly on the values of thermal conductivity and mass accretion rate. We apply our model to the dwarf nova system VW Hyi and compare the spectra predicted for different values of the thermal conductivity with the observed spectrum. We find a significant deviation for all values of thermal conductivity that are a sizable fraction of the Spitzer c...

  1. A New Small-Amplitude Variable Hot DQ White Dwarf

    OpenAIRE

    Dunlap, B. H.; Barlow, B. N.; Clemens, J. C.

    2010-01-01

    We present the discovery of photometric variations in the carbon-dominated atmosphere (hot DQ) white dwarf star SDSS J133710.19-002643.6. We find evidence for two low-amplitude, harmonically-related periodicities at 169 s and 339 s, making it the fastest and smallest amplitude of the four known hot DQ variables and the only variable whose spectrum suggests the presence of hydrogen. The star's fundamental and harmonic amplitudes are roughly equal, and its pulse shape is simil...

  2. A Progress Report on the Carbon Dominated Atmosphere White Dwarfs

    OpenAIRE

    Dufour, P.; Liebert, J.; Swift, B.; Fontaine, G.; Sukhbold, T.

    2009-01-01

    Recently, Dufour et al. (2007) reported the unexpected discovery that a few white dwarfs found in the Sloan Digital Sky Survey had an atmosphere dominated by carbon with little or no trace of hydrogen and helium. Here we present a progress report on these new objects based on new high signal-to-noise follow-up spectroscopic observations obtained at the 6.5m MMT telescope on Mount Hopkins, Arizona.

  3. Heavy element abundance patterns in hot DA white dwarfs

    OpenAIRE

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

    2003-01-01

    We present a series of systematic abundance measurements for 25 hot DA white dwarfs in the temperature range ~20000-110000K, based on far-UV spectroscopy with STIS/GHRS on HST, IUE and FUSE. Using our latest heavy element blanketed non-LTE stellar atmosphere calculations we have addressed the heavy element abundance patterns making completely objective measurements of abundance values and upper limits using a 2 fitting technique to determine the uncertainties in the abundanc...

  4. Magnetic Accreting White Dwarfs in the XMM-Newton Era

    Science.gov (United States)

    de Martino, D.; Bonnet-Bidaud, J.-M.; Falanga, M.; Masetti, N.; Matt, G.; Mouchet, M.; Mukai, K.

    2010-07-01

    Magnetic Cataclysmic Variables (MCVs) are the brightest X-ray sources among accreting white dwarf binaries. They were recently found to constitute a non-negligible fraction of galactic hard (>20 keV) X-ray sources suggesting a still hidden but potentially important population. XMM-Newton allowed to identify the true nature of many new CV candidates, to infer unexpected X-ray properties and their link with fundamental parameters of the accreting primaries.

  5. Merging Binary Stars and the magnetic white dwarfs

    OpenAIRE

    Briggs, Gordon P.; Ferrario, Lilia; Tout, Christopher A.; Wickramasinghe, Dayal T.; Hurley, Jarrod R.

    2014-01-01

    A magnetic dynamo driven by differential rotation generated when stars merge can explain strong fields in certain classes of magnetic stars, including the high field magnetic white dwarfs (HFMWDs). In their case the site of the differential rotation has been variously proposed to be within a common envelope, the massive hot outer regions of a merged degenerate core or an accretion disc formed by a tidally disrupted companion that is subsequently incorporated into a degenerat...

  6. Astrometric determination of white dwarf radial velocities with Gaia?

    OpenAIRE

    Jordan, Stefan; Bruijne, Jos

    2012-01-01

    Usually, the determination of radial velocities of stars relies on the shift of spectral lines by the Doppler effect. Russel & Atkinson (1931) and Oort (1932) already noted that due to the large proper motion and parallax of the white dwarf (WD) van Maanen 2, a determination of the perspective acceleration of the proper motion would provide a direct astrometric determination of the radial velocity which is independent of the gravitational redshift. If spectroscopic redshift ...

  7. S. Chandrasekhar: White Dwarfs, $H^-$ ion,.., Black holes

    CERN Document Server

    Gupta, Patrick Das

    2011-01-01

    This is a concise review, addressed to undergraduate students, of S. Chandrasekhar's oeuvre in astrophysics, ranging from his early studies on white dwarfs using relativistic quantum statistics to topics as diverse as dynamical friction, negative hydrogen ion, fluid dynamical instabilities, black holes and gravitational waves. The exposition is based on simple physical explanations in the context of observational astronomy. Black holes and their role as central engines of active, compact, high energy sources have been discussed.

  8. White dwarf stars as strange quark matter detectors

    International Nuclear Information System (INIS)

    We show that the presence of a strange matter core inside a white dwarf (WD) star produces a drastic change in the spectrum of non-radial oscillations in the range of periods corresponding to gravity modes. The distinctive, observable signal for such a core is a very short period spacing between consecutive modes, far shorter than in the case of pulsating WDs without any compact core. (letter to the editor)

  9. The white dwarf luminosity function – I. Statistical errors and alternatives

    OpenAIRE

    Geijo, Enrique M.; Torres Gil, Santiago; Isern Vilaboy, Jordi; Garci?a-berro Montilla, Enrique

    2005-01-01

    The white dwarf luminosity function is an important tool for the study of the solar neighbourhood, since it allows the determination of the age of the Galactic disc. Over the years, several methods have been proposed to compute galaxy luminosity functions, from the most simple ones – counting sample objects inside a given volume – to very sophisticated ones – like the C- method, the STY method or the Choloniewski method, among others. However, only the 1/Vmax method is usually employed ...

  10. Weak magnetic fields in white dwarfs and their direct progenitors?

    OpenAIRE

    Jordan, S.; Bagnulo, S.; Landstreet, J.; Fossati, L.; Valyanin, G. G.; Monin, D.; Wade, G. A.; Werner, K.; O Toole, S. J.

    2012-01-01

    We have carried out a re-analysis of polarimetric data of central stars of planetary nebulae, hot subdwarfs, and white dwarfs taken with FORS1 (FOcal Reducer and low dispersion Spectrograph) on the VLT (Very Large Telescope), and added a large number of new observations in order to increase the sample. A careful analysis of the observations using only one wavelength calibration for the polarimetrically analysed spectra and for all positions of the retarder plate of the spect...

  11. A Spectroscopic Analysis of White Dwarfs in the Kiso Survey

    OpenAIRE

    Limoges, M. -m; Bergeron, P.

    2010-01-01

    We present a spectroscopic analysis of white dwarfs found in the Kiso survey. Spectroscopic observations at high signal-to-noise ratio have been obtained for all DA and DB stars in the Kiso Schmidt ultraviolet excess survey (KUV stars). These observations led to the reclassification of several KUV objects, including the discovery of three unresolved DA+DB double degenerate binaries. The atmospheric parameters (Teff and log g) are obtained from detailed model atmosphere fits ...

  12. Evidence of Precession of the White Dwarf in Cataclysmic Variables

    OpenAIRE

    Tovmassian, G.; Zharikov, S.; Neustroev, V.

    2006-01-01

    Cataclysmic Variables (CV) are close binary systems, in which the primary, the more massive star, is a white dwarf. CVs usually exhibit a number of periodicities, most of which are now understood. However, recently, a new phenomenon was discovered that does not fit the standard picture. Two objects have been discovered to show periods that are much longer than orbital, and have no relation to it, either in light curves or in radial velocity (RV) variations measured from spec...

  13. Combination Frequencies in the Fourier Spectra of White Dwarfs

    OpenAIRE

    Wu, Yanqin

    2000-01-01

    Combination frequencies are observed in the Fourier spectra of pulsating DA and DB white dwarfs. They appear at sums and differences of frequencies associated with the stellar gravity-modes. Brickhill (1992) proposed that the combination frequencies result from mixing of the eigenmode signals as the surface convection zone varying in depth when undergoing pulsation. This depth changes cause time-dependent thermal impedance, which mix different harmonic frequencies in the lig...

  14. The (Double) White Dwarf Binary SDSS 1257+5428

    CERN Document Server

    Kulkarni, S R

    2010-01-01

    SDSS 1257+5428 is a white dwarf in a close orbit with a companion that has been suggested to be a neutron star. If so, it hosts the closest known neutron star, and its existence implies a great abundance of similar systems and a rate of white-dwarf neutron-star mergers similar to that of the type Ia supernova rate. Here, we present high signal-to-noise spectra of SDSS 1257+5428, which confirm an independent finding that the system is in fact composed of two white dwarfs, one relatively cool and with low mass, and the other hotter and more massive. With this, the demographics and merger rate are no longer puzzling (various factors combine to lower the latter by more than two orders of magnitude). We show that the spectra are fit well with a combination of two hydrogen model atmospheres, as long as the lines of the higher-gravity component are broadened significantly relative to what is expected from just pressure broadening. Interpreting this additional broadening as due to rotation, the inferred spin period i...

  15. The age-metallicity dependence for white dwarfs

    CERN Document Server

    Romero, A D; Kepler, S O

    2015-01-01

    We present a theoretical study on the metallicity dependence of the initial$-$to$-$final mass relation and its influence on white dwarf age determinations. We compute a grid of evolutionary sequences from the main sequence to $\\sim 3\\, 000$ K on the white dwarf cooling curve, passing through all intermediate stages. During the thermally-pulsing asymptotic giant branch no third dredge-up episodes are considered and thus the photospheric C/O ratio is below unity for sequences with metallicities larger than $Z=0.0001$. We consider initial metallicities from $Z=0.0001$ to $Z=0.04$, accounting for stellar populations in the galactic disk and halo, with initial masses below $\\sim 3M_{\\odot}$. We found a clear dependence of the shape of the initial$-$to$-$final mass relation with the progenitor metallicity, where metal rich progenitors result in less massive white dwarf remnants, due to an enhancement of the mass loss rates associated to high metallicity values. By comparing our theoretical computations with semi em...

  16. THE SPECTRAL TYPES OF WHITE DWARFS IN MESSIER 4

    International Nuclear Information System (INIS)

    We present the spectra of 24 white dwarfs in the direction of the globular cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs. Determining the spectral types of the stars in this sample, we find 24 type DA and 0 type DB (i.e., atmospheres dominated by hydrogen and helium, respectively). Assuming the ratio of DA/DB observed in the field with effective temperature between 15,000-25,000 K, i.e., 4.2:1, holds for the cluster environment, the chance of finding no DBs in our sample simply due to statistical fluctuations is only 6 x 10-3. The spectral types of the ?100 white dwarfs previously identified in open clusters indicate that DB formation is strongly suppressed in that environment. Furthermore, all the ?10 white dwarfs previously identified in other globular clusters are exclusively type DA. In the context of these two facts, this finding suggests that DB formation is suppressed in the cluster environment in general. Though no satisfactory explanation for this phenomenon exists, we discuss several possibilities.

  17. Describing SGRs/AXPs as fast and magnetized white dwarfs

    CERN Document Server

    Malheiro, M

    2013-01-01

    Over the last decade, observational evidence has mounted that SGRs/AXPs belong to a particular class of pulsars. Furthermore, fast and very magnetic white dwarfs have been observed, and recently two SGRs with low magnetic fields $B\\sim(10^{12}-10^{13})$ G, namely SGR 0418+5729 and Swift J1822.3-1606 were discovered with a period of $P\\sim9.08$ s and $P\\sim8.44$ s, respectively. These new discoveries opens the question of the nature of SGRs/AXPs, emerging alternative scenarios, in particular the white dwarf (WD) pulsar model. These astronomical observations have based an alternative description of the SGRs/AXPs expressed on rotating highly magnetized and very massive WDs. In this new description, several observational properties are easy understood and well explained as a consequence of the large radius of a massive white dwarf that manifests a new scale of mass density, moment of inertia, rotational energy, and magnetic dipole moment in comparison with the case of neutron stars. In this contribution, we will ...

  18. Atmospheric parameters and carbon abundance for hot DB white dwarfs

    CERN Document Server

    Koester, Detlev; Gänsicke, Boris T

    2014-01-01

    Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near effective temperatures of 25000K are extremely difficult to determine from optical spectroscopy. This is particularly unfortunate, because this is the range of variable DBV or V777 Her stars. Accurate atmospheric parameters are needed to help or confirm the asteroseismic analysis of these objects. Another important aspect is the new class of white dwarfs - the hot DQ - detected by Dufour et al. (2007), with spectra dominated by carbon lines. The analysis shows that their atmospheres are pure carbon. The origin of these stars is not yet understood, but they may have an evolutionary link with the hotter DBs as studied here. Our aim is to determine accurate atmospheric parameters and element abundances and study the implications for the evolution white dwarfs of spectral classes DB and hot DQ. High resolution UV spectra of five DBs are studied with model atmospheres. We determine stellar parameters and abundances or upper limits of C and Si....

  19. ON THE FORMATION OF HOT DQ WHITE DWARFS

    International Nuclear Information System (INIS)

    We present the first full evolutionary calculations aimed at exploring the origin of hot DQ white dwarfs. These calculations consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Our calculations provide strong support for the diffusive/convective mixing picture for the formation of hot DQs. We find that the hot DQ stage is a short-lived stage and that the range of effective temperatures where hot DQ stars are found can be accounted for by different masses of residual helium and/or different initial stellar masses. In the frame of this scenario, a correlation between the effective temperature and the surface carbon abundance in DQs should be expected, with the largest carbon abundances expected in the hottest DQs. From our calculations, we suggest that most of the hot DQs could be the cooler descendants of some PG 1159 stars characterized by He-rich envelopes markedly smaller than those predicted by the standard theory of stellar evolution. At least for one hot DQ, the high-gravity white dwarf SDSS J142625.70+575218.4, an evolutionary link between this star and the massive PG 1159 star H1504+65, is plausible.

  20. Thermally Stable Nuclear Burning on Accreting White Dwarfs

    CERN Document Server

    Shen, K J; Shen, Ken J.; Bildsten, Lars

    2007-01-01

    One of the challenges to increasing the mass of a white dwarf through accretion is the tendency for the accumulating hydrogen to ignite unstably and potentially trigger mass loss. It has been known for many years that there is a narrow range of accretion rates for which the hydrogen can burn stably, allowing for the white dwarf mass to increase as a pure helium layer accumulates. We first review the physics of stable burning, providing a clear explanation for why radiation pressure stabilization leads to a narrow range of accretion rates for stable burning near the Eddington limit, confirming the recent work of Nomoto and collaborators. We also explore the possibility of stabilization due to a high luminosity from beneath the burning layer. We then examine the impact of the beta-decay-limited ``hot'' CNO cycle on the stability of burning. Though this plays a significant role for accreting neutron stars, we find that for accreting white dwarfs, it can only increase the range of stably-burning accretion rates f...

  1. The Ultramassive White Dwarf EUVE J1746--706

    Science.gov (United States)

    Dupuis, Jean; Vennes, Stéphane

    1997-02-01

    We have obtained new optical and extreme ultraviolet (EUV) spectroscopy of the ultramassive white dwarf EUVE J1746-706. We revise Vennes et al.'s original estimates of the atmospheric parameters, and we measure an effective temperature of 46,500 +/- 700 K and a surface gravity log g = 9.05 +/- 0.15 (~1.2 Msolar), in agreement with Balmer line profiles and the EUV continuum. We derive an upper limit on the atmospheric abundance of helium of He/H = 1.3 × 10-4 and a neutral hydrogen column density in the local interstellar medium NH I = 1.8 +/- 0.4 × 1019 cm-2 from the EUV spectrum. Our upper limit corresponds to half the helium abundance observed in the atmosphere of the ultramassive white dwarf GD 50. We discuss the possibility that EUVE J1746-706 represents an earlier phase of evolution relative to GD 50 and may, therefore, help us understand the origin and evolution of massive white dwarfs. Based on observations obtained with NASA's Extreme Ultraviolet Explorer and Mount Stromlo Observatory's 74 inch telescope.

  2. 56Ni Production in Double Degenerate White Dwarf Collisions

    CERN Document Server

    Raskin, Cody; Rockefeller, Gabriel; Fryer, Chris; Diehl, Steven; Timmes, F X

    2010-01-01

    We present a comprehensive study of white dwarf collisions as an avenue for creating type Ia supernovae. Using a smooth particle hydrodynamics code with a 13-isotope, {\\alpha}-chain nuclear network, we examine the resulting 56Ni yield as a function of total mass, mass ratio, and impact parameter. We show that several combinations of white dwarf masses and impact parameters are able to produce sufficient quantities of 56Ni to be observable at cosmological distances. We find the 56Ni production in double-degenerate white dwarf collisions ranges from sub-luminous to the super-luminous, depending on the parameters of the collision. For all mass pairs, collisions with small impact parameters have the highest likelihood of detonating, but 56Ni production is insensitive to this parameter in high-mass combinations, which significantly increases their likelihood of detection. We also find that the 56Ni dependence on total mass and mass ratio is not linear, with larger mass primaries producing disproportionately more 5...

  3. Evidence for Terrestrial Planetary System Remnants at White Dwarfs

    CERN Document Server

    Farihi, J

    2010-01-01

    The last several years have brought about a dynamic shift in the view of exoplanetary systems in the post-main sequence, perhaps epitomized by the evidence for surviving rocky planetary bodies at white dwarfs. Coinciding with the launch of the Spitzer Space Telescope, both space- and ground-based data have supported a picture whereby asteroid analogs persist at a significant fraction of cool white dwarfs, and are prone to tidal disruption when passing close to the compact stellar remnant. The ensuing debris can produce a detectable infrared excess, and the material gradually falls onto the star, polluting the atmosphere with heavy elements that can be used to determine the bulk composition of the destroyed planetary body. Based on the observations to date, the parent bodies inferred at white dwarfs are best described as asteroids, and have a distinctly rocky composition similar to material found in the inner Solar System. Their minimum masses are typical of large asteroids, and can approach or exceed the mass...

  4. Latest results from the DODO survey: Imaging planets around white dwarfs

    OpenAIRE

    Hogan, E.; Burleigh, MR; Clarke, Fj

    2011-01-01

    The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures lower (< 500 K) than the coolest brown dwarfs currently observed. The discovery of planets around white dwarfs would prove that such objects can survive the final stages of stellar evoluti...

  5. Peculiar variations of white dwarf pulsation frequencies and maestro

    Science.gov (United States)

    Dalessio, James Ruland

    In Part I we report on variations of the normal mode frequencies of the pulsating DB white dwarfs EC 20058-5234 and KIC 8626021 and the pulsating DA white dwarf GD 66. The observations of EC 20058-5234 and KIC 8626021 were motivated by the possibility of measuring the plasmon neutrino production rate of a white dwarf, while the observations of GD 66 were part of a white dwarf pulsation timing based planet search. We announce the discovery of periodic and quasi-periodic variations of multiple normal mode frequencies that cannot be due to the presence of planetary companions. We note the possible signature of a planetary companion to EC 20058-5234 and show that GD 66 cannot have a planet in a several AU orbit down to half a Jupiter mass. We also announce the discovery of secular variations of the normal mode frequencies of all three stars that are inconsistent with cooling alone. Importantly, the rates of period change of several modes of KIC 8626021 are consistent with evolutionary cooling, but are not yet statistically significant. These modes offer the best possibility of measuring the neutrino production rate in a white dwarf. We also observe periodic and secular variations in the frequency of a combination mode that exactly matches the variations predicted by the parent modes, strong observational evidence that combination modes are created by the convection zone and are not normal modes. Periodic variations in the amplitudes of many of these modes is also noted. We hypothesize that these frequency variations are caused by complex variations of the magnetic field strength and geometry, analogous to behavior observed in the Sun. In Part II we describe the MAESTRO software framework and the MAESTRO REDUCE algorithm. MAESTRO is a collection of astronomy specific MatLab software developed by the Whole Earth Telescope. REDUCE is an an algorithm that can extract the brightness of stars on a set of CCD images with minimal configuration and human interaction. The key to this algorithm is automatic identification of stars and a sophisticated implementation of geometric hashing.

  6. Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey

    CERN Document Server

    Gates, E; Harris, H C; Subba-Rao, M; Anderson, S; Kleinman, S J; Liebert, J; Brewington, H; Brinkmann, J; Harvanek, M; Krzesínski, J; Lamb, D Q; Long, D; Neilsen, E H; Newman, P R; Nitta, A; Snedden, S A; Gates, Evalyn; Gyuk, Geza; Harris, Hugh C.; Subbarao, Mark; Anderson, Scott; Liebert, James; Brewington, Howard; Harvanek, Michael; Krzesinski, Jurek; Lamb, Don Q.; Long, Dan; Neilsen, Eric H.; Newman, Peter R.; Nitta, Atsuko; Snedden, Stephanie A.

    2004-01-01

    We report the discovery of five very cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced absorption (CIA) from molecular hydrogen and are similar in color to the three previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402. The fifth, an ultracool white dwarf candidate, shows milder CIA flux suppression and has a color and spectral shape similar to WD 0346+246. All five new white dwarfs are faint (g > 18.9) and have significant proper motions. One of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.

  7. Observed properties and long-term structure and evolution of white dwarfs in cataclysmic variables

    International Nuclear Information System (INIS)

    An attempt is made to explain the observed surface temperatures and luminosities of cataclysmic variable white dwarfs based on the cooling physics and time-averaged structure of the white dwarf in response to accretion. The evolutionary changes in core temperature and core luminosity as an accreting white dwarf evolves quasi-statically are compared with the cooling evolution of a nonaccreting, one solar mass, pure C-12 core. Then, the evolutionary behavior of the white dwarf envelope in mass and temperature, in response to long-term accretion, is used to predict temperatures and luminosities of white dwarfs during quiescent intervals between nova outbursts. It is shown that the observed luminosities and effective temperatures of the bare white dwarfs detected in some cataclysmic variables are the expected intrinsic values associated with classical nova thermonuclear outburst cycles. Two alternate interpretations of their observed surface temperatures and luminosities are assessed. 22 references

  8. Constraining white-dwarf kicks in globular clusters : IV. Retarding Core Collapse

    CERN Document Server

    Heyl, Jeremy S

    2009-01-01

    Observations of white dwarfs in the globular clusters NGC 6397 and Omega Centauri indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant branch is slightly asymmetric. The kicks may be large enough to dramatically change the radial distribution of young white dwarfs, giving them larger energies than other stars in the cluster. As these energetic white dwarfs travel through the cluster they can impart their excess energy on the other stars in the cluster. A Monte-Carlo simualtion of the white-dwarfs kicks combined with estimate of the phase-space diffusion of the white dwarfs reveals that as the white dwarfs equilibrate, they lose most of their energy in the central region of the cluster. They could possibly mimic the effect of binaries, puffing up the cluster and delaying core collapse.

  9. Understanding the Cool DA White Dwarf Pulsator, G29–38

    OpenAIRE

    Kleinman, S. J.; Solheim, Jan Erik; Nather, R. E.; Bruvold, A.; Winget, D. E.; Clemens, J. C.; Bradley, A.; Kanaan, A.; Claver, C. F.; Watson, T. K.; Yanagida, K.; Nitta, A.; Dixson, J. S.; Grauer, A. D.; Hine, B. P.

    1997-01-01

    The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post main sequence evolution, along with their cooling rates, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously been...

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

    OpenAIRE

    Kilic, Mukremin; Leggett, S. K.; Tremblay, P. -e; Von Hippel, Ted; Bergeron, P.; Harris, Hugh C.; Munn, Jeffrey A.; Williams, Kurtis A.; Gates, Evalyn; Farihi, J.

    2010-01-01

    We present optical spectroscopy and near-infrared photometry of 126 cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Our sample includes high proper motion targets selected using the SDSS and USNO-B astrometry and a dozen previously known ultracool white dwarf candidates. Our optical spectroscopic observations demonstrate that a clean selection of large samples of cool white dwarfs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic Survey Telescope d...

  11. Spectroscopic Identification of Faint White Dwarf Candidates in the Praesepe Open Star Cluster

    OpenAIRE

    Williams, Kurtis A.; Bolte, Michael; Liebert, James

    2004-01-01

    We present spectroscopic observations of the remaining four candidate white dwarfs in Praesepe. All four candidates are quasars with redshifts between 0.8 and 2.8. One quasar, LB 6072, is observed to have a strong metal-line absorption system blueward of the quasar redshift. The lack of additional white dwarfs in Praesepe leaves the total known white-dwarf population of the cluster at five, well below the number expected from commonly-assumed initial mass functions, though s...

  12. Testing Fundamental Particle Physics with the Galactic White Dwarf Luminosity Function

    OpenAIRE

    Bertolami, Marcelo M. Miller; Melendez, Brenda E.; Althaus, Leandro G.; Isern, Jordi

    2014-01-01

    Recent determinations of the white dwarf luminosity function (WDLF) from very large surveys have extended our knowledge of the WDLF to very high luminosities. It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to test the possible properties and existence of fundamental weakly interacting subelectronvolt particles. This, together with the availability of new full evolutionary white dwarf models that are reliable at high lumi...

  13. The fate of accreting white dwarfs: type I supernovae vs. collapse

    International Nuclear Information System (INIS)

    The fate of accreting white dwarfs is examined with respect to thermonuclear explosion or collapse. The paper was presented to the conference on ''The early universe and its evolution'', Erice, Italy 1986. Effects of accretion and the fate of white dwarfs, models for type 1a and 1b supernovae, collapse induced by carbon deflagration at high density, and fate of double white dwarfs, are all discussed. (U.K.)

  14. Strongly magnetized electron degenerate gas: Highly super-Chandrasekhar white dwarfs

    OpenAIRE

    Das, Upasana; Mukhopadhyay, Banibrata

    2013-01-01

    We consider a relativistic, degenerate, electron gas under the influence of a strong magnetic field, which describes magnetized white dwarfs. Landau quantization changes the density of states available to the electrons, thus modifying the underlying equation of state. We obtain the mass-radius relations for such white dwarfs and show that it is possible to have magnetized white dwarfs with a mass significantly greater than the Chandrasekhar limit in the range 2.3 - 2.6 M_sun...

  15. Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid

    CERN Document Server

    Tremblay, P -E; Freytag, B; Steffen, M; Caffau, E

    2013-01-01

    3D model atmospheres for giants, dwarfs, and white dwarfs, computed with the CO5BOLD code and part of the CIFIST grid, have been used for spectroscopic and asteroseismic studies. Unlike existing plane-parallel 1D structures, these simulations predict the spatially and temporally resolved emergent intensity so that granulation can be analysed, which provides insights on how convective energy transfer operates in stars. The wide range of atmospheric parameters of the CIFIST 3D simulations (3600 < Teff (K) < 13,000 and 1 < log g < 9) allows the comparison of convective processes in significantly different environments. We show that the relative intensity contrast is correlated with both the Mach and Peclet numbers in the photosphere. The horizontal size of granules varies between 3 and 10 times the local pressure scale height, with a tight correlation between the factor and the Mach number of the flow. Given that convective giants, dwarfs, and white dwarfs cover the same range of Mach and Peclet numb...

  16. Hot C-rich white dwarfs: testing the DB-DQ transition through pulsations

    CERN Document Server

    Córsico, A H; Althaus, L G; García-Berro, E

    2009-01-01

    Hot DQ white dwarfs constitute a new class of white dwarf stars, uncovered recently within the framework of the SDSS project. Recently, three hot DQ white dwarfs have been reported to exhibit photometric variability with periods compatible with pulsation g-modes. Here, we present a nonadiabatic pulsation analysis of the recently discovered carbon-rich hot DQ white dwarf stars. One of our main aims is to test the convective-mixing picture for the origin of hot DQs through the pulsational properties. Our study relies on the full evolutionary models of hot DQ white dwarfs recently developed by Althaus et al. (2009), that consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Specifically, we present a stability analysis of white dwarf models from stages before the blue edge of the DBV instability strip until the domain of the hot DQ white dwarfs, including the transition DB --> hot DQ white dwarf. We found that at evolutionary phases in which the models are characteriz...

  17. Gaseous Material Orbiting the Polluted, Dusty White Dwarf HE1349-2305

    CERN Document Server

    Melis, Carl; Farihi, J; Bochanski, J; Burgasser, Adam J; Parsons, S; Gaensicke, B; Koester, D; Swift, Brandon

    2012-01-01

    We present new spectroscopic observations of the polluted, dusty, helium-dominated atmosphere white dwarf star HE1349-2305. Optical spectroscopy reveals weak CaII infrared triplet emission indicating that metallic gas debris orbits and is accreted by the white dwarf. Atmospheric abundances are measured for magnesium and silicon while upper limits for iron and oxygen are derived from the available optical spectroscopy. HE1349-2305 is the first gas disk-hosting white dwarf star identified amongst previously known polluted white dwarfs. Further characterization of the parent body polluting this star will require ultraviolet spectroscopy.

  18. The Accreting White Dwarf in SS Cygni Revealed

    CERN Document Server

    Sion, Edward; Myzcka, Janine; Blair, William

    2010-01-01

    We have carried out a combined Hubble Space Telescope (HST/GHRS) and Far Ultraviolet Spectroscopic Explorer FUSE) analysis of the prototype dwarf nova SS Cygni during quiescence. The FUSE and HST spectra were obtained at comparable times after outburst and have matching flux levels where the two spectra overlap. In our synthetic spectral analysis, we have used SS Cygni's accurate HST FGS parallax giving d = 166pc, a newly determined mass for the accreting white dwarf (Bitner et al. 2007) of Mwd=0.81Msun (lower than the previous, widely used 1.2 Msun) and the reddening E_{B-V} values 0.04 (Verbunt 1987; La Dous 1991) and 0.07 (Bruch and Engel 1994) derived from the 2175A absorption feature in the IUE LWP spectra. From the best-fit model solutions to the combined HST + FUSE spectral energy distribution, we find that the white dwarf is reaching a temperature Teff of 45-55,000K in quiescence, assuming Log(g)= 8.3 with a solar composition accreted atmosphere. The exact temperature of the WD depends on the reddenin...

  19. Measuring White Dwarf Accretion Rates via Their Effective Temperatures

    Science.gov (United States)

    Townsley, Dean M.; Bildsten, Lars

    2003-10-01

    Our previous theoretical study of the impact of an accreting envelope on the thermal state of an underlying white dwarf (WD) has yielded equilibrium core temperatures, classical nova ignition masses, and thermal luminosities for WDs accreting at time-averaged rates of =10-11 to 10-8 Msolar yr-1. These values are appropriate to WDs in cataclysmic variables (CVs) of Porb predicted for CVs as they transit the period gap. For dwarf novae below the period gap, we show that if is that given by gravitational radiation losses alone, then the WD masses are greater than 0.8 Msolar. An alternative conclusion is that the masses are closer to 0.6 Msolar and is 3-4 times larger than that expected from gravitational radiation losses. In either case, it is very plausible that a subset of CVs with PorbZyl and collaborators for GW Lib.

  20. Detection of spatial variations in the (D/H) ratio in the local interstellar medium

    CERN Document Server

    Vidal-Madjar, A; Ferlet, R; Hébrard, G; Köster, D; Audouze, Jean; Cassé, M; Vangioni-Flam, E; Webb, J K

    1998-01-01

    We present high spectral resolution HST-GHRS observations of the white dwarf G191-B2B, and derive the interstellar D/H ratio on the line of sight. We have observed and analysed simultaneously the interstellar lines of HI, DI, NI, OI, SiII, and SiIII. We detect three absorbing clouds, and we measure an average (D/H) ratio: N(DI)_Total / N(HI)_Total = 1.12 +/- 0.08 x 10**-5, in disagreement with the value of the D/H ratio as measured toward Capella by Linsky et al. (1993, 1995). We re-analyze the GHRS data of the Capella line of sight, and confirm their estimate, as we find: (D/H)_Capella = 1.56 +/- 0.1 x 10**-5, in the Local Interstellar Cloud in which the Sun is embedded. This shows that the D/H ratio varies by at least 30% within the local interstellar medium (not to be confused with the Local Interstellar Cloud). Furthermore, the Local Interstellar Cloud is also detected toward our target G191-B2B, and we show that the D/H ratio in this component, toward G191-B2B, can be made compatible with that derived to...

  1. The fraction of circumstellar debris at white dwarfs

    Science.gov (United States)

    Rocchetto, M.; Farihi, J.; Gänsicke, B. T.; Bergfors, C.

    2014-04-01

    We present results of the first unbiased Spitzer IRAC survey of a homogeneous and well-defined sample of 134 single white dwarfs in search of infrared excesses compatible with the presence of circumstellar dust formed from the disruption of planetesimals. The stars were selected without regard to atmospheric metal content but were chosen to have 1) hydrogen rich atmospheres, 2) 17 000K atmospheric pollution, corroborate this interpretation as do the distribution of LIR/L* values as a function of cooling age.

  2. Cool White Dwarfs Selection with Pan-STARRS Proper Motions

    CERN Document Server

    Lam, M C

    2015-01-01

    The use of Reduced Proper Motion in identifying isolated white dwarfs has long been used as a proxy for the absolute magnitude in a population with known kinematics. This, however, introduces a proper motion detection limit on top of the existing photometric limit. How the survey volume is hampered by this extra parameter is discussed in Hambly et al. 2012. In this work, we discuss some robust outlier rejection methods in order to minimise the proper motion limit and hence maximise the survey volume. The generalised volume, corrected for the distance of the Sun from the Galactic Plane, is integrated explicitly.

  3. Electrical conductivity of plasmas of DB white dwarf atmospheres

    OpenAIRE

    Sreckovic, V. A.; Ignjatovic, Lj M.; Mihajlov, A. A.; Dimitrijevic, M. S.

    2012-01-01

    The static electrical conductivity of non-ideal, dense, partially ionized helium plasma was calculated over a wide range of plasma parameters: temperatures $1\\cdot 10^{4}\\textrm{K} \\lesssim T \\lesssim 1\\cdot 10^{5}\\textrm{K}$ and mass density $1 \\times 10^{-6} \\textrm{g}/\\textrm{cm}^{3} \\lesssim \\rho \\lesssim 2 \\textrm{g}/\\textrm{cm}^{3}$. Calculations of electrical conductivity of plasma for the considered range of plasma parameters are of interest for DB white dwarf atmosp...

  4. Hot DB White Dwarfs from the Sloan Digital Sky Survey

    OpenAIRE

    Eisenstein, Daniel J.; Liebert, James; Koester, Detlev; Kleinmann, S. J.; Nitta, Atsuko; Smith, Paul S.; Barentine, J. C.; Brewington, Howard J.; Brinkmann, J.; Harvanek, Michael; Krzesinski, Jurek; Neilsen Jr, Eric H.; Long, Dan; Schneider, Donald P.; Snedden, Stephanie A.

    2006-01-01

    We present ugriz photometry and optical spectroscopy for 28 DB and DO white dwarfs with temperatures between 28,000K and 45,000K. About 10 of these are particularly well-observed; the remainder are candidates. These are the hottest DB stars yet found, and they populate the "DB gap" between the hotter DO stars and the familiar DB stars cooler than 30,000K. Nevertheless, after carefully matching the survey volumes, we find that the ratio of DA stars to DB/DO stars is a factor ...

  5. Deriving Precise Ages for Field White Dwarfs using Bayesian Techniques

    Science.gov (United States)

    Webster, Aaron; von Hippel, Ted; Bayesian Analysis of Stellar Evolution (BASE)

    2015-01-01

    We apply a self-consistent and robust Bayesian statistical approach along with a range of modern model ingredients to determine the ages, distances, and ZAMS masses of old field white dwarfs with hydrogen, helium, and mixed atmosphere types. 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. In cases where we can determine the composition of the atmosphere, our technique requires only quality optical and near-IR photometry to derive ages with GAIA era, when thousands of WDs will have distances accurate to <2%.

  6. The movie about the magnetism in isolated white dwarfs

    Science.gov (United States)

    Sliwinski, M. S.; Krzyczkowska, L. I.

    2004-10-01

    We present the movie about magnetism in the isolated white dwarfs. We tried to create a brand new form of presentation of astronomical knowledge. This one was made to show it to the scientists, so we assumed some level of knowledge that was needed to understand the movie. However it still can be a way to arouse interest of non-astronomers (even children) in astronomical research. This could also be the best way to teach young people or help them to understand astronomy and astrophysics that they have to learn at school. Below we report the plot of the movie adding, in some parts, comments that will follow.

  7. Suppression of cooling by strong magnetic fields in white dwarf stars.

    Science.gov (United States)

    Valyavin, G; Shulyak, D; Wade, G A; Antonyuk, K; Zharikov, S V; Galazutdinov, G A; Plachinda, S; Bagnulo, S; Machado, L Fox; Alvarez, M; Clark, D M; Lopez, J M; Hiriart, D; Han, Inwoo; Jeon, Young-Beom; Zurita, C; Mujica, R; Burlakova, T; Szeifert, T; Burenkov, A

    2014-11-01

    Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young. PMID:25327247

  8. 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.

  9. Interferometric Astrometry of the Detached White Dwarf - M Dwarf Binary Feige 24 Using Hubble Space Telescope Fine Guidance Sensor; 3, White Dwarf Radius and Component Mass Estimates

    CERN Document Server

    Benedict, G F; Franz, O G; Wasserman, L H; Nelan, E; Lee, J; Fredrick, L W; Jefferys, W H; Van Altena, W F; Robinson, E L; Spiesman, W J; Shelus, P J; Hemenway, P D; Duncombe, R L; Story, D; Whipple, A L; Bradley, A

    2000-01-01

    With HST FGS 3 we have determined a parallax for the white dwarf - M dwarf interacting binary, Feige 24. The white dwarf (DA) component has an effective temperature, T_eff of approximately 56,000 K degrees. A weighted average with past parallax determinations (pi_abs = 14.6 +- 0.4 milliseconds of arc) narrows the range of possible radius values, compared to past estimates. We obtain R_DA = 0.0185 +- 0.0008 R_solar with uncertainty in the temperature and bolometric correction the dominant contributors to the error. FGS photometry provides a light curve entirely consistent with reflection effects. A recently refined model Mass-Luminosity Relation (Baraffe et al. 1998) for low mass stars provides a mass estimate for the M dwarf companion, M_dM = 0.37 +- 0.20 M_solar, where the mass range is due to metallicity and age uncertainties. Radial velocities from Vennes and Thorstensen (1994) provide a mass ratio from which we obtain M_DA = 0.49+0.19-0.05 ~ M_solar. Independently, our radius and recent log_g determinatio...

  10. Excess infrared radiation from a white dwarf - an orbiting brown dwarf

    International Nuclear Information System (INIS)

    The authors have discovered that the white dwarf star Giclas 29-38 appears to emit substantial radiation at wavelengths between 2 and 5 ?m, far in excess of that expected from an extrapolation of the visual and near-infrared spectrum of the star. The infrared colour temperature of the excess radiation is 1,200 ± 200 K and, at the distance of G29-38, corresponds to a total luminosity of 5 x 10-5 solar luminosities. If the excess 3.5-?m radiation is emitted by a single spherical body at 1,200 K, then its radius is 0.15 solar radii. The most natural interpretation of the observations is that there is a substellar, somewhat Jupiter-like brown dwarf in orbit around G29-38. (author)

  11. Near-UV absorption in very cool DA white dwarfs

    CERN Document Server

    Saumon, D; Kowalski, P M

    2014-01-01

    The atmospheres of very cool, hydrogen-rich white dwarfs (Teff <6000 K) are challenging to models because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, weakly ionized dense gas. In particular, many models that assume relatively simple models for the broadening of atomic levels and mostly ideal gas physics overestimate the flux in the blue part of their spectra. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman alpha line of atomic H by collisions primarily with H2. For the purpose of validating this model more rigorously, we acquired Hubble Space Telescope STIS spectra of 8 very cool white dwarfs (5 DA and 3 DC stars). Combined with their known parallaxes, BVRIJHK and Spitzer IRAC photometry, we analyze their entire spectral energy distribution (from 0.24 to 9.3 micron) with a large grid of model atmospheres and synthetic spectra. We f...

  12. SGRs and AXPs: Massive Rotating White Dwarfs versus Magnetars

    CERN Document Server

    Malheiro, Manuel; Ruffini, Remo

    2011-01-01

    The recent observations of SGR 0418+5729 offer an authentic Rosetta Stone for deciphering the energy source of Soft Gamma Ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). The "magnetar" model, appeals to a yet untested new energy source in astrophysical systems: bulk magnetic energy. It leads for SGR 0418+5729 to results in contradiction with observations. It is shown how a consistent model for SGRs and AXPs can be expressed in terms of canonical physics and astrophysics within massive, fast rotating, and highly magnetized white dwarfs. The pioneering work of B. Paczynski (1990) on 1E 2259+586 is extended and further developed to describe the observed properties of all known SGRS and AXPs by assuming spin-down powered massive, fast rotating, and highly magnetized white dwarfs. Within this model, we obtain the theoretical prediction for the lower limit of the first time derivative of the rotational period of SGR 0418+5729, $\\dot{P} \\geq L_X P^3/(4\\pi^2 I)=1.18\\times 10^{-16}$ s/s being $L_X$ and $P$ th...

  13. The progenitors of magnetic white dwarfs in open clusters

    CERN Document Server

    Külebi, B; Jordan, S; Euchner, F

    2013-01-01

    According to the fossil-field hypothesis magnetic fields are remnants of the previous stages of evolution. However, population synthesis calculations are unable to reproduce the magnetic white dwarf (MWD) sample without binary interaction or inclusion of a population of progenitor with unobservable small-scale fields. One necessary ingredient in population synthesis is the initial-to-final-mass relation (IFMR) which describes the mass-loss processes during the stellar evolution. When white dwarfs are members of open clusters, their evolutionary histories can be assessed through the use of cluster properties. In this work, we assess the cluster membership by correlating the proper-motion of MWDs with the cluster proper-motion and by analyzing the candidates spectroscopically with our magnetic model spectra in order to estimate the effective temperature and radii. We identified SDSS J085523.87+164059.0 to be a proper-motion member of Praesepe. We also included the data of the formerly identified cluster members...

  14. Merging binary stars and the magnetic white dwarfs

    Science.gov (United States)

    Briggs, Gordon P.; Ferrario, Lilia; Tout, Christopher A.; Wickramasinghe, Dayal T.; Hurley, Jarrod R.

    2015-02-01

    A magnetic dynamo driven by differential rotation generated when stars merge can explain strong fields in certain classes of magnetic stars, including the high field magnetic white dwarfs (HFMWDs). In their case the site of the differential rotation has been variously proposed to be within a common envelope, the massive hot outer regions of a merged degenerate core or an accretion disc formed by a tidally disrupted companion that is subsequently incorporated into a degenerate core. We synthesize a population of binary systems to investigate the stellar merging hypothesis for observed single HFMWDs. Our calculations provide mass distribution and the fractions of white dwarfs that merge during a common envelope phase or as double degenerate systems in a post-common-envelope phase. We vary the common envelope efficiency parameter ? and compare with observations. We find that this hypothesis can explain both the observed incidence of magnetism and the mass distribution of HFMWDs for a wide range of ?. In this model, the majority of the HFMWDs are of the carbon-oxygen type and merge within a common envelope. Less than about a quarter of a per cent of HFMWDs originate from double degenerate stars that merge after common envelope evolution and these populate the high-mass tail of the HFMWD mass distribution.

  15. Minimal dielectric polarization stopping power in white dwarfs

    Science.gov (United States)

    Akbari-Moghanjoughi, M.

    2015-02-01

    In this paper, we investigate the energy loss of ions by arbitrarily degenerate electron fluid, in the framework of hydrodynamic model by incorporating the generalized relativistic degeneracy pressure, Wigner-Seitz cell Coulomb interactions, and electron spin-exchange pressures for a wide range of electron number-density regimes relevant to the solid density (SD), inertial confinement fusion (ICF), warm dense matter (WDM), and super-dense astrophysical objects, such as white dwarf (WD) stars. It is found that the use of non-relativistic degeneracy pressure for electron fluid, instead of the exact Chandrasekhar relativistic degeneracy pressure, for the ICF density regime and beyond can introduce significant relative error to the stopping power calculation. Therefore, current study may introduce a significant change to the ICF scheme of super-compressed fuel. It is further revealed that the relativistic degeneracy parameter, R 0, and the atomic number of constituent ions, Z, significantly affect the maximum stopping power velocity of ions. We also discover that the velocity-averaged energy loss function becomes minimal in electron number density typical of white dwarf stars, n 0?2×1030 cm-3. It is found that the characteristic density for the minimal ion beam energy loss does not depend on the value of other plasma parameters, such as the ion-electron collision rate and the ion temperature or its atomic number. The latter finding, in particular, may help in better understanding of fusion-burning waves in dense compact stars and their cooling mechanisms.

  16. DA WHITE DWARFS OBSERVED IN THE LAMOST PILOT SURVEY

    Energy Technology Data Exchange (ETDEWEB)

    Zhang Yueyang; Deng Licai; Liu Chao; Carrell, Kenneth; Yang Fan; Gao Shuang; Xu Yan; Li Jing; Zhang Haotong; Zhao Yongheng; Luo Ali; Bai Zhongrui; Yuan Hailong [Key Lab for Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 (China); Lepine, Sebastien [Department of Astrophysics, Division of Physical Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY (United States); Newberg, Heidi Jo; Carlin, Jeffrey L. [Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180 (United States); Jin Ge [University of Science and Technology of China, Hefei 230026 (China)

    2013-08-01

    A total of {approx}640, 000 objects from the LAMOST pilot survey have been publicly released. In this work, we present a catalog of DA white dwarfs (DAWDs) from the entire pilot survey. We outline a new algorithm for the selection of white dwarfs (WDs) by fitting Sersic profiles to the Balmer H{beta}, H{gamma}, and H{delta} lines of the spectra, and calculating the equivalent width of the Ca II K line. Two thousand nine hundred sixty-four candidates are selected by constraining the fitting parameters and the equivalent width of the Ca II K line. All the spectra of candidates are visually inspected. We identify 230 DAWDs (59 of which are already included in the Villanova and SDSS WD catalogs), 20 of which are DAWDs with non-degenerate companions. In addition, 128 candidates are classified as DAWDs/subdwarfs, which means the classifications are ambiguous. The result is consistent with the expected DAWD number estimated based on the LEGUE target selection algorithm.

  17. Lyman-alpha wing absorption in cool white dwarf stars

    CERN Document Server

    Rohrmann, R D; Kepler, S O

    2010-01-01

    Kowalski & Saumon (2006) identified the missing absorption mechanism in the observed spectra of cool white dwarf stars as the Ly-alpha red wing formed by the collisions between atomic and molecular hydrogen and successfully explained entire spectra of many cool DA-type white dwarfs. Owing to the important astrophysical implications of this issue, we present here an independent assessment of the process. For this purpose, we compute free-free quasi-molecular absorption in Lyman-alpha due to collisions with H and H2 within the one-perturber, quasi-static approximation. Line cross-sections are obtained using theoretical molecular potentials to describe the interaction between the radiating atom and the perturber. The variation of the electric-dipole transition moment with the interparticle distance is also considered. Six and two allowed electric dipole transitions due to H-H and H-H2 collisions, respectively, are taken into account. The new theoretical Lyman-alpha line profiles are then incorporated in our ...

  18. Simplified Hydrostatic Carbon Burning in White Dwarf Interiors

    CERN Document Server

    Förster, Francisco; Podsiadlowski, Philipp

    2010-01-01

    We introduce two simplified nuclear networks that can be used in hydrostatic carbon burning reactions occurring in white dwarf interiors. They model the relevant nuclear reactions in carbon-oxygen white dwarfs (COWDs) approaching ignition in Type Ia supernova (SN Ia) progenitors, including the effects of the main e-captures and \\beta-decays that drive the convective Urca process. They are based on studies of a detailed nuclear network compiled by the authors and are defined by approximate sets of differential equations whose derivations are included in the text. The first network, N1, provides a good first order estimation of the distribution of ashes and it also provides a simple picture of the main reactions occurring during this phase of evolution. The second network, N2, is a more refined version of N1 and can reproduce the evolution of the main physical properties of the full network to the 5% level. We compare the evolution of the mole fraction of the relevant nuclei, the neutron excess, the photon ener...

  19. Calcium and magnesium in the helium white dwarf GD 401

    International Nuclear Information System (INIS)

    A calcium abundance (Ca/Heapprox.6--9 x 10-9) has been derived for the DB degenerate object GD 401 using spectrum synthesis techniques and observational data acquired with the SIT spectrograph. From these data we have also derived upper limits for the Mg/He and Fe/He ratios of approx.1 x 10-8 and approx.6 x 10-9, respectively. The errors on these abundances are relatively high (factors of approx.5--10), since we used a cooler model (T/sub e/=8800 K; cf. 9500 K, Greenstein) and abundances of the electron donors (C, N, O, H) which are uncertain because of the lack of observable features. However, two conclusions can be made from this analysis: (1) calcium is deficient relative to the Sun, contrary to the estimates of Liebert et al., and (2) the Mg/Ca ratio limit is much smaller than the solar value, in contrast to the results of the two other well-studied metal-line white dwarfs (Ross 640, van Maanen 2). The second result for GD 401 is at variance with recent diffusion theories for white dwarfs, which generally predict light-to-heavy element ratios larger than the Sun. If more detailed model atmospheres confirm these results, GD 401 may reflect abundance ratios from recently accreted interstellar material which has noncosmic abundances

  20. Photometric variability of the unique magnetic white dwarf GD356

    CERN Document Server

    Brinkworth, C S; Wynn, G A; Marsh, T R

    2004-01-01

    GD356 is a magnetic white dwarf (B = 13MG) that uniquely displays weak resolved Zeeman triplets of Halpha and Hbeta in emission. As such, GD356 may be the only known white dwarf with some kind of chromosphere, although accretion from the interstellar medium or more exotic mechanisms cannot be ruled out. Here, we report the detection of low amplitude (+/-~0.2%) near-sinusoidal photometric (V-band) variability in GD356, with a period of 0.0803 days (~115 minutes). We interpret this as the rotation period of the star. We model the variability with a dark spot (by analogy with star spots) covering 10% of the stellar surface. It seems likely that this spot is also the site of the Zeeman emission, requiring the presence of a temperature inversion. We show that the spot is never totally visible or obscured, and that both polar and equatorial spots produce good fits to the data when viewed at high and low inclination respectively.

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

    CERN Document Server

    Hoard, D W; Wachter, Stefanie; Leisawitz, David T; Cohen, Martin

    2013-01-01

    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 photosphere alone. Seven of these are previously known white dwarfs 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 white dwarf 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 white dwarfs with dust disks....

  2. Spitzer Observations of White Dwarfs: the Missing Planetary Debris Around DZ Stars

    CERN Document Server

    Xu, S

    2011-01-01

    We report a Spitzer/IRAC search for infrared excesses around white dwarfs, including 14 newly-observed targets and 16 unpublished archived stars. We find a substantial infrared excess around two warm white dwarfs --- J220934.84+122336.5 and WD 0843+516, the latter apparently being the hottest white dwarf known to display a close-in dust disk. Extending previous studies, we find that the fraction of white dwarfs with dust disks increases as the star's temperature increases; for stars cooler than 10,000 K, even the most heavily polluted ones do not have ~1000 K dust. There is tentative evidence that the dust disk occurrence is correlated with the volatility of the accreted material. In the Appendix, we modify a previous analysis to show that Poynting-Robertson drag might play an important role in transferring materials from a dust disk into a white dwarf's atmosphere.

  3. Radius constraints from high-speed photometry of 20 low-mass white dwarf binaries

    CERN Document Server

    Hermes, J J; Kilic, Mukremin; Gianninas, A; Chote, Paul; Sullivan, D J; Winget, D E; Bell, Keaton J; Falcon, R E; Winget, K I; Mason, Paul A; Harrold, Samuel T; Montgomery, M H

    2014-01-01

    We carry out high-speed photometry on 20 of the shortest-period, detached white dwarf binaries known and discover systems with eclipses, ellipsoidal variations (due to tidal deformations of the visible white dwarf), and Doppler beaming. All of the binaries contain low-mass white dwarfs with orbital periods less than 4 hr. Our observations identify the first eight tidally distorted white dwarfs, four of which are reported for the first time here, which we use to put empirical constraints on the mass-radius relationship for extremely low-mass (<0.30 Msun) white dwarfs. We also detect Doppler beaming in several of these binaries, which confirms the high-amplitude radial-velocity variability. All of these systems are strong sources of gravitational radiation, and long-term monitoring of those that display ellipsoidal variations can be used to detect spin-up of the tidal bulge due to orbital decay.

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

    CERN Document Server

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

    2010-01-01

    NGC 6791 is a well studied open cluster1 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 termination of the white dwarf cooling sequence (~6 Gyr) are significantly different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallise and phase separation of the main constituents of the core of a typical white dwarf, 12C and 16O, is expected to occur. This sequence of events is expected to introduce significant 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, solving the age discrepancy for NGC 6791.

  5. A search for kilogauss magnetic fields in white dwarfs and hot subdwarf stars

    CERN Document Server

    Valyavin, G; Fabrika, S; Reisenegger, A; Wade, G A; Han, I; Monin, D; Han, Inwoo

    2006-01-01

    We present new results of a survey for weak magnetic fields among DA white dwarfs with inclusion of some brighter hot subdwarf stars. We have detected variable circular polarization in the Halpha line of the hot subdwarf star Feige 34 (SP: sdO). From these data, we estimate that the longitudinal magnetic field of this star varies from -1.1 +/- 3.2 kG to +9.6 +/- 2.6 kG, with a mean of about +5 kG and a period longer than 2 h. In this study, we also confirm the magnetic nature of white dwarf WD1105-048 and present upper limits of kilogauss longitudinal magnetic fields of 5 brightest DA white dwarfs. Our data support recent finding that 25% of white dwarfs have kilogauss magnetic fields. This frequency also confirms results of early estimates obtained using the magnetic field function of white dwarfs.

  6. On the evolution of rapidly rotating massive white dwarfs towards supernovae or collapses

    CERN Document Server

    Yoon, S C; Yoon, Sung-Chul; Langer, Norbert

    2005-01-01

    A recent study by Yoon & Langer (2004a) indicated that the inner cores of rapidly accreting (Mdot > 10^{-7} M_sun/yr) CO white dwarfs may rotate differentially, with a shear rate near the threshold value for the onset of the dynamical shear instability. Such differentially rotating white dwarfs obtain critical masses for thermonuclear explosion or electron-capture induced collapse which significantly exceed the canonical Chandrasekhar limit. Here, we construct two-dimensional differentially rotating white dwarf models with rotation laws resembling those of the one-dimensional models of Yoon & Langer (2004a). We derive analytic relations between the white dwarf mass, its angular momentum, and its rotational-, gravitational- and binding energy. We show that these relations are applicable for a wide range of angular velocity profiles, including solid body rotation. We demonstrate that pre-explosion and pre-collapse conditions of both, rigidly and differentially rotating white dwarfs are well established ...

  7. A stellar prominence in the white dwarf/red dwarf binary QS Vir: evidence for a detached system

    CERN Document Server

    Parsons, S G; Gänsicke, B T; Tappert, C

    2010-01-01

    Using high resolution UVES spectra of the eclipsing Post Common Envelope Binary QS Vir we detect material along the line of sight to the white dwarf at orbital phase $\\phi=0.16$. We ascribe this to a stellar prominence originating from the M dwarf secondary star which passes in front of the white dwarf at this phase. This creates sharp absorption features in the hydrogen Balmer series and Ca II H and K lines. The small size of the white dwarf allows us to place tight constraints on the column density of hydrogen in the n=2 level of log_(10)(N_2) = 14.10 +/- 0.03 cm^(-2) and, assuming local thermodynamical equilibrium, the temperature of the prominence material of ~9000K. The prominence material is at least 1.5 stellar radii from the surface of the M dwarf. The location of the prominence is consistent with emission features previously interpreted as evidence for Roche lobe overflow in the system. We also detect Mg II 4481A absorption from the white dwarf. The width of the Mg II line indicates that the white dw...

  8. A connection between the instability strips of ZZ Ceti and V777 Herculis white dwarfs. Pulsating accreting GW Lib white dwarfs

    Science.gov (United States)

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

    2015-03-01

    Aims: We aim to determine the theoretical instability strips of white dwarfs with diverse H and He content in their atmospheres, from a solar composition to a H-depleted atmosphere. Pulsators with mixed H-He atmospheres are indeed known to exist, and these are the white dwarfs in cataclysmic accreting systems of the GW Lib type. We thus also aim to determine the range of periods of excited pulsation modes, and to qualitatively compare these to the observed periods in GW Lib white dwarf pulsators. Methods: In the first full nonadiabatic stability analysis of pulsators of this kind, we applied a time-dependent convection treatment and an energy leakage argument to compute, for cooling models of white dwarfs with various masses and envelope compositions, the location of the blue and the red edges, as well as the properties of pulsation modes. Results: We find that our derived instability strips form a true continuum in the log g-Teff plane and that their individual location depends uniquely on the assumed atmospheric composition, from the solar composition models at low effective temperatures to the H-depleted models at much higher temperatures. Taking into account our previous results from the ZZ Ceti (pure H atmosphere) and V777 Her (pure He atmosphere) white dwarf pulsators, this implies that all of these instability domains are connected via the same fundamental driving mechanism. Applying our results to the case of white dwarf pulsators of the GW Lib type, we find that our theoretical instability strips can qualitatively account for all of the known cases. The computed range of periods of excited modes also compares qualitatively very well to the observed ones. Conclusions: The GW Lib pulsators are very similar in nature to ZZ Ceti and V777 Her white dwarfs. It is the diverse chemical compositions in their atmosphere and envelope that defines their specific pulsation properties. Beyond GW Lib pulsators, white dwarfs can sometimes exhibit mixed H-He atmospheres, such as in the recently found proto-He white dwarf pulsators. Our results open the way towards quantitative asteroseismology of these various kinds of white dwarfs.

  9. A photometric selection of white dwarf candidates in Sloan Digital Sky Survey Data Release 10

    Science.gov (United States)

    Gentile Fusillo, Nicola Pietro; Gänsicke, Boris T.; Greiss, Sandra

    2015-04-01

    We present a method which uses cuts in colour-colour and reduced proper motion-colour space to select white dwarfs without the recourse to spectroscopy while allowing an adjustable compromise between completeness and efficiency. Rather than just producing a list of white dwarf candidates, our method calculates a probability of being a white dwarf (PWD) for any object with available multiband photometry and proper motion. We applied this method to all objects in the Sloan Digital Sky Survey (SDSS) Data Release 10 (DR10) photometric footprint and to a few selected sources in DR7 which did not have reliable photometry in DR9 or DR10. This application results in a sample of 61 969 DR10 and 3799 DR7 photometric sources with calculated PWD from which it is possible to select a sample of ˜23 000 high-fidelity white dwarf candidates with Teff ? 7000 K and g ? 19. This sample contains over 14 000 high confidence white dwarfs candidates which have not yet received spectroscopic follow-up. These numbers show that, to date, the spectroscopic coverage of white dwarfs in the SDSS photometric footprint is, on average, only ˜40 per cent complete. While we describe here in detail the application of our selection to the SDSS catalogue, the same method could easily be applied to other multicolour, large area surveys. We also publish a list of 8701 bright (g ? 19) white dwarfs with SDSS spectroscopy, of which 1781 are new identifications in DR9/DR10.

  10. TRANSIT SURVEYS FOR EARTHS IN THE HABITABLE ZONES OF WHITE DWARFS

    International Nuclear Information System (INIS)

    To date the search for habitable Earth-like planets has primarily focused on nuclear burning stars. I propose that this search should be expanded to cool white dwarf stars that have expended their nuclear fuel. I define the continuously habitable zone of white dwarfs and show that it extends from ?0.005 to 0.02 AU for white dwarfs with masses from 0.4 to 0.9 Msun, temperatures less than ?104 K, and habitable durations of at least 3 Gyr. As they are similar in size to Earth, white dwarfs may be deeply eclipsed by terrestrial planets that orbit edge-on, which can easily be detected with ground-based telescopes. If planets can migrate inward or reform near white dwarfs, I show that a global robotic telescope network could carry out a transit survey of nearby white dwarfs placing interesting constraints on the presence of habitable Earths. If planets were detected, I show that the survey would favor detection of planets similar to Earth: similar size, temperature, and rotation period, and host star temperatures similar to the Sun. The Large Synoptic Survey Telescope could place even tighter constraints on the frequency of habitable Earths around white dwarfs. The confirmation and characterization of these planets might be carried out with large ground and space telescopes.

  11. New phase diagrams for dense carbon-oxygen mixtures and white dwarf evolution

    CERN Document Server

    Althaus, Leandro G; Isern, Jordi; Córsico, Alejandro H; Bertolami, Marcelo M Miller

    2011-01-01

    Cool white dwarfs are reliable and independent stellar chronometers. The most common white dwarfs have carbon-oxygen dense cores. Consequently, the cooling ages of very cool white dwarfs sensitively depend on the adopted phase diagram of the carbon-oxygen binary mixture. A new phase diagram of dense carbon-oxygen mixtures appropriate for white dwarf interiors has been recently obtained using direct molecular dynamics simulations. In this paper, we explore the consequences of this phase diagram in the evolution of cool white dwarfs. To do this we employ a detailed stellar evolutionary code and accurate initial white dwarf configurations, derived from the full evolution of progenitor stars. We use two different phase diagrams, that of Horowitz et al. (2010), which presents an azeotrope, and the phase diagram of Segretain & Chabrier (1993), which is of the spindle form. We computed the evolution of 0.593 and 0.878M_sun white dwarf models during the crystallization phase, and we found that the energy released...

  12. On the incidence of weak magnetic fields in DA white dwarfs

    CERN Document Server

    Landstreet, J D; Valyavin, G G; Fossati, L; Jordan, S; Monin, D; Wade, G

    2012-01-01

    Context: About 10% of white dwarfs have magnetic fields with strength in the range between about 10^5 and 3x10^8 G. It is not known whether the remaining white dwarfs are not magnetic, or if they have a magnetic field too weak to be detected with the techniques adopted in the large surveys. Aims. We describe the results of the first survey specifically devised to clarify the detection frequency of kG-level magnetic fields in cool DA white dwarfs. Methods: Using the FORS1 instrument of the ESO VLT, we have obtained Balmer line circular spectropolarimetric measurements of a small sample of cool (DA6 - DA8) white dwarfs. Using FORS and UVES archive data, we have also revised numerous white dwarf field measurements previously published in the literature. Results: We have discovered an apparently constant longitudinal magnetic field of \\sim9.5 kG in the DA6 white dwarf WD2105-820. This star is the first weak-field white dwarf that has been observed sufficiently to roughly determine the characteristics of its field...

  13. Remarkable spectral variability on the spin period of the accreting white dwarf in V455 And

    CERN Document Server

    Bloemen, S; De Smedt, K; Vos, J; Gänsicke, B T; Marsh, T R; Rodriguez-Gil, P

    2012-01-01

    We present spin-resolved spectroscopy of the accreting white dwarf binary V455 And. With a suggested spin period of only 67s, it has one of the fastest spinning white dwarfs known. To study the spectral variability on the spin period of the white dwarf, we observed V455 And with 2s integration times, which is significantly shorter than the spin rate of the white dwarf. To achieve this cadence, we used the blue arm of the ISIS spectrograph at the 4.2-m William Herschel Telescope, equipped with an electron multiplying CCD (EMCCD). Strong coherent signals were detected in our time series, which lead to a robust determination of the spin period of the white dwarf (Pspin=67.619 +/- 0.002 s). Folding the spectra on the white dwarf spin period uncovered very complex emission line variations in Hgamma, He I 4472 and He II 4686. We attribute the observed spin phase dependence of the emission line shape to the presence of magnetically controlled accretion onto the white dwarf via accretion curtains, consistent with an ...

  14. Properties of Hot White Dwarfs in Extreme-Ultraviolet/Soft X-Ray Surveys

    Science.gov (United States)

    Vennes, Stéphane

    1999-11-01

    Intermediate-dispersion spectroscopy (?~6 Å) of 38 ultrasoft X-ray sources (ROSAT PSPC/WFC and EUVE)--27 H-rich white dwarfs (DA), one magnetic white dwarf (DAp), eight active galactic nuclei, a new cataclysmic variable, and an active late-type star--is presented. Atmospheric (Teff, log g) and stellar (age, mass) parameters of the DA white dwarfs are determined, for the first time in the case of 12 objects. Adding the present sample to the EUV-selected sample of previous studies by Vennes et al., I define an enlarged population of 141 hot white dwarfs and redetermine the DA white dwarf mass distribution taking into account improved mass measurements for ultramassive white dwarfs. High-dispersion spectroscopy (?~1.0-1.4 Å) of the H? line core in a representative collection of white dwarfs (the ultramassive DA GD 50, two hot DAs, and the low-mass DA EUVE J0512-006) indicate low projected rotation velocity vrot sin iMount Stromlo Observatory 74 inch telescope, the Siding Spring Observatory 2.3 m telescope, and the Anglo-Australian Observatory 4 m telescope.

  15. Concequences of accretion in a carbon-oxygen white dwarf double system

    International Nuclear Information System (INIS)

    Evolution of 0.838 Msub(sun) carbon-oxygen white dwarf, which accretes C+O matter, is calculated. If accretion rate M-6 Msub(sun)/yr, then carbon ignits at the center of the dwarf. This should results in a thermonuclear explosion and a SNI outburst. If M>Msub(cr) carbon ignits near the surface of the dwarf. The rate Msub(cr) is close to the Eddinqton accretion rate. Thus io.nition of the carbon-oxygen white dwarf at the center and SNI outburst is possible practically for any admissible rate of C+O accretion

  16. Dynamical Tides in Compact White Dwarf Binaries: Influence of Rotation

    CERN Document Server

    Fuller, Jim

    2014-01-01

    Tidal interactions play an important role in the evolution and ultimate fate of compact white dwarf (WD) binaries. Not only do tides affect the pre-merger state (such as temperature and rotation rate) of the WDs, but they may also determine which systems merge and which undergo stable mass transfer. In this paper, we attempt to quantify the effects of rotation on tidal angular momentum transport in binary stars, with specific calculations applied to WD stellar models. We incorporate the effect of rotation using the traditional approximation, in which the dynamically excited gravity waves within the WDs are transformed into gravito-inertial Hough waves. The Coriolis force has only a minor effect on prograde gravity waves, and previous results predicting the tidal spin-up and heating of inspiraling WDs are not significantly modified. However, rotation strongly alters retrograde gravity waves and inertial waves, with important consequences for the tidal spin-down of accreting WDs. We identify new dynamical tidal...

  17. V445 Puppis -- Helium nova on a massive white dwarf

    CERN Document Server

    Kato, M; Kato, Mariko; Hachisu, Izumi

    2003-01-01

    The 2000 outburst of V445 Puppis shows unique properties, such as absence of hydrogen, enrichment of helium and carbon, slow development of the light curve with a small amplitude that does not resemble any classical novae. This object has been suggested to be the first example of helium novae. We calculate theoretical light curves of helium novae and reproduce the observational light curve of V445 Pup. Modeling indicates a very massive white dwarf (WD), more massive than 1.3 Msun. The companion star is possibly either a helium star or a helium-rich main-sequence star. We estimate the ignition mass as several times 10^{-5} Msun, the corresponding helium accretion rate as several times 10^{-7} Msun yr^{-1}, and the recurrence period as several tens of years. These values suggest that the WD is growing in mass and ends up either a Type Ia supernova or an accretion induced collapse to a neutron star.

  18. Neutron star formation by collapse of white dwarfs

    International Nuclear Information System (INIS)

    Mass-accreting carbon-oxygen white dwarfs become thermally and dynamically unstable when they reach high enough central densities. Carbon ignition at the star's center likely propagates subsonically and, in the case of an initially solid core, leads to collapse if the rate of increase of the core's mass is sufficiently fast. Recent results indicate, however, that solidification of the core induces carbon-oxygen separation. The central regions are then made of pure oxygen while carbon is rejected to lower-density layers. Carbon ignition happens only after neutronization of the central (oxygen) regions. Collapse to a neutron star is then independent from the rate of mass increase and the only possible restrictions are set by the behaviour of the outer, accreted layers. X-ray sources, pulsars and Type I supernovae are likely outcomes of this process. (orig.)

  19. The pulsating low-mass He-core white dwarfs

    CERN Document Server

    Córsico, Alejandro H; Romero, Alejandra D

    2013-01-01

    Recent years have witnessed the discovery of many low-mass ($\\lesssim 0.45 M_{\\odot}$) white dwarf (WD) stars --- expected to harbor He cores--- in the field of the Milky Way and in several galactic globular and open clusters. Recently, three pulsating objects of this kind have been discovered: SDSS J1840+6423, SDSS J1112+1117, and SDSS J1518+0658. Motivated by these very exciting findings, and in view of the valuable asteroseismological potential of these objects, we present here the main outcomes of a detailed theoretical study on the seismic properties of low-mass He-core WDs based on fully evolutionary models representative of these objects. This study is aimed to provide a theoretical basis from which to interpret present and future observations of variable low-mass WDs.

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

    International Nuclear Information System (INIS)

    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

  1. 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.

  2. THE ELM SURVEY. V. MERGING MASSIVE WHITE DWARF BINARIES

    Energy Technology Data Exchange (ETDEWEB)

    Brown, Warren R.; Kenyon, Scott J. [Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 (United States); Kilic, Mukremin; Gianninas, A. [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK, 73019 (United States); Allende Prieto, Carlos, E-mail: wbrown@cfa.harvard.edu, E-mail: skenyon@cfa.harvard.edu, E-mail: kilic@ou.edu, E-mail: alexg@nhn.ou.edu, E-mail: callende@iac.es [Instituto de Astrofisica de Canarias, E-38205, La Laguna, Tenerife (Spain)

    2013-05-20

    We present the discovery of 17 low-mass white dwarfs (WDs) in short-period (P {<=} 1 day) binaries. Our sample includes four objects with remarkable log g {approx_equal} 5 surface gravities and orbital solutions that require them to be double degenerate binaries. All of the lowest surface gravity WDs have metal lines in their spectra implying long gravitational settling times or ongoing accretion. Notably, six of the WDs in our sample have binary merger times <10 Gyr. Four have {approx}>0.9 M{sub Sun} companions. If the companions are massive WDs, these four binaries will evolve into stable mass transfer AM CVn systems and possibly explode as underluminous supernovae. If the companions are neutron stars, then these may be millisecond pulsar binaries. These discoveries increase the number of detached, double degenerate binaries in the ELM Survey to 54; 31 of these binaries will merge within a Hubble time.

  3. Calibrating cosmological chronometers: White dwarf masses via astrometry

    International Nuclear Information System (INIS)

    In an effort to increase the number of accurate dynamical masses for white dwarfs (WDs), we have begun an initiative using Hubble Space Telescope's Fine Guidance Sensors (FGS) to resolve suspected binary WDs. With the increasing number of WD trigonometric parallaxes becoming available via CTIO's and the USNO's ongoing parallax programs, we have targeted objects that are overluminous at V magnitude and are presumably unresolved multiple systems. A few targets were selected because of spectral anomalies or possible perturbations evident in the residuals of the trigonometric parallax solutions. A total of 16 HST orbits were designated to this program and 12 are completed. Of the eleven WDs observed thus far (one object was observed twice), all but one were unresolved. Analysis of a recent orbit's data indicate a pair was resolved with a separation of 70 mas and a delta V magnitude of ?1.4. Coupled with astrometric data from the USNO parallax program, we have obtained preliminary constraints on component masses.

  4. Detectability of substellar companions around white dwarfs with Gaia

    CERN Document Server

    Silvotti, Roberto; Lattanzi, Mario; Morbidelli, Roberto

    2014-01-01

    To date not a single-bona fide planet has been identified orbiting a single white dwarf. In fact we are ignorant about the final configuration of >95% of planetary systems. Theoretical models predict a gap in the final distribution of orbital periods, due to the opposite effects of stellar mass loss (planets pushed outwards) and tidal interactions (planets pushed inwards) during the RGB and the AGB stellar expansions. Over its five year primary mission, Gaia is expected to astrometrically detect the first (few tens of) WD massive planets/BDs giving first evidence that WD planets exist, at least those in wide orbits. In this article we present preliminary results of our simulations of what Gaia should be able to find in this field.

  5. Tidal disruption of white dwarfs by intermediate mass black holes

    Directory of Open Access Journals (Sweden)

    Bode T.

    2012-12-01

    Full Text Available Modeling ultra-close encounters between a white dwarf and a spinning, intermediate mass black hole requires a full general relativistic treatment of gravity. This paper summarizes results from such a study. Our results show that the disruption process and prompt accretion of the debris strongly depend on the magnitude and orientation of the black hole spin. On the other hand, the late-time accretion onto the black hole follows the same decay, ? ? ?t?5/3, estimated from Newtonian gravity disruption studies. The spectrum of the fallback material peaks in the soft X-rays and sustains Eddington luminosity for 1–3?yrs after the disruption. The orientation of the black hole spin has also a profound effect on how the outflowing debris obscures the central region. The disruption produces a burst of gravitational radiation with characteristic frequencies of ?3.2?Hz and strain amplitudes of ?10?18 for galactic intermediate mass black holes.

  6. LOW MASS STELLAR AND SUBSTELLAR COMPANIONSHIP AMONG NEARBY WHITE DWARFS

    Directory of Open Access Journals (Sweden)

    M. Radiszcz

    2009-01-01

    Full Text Available This work is a systematic, deep search for stellar and substellar objects orbiting nearby white dwarfs (WDs. The scienti c interest spans testing speci c predictions of common envelope evolutionary phase models, as well as providing constraints to planetary system evolution in advanced stages of its parent star (Livio & Soker 1984; Willes & Wu 2005. Additionally, we seek to explore the hypothesis about the origin of metal lines in hydrogen WDs, produced by the accretion of tidal disturbed asteroidal or cometary material. This could be linked to the presence of a undetected substellar object that perturbed the orbits of these asteroids or comets (Debes & Sigurdsson 2002. Here, we show preliminary results of this project.

  7. Cosmological Fast Radio Bursts from Binary White Dwarf Mergers

    CERN Document Server

    Kashiyama, Kazumi; Mészáros, Peter

    2013-01-01

    Recently, Thornton et al. (2013) reported the detections of 4 fast radio bursts (FRB). The dispersion measures indicate that the sources are at cosmological distance. Given the large full sky event rate ~ 10^4 sky^-1 day^-1, the FRBs are a promising target of multi-messenger astronomy in the coming years. Here we propose double degenerate, binary white-dwarf (WD) mergers as the source of FRBs, which is produced by coherent emission from the polar region of a rapidly rotating, magnetized massive WD formed after the merger. The basic characteristics of the FRBs, such as the energetics, the emission duration and the event rate can be consistently explained in this scenario. As a result, we predict that some FRBs can accompany type Ia supernovae (SNIa). The simultaneous detection could test our scenario, may probe the progenitor of SNIa, and moreover would give a novel constraint on the cosmological parameters. We strongly encourage SN surveys following up FRBs.

  8. Masses, radii, temperatures, luminosities and surface compositions of white dwarfs

    Science.gov (United States)

    Greenstein, J. L.

    1979-01-01

    The masses, radii, temperature, luminosities and compositions of white dwarf stars are discussed, with particular emphasis on those areas in which considerable work remains to be done. It is pointed out that radii for stars of known parallax are available, together with masses and their variance. Consideration is then given to the difficulties involved in the determination of the temperatures of the hottest stars and helium stars by means of Stromgren or multichannel photometry, and it is noted that the frequency of binaries among the hottest degenerates seems too high to be due to chance. Results of IUE observations of the UV spectra of three DA and one DO star are discussed, with attention given to the similarities between UV-derived and ground-based determinations of temperatures and surface gravities. The metal abundances of hydrogen and helium degenerate stars are finally considered, and spectral classes and abundances of the helium atmosphere degenerates are tabulated.

  9. THE FREQUENCY OF DEBRIS DISKS AT WHITE DWARFS

    International Nuclear Information System (INIS)

    We present near- and mid-infrared photometry and spectroscopy from PAIRITEL, IRTF, and Spitzer of a metallicity-unbiased sample of 117 cool, hydrogen-atmosphere white dwarfs (WDs) from the Palomar-Green survey and find five with excess radiation in the infrared, translating to a 4.3+2.7 –1.2% frequency of debris disks. This is slightly higher than, but consistent with the results of previous surveys. Using an initial-final mass relation, we apply this result to the progenitor stars of our sample and conclude that 1-7 M ? stars have at least a 4.3% chance of hosting planets; an indirect probe of the intermediate-mass regime eluding conventional exoplanetary detection methods. Alternatively, we interpret this result as a limit on accretion timescales as a fraction of WD cooling ages; WDs accrete debris from several generations of disks for ?10 Myr. The average total mass accreted by these stars ranges from that of 200 km asteroids to Ceres-sized objects, indicating that WDs accrete moons and dwarf planets as well as solar system asteroid analogs.

  10. THE FREQUENCY OF DEBRIS DISKS AT WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Barber, Sara D.; Patterson, Adam J.; Kilic, Mukremin [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019 (United States); Leggett, S. K. [Gemini Observatory, 670 N. A' ohoku Place, Hilo, HI 96720 (United States); Dufour, P. [Departement de Physique, Universite de Montreal, C.P. 6128, Succursale Centre-Ville, Montreal, Quebec H3C 3J7 (Canada); Bloom, J. S.; Starr, D. L., E-mail: barber@nhn.ou.edu [Department of Astronomy, University of California, Berkeley, CA 94720 (United States)

    2012-11-20

    We present near- and mid-infrared photometry and spectroscopy from PAIRITEL, IRTF, and Spitzer of a metallicity-unbiased sample of 117 cool, hydrogen-atmosphere white dwarfs (WDs) from the Palomar-Green survey and find five with excess radiation in the infrared, translating to a 4.3{sup +2.7} {sub -1.2}% frequency of debris disks. This is slightly higher than, but consistent with the results of previous surveys. Using an initial-final mass relation, we apply this result to the progenitor stars of our sample and conclude that 1-7 M {sub Sun} stars have at least a 4.3% chance of hosting planets; an indirect probe of the intermediate-mass regime eluding conventional exoplanetary detection methods. Alternatively, we interpret this result as a limit on accretion timescales as a fraction of WD cooling ages; WDs accrete debris from several generations of disks for {approx}10 Myr. The average total mass accreted by these stars ranges from that of 200 km asteroids to Ceres-sized objects, indicating that WDs accrete moons and dwarf planets as well as solar system asteroid analogs.

  11. Calcium-Rich Gap Transients: Tidal Detonations of White Dwarfs?

    CERN Document Server

    Sell, P H; Kotak, R; Knigge, C; Sand, D J

    2015-01-01

    We hypothesize that at least some of the recently discovered class of calcium-rich gap transients are tidal detonation events of white dwarfs (WDs) by black holes (BHs) or possibly neutron stars. We show that the properties of the calcium-rich gap transients agree well with the predictions of the tidal detonation model. Under the predictions of this model, we use a follow-up X-ray observation of one of these transients, SN 2012hn, to place weak upper limits on the detonator mass of this system that include all intermediate-mass BHs (IMBHs). As these transients are preferentially in the stellar haloes of galaxies, we discuss the possibility that these transients are tidal detonations of WDs caused by random flyby encounters with IMBHs in dwarf galaxies or globular clusters. This possibility has been already suggested in the literature but without connection to the calcium-rich gap transients. In order for the random flyby cross-section to be high enough, these events would have to be occurring inside these den...

  12. Type Ia Supernovae from Merging White Dwarfs I. Prompt Detonations

    CERN Document Server

    Moll, Rainer; Kasen, Daniel; Woosley, Stan

    2013-01-01

    Merging white dwarfs are a possible progenitor of Type Ia supernovae (SNe Ia). While it is not entirely clear if and when an explosion is triggered in such systems, numerical models suggest that a detonation might be initiated before the stars have coalesced to form a single compact object. Here we study such "peri-merger" detonations by means of numerical simulations, modeling the disruption and nucleosynthesis of the stars until the ejecta reach the coasting phase. Synthetic light curves and spectra are generated for comparison with observations. Three models are considered with primary masses 0.96 Msun, 1.06 Msun, and 1.20 Msun. Of these, the 0.96 Msun dwarf merging with an 0.81 Msun companion, with a Ni56 yield of 0.58 Msun, is the most promising candidate for reproducing common SNe Ia. The more massive mergers produce unusually luminous SNe Ia with peak luminosities approaching those attributed to "super-Chandrasekhar" mass SNe Ia. While the synthetic light curves and spectra of some of the models resemb...

  13. White Dwarfs in Globular Clusters HST Observations of M4

    CERN Document Server

    Richer, H B; Ibata, R A; Pryor, C E; Bell, R A; Bolte, M; Bond, H E; Harris, W E; Hesser, J E; Holland, S; Ivanans, N; Mandushev, G I; Stetson, P B; Wood, M A; Richer, Harvey B.; Fahlman, Gregory G.; Ibata, Rodrigo A.; Pryor, Carlton; Bell, Roger A.; Bolte, Michael; Bond, Howard E.; Harris, William E.; Hesser, James E.; Holland, Steve; Ivanans, Nicholas; Mandushev, Georgi; Stetson, Peter B.; Wood, Matt A.

    1997-01-01

    Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed and sizeable WD populations were found in all three. The location of these WDs in the color-magnitude diagram, their mean mass of 0.51($ \\pm 0.03$)M$_{\\odot}$, and their luminosity function confirm basic tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are about 9 Gyr old, a level which is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turnoff ages of globular clusters a...

  14. DUSTY DISKS AROUND WHITE DWARFS. I. ORIGIN OF DEBRIS DISKS

    International Nuclear Information System (INIS)

    A significant fraction of the mature FGK stars have cool dusty disks at least an order of magnitude brighter than the solar system's outer zodiacal light. Since such dusts must be continually replenished, they are generally assumed to be the collisional fragments of residual planetesimals analogous to the Kuiper-Belt objects. At least 10% of solar-type stars also bear gas giant planets. The fraction of stars with known gas giants or detectable debris disks (or both) appears to increase with the stellar mass. Here, we examine the dynamical evolution of systems of long-period gas giant planets and residual planetesimals as their host stars evolve off the main sequence, lose mass, and form planetary nebula around remnant white dwarf cores. The orbits of distant gas giant planets and super-km-size planetesimals expand adiabatically. During the most intense asymptotic giant branch mass-loss phase, sub-meter-size particles migrate toward their host stars due to the strong hydrodynamical drag by the intense stellar wind. Along their migration paths, gas giant planets capture and sweep up sub-km-size planetesimals onto their mean-motion resonances. These planetesimals also acquire modest eccentricities which are determined by the mass of the perturbing planets, and the rate and speed of stellar mass loss. The swept-up planetesimals undergo disruptive collisions which lead to the production of grains with an extended size range. The radiation drag on these particles is ineffecdiation drag on these particles is ineffective against the planets' resonant barrier and they form 30-50 AU size rings which can effectively reprocess the stellar irradiation in the form of FIR continuum. We identify the recently discovered dust ring around the white dwarf WD 2226-210 at the center of the Helix nebula as a prototype of such disks and suggest such rings may be common.

  15. The dynamics of polluting white dwarfs with remnant extrasolar Oort Cloud comets

    Science.gov (United States)

    Veras, D.; Shannon, A.; Gaensicke, B.

    2014-07-01

    The progenitors of the abundant metal pollution detected in white dwarf atmospheres is widely thought to be asteroidal, based on measured abundances of heavy elements. Not a single polluted white dwarf yet harbours abundances which are compositionally consistent with the ingestion of cometary material. This finding is surprising given the high frequency of observed Sun-grazing comets in our Solar System from the coronagraphs on SoHO's LASCO. Here, we consider this puzzle from a dynamical perspective. We determine how often remnant extrasolar 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 (0.52-1.00M_?) 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 the 5 Gyr ages of the oldest-known polluted white dwarfs. We find that the collision rates with giant branch stars and white dwarfs are roughly one comet per 10^{3} and 10^{4} yr, respectively (see the Figure). This rate is insufficiently low to account for all of the mass found in the deep convective layers of many polluted DBZ white dwarfs, but high enough to be observable with our current sample of polluted white dwarfs. Hence, we conclude that both the pollution process is not dominated by long-period comets and, alternatively, that we should see some evidence of their accretion as the polluted white dwarf sample increases over the next few years.

  16. ENERGY DISSIPATION THROUGH QUASI-STATIC TIDES IN WHITE DWARF BINARIES

    International Nuclear Information System (INIS)

    We present a formalism to study tidal interactions in white dwarf binaries in the limiting case of quasi-static tides, in which the tidal forcing frequencies are small, compared to the inverse of the white dwarf's dynamical timescale. The formalism is valid for arbitrary orbital eccentricities and therefore applicable to white dwarf binaries in the Galactic disk as well as globular clusters. In the quasi-static limit, the total perturbation of the gravitational potential shows a phase shift with respect to the position of the companion, the magnitude of which is determined primarily by the efficiency of energy dissipation through convective damping. We determine rates of secular evolution of the orbital elements and white dwarf rotational angular velocity for a 0.3 Msun helium white dwarf in binaries with orbital frequencies in the Laser Interferometer Space Antenna (LISA) gravitational wave frequency band and companion masses ranging from 0.3 Msun to 105 Msun. The resulting tidal evolution timescales for the orbital semimajor axis are longer than a Hubble time, so that convective damping of quasi-static tides need not be considered in the construction of gravitational wave templates of white dwarf binaries in the LISA band. Spin-up of the white dwarf, on the other hand, can occur on timescales of less than 10 Myr, provided that the white dwarf is initially rotating with a frequency much smaller than the orbital frequency. Fmuch smaller than the orbital frequency. For semi-detached white dwarf binaries spin-up can occur on timescales of less than 1 Myr. Nevertheless, the timescales remain longer than the orbital inspiral timescales due to gravitational radiation, so that the degree of asynchronism in these binaries increases. As a consequence, tidal forcing eventually occurs at forcing frequencies beyond the quasi-static tide approximation. For the shortest period binaries, energy dissipation is therefore expected to take place through dynamic tides and resonantly excited g-modes.

  17. Multi-site Observations Of The Accreting White Dwarf Pulsator SDSSJ161033.64-010223.3

    Science.gov (United States)

    Mukadam, Anjum S.; Gaensicke, B.; Townsley, D. M.; Szkody, P.; Bildsten, L.; Sion, E. M.

    2008-09-01

    Accreting white dwarfs in cataclysmic variables (CVs) have also recently been shown to exhibit non-radial pulsations similar to their non-interacting counterparts; GW Librae was the first accreting pulsator discovered in 1998 (Warner & van Zyl 1998; van Zyl et al. 2000, 2004). When the orbital period of a CV is close to the evolutionary period minimum, the accretion rate is small (Gaensicke et al. 2006). The system is in quiescence and the white dwarf flux dominates the light from the CV (Sion 1999). A search for non-radial pulsations among suitable candidates close to the period minimum has led to the discovery of 12 such systems known to date. These unique systems provide us with an opportunity to learn about the physical parameters of the accreting and pulsating white dwarf using the technique of asteroseismology. Accreting pulsators are of interest as they have likely undergone a few billion years of accretion and thousands of thermonuclear runaways. Questions we can address by studying these systems will be to what extent accretion affects the white dwarf mass, temperature, and composition and how efficiently angular momentum is transferred into the core of the white dwarf. Accreting pulsators in non-eclipsing CVs may prove to be our only opportunity to obtain meaningful mass constraints for the primary white dwarfs. Constraining the population, mass distribution, and evolution of accreting white dwarfs is also important for studying supernovae Type Ia systematics. We present the results from our multi-site observations of the accreting white dwarf pulsator SDSSJ161033.64-010223.3 (Woudt & Warner 2004), acquired in May 2007. The pulsation spectrum shows two independent periods at 609s and 347s. The light curve also allows us to measure the orbital period of the pulsating white dwarf using two different methods. We thank NSF for partial support of this project through the grant AST-0607840.

  18. A Hot DQ White Dwarf in the Open Star Cluster M35

    OpenAIRE

    Williams, Kurtis A.; Liebert, James; Bolte, Michael; Hanson, Robert B.

    2006-01-01

    We report the discovery of a hot DQ white dwarf, NGC 2168:LAWDS 28, that is a likely member of the 150-Myr old cluster NGC 2168 (Messier 35). The spectrum of the white dwarf is dominated by CII features. The effective temperature is difficult to estimate but likely > 20,000 K based on the temperatures of hot DQs with similar spectra. NGC2168:LAWDS 28 provides further evidence that hot DQs may be the ``missing'' high-mass helium-atmosphere white dwarfs. Based on published stu...

  19. Accretion of Debris-Disk Matter onto White Dwarfs: Stellar Consequences and Derived Accretion Rates

    Science.gov (United States)

    Deal, M.; Vauclair, S.; Vauclair, G.

    2013-12-01

    Heavy elements are observed in the atmospheres of many DA and DB white dwarfs, and their presence is attributed to the accretion of matter coming from debris disks. Several authors have deduced accretion rates from observed abundances, taking into account the mixing induced by convective zones and gravitational settling. The values obtained are different for DA and DB white dwarfs. Here we show that an important process was forgotten in all these computations: thermohaline mixing, induced by the inverse ?-gradient which is established during an accretion process. Taking this mixing into account leads to an increase in the derived accretion rates, especially for DA white dwarfs, and modifies the conclusions.

  20. Thermohaline Instabilities Induced by Heavy Element Accretion onto White Dwarfs: Consequences on the Derived Accretion Rates

    CERN Document Server

    Deal, M; Vauclair, G

    2012-01-01

    Heavy elements are observed in the atmospheres of many DA and DB white dwarfs, and their presence is attributed to the accretion of matter coming from debris disks. Several authors have deduced accretion rates from the observed abundances, taking into account the mixing induced by the convective zones and the gravitational settling. The obtained values are different for DA and DB white dwarfs. Here we show that an important process was forgotten in all these computations: thermohaline mixing, induced by the inverse $\\mu$-gradient built during the accretion process. Taking this mixing into account leads to an increase of the derived accretion rates, specially for DA white dwarfs, and modifies the conclusions.

  1. Thermohaline Instabilities Induced by Heavy Element Accretion onto White Dwarfs: Consequences on the Derived Accretion Rates

    Science.gov (United States)

    Deal, M.; Vauclair, S.; Vauclair, G.

    2013-01-01

    Heavy elements are observed in the atmospheres of many DA and DB white dwarfs, and their presence is attributed to the accretion of matter coming from debris disks. Several authors have deduced accretion rates from the observed abundances, taking into account the mixing induced by the convective zones and the gravitational settling. The obtained values are different for DA and DB white dwarfs. Here we show that an important process was forgotten in all these computations: thermohaline mixing, induced by the inverse ?-gradient built during the accretion process. Taking this mixing into account leads to an increase of the derived accretion rates, specially for DA white dwarfs, and modifies the conclusions.

  2. Accretion of planetary matter from debris disks around white dwarfs: the fate of planetary systems

    Science.gov (United States)

    Deal, M.; Deheuvels, S.; Vauclair, G.; Vauclair, S.; Wachlin, F. C.

    2013-12-01

    Heavy elements are observed in the atmospheres of many DA and DB white dwarfs, and their presence is attributed to the accretion of matter coming from debris disks. Several authors have deduced accretion rates from the observed abundances, taking into account the mixing induced by the convective zones and the gravitational settling. The obtained values are different for DA and DB white dwarfs. Here we show that an important process was forgotten in all these computations: thermohaline mixing, induced by the inverse ?-gradient built during the accretion process. Taking this mixing into account leads to an increase of the derived accretion rates, specially for DA white dwarfs, and modifies the conclusions.

  3. Limits on the neutrino magnetic dipole moment from the luminosity function of hot white dwarfs

    CERN Document Server

    Bertolami, Marcelo Miguel Miller

    2014-01-01

    Recent determinations of the white dwarf luminosity function (WDLF) from very large surveys have extended our knowledge of the WDLF to very high luminosities. This, together with the availability of new full evolutionary white dwarf models that are reliable at high luminosities, have opened the possibility of testing particle emission in the core of very hot white dwarfs, where neutrino processes are dominant. We use the available WDLFs from the Sloan Digital Sky Survey and the SuperCOSMOS Sky Survey to constrain the value of the neutrino magnetic dipole moment ($\\mu_\

  4. A search for a new class of pulsating DA white dwarf stars in the DB gap

    Science.gov (United States)

    Kurtz, D. W.; Shibahashi, H.; Dhillon, V. S.; Marsh, T. R.; Littlefair, S. P.

    2008-10-01

    While white dwarf stars are classified into many subgroups based on the appearance of hydrogen, helium, carbon, oxygen and other spectral lines - or even pure continuum with no lines in the case of the DC stars - the vast majority fall into two major subgroups: those with hydrogen atmospheres (the DA white dwarfs), and those with helium atmospheres (the DO and DB white dwarfs). Remarkably, in the range 45000 >= Teff >= 30000K there are only a few white dwarfs with helium atmospheres to be found - the vast majority are DAs in this temperature range - although white dwarfs with helium atmospheres are found at both hotter (DO) and cooler (DB) effective temperatures. This dearth of helium atmosphere white dwarfs in this temperature range is known as the `DB gap' and is understood in terms convective mixing of the outer atmospheres at the hot and cool ends of the gap, while radiative stability allows the lighter hydrogen to float to the top in the DB gap, so the stars are seen to be DA hydrogen atmosphere white dwarfs. Asteroseismology is an important tool for probing stellar interiors, and white dwarf stars are the most successfully studied group using this technique. In a stability analysis of the stars in the DB gap, Shibahashi has recently predicted the existence of a new class of pulsating white dwarf stars. He finds from models that DA white dwarfs near the red edge of the DB gap have convectively stable outer atmospheres because of a steep mean molecular weight gradient, yet nevertheless have a superadiabatic layer that renders them pulsationally unstable due to radiative heat exchange. There have been very few observational tests for pulsation among stars of this type. We have initiated a survey to search for the predicted pulsators and report here our first observations of five stars with the South African Astronomical Observatory 1.9-m telescope and University of Cape Town CCD photometer, and two stars with the William Herschel Telescope 4.2-m telescope and the ULTRACAM photometer. We have two detections at formal significance levels greater then 4? the rest are null results with upper limits of about 6-8mmag with the 1.9-m telescope and about 3mmag with the 4.2-m telescope. The two formally significant detections need confirmation, but the cases for them are good. Should they be confirmed, a new class of pulsating white dwarfs will become available for asteroseismic investigation, providing new insight into white dwarfs in general and into the DB gap in particular.

  5. Features of the mass transfer in magnetic cataclysmic variables with fast-rotating white dwarfs

    Directory of Open Access Journals (Sweden)

    Isakova Polina

    2014-01-01

    Full Text Available The flow structure in magnetic cataclysmic variables was investigated taking into account the effects of strong magnetic field and fast rotation of the white dwarf. We modeled the AE Aqr system as a unique object that has the rotation period of the white dwarf is about 1000 times shorter than the orbital period of the binary system. Observations show that in spite of fast rotation of the white dwarf some part of the stream from the inner Lagrange point comes into the Roche lobe region. We analyzed possible mechanisms preventing material to outflow from the system.

  6. On the incidence of weak magnetic fields in DA white dwarfs

    OpenAIRE

    Landstreet, J. D.; Bagnulo, S.; Valyavin, G. G.; Fossati, L.; Jordan, S.; Monin, D.; Wade, G.

    2012-01-01

    Context: About 10% of white dwarfs have magnetic fields with strength in the range between about 10^5 and 3x10^8 G. It is not known whether the remaining white dwarfs are not magnetic, or if they have a magnetic field too weak to be detected with the techniques adopted in the large surveys. Aims. We describe the results of the first survey specifically devised to clarify the detection frequency of kG-level magnetic fields in cool DA white dwarfs. Methods: Using the FORS1 ins...

  7. Violation of Chandrasekhar Mass Limit: The Exciting Potential of Strongly Magnetized White Dwarfs

    CERN Document Server

    Das, Upasana

    2012-01-01

    We consider a relativistic, degenerate, electron gas under the influence of a strong magnetic field, which describes magnetized white dwarfs. Landau quantization changes the density of states available to the electrons, thus modifying the underlying equation of state. In the presence of very strong magnetic fields a maximum of either one, two or three Landau level(s) is/are occupied. We obtain the mass-radius relations for such white dwarfs and their detailed investigation leads us to propose the existence of white dwarfs having a mass ~2.3M_Sun, which overwhelmingly exceeds the Chandrasekhar mass limit.

  8. Constraining white-dwarf kicks in globular clusters : III. Cluster Heating

    CERN Document Server

    Heyl, Jeremy S

    2008-01-01

    Recent observations of white dwarfs in globular clusters indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant branch is slightly asymmetric. The kicks may be large enough to dramatically change the radial distribution of young white dwarfs, giving them typically larger velocities than other stars of similar mass in the cluster. As these fast moving white dwarfs travel through the cluster they can impart their excess kinetic energy on the other stars in the cluster, providing a new heat source for globular clusters, especially during their youth.

  9. The habitability and detection of Earth-like planets orbiting cool white dwarfs

    CERN Document Server

    Fossati, L; Haswell, C A; Patel, M R; Busuttil, R; Kowalski, P M; Shulyak, D V; Sterzik, M F

    2012-01-01

    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 ~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, hence non-magnetic white dwarfs are compatible with the persistence of complex life. Polarisation due to a terrestrial planet in the CHZ of a cool white dwarf is 10^2 (10^4) 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 reveal the presence of a planet atmosphere, providing a first characterisation. Planets in the CHZ of a 0.6 M_sun white dwarf will be distorted by...

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

    International Nuclear Information System (INIS)

    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 ?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 102 (104) 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? 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.

  11. 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.

  12. A Second Stellar Color Locus: a Bridge from White Dwarfs to M stars

    CERN Document Server

    Smolcic, V; Knapp, G R; Lupton, R H; Pavlovski, K; Ilijic, S; Schlegel, D J; Smith, J A; McGehee, P M; Silvestri, N M; Hawley, S L; Rockosi, C M; Gunn, J E; Strauss, M A; Fan, X; Eisenstein, D J; Harris, H

    2004-01-01

    We report the discovery of a locus of stars in the SDSS g-r vs. u-g color-color diagram that connects the colors of white dwarfs and M dwarfs. While its contrast with respect to the main stellar locus is only ~1:2300, this previously unrecognized feature includes 863 stars from the SDSS Data Release 1. The position and shape of the feature are in good agreement with predictions of a simple binary star model that consists of a white dwarf and an M dwarf, with the components' luminosity ratio controlling the position along this binary system locus. SDSS DR1 spectra for 47 of these objects strongly support this model. The absolute magnitude--color distribution inferred for the white dwarf component is in good agreement with the models of Bergeron et al. (1995).

  13. KNOW YOUR NEIGHBORHOOD: A DETAILED MODEL ATMOSPHERE ANALYSIS OF NEARBY WHITE DWARFS

    International Nuclear Information System (INIS)

    We present improved atmospheric parameters of nearby white dwarfs lying within 20 pc of the Sun. The aim of the current study is to obtain the best statistical model of the least-biased sample of the white dwarf population. A homogeneous analysis of the local population is performed combining detailed spectroscopic and photometric analyses based on improved model atmosphere calculations for various spectral types including DA, DB, DC, DQ, and DZ stars. The spectroscopic technique is applied to all stars in our sample for which optical spectra are available. Photometric energy distributions, when available, are also combined to trigonometric parallax measurements to derive effective temperatures, stellar radii, as well as atmospheric compositions. A revised catalog of white dwarfs in the solar neighborhood is presented. We provide, for the first time, a comprehensive analysis of the mass distribution and the chemical distribution of white dwarf stars in a volume-limited sample.

  14. Neutrino emission and oscillations in white dwarf matter accreting onto a primordial black hole

    International Nuclear Information System (INIS)

    Properties of the neutrinos emitted during an accretion of a white dwarf substance by a primordial black hole are considered. The possibility to detect these neutrinos and their oscillations is discussed

  15. The polluted atmospheres of cool white dwarfs and the magnetic field connection

    CERN Document Server

    Kawka, A

    2014-01-01

    We present an analysis of X-Shooter spectra of the polluted hydrogen-rich white dwarfs (DAZ) NLTT 888 and NLTT 53908. The spectra of NLTT 53908 show strong, Zeeman-split calcium lines (CaII H&K and Ca I $\\lambda 4226$) and the star appears to be a close relative of the polluted magnetic white dwarf (DAZH) NLTT 10480, while the spectra of NLTT 888 show narrow lines of calcium and iron. A comparison of the DAZ NLTT 888 and the DAZH NLTT 53908 with other class members illustrates the diversity of environment and formation circumstances surrounding these objects. In particular, we find that the incidence of magnetism in old, polluted white dwarfs significantly exceeds that found in the general white dwarf population which suggests an hypothetical link between a crowded planetary system and magnetic field generation.

  16. An upper limit to the secular variation of the gravitational constant from white dwarf stars

    International Nuclear Information System (INIS)

    A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound ?/G ? ?1.8 × 10?12 yr?1. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function

  17. A determination of the white-dwarf masses in wide binary radio-pulsar systems

    International Nuclear Information System (INIS)

    The paper concerns a determination of the mass of the white dwarf in wide binary radio-pulsar systems, including the mass of neutron star PSR 1855 + 09. The binary pulsars considered consist of systems with long orbital periods, low mass functions and circular orbits. The determination of the white-dwarf mass is based on a constraint to the orbital solution, i.e. that the giant progenitor of the present white dwarf secondary should have filled its Roche lobe while spinning up the pulsar companion by mass transfer. Employment of this constraint leads to the mass of the white dwarf in these type of binary pulsars, and to an upper limit of approx. 1.2 M solar mass to PSR 1855 + 09. (U.K.)

  18. KNOW YOUR NEIGHBORHOOD: A DETAILED MODEL ATMOSPHERE ANALYSIS OF NEARBY WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Giammichele, N.; Bergeron, P. [Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation (United States); Dufour, P., E-mail: noemi.giammichele@astro.umontreal.ca, E-mail: pierre.bergeron@astro.umontreal.ca, E-mail: patrick.dufour@astro.umontreal.ca [Departement de Physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada)

    2012-04-01

    We present improved atmospheric parameters of nearby white dwarfs lying within 20 pc of the Sun. The aim of the current study is to obtain the best statistical model of the least-biased sample of the white dwarf population. A homogeneous analysis of the local population is performed combining detailed spectroscopic and photometric analyses based on improved model atmosphere calculations for various spectral types including DA, DB, DC, DQ, and DZ stars. The spectroscopic technique is applied to all stars in our sample for which optical spectra are available. Photometric energy distributions, when available, are also combined to trigonometric parallax measurements to derive effective temperatures, stellar radii, as well as atmospheric compositions. A revised catalog of white dwarfs in the solar neighborhood is presented. We provide, for the first time, a comprehensive analysis of the mass distribution and the chemical distribution of white dwarf stars in a volume-limited sample.

  19. Testing Fundamental Particle Physics with the Galactic White Dwarf Luminosity Function

    CERN Document Server

    Bertolami, Marcelo M Miller; Althaus, Leandro G; Isern, Jordi

    2014-01-01

    Recent determinations of the white dwarf luminosity function (WDLF) from very large surveys have extended our knowledge of the WDLF to very high luminosities. It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to test the possible properties and existence of fundamental weakly interacting subelectronvolt particles. This, together with the availability of new full evolutionary white dwarf models that are reliable at high luminosities, have opened the possibility of testing particle emission in the core of very hot white dwarfs. We use the available WDLFs from the Sloan Digital Sky Survey and the SuperCOSMOS Sky Survey to constrain the values of the neutrino magnetic dipole moment ($\\mu_\

  20. Spiral Disk Instability Can Drive Thermonuclear Explosions in Binary White Dwarf Mergers

    CERN Document Server

    Kashyap, Rahul; García-Berro, Enrique; Aznar-Siguán, Gabriela; Ji, Suoqing; Lorén-Aguilar, Pablo

    2015-01-01

    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 which 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 which 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.

  1. Spiral Instability Can Drive Thermonuclear Explosions in Binary White Dwarf Mergers

    Science.gov (United States)

    Kashyap, Rahul; Fisher, Robert; García-Berro, Enrique; Aznar-Siguán, Gabriela; Ji, Suoqing; Lorén-Aguilar, Pablo

    2015-02-01

    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.

  2. Know Your Neighborhood: A Detailed Model Atmosphere Analysis of Nearby White Dwarfs

    CERN Document Server

    Giammichele, Noemi; Dufour, Patrick

    2012-01-01

    We present improved atmospheric parameters of nearby white dwarfs lying within 20 pc of the Sun. The aim of the current study is to obtain the best statistical model of the least-biased sample of the white dwarf population. A homogeneous analysis of the local population is performed combining detailed spectroscopic and photometric analyses based on improved model atmosphere calculations for various spectral types including DA, DB, DC, DQ, and DZ stars. The spectroscopic technique is applied to all stars in our sample for which optical spectra are available. Photometric energy distributions, when available, are also combined to trigonometric parallax measurements to derive effective temperatures, stellar radii, as well as atmospheric compositions. A revised catalog of white dwarfs in the solar neighborhood is presented. We provide, for the first time, a comprehensive analysis of the mass distribution and the chemical distribution of white dwarf stars in a volume-limited sample.

  3. On the Interpretation of High Velocity White Dwarfs as Members of the Galactic Halo

    CERN Document Server

    Bergeron, P; Hamuy, M; Leggett, S K; Currie, M J; Lajoie, C P; Dufour, P; Ruiz, Maria Teresa

    2005-01-01

    A detailed analysis of 32 of the 38 halo white dwarf candidates identified by Oppenheimer et al. is presented, based on model atmosphere fits to observed energy distributions built from optical BVRI and infrared JHK CCD photometry. Effective temperatures and atmospheric compositions are determined for all objects, as well as masses and cooling ages when trigonometric parallax measurements are available. This sample is combined with that of other halo white dwarf candidates and disk white dwarfs to study the nature of these objects in terms of reduced proper motion diagrams, tangential velocities, and stellar ages. We reaffirm the conclusions of an earlier analysis based on photographic magnitudes of the same sample that total stellar ages must be derived in order to associate a white dwarf with the old halo population, and that this can only be accomplished through precise mass and distance determinations.

  4. An upper limit to the secular variation of the gravitational constant from white dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    García-Berro, Enrique; Lorén-Aguilar, Pablo; Torres, Santiago [Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades, 5, 08860 Castelldefels (Spain); Althaus, Leandro G. [Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, (1900) La Plata (Argentina); Isern, Jordi, E-mail: garcia@fa.upc.edu, E-mail: loren@fa.upc.edu, E-mail: santi@fa.upc.edu, E-mail: althaus@fcaglp.fcaglp.unlp.edu.ar, E-mail: isern@ieec.cat [Institut de Ciències de l' Espai (CSIC), Campus UAB, 08193 Bellaterra (Spain)

    2011-05-01

    A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound ?/G ? ?1.8 × 10{sup ?12} yr{sup ?1}. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.

  5. Strongly magnetized electron degenerate gas: Highly super-Chandrasekhar white dwarfs

    CERN Document Server

    Das, Upasana

    2013-01-01

    We consider a relativistic, degenerate, electron gas under the influence of a strong magnetic field, which describes magnetized white dwarfs. Landau quantization changes the density of states available to the electrons, thus modifying the underlying equation of state. We obtain the mass-radius relations for such white dwarfs and show that it is possible to have magnetized white dwarfs with a mass significantly greater than the Chandrasekhar limit in the range 2.3 - 2.6 M_sun. Recent observations of peculiar type Ia supernovae - SN 2006gz, SN 2007if, SN 2009dc, SN 2003fg - seem to suggest super-Chandrasekhar-mass white dwarfs with masses up to 2.4 - 2.8 M_sun, as their most likely progenitors and interestingly our results lie within the observational limits.

  6. On Type Ia Supernovae From The Collisions of Two White Dwarfs

    CERN Document Server

    Raskin, Cody; Scannapieco, Evan; Diehl, Steven; Fryer, Chris

    2009-01-01

    We explore collisions between two white dwarfs as a pathway for making Type Ia Supernovae (SNIa). White dwarf number densities in globular clusters allow 10-100 redshift <1 collisions per year, and observations by (Chomiuk et al. 2008) of globular clusters in the nearby S0 galaxy NGC 7457 have detected what is likely to be a SNIa remnant. We carry out simulations of the collision between two 0.6 solar mass white dwarfs at various impact parameters and mass resolutions. For impact parameters less than half the radius of the white dwarf, we find such collisions produce approximately 0.4 solar masses of Ni56, making such events potential candidates for underluminous SNIa or a new class of transients between Novae and SNIa.

  7. Elements separation and mixing processes in the envelopes of white dwarfs

    International Nuclear Information System (INIS)

    The discovery of very hot hydrogen rich and helium rich white dwarfs suggests that the separation into these two chemically different groups occurs very early in the evolution of the white dwarfs. In this paper, we investigate the possibility that helium rich white dwarfs could have evolved from initially hydrogen rich white dwarfs. We first estimate the time scale for helium diffusion in the envelope of a white dwarf. The properties of the variable mean molecular weight region (the ?-barrier) built by the diffusion processes are discussed. Then we investigate two possible sources of mixing: first, we analyze the development of the diferential rotation induced by the meridional circulation in rotating white dwarfs. We estimate how long it takes to the shear flow of this differential rotation to develop instabilities, according to the Richardson criteria. The second mixing mechanism examined here is convection. We calculate convection zones in a number of models of inhomogeneous envelopes, consisting of hydrogen floating on a helium rich interior with a realistic ?-barrier between. We find that mixing does not occur at effective temperatures higher than Tsub(e) approximately 11,000-12,0000K. The mixing does occur at lower temperatures depending on the hydrogen content. It is shown that the DB white dwarfs, as a whole, could not be produced by such convective mixing in initially DA white dwarfs. At cooler effective temperatures,the helium rich sequence coulmperatures,the helium rich sequence could be mixture of both populations: one restulting from the cooling of the DBs, the other from the mixing of the DAs. (orig./WL)

  8. Might Carbon-Atmosphere White Dwarfs Harbour a New Type of Pulsating Star?

    OpenAIRE

    Fontaine, G.; Brassard, P.; Dufour, P.

    2008-01-01

    In the light of the recent and unexpected discovery of a brand new type of white dwarfs, those with carbon-dominated atmospheres, we examine the asteroseismological potential of such stars. The motivation behind this is based on the observation that past models of carbon-atmosphere white dwarfs have partially ionized outer layers that bear strong resemblance with those responsible for mode excitation in models of pulsating DB (helium-atmosphere) and pulsating DA (hydrogen-at...

  9. White dwarfs and neutron stars in globular cluster X-ray sources

    International Nuclear Information System (INIS)

    It is predicted that globular clusters contain at least as many cataclysmic variables, which contain white dwarfs, as bright X-ray sources, which contain neutron stars. The consequences of the frequent formation of temporarily bound triple systems (resonance scattering) are examined including; the extra enhancement of neutron star capture with respect to white dwarf capture, the considerable probability of actual collisions between stars, and the presence of cataclysmic variables in the outer regions of globular clusters. (U.K.)

  10. The Physics of Crystallization from Globular Cluster White Dwarf Stars in NGC 6397

    OpenAIRE

    Winget, D. E.; Kepler, S. O.; Campos, Fabiola; Montgomery, M. H.; Girardi, Leo; Bergeron, P.; Williams, Kurtis

    2009-01-01

    We explore the physics of crystallization in the deep interiors of white dwarf stars using the color-magnitude diagram and luminosity function constructed from proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with the theory of crystallization of the ions in the interior of white dwarf stars and provide the first empirical evidence that the phase transition is first order: latent heat is rel...

  11. Temperature and Gravity Dependence of Trace Element Abundances in Hot DA White Dwarfs (94-EUVE-094)

    Science.gov (United States)

    Finley, David S.

    1998-01-01

    EUV spectroscopy has shown that DA white dwarfs hotter than about 45,000 K may contain trace heavy elements, while those hotter than about 50,000 K almost always have significant abundances of trace heavy elements. One of our continuing challenges is to identify and determine the abundances of these trace constituents, and then to relate the observed abundance patterns to the present conditions and previous evolutionary histories of the hot DA white dwarfs.

  12. Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey

    OpenAIRE

    Leggett, S. K.; Lodieu, N.; Tremblay, P. -e; Bergeron, P.; Nitta, A.

    2011-01-01

    We present the results of a search for cool white dwarfs in the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky Survey (SDSS) to identify cool hydrogen-rich white dwarf candidates by their neutral optical colors and blue near-infrared colors, as well as faint Reduced Proper Motion magnitudes. Optical spectroscopy was obtained at Gemini Observatory, and showed ...

  13. Stars of type MS with evidence of white dwarf companions. [IUE, Main Sequence (MS)

    Science.gov (United States)

    Peery, Benjamin F., Jr.

    1986-01-01

    A search for white dwarf companions of MS-type stars was conducted, using IUE. The overendowments of these stars in typical S-process nuclides suggest that they, like the Ba II stars, may owe their peculiar compositions to earlier mass transfer. Short-wavelength IUE spectra show striking emission line variability in HD35155, HD61913, and 4 Ori; HD35155 and 4 Ori show evidence of white dwarf companions.

  14. The Ratio of Helium- to Hydrogen-Atmosphere White Dwarfs: Direct Evidence for Convective Mixing

    OpenAIRE

    Tremblay, P. -e; Bergeron, P.

    2007-01-01

    We determine the ratio of helium- to hydrogen-atmosphere white dwarf stars as a function of effective temperature from a model atmosphere analysis of the infrared photometric data from the Two Micron All Sky Survey combined with available visual magnitudes. Our study surpasses any previous analysis of this kind both in terms of the accuracy of the Teff determinations as well as the size of the sample. We observe that the ratio of helium- to hydrogen-atmosphere white dwarfs i...

  15. Observing white dwarfs orbiting massive black holes in the gravitational wave and electro-magnetic window

    OpenAIRE

    Sesana, A.; Vecchio, A.; Eracleous, M.; Sigurdsson, S.

    2008-01-01

    We consider a potentially new class of gravitational wave sources consisting of a white dwarf coalescing into a massive black hole in the mass range ~10^4-10^5\\msun. These sources are of particular interest because the gravitational wave signal produced during the inspiral phase can be detected by the Laser Interferometer Space Antenna (LISA) and is promptly followed, in an extended portion of the black hole and white dwarf mass parameter space, by an electro-magnetic signal...

  16. On the distance scale of planetary nebulae and white dwarf birth rates

    International Nuclear Information System (INIS)

    Arguments are presented which favor an increase of the distance scale of planetary nebulae by 30% compared to the Seaton-Webster scale. The consequences for evolutionary tracks, PN and white dwarf relations, and birth rates are discussed. It is concluded that opposite to Smith jr. (1976) underestimated, and that the proposed change in distance scale of PN brings white dwarf and PN birth rates into almost complete agreement. (orig.)

  17. Comment on "Strong constraints on magnetized white dwarfs surpassing the Chandrasekhar mass limit"

    OpenAIRE

    Das, Upasana; Mukhopadhyay, Banibrata

    2014-01-01

    We show that the upper bound for the central magnetic field of a super-Chandrasekhar white dwarf calculated by Nityananda and Konar [Phys. Rev. D 89, 103017 (2014)] is completely erroneous. This in turn strengthens the argument in favor of the stability of the recently proposed magnetized super-Chandrasekhar white dwarfs. We also point out several other numerical errors in their work. Overall we conclude, based on our calculations, that the arguments put forth by Nityananda ...

  18. Violation of Chandrasekhar Mass Limit: The Exciting Potential of Strongly Magnetized White Dwarfs

    OpenAIRE

    Das, Upasana; Mukhopadhyay, Banibrata

    2012-01-01

    We consider a relativistic, degenerate, electron gas under the influence of a strong magnetic field, which describes magnetized white dwarfs. Landau quantization changes the density of states available to the electrons, thus modifying the underlying equation of state. In the presence of very strong magnetic fields a maximum of either one, two or three Landau level(s) is/are occupied. We obtain the mass-radius relations for such white dwarfs and their detailed investigation l...

  19. Revisiting some physics issues related to the new mass limit for magnetized white dwarfs

    OpenAIRE

    Das, Upasana; Mukhopadhyay, Banibrata

    2013-01-01

    We clarify important physics issues related to the recently established new mass limit for magnetized white dwarfs which is significantly super-Chandrasekhar. The issues include, justification of high magnetic field and the corresponding formation of stable white dwarfs, contribution of the magnetic field to the total density and pressure, flux freezing, variation of magnetic field and related currents therein. We also attempt to address the observational connection of such ...

  20. Does an Average White Dwarf Have Enough Mass to Prevent an Accretion Disk Tilt?

    OpenAIRE

    Montgomery, M. M.

    2010-01-01

    In a recent publication, we introduce the lift force as a common source to accretion disk tilt that is likely relevant to accretion disk systems. Lift is generated by slightly different supersonic gas stream speeds flowing over and under the disk at the bright spot. In this conference proceeding, we focus on whether the average white dwarf has enough mass to prevent a disk tilt in non-magnetic Cataclysmic Variables (CVs) with accretion disks. Assuming a white dwarf mass of 0...

  1. Energy Dissipation through Quasi-Static Tides in White Dwarf Binaries

    OpenAIRE

    Willems, B.; Deloye, C. J.; Kalogera, V.

    2009-01-01

    We study tidal interactions in white dwarf binaries in the limiting case of quasi-static tides. The formalism is valid for arbitrary orbital eccentricities and therefore applicable to white dwarf binaries in the Galactic disk as well as globular clusters. In the quasi-static limit, the total perturbation of the gravitational potential shows a phase shift with respect to the position of the companion, the magnitude of which is determined primarily by the efficiency of energy ...

  2. Supernova Type Ia progenitors from merging double white dwarfs: Using a new population synthesis model

    OpenAIRE

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

    2012-01-01

    The study of Type Ia supernovae (SNIa) has lead to greatly improved insights into many fields in astrophysics, however a theoretical explanation of the origin of these events is still lacking. We investigate the potential contribution to the SNIa rate from the population of merging double carbon-oxygen white dwarfs. We aim to develope a model that fits the observed SNIa progenitors as well as the observed close double white dwarf population. We differentiate between two scen...

  3. Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfs

    OpenAIRE

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

    2007-01-01

    We report the first discovery of argon in hot evolved stars and white dwarfs. We have identified the ArVII 1063.55A line in some of the hottest known (Teff=95000-110000 K) central stars of planetary nebulae and (pre-) white dwarfs of various spectral type. We determine the argon abundance and compare it to theoretical predictions from stellar evolution theory as well as from diffusion calculations. We analyze high-resolution spectra taken with the Far Ultraviolet Spectroscop...

  4. Echelle Studies of Helium-Rich White Dwarfs

    Science.gov (United States)

    Sion, Edward M.

    We propose to observe two different spectroscopic types of hot helium-rich white dwarfs, a DB GD358, and a DO HZ21, in the high dispersion mode. Our targets represent the first helium-rich degenerates, below 50000K to be observed with the IUE echelle. one target, GD358 (the first DB ever to be observed with the echelle) is the pulsating prototype of a group of DB stars exhibiting non-radial, g-mode oscillations and lies in temperature near the blue edge of the DB instability strip at the lower boundary of the DO/DB gap. The other target, HZ21 lies at the upper boundary of the DO/DB gap and represents the coolest of only four DO stars that were accessible to IUE echelle observations. This downward extension in temperature of our echelle studies of helium-rich white dwarfs, has the following scientific objectives: (1) to search for evidence of recent or ongoing mass loss undetectable at low IUE resolution but detected with the IUE echelle among hotter helium-rich (DO) stars and some DA stars of comparable Teff as our targets; (2) to search for evidence of sharp ionized metal absorption features formed in and/or far above the photosphere due either to radiative forces levitation, convective dredge-up or interstellar accretion and look for evidence of a sharp non-LTE line core at HeII (1640) in HZ21; (3) to derive metal abundances of detected features thus providing a critical test of radiative forces/diffusion theory in hot, helium-rich degenerates at lower temperatures; to derive a more stringent upper limit carbon/helium abundance for a DB star; (4) to use the abundances and mix of detected ions together with upper limit rotational velocities from their FWHM to test theoretical scenarios for DB formation, the DO/DB gap and the DO ----> DB evolutionary link. A comparison of HZ21 with the hotter DO stars is crucial to identifying its progenitor channel.

  5. 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.

  6. Finding rocky asteroids around white dwarfs by their periodic thermal emission

    CERN Document Server

    Lin, Henry

    2014-01-01

    Since old white dwarfs are exceptionally dim, the contrast between the thermal emission of an orbiting object and a white dwarf is dramatically enhanced compared to a main sequence host. Furthermore, rocky objects much smaller than the moon have no atmospheres and are tidally locked to the white dwarf if they orbit near the Roche zone. We show that this leads to temperature contrasts between their day and night side of order unity that should lead to temporal variations in infrared flux over an orbital period of ~ 0.2 to ~ 2 days. Ground based telescopes could detect objects with a mass as small as 1% of the lunar mass $M_L$ around Sirius B with a few hours of exposure. The James Webb Space Telescope (JWST) may be able to detect objects as small as $10^{-3} M_L$ around most nearby white dwarfs. The tightest constraints will typically be placed on 12,000 K white dwarfs, whose Roche zone coincides with the dust sublimation zone. Constraining the abundance of minor planets around white dwarfs as a function of th...

  7. Fundamental properties of the pulsar/white-dwarf binary PSR B1855+09

    Science.gov (United States)

    van Kerkwijk, Marten

    1996-07-01

    We propose to obtain a broad-band image of the field of the millisecond pulsar B1855+09, to identify the white-dwarf companion. The PSR B1855+09 system is unique. It is the only one for which Shapiro delay of the pulsed signal has yielded an accurate mass determination of the white dwarf {M_WD=0.258^+0.028_-0.016 Msun}, and it is the only pulsar for which a parallax has been measured. If the temperature of the white dwarf were known as well, then the system would offer an unprecedented opportunity for quantitative comparisons between the theoretical and observational properties of a low-mass helium white dwarf, which would allow one to constrain the interior structure and atmosphere of the white dwarf, to determine the cooling age of the white dwarf, and to constrain the age of the pulsar. At its low Galactic latitude, the companion has eluded detection until the appearance of the Keck telescope, with which we have found a candidate counterpart. We hope to confirm the identification with this HST observation.

  8. 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. PMID:20164924

  9. Precise parameters for both white dwarfs in the eclipsing binary CSS 41177

    Science.gov (United States)

    Bours, M. C. P.; Marsh, T. R.; Parsons, S. G.; Copperwheat, C. M.; Dhillon, V. S.; Littlefair, S. P.; Gänsicke, B. T.; Gianninas, A.; Tremblay, P.-E.

    2014-03-01

    We present ULTRACAM photometry and X-Shooter spectroscopy of the eclipsing double white dwarf binary CSS 41177, the only such system that is also a double-lined spectroscopic binary. Combined modelling of the light curves and radial velocities yield masses and radii for both white dwarfs without the need to assume mass-radius relations. We find that the primary white dwarf has a mass of M1 = 0.38 ± 0.02 M? and a radius of R1 = 0.0222 ± 0.0004 R?. The secondary white dwarf's mass and radius are M2 = 0.32 ± 0.01 M? and R2 = 0.0207 ± 0.0004 R?, and its temperature and surface gravity (T2 = 11678 ± 313 K, log(g2) = 7.32 ± 0.02) put it close to the white dwarf instability strip. However, we find no evidence for pulsations to roughly 0.5 per cent relative amplitude. Both masses and radii are consistent with helium white dwarf models with thin hydrogen envelopes of ?10-4 M*. The two stars will merge in 1.14 ± 0.07 Gyr due to angular momentum loss via gravitational wave emission.

  10. Precise parameters for both white dwarfs in the eclipsing binary CSS 41177

    CERN Document Server

    Bours, M C P; Parsons, S G; Copperwheat, C M; Dhillon, V S; Littlefair, S P; Gänsicke, B T; Gianninas, A; Tremblay, P -E

    2014-01-01

    We present ULTRACAM photometry and X-Shooter spectroscopy of the eclipsing double white dwarf binary CSS 41177, the only such system that is also a double-lined spectroscopic binary. Combined modelling of the light curves and radial velocities yield masses and radii for both white dwarfs without the need to assume mass-radius relations. We find that the primary white dwarf has a mass of M1 = 0.38(2) Msun and a radius of R1 = 0.0222(4) Rsun. The secondary white dwarf's mass and radius are M2 = 0.32(1) Msun and R2 = 0.0207(4) Rsun, and its temperature and surface gravity (T2 = 11678(313) K, log(g2) = 7.32(2)) put it close to the white dwarf instability strip. However, we find no evidence for pulsations to roughly 0.5% relative amplitude. Both masses and radii are consistent with helium white dwarf models with thin hydrogen envelopes of 0.0001 Mstar. The two stars will merge in 1.14 Gyr due to angular momentum loss via gravitational wave emission.

  11. Double white dwarf mergers and elemental surface abundances in extreme helium and R Coronae Borealis stars

    CERN Document Server

    Jeffery, C Simon; Saio, Hideyuki

    2011-01-01

    The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB) stars are discussed in terms of the merger of a carbon-oxygen white dwarf with a helium white dwarf. The model is expressed as a linear mixture of the individual layers of both constituent white dwarfs, taking account of the specific evolution of each star. In developing this recipe from previous versions, particular attention has been given to the inter-shell abundances of the asymptotic giant branch star which evolved to become the carbon-oxygen white dwarf. Thus the surface composition of the merged star is estimated as a function of the initial mass and metallicity of its progenitor. The question of whether additional nucleosynthesis occurs during the white dwarf merger has been examined. The high observed abundances of carbon and oxygen must either originate by dredge-up from the core of the carbon-oxygen white dwarf during a cold merger or be generated directly by alpha-burning during a hot merger. The presence of large quantiti...

  12. THE HYADES CLUSTER: IDENTIFICATION OF A PLANETARY SYSTEM AND ESCAPING WHITE DWARFS

    International Nuclear Information System (INIS)

    Recently, some hot DA-type white dwarfs have been proposed to plausibly be escaping members of the Hyades. We used hydrogen Balmer lines to measure the radial velocities of seven such stars and confirm that three, and perhaps two others, are/were indeed cluster members and one is not. The other candidate Hyad is strongly magnetic and its membership status remains uncertain. The photospheres of at least one quarter of field white dwarf stars are ''polluted'' by elements heavier than helium that have been accreted. These stars are orbited by extended planetary systems that contain both debris belts and major planets. We surveyed the seven classical single Hyades white dwarfs and the newly identified (escaping) Hyades white dwarfs and found calcium in the photosphere of LP 475-242 of type DBA (now DBAZ), thus implying the presence of an orbiting planetary system. The spectrum of white dwarf GD 31, which may be, but probably is not, an escaping member of the Hyades, displays calcium absorption lines; these originate either from the interstellar medium or, less likely, from a gaseous circumstellar disk. If GD 31 was once a Hyades member, then it would be the first identified white dwarf Hyad with a cooling age >340 Myr.

  13. Testing Stellar Evolution Theory with the Binary Fraction of Low-Mass White Dwarfs

    Science.gov (United States)

    Brown, Justin; Kilic, M.; Brown, W.

    2011-01-01

    We describe spectroscopic observations of 21 low-mass (< 0.45 Msolar) white dwarfs from the Palomar-Green Survey obtained over a period of three years. We use both radial velocity analysis and infrared photometry to identify binary systems, and find that the fraction of single, low-mass white dwarfs is 30% with an uncertainty of 18%. We discuss the inverse relationship between the mass and the binary fraction of white dwarfs and compare this relationship to existing theoretical models. We also compare the period distribution of 0.2 and 0.4 solar mass white dwarfs and find that lower mass white dwarfs have systematically shorter orbital periods. Our results support the idea that single low-mass white dwarfs can be formed through enhanced mass-loss from a metal-rich progenitor star. This work is supported in part by the NSF REU and DOD ASSURE programs under NSF grant no. 0754568 and by the Smithsonian Institution.

  14. Orbital Parameters and Chemical Composition of Four White Dwarfs in Post-Common-Envelope Binaries

    Science.gov (United States)

    Kawka, Adela; Vennes, Stéphane; Dupuis, Jean; Chayer, Pierre; Lanz, Thierry

    2008-03-01

    We present FUSE observations of the hot white dwarfs in the post-common-envelope binaries Feige 24, EUVE J0720-317, BPM 6502, and EUVE J2013+400. The spectra show numerous photospheric absorption lines, which trace the white dwarf orbital motion. We report the detection of C III, O VI, P V, and Si IV in the spectra of Feige 24, EUVE J0720-317, and EUVE J2013+400 and the detection of C III, N II, Si III, Si IV, and Fe III in the spectra of BPM 6502. Abundance measurements support the possibility that white dwarfs in post-common-envelope binaries accrete material from the secondary star wind. The FUSE observations of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We used the FUSE spectra to measure the radial velocities traced by the white dwarf in the four binaries, where the zero-point velocities were fixed using the ISM velocities in the line of sight of the stellar systems. For BPM 6502 we determined a white dwarf velocity semiamplitude of KWD=18.6+/-0.5 km s-1, and with the velocity semiamplitude of the red dwarf companion (KRD=75.2+/-3.1 km s-1), we estimate the mass ratio to be q=0.25+/-0.01. Adopting a spectroscopic mass determination for the white dwarf, we infer a low secondary mass of MRD=0.14+/-0.01 Msolar. For EUVE J2013+400 we determine a white dwarf velocity semiamplitude of KWD=36.7+/-0.7 km s-1. The FUSE observations of EUVE J0720-317 cover approximately 30% of the binary period and, combined with the HST GHRS measurements, we update the binary properties. FUSE observations of Feige 24 cover approximately 60% of the orbit, and we combine this data set with HST STIS data to update the binary properties.

  15. The not-so-extreme white dwarf of the CV GD 552

    International Nuclear Information System (INIS)

    GD 552 is a cataclysmic binary which was previously believed to be composed of an M-star and a white dwarf, the latter having an extreme mass of 1.4 solar masses. In a recent paper we showed that this is not compatible with new observational evidence and presented an alternative model in which the white dwarf has a typical mass and the companion is a brown dwarf, making the system a likely member of the elusive group of CVs which have already evolved through minimum orbital period. Here we present additional spectroscopical evidence supporting this conclusion by means of skew mapping.

  16. Dynamical tides in compact white dwarf binaries: influence of rotation

    Science.gov (United States)

    Fuller, Jim; Lai, Dong

    2014-11-01

    Tidal interactions play an important role in the evolution and ultimate fate of compact white dwarf (WD) binaries. Not only do tides affect the pre-merger state (such as temperature and rotation rate) of the WDs, but they may also determine which systems merge and which undergo stable mass transfer. In this paper, we attempt to quantify the effects of rotation on tidal angular momentum transport in binary stars, with specific calculations applied to WD stellar models. We incorporate the effect of rotation using the traditional approximation, in which the dynamically excited gravity waves within the WDs are transformed into gravitoinertial Hough waves. The Coriolis force has only a minor effect on prograde gravity waves, and previous results predicting the tidal spin-up and heating of inspiraling WDs are not significantly modified. However, rotation strongly alters retrograde gravity waves and inertial waves, with important consequences for the tidal spin-down of accreting WDs. We identify new dynamical tidal forcing terms that arise from a proper separation of the equilibrium and dynamical tide components; these new forcing terms are very important for systems near synchronous rotation. Additionally, we discuss the impact of Stokes drift currents on the wave angular momentum flux. Finally, we speculate on how tidal interactions will affect supersynchronously rotating WDs in accreting systems.

  17. A Third Hot White Dwarf Companion Detected by Kepler

    CERN Document Server

    Carter, Joshua A; Fabrycky, Daniel

    2010-01-01

    We have found a system listed in the Kepler Binary Catalog (3.273 day period; Prsa et al. 2010) that we have determined is comprised of a low-mass, thermally-bloated, hot white dwarf orbiting an A star of about 2.3 solar masses. In this work we designate the object, KIC 10657664, simply as KHWD3. We use the transit depth of ~0.66%, the eclipse depth of ~1.9%, and regular smooth periodic variations at the orbital frequency and twice the orbital frequency to analyze the system parameters. The smooth periodic variations are identified with the classical ellipsoidal light variation and illumination effects, and the newly utilized Doppler boosting effect. Given the measured values of R/a and inclination angle of the binary, both the ELV and DB effects are mostly sensitive to the mass ratio, q = M_2/M_1, of the binary. The two effects yield values of q which are somewhat inconsistent - presumably due to unidentified systematic effects - but which nonetheless provide a quite useful set of possibilities for the mass ...

  18. The white dwarf cooling sequence of the Galactic bulge

    CERN Document Server

    Calamida, A; Anderson, J; Casertano, S; Brown, T; Cassisi, S; Sokol, J; Bond, H; Ferguson, H; Livio, M; Salaris, M; Valenti, J

    2013-01-01

    We collected F606W- and F814W-band time-series data of the Sagittarius low-reddening window in the Galactic bulge with the Advanced Camera far Surveys mounted on the Hubble Space Telescope. We sampled the region approximately every two weeks for one year, with the principal aim to detect a hidden population of isolated black holes and neutron stars in the Galactic disk through astrometric microlensing. We present preliminary results here based on a photometric catalog including ~3*10^5 stars down to F606W ~ 31 mag. Proper motions were also measured, with an accuracy of better than ~ 0.5 mas/yr at F606W ~ 28 mag in both coordinates. We were then able to separate disk and bulge stars and to obtain a clean bulge color-magnitude diagram. Together with a dozen candidate extreme horizontal branch stars we were able to identify for the first time a clearly defined white dwarf (WD) cooling sequence in the bulge. The comparison between theory and observations shows that a substantial fraction of the WDs (~40%) is syst...

  19. THE ELM SURVEY. IV. 24 WHITE DWARF MERGER SYSTEMS

    Energy Technology Data Exchange (ETDEWEB)

    Kilic, Mukremin [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 West Brooks Street, Norman, OK 73019 (United States); Brown, Warren R.; Kenyon, S. J. [Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 (United States); Allende Prieto, Carlos [Instituto de Astrofisica de Canarias, E-38205 La Laguna, Tenerife (Spain); Heinke, Craig O. [Department of Physics, CCIS 4-183, University of Alberta, Edmonton, AB, T6G 2E1 (Canada); Agueeros, M. A. [Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027 (United States); Kleinman, S. J., E-mail: kilic@ou.edu [Gemini Observatory, 670 North A' ohoku Place, Hilo, HI 96720 (United States)

    2012-06-01

    We present new radial velocity and X-ray observations of extremely low mass (ELM, {approx}0.2 M{sub Sun }) white dwarf (WD) candidates in the Sloan Digital Sky Survey Data Release 7 area. We identify seven new binary systems with 1-18 hr orbital periods. Five of the systems will merge due to gravitational wave radiation within 10 Gyr, bringing the total number of merger systems found in the ELM Survey to 24. The ELM Survey has now quintupled the known merger WD population. It has also discovered the eight shortest period detached binary WD systems currently known. We discuss the characteristics of the merger and non-merger systems observed in the ELM Survey, including their future evolution. About half of the systems have extreme mass ratios. These are the progenitors of the AM Canum Venaticorum systems and Type Ia supernovae. The remaining targets will lead to the formation of extreme helium stars, subdwarfs, or massive WDs. We identify three targets that are excellent gravitational wave sources. These should be detected by the Laser Interferometer Space Antenna like missions within the first year of operation. The remaining targets are important indicators of what the Galactic foreground may look like for gravitational wave observatories.

  20. Global Models of Runaway Accretion in White Dwarf Debris Disks

    CERN Document Server

    Metzger, Brian D; Bochkarev, Konstantin V

    2012-01-01

    A growing sample of white dwarfs (WDs) with metal-enriched atmospheres are accompanied by excess infrared emission, indicating that they are encircled by a compact dusty disk of solid debris. Such `WD debris disks' are thought to originate from the tidal disruption of asteroids or other minor bodies, but the precise mechanism(s) responsible for transporting matter to the WD surface remains unclear, especially in those systems with the highest inferred metal accretion rates dM_Z/dt ~ 1e8-1e10 g/s. Here we present global time-dependent calculations of the coupled evolution of the gaseous and solid components of WD debris disks. Solids transported inwards (initially due to PR drag) sublimate at tens of WD radii, producing a source of gas that accretes onto the WD surface and viscously spreads outwards in radius, where it overlaps with the solid disk. If the aerodynamic coupling between the solids and gaseous disks is sufficiently strong (and/or the gas viscosity sufficiently weak), then gas builds up near the su...

  1. SEISMOLOGY OF A MASSIVE PULSATING HYDROGEN ATMOSPHERE WHITE DWARF

    Energy Technology Data Exchange (ETDEWEB)

    Kepler, S. O.; Pelisoli, Ingrid; Pecanha, Viviane; Costa, J. E. S. [Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS (Brazil); Fraga, Luciano [Southern Observatory for Astrophysical Research, Casilla 603, La Serena (Chile); Hermes, J. J.; Winget, D. E.; Castanheira, Barbara [Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712-1083 (United States); Corsico, A. H.; Romero, A. D.; Althaus, Leandro [Facultad de Ciencias Astronomicas y Geofisicas, Universidad Nacional de La Plata (Argentina); Kleinman, S. J.; Nitta, A. [Gemini Observatory, Northern Operations Center, 670 North A' ohoku Place, Hilo, HI 96720 (United States); Koester, D. [Institut fuer Theoretische Physik und Astrophysik, Universitaet Kiel, D-24098 Kiel (Germany); Kuelebi, Baybars [Institut de Ciencies de L' Espai, Universitat Autonoma de Barcelon and Institute for Space Studies of Catalonia, c/Gran Capita 2-4, Edif. Nexus 104, E-08034 Barcelona (Spain); Jordan, Stefan [Astronomisches Rechen-Institut, ZAH, Moenchhofstr. 12-14, D-69120 Heidelberg (Germany); Kanaan, Antonio, E-mail: kepler@if.ufrgs.br [Departamento de Fisica, Universidade Federal de Santa Catarina, Florianopolis, SC (Brazil)

    2012-10-01

    We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1 m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1 m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M{sub *} = 0.88 {+-} 0.02 M{sub Sun} and T{sub eff} = 12, 100 {+-} 140 K. These values are consistent with those derived from the optical spectra and photometric colors.

  2. Death by Dynamics: Planetoid-Induced Explosions on White Dwarfs

    CERN Document Server

    Di Stefano, Rosanne; Guillochon, James; Steiner, James F

    2015-01-01

    At intervals as short as ten thousand years, each white dwarf (WD) passes within a solar radius of a planetoid, i.e., a comet, asteroid, or planet. Gravitational tidal forces tear the planetoid apart; its metal-rich debris falls onto the WD, enriching the atmosphere. A third of WDs exhibit atmospheric "pollution". For roughly every hundred planetoid disruptions, a planetoid collides with a WD. We simulate a small number of collisions, in which "death-by-dynamics" refers to the fate of the planetoid. We also compute the energies and likely durations of a broad sample of collision events, and identify detection strategies at optical and X-ray wavelengths. Collisions with the most massive planetoids can be detected in external galaxies. Some may trigger nuclear burning. If one in $\\sim 10^7-10^8$ of WD-planetoid collisions creates the conditions needed for a Type Ia supernova (SN~Ia), "death-by-dynamics" would also refer to the fate of the WD, and could provide a novel channel for the production of SN~Ia. We con...

  3. White dwarfs as the maximal soft x-ray scatterers

    International Nuclear Information System (INIS)

    In this paper, we explore the effect of density on the structure formation and the electromagnetic wave (EMw) elastic scattering on quantum plasmas, using the generalized quantum hydrodynamic model valid for a wide range of the plasma density and relativistic degeneracy. It is found that the electron quantum diffraction effect caused by the Bohm potential has a fundamental effect on the ion correlations in a degenerate electron fluid and crystallization in quantum plasmas in the solid-density regime and beyond. The ion correlations and structure formation are shown to be fundamentally affected by the plasma density and the relativistic degeneracy parameters. Moreover, distinct behavior is shown to exist between the non-relativistic and relativistic matter density regimes, regarding the normalized EMw elastic scattering cross-sections. It is theoretically discovered that the maximal Thomson scattering coincides with the average density of a typical white dwarf corresponding to the soft X-ray wavelength regime. Current research can be very useful in plasma optical diagnostic methods for a wide range of electron number-density from warm dense matter and inertial confinement fusion to the astrophysical compact objects

  4. COSMOLOGICAL FAST RADIO BURSTS FROM BINARY WHITE DWARF MERGERS

    International Nuclear Information System (INIS)

    Recently, Thornton et al. reported the detection of four fast radio bursts (FRBs). The dispersion measures indicate that the sources of these FRBs are at cosmological distance. Given the large full sky event rate ?104 sky–1 day–1, the FRBs are a promising target for multi-messenger astronomy. Here we propose double degenerate, binary white-dwarf (WD) mergers as the source of FRBs, which are produced by coherent emission from the polar region of a rapidly rotating, magnetized massive WD formed after the merger. The basic characteristics of the FRBs, such as the energetics, emission duration and event rate, can be consistently explained in this scenario. As a result, we predict that some FRBs can accompany type Ia supernovae (SNe Ia) or X-ray debris disks. Simultaneous detection could test our scenario and probe the progenitors of SNe Ia, and moreover would provide a novel constraint on the cosmological parameters. We strongly encourage future SN and X-ray surveys that follow up FRBs

  5. A SPECTROSCOPIC ANALYSIS OF WHITE DWARFS IN THE KISO SURVEY

    International Nuclear Information System (INIS)

    We present a spectroscopic analysis of white dwarfs found in the Kiso survey. Spectroscopic observations at high signal-to-noise ratio have been obtained for all DA and DB stars in the Kiso Schmidt ultraviolet excess survey (KUV stars). These observations led to the reclassification of several KUV objects, including the discovery of three unresolved DA+DB double-degenerate binaries. The atmospheric parameters (Teff and log g) are obtained from detailed model atmosphere fits to optical spectroscopic data. The mass distribution of our sample is characterized by a mean value of 0.606 Msun and a dispersion of 0.135 Msun for DA stars, and 0.758 Msun and a dispersion of 0.192 Msun for DB stars. Absolute visual magnitudes obtained from our spectroscopic fits allow us to derive an improved luminosity function for the DA and DB stars identified in the Kiso survey. Our luminosity function is found to be significantly different from earlier estimates based on empirical photometric calibrations of MV for the same sample. The results for the DA stars now appear entirely consistent with those obtained for the PG survey using the same spectroscopic approach. The space density for DA stars with MV ? 12.75 is 2.80 x 10-4 pc-3 in the Kiso survey, which is 9.6% smaller than the value found in the PG survey. The completeness of both surveys is briefly discussed.ly discussed.

  6. A Double White-Dwarf Cooling Sequence in {\\omega} Centauri

    CERN Document Server

    Bellini, A; Salaris, M; Cassisi, S; Bedin, L R; Piotto, G; Bergeon, P

    2013-01-01

    We have applied our empirical-PSF-based photometric techniques on a large number of calibration-related WFC3/UVIS UV-B exposures of the core of {\\omega} Cen, and found a well-defined split in the right part of the white-dwarf cooling sequence (WDCS). The redder sequence is more populated by a factor of ~2. We can explain the separation of the two sequences and their number ratio in terms of the He-normal and He-rich subpopulations that had been previously identified along the cluster main sequence. The blue WDCS is populated by the evolved stars of the He-normal component (~0.55 Msun CO-core DA objects) while the red WDCS hosts the end-products of the He-rich population (~0.46 Msun objects, ~10% CO-core and ~90% He-core WDs). The He-core WDs correspond to He-rich stars that missed the central He-ignition, and we estimate their fraction by analyzing the population ratios along the cluster horizontal branch.

  7. A DOUBLE WHITE-DWARF COOLING SEQUENCE IN {omega} CENTAURI

    Energy Technology Data Exchange (ETDEWEB)

    Bellini, A.; Anderson, J. [Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218 (United States); Salaris, M. [Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD (United Kingdom); Cassisi, S. [Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Collurania, via Mentore Maggini, I-64100 Teramo (Italy); Bedin, L. R. [Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, v.co dell' Osservatorio 5, I-35122, Padova (Italy); Piotto, G. [Dipartimento di Fisica e Astronomia ' Galileo Galilei' , Universita di Padova, v.co dell' Osservatorio 3, I-35122, Padova (Italy); Bergeron, P., E-mail: bellini@stsci.edu [Departement de Physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada)

    2013-06-01

    We have applied our empirical-point-spread-function-based photometric techniques on a large number of calibration-related WFC3/UVIS UV-B exposures of the core of {omega} Cen, and found a well-defined split in the bright part of the white-dwarf cooling sequence (WDCS). The redder sequence is more populated by a factor of {approx}2. We can explain the separation of the two sequences and their number ratio in terms of the He-normal and He-rich subpopulations that had been previously identified along the cluster main sequence. The blue WDCS is populated by the evolved stars of the He-normal component ({approx}0.55 M{sub Sun} CO-core DA objects), while the red WDCS hosts the end products of the He-rich population ({approx}0.46 M{sub Sun} objects, and {approx}10% CO-core and {approx}90% He-core WDs). The He-core WDs correspond to He-rich stars that missed the central He ignition, and we estimate their fraction by analyzing the population ratios along the cluster horizontal branch.

  8. Mass-radius relations for helium white dwarfs

    CERN Document Server

    Civelek, R

    1998-01-01

    In recent years, white dwarf stars are receiving increasing attension. The recent studies on the relation of M-R for the WDs are those of Wood (1990), Vennes et al. (1995) and Althaus and Benvenuto (1997, 1998). Vennes et al. (1995) computed static M-R relation for masses between 0.4 $M_\\odot$ and 0.7 $M_\\odot$ assuming non zero temperature effects. They assumed the luminosity is proportional to the mass which works for cool WDs but their results are in the range of high effective temperatures. As Althaus and Benvenuto (1997) mentioned, luminosity is not proportional to a constant for hot WD interiors because of neutrino emission. The purpose of this study is to present the effect of neutrino emission at finite temperatures. We considered fully degenerate configuration for WDs with pure helium composition to obtain static mass-radius relation for masses greater than 0.4 $M_\\odot$ with luminosity mass ratio due to neutrino energy loss.

  9. Ultraviolet spectrum synthesis of the helium white dwarf Ross 640

    International Nuclear Information System (INIS)

    The ultraviolet spectrum of the helium-rich white dwarf Ross 640 has been used to derive Mg, Si, and Fe abundances by a spectrum synthesis technique. The visible spectrum provides a Ca abundance and another estimate of the Mg abundance. The Values obtained were: Mg/He=6.8 x 10-8: Si/He=3.2 x 10-8; Ca/He=7.6 x 10-10; Fe/He=2.0 x 10-9. These are far below the solar abundances, and the metals have different ratios relative to the Sun. The estimated cooling and diffusion times for Ross 640 predict that no metals should be observed. Our abundances indicate the operation of diffusion processes (Mg/Ca>> cosmic ratio) within a helium-rich atmosphere, provided some mechanism exists to deposit metals in the observable layers. However, no firm conclusion can yet be reached as to whether the metals have somehow been convectively mixed into the atmosphere or accreted from the interstellar medium

  10. Electrical conductivity of plasmas of DB white dwarf atmospheres

    CERN Document Server

    Sreckovic, V A; Mihajlov, A A; Dimitrijevic, M S; 10.1111/j.1365-2966.2010.16702.x

    2012-01-01

    The static electrical conductivity of non-ideal, dense, partially ionized helium plasma was calculated over a wide range of plasma parameters: temperatures $1\\cdot 10^{4}\\textrm{K} \\lesssim T \\lesssim 1\\cdot 10^{5}\\textrm{K}$ and mass density $1 \\times 10^{-6} \\textrm{g}/\\textrm{cm}^{3} \\lesssim \\rho \\lesssim 2 \\textrm{g}/\\textrm{cm}^{3}$. Calculations of electrical conductivity of plasma for the considered range of plasma parameters are of interest for DB white dwarf atmospheres with effective temperatures $1\\cdot 10^{4}\\textrm{K} \\lesssim T_{eff} \\lesssim 3\\cdot 10^{4}\\textrm{K}$. Electrical conductivity of plasma was calculated by using the modified random phase approximation and semiclassical method, adapted for the case of dense, partially ionized plasma. The results were compared with the unique existing experimental data, including the results related to the region of dense plasmas. In spite of low accuracy of the experimental data, the existing agreement with them indicates that results obtained in th...

  11. On the white dwarf cooling sequence with extremely large telescopes

    CERN Document Server

    Bono, G; Gilmozzi, R

    2012-01-01

    We present new diagnostics of white dwarf (WD) cooling sequences and luminosity functions (LFs) in the near-infrared (NIR) bands that will exploit the sensitivity and resolution of future extremely large telescopes. The collision-induced absorption (CIA) of molecular hydrogen causes a clearly defined blue turn-off along the WD (WDBTO) cooling sequences and a bright secondary maximum in the WD LFs. These features are independent of age over a broad age range and are minimally affected by metal abundance. This means that the NIR magnitudes of the WDBTO are very promising distance indicators. The interplay between the cooling time of progressively more massive WDs and the onset of CIA causes a red turn-off along the WD (WDRTO) cooling sequences and a well defined faint peak in the WD LFs. These features are very sensitive to the cluster age, and indeed the K-band magnitude of the faint peak increases by 0.2--0.25 mag/Gyr for ages between 10 and 14 Gyr. On the other hand, the faint peak in the optical WD LF incre...

  12. The Long-Term Evolution of Double White Dwarf Mergers

    CERN Document Server

    Shen, Ken J; Kasen, Daniel; Quataert, Eliot

    2011-01-01

    In this paper, we present a model for the long-term evolution of the merger of two unequal mass C/O white dwarfs (WDs). After the dynamical phase of the merger, magnetic stresses rapidly redistribute angular momentum, leading to nearly solid body rotation on a viscous timescale, 1e4-1e8 s, long before significant cooling can occur. Because of heating during the dynamical and viscous phases, the less massive WD is transformed into a hot radially extended envelope supported by thermal pressure and with the majority of the mass having negligible rotational support. This extended envelope then undergoes Kelvin-Helmholtz contraction on a thermal timescale of ~1e4 yr; during this period, the merger remnant radiates near the Eddington limit. Given the double WD merger rate of a few per 1000 yr, a few dozen of these near-Eddington sources should exist in a Milky Way-type galaxy. In our calculations, the contraction of the cooling envelope is relatively rapid, and the base of the envelope is compressed until off-cente...

  13. The Viscous Evolution of White Dwarf Merger Remnants

    CERN Document Server

    Schwab, Josiah; Quataert, Eliot; Dan, Marius; Rosswog, Stephan

    2012-01-01

    The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multi-dimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an $\\alpha$-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al. (2011), which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towards spherical states on time-scales of hours, even though a significant fraction of the mass is initially rotationally supported. The viscous evolution unbinds only a very small amount of mass $(< 10^{-5} M_\\odot)$. Viscous heating causes some of the systems we study with He WD secondaries to reach conditions of nearly dynamical burning. It is thus possible that the post-merger viscous phase triggers detonation of...

  14. Asteroseismology of white dwarf stars. I - Adiabatic results

    International Nuclear Information System (INIS)

    A preliminary investigation of the asteroseismological properties of chemically stratified evolutionary DA and DB white dwarf models is reported. The period and kinetic energy distributions for nonradial g-modes of spherical harmonic indices l = 1-3 are computed in the adiabatic approximation, and the effects of varying the total stellar masss and the surface layer masses on the pulsation properties are studied using an extensive grid of models. Significant resonant mode trapping due to chemical stratification is found. Modes trapped in the outer layers have much lower kinetic energies; these minima also show up as minima in the period spacing between modes of consecutive radial overtone k. Mode trapping occurs at the same or nearly the same value of k for different l-values. Thus, l-values of trapped modes may be identified on the basis of their period ratios. It is shown that observational identification of these period ratios can be used to constrain the mass of the star and its surface layer. 68 refs

  15. He-accreting white dwarfs: accretion regimes and final outcomes

    Science.gov (United States)

    Piersanti, L.; Tornambé, A.; Yungelson, L. R.

    2014-12-01

    The behaviour of carbon-oxygen (CO) white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyse the thermal response of an accreting WD to mass deposition at different timescales. The analysis has been performed for initial WD masses and accretion rates in the range 0.60-1.02 M? and 10-9-10-5 M? yr-1, respectively. Thermal regimes in the parameter space MWD-dot{M}_He leading to formation of red-giant-like structures, steady burning of He, and mild, strong and dynamical flashes have been identified and the transition between these regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building up of a He-rich layer via H burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomial fits to the obtained results are provided for use in binary population synthesis computations. Several applications for close binary systems with He-rich donors and CO WD accretors are considered and the relevance of the results for interpreting He novae is discussed.

  16. Dusty Disks around White Dwarfs I: Origin of Debris Disks

    CERN Document Server

    Dong, Ruobing; Lin, D N C; Liu, X -W

    2010-01-01

    A significant fraction of the mature FGK stars have cool dusty disks at least an orders of magnitudes brighter than the solar system's outer zodiacal light. Since such dusts must be continually replenished, they are generally assumed to be the collisional fragments of residual planetesimals analogous to the Kuiper Belt objects. At least 10% of solar type stars also bear gas giant planets. The fraction of stars with known gas giants or detectable debris disks (or both) appears to increase with the stellar mass. Here, we examine the dynamical evolution of systems of long-period gas giant planets and residual planetesimals as their host stars evolve off the main sequence, lose mass, and form planetary nebula around remnant white dwarf cores. The orbits of distant gas giant planets and super-km-size planetesimals expand adiabatically. During the most intense AGB mass loss phase, sub-meter-size particles migrate toward their host stars due to the strong hydrodynamical drag by the intense stellar wind. Along their ...

  17. Graviton mass from close white dwarf binaries detectable with LISA

    International Nuclear Information System (INIS)

    The arrival times of gravitational waves and optical light from orbiting binaries provide a mechanism to understand the propagation speed of gravity when compared to that of light or electromagnetic radiation. This is achieved with a measurement of any offset between the optically derived orbital phase and that derived from gravitational wave data, at a specified location of one binary component with respect to the other. Using a sample of close white dwarf binaries (CWDBs) detectable with the Laser Interferometer Space Antenna and optical light curve data related to binary eclipses from meter-class telescopes for the same sample, we determine the accuracy to which orbital phase differences can be extracted. We consider an application of these measurements involving a variation of the speed of gravity, when compared to the speed of light, due to a massive graviton. For a subsample of ?400 CWDBs with high signal-to-noise ratio gravitational wave and optical data with magnitudes brighter than 25, the combined upper limit on the graviton mass is at the level of ?6x10-24 eV. This limit is two orders of magnitude better than the present limit derived by Yukawa-correction arguments related to the Newtonian potential and applied to the Solar System

  18. Graviton Mass from Close White Dwarf Binaries Detectable with LISA

    CERN Document Server

    Cooray, A R; Cooray, Asantha; Seto, Naoki

    2004-01-01

    The arrival times of gravitational waves and optical light from orbiting binaries provide a mechanism to understand the propagation effects of gravity when compared to light. This is achieved with a measurement of any offset between optically derived orbital phase related to that derived from gravitational wave data, at a specified location of one binary component with respect to the other. Using a sample of close white dwarf binaries (CWDBs) detectable with the Laser Interferometer Space Antenna (LISA) and optical light curve data related to binary eclipses from meter-class telescopes for the same sample, we determine the accuracy to which orbital phase differences can be extracted. We consider an application of these measurements, involving a variation to the speed of gravity, when compared to the speed of light, due to a massive graviton. For a subsample of $\\sim$ 400 CWDBs with high signal-to-noise gravitational wave and optical data with magnitudes brighter than 25, the combined upper limit on the gravit...

  19. Calibrating cosmological chronometers: White dwarf masses via astrometry

    Energy Technology Data Exchange (ETDEWEB)

    Subasavage, John P; Henry, Todd J; Jao, W-C [Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30302-4106 (United States); Nelan, Edmund P [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Harris, Hugh C; Dahn, Conard C, E-mail: subasavage@chara.gsu.ed, E-mail: thenry@chara.gsu.ed, E-mail: jao@chara.gsu.ed, E-mail: nelan@stsci.ed, E-mail: hch@nofs.navy.mi, E-mail: dahn@nofs.navy.mi [US Naval Observatory, Flagstaff Station, PO Box 1149, Flagstaff, AZ 86002-1149 (United States)

    2009-06-01

    In an effort to increase the number of accurate dynamical masses for white dwarfs (WDs), we have begun an initiative using Hubble Space Telescope's Fine Guidance Sensors (FGS) to resolve suspected binary WDs. With the increasing number of WD trigonometric parallaxes becoming available via CTIO's and the USNO's ongoing parallax programs, we have targeted objects that are overluminous at V magnitude and are presumably unresolved multiple systems. A few targets were selected because of spectral anomalies or possible perturbations evident in the residuals of the trigonometric parallax solutions. A total of 16 HST orbits were designated to this program and 12 are completed. Of the eleven WDs observed thus far (one object was observed twice), all but one were unresolved. Analysis of a recent orbit's data indicate a pair was resolved with a separation of 70 mas and a delta V magnitude of approx1.4. Coupled with astrometric data from the USNO parallax program, we have obtained preliminary constraints on component masses.

  20. Hubble Space Telescope observations of white dwarfs in detached binaries

    International Nuclear Information System (INIS)

    I describe three programs of Hubble Space Telescope (HST) observations of white dwarfs (WDs) in detached binaries. (1) I summarize the HST UV spectroscopy of the Hyades eclipsing binary V471 Tauri, containing a dK main-sequence star and a hot WD. By phasing time-resolved STIS spectra on the 9.25-min rotation period of the WD, we have shown that there are two opposite spots on the WD where there is magnetic accretion from the K star's wind. (2) Several programs of direct imaging and FGS astrometry are yielding high-precision visual orbits and dynamical masses, including the famous systems of Sirius and Procyon, as well as three fainter binaries in which both components are DC WDs. (3) Finally I discuss the bizarre nucleus of the low-surface-brightness planetary nebula EGB 6. The central star is associated with a compact emission-line nebula, which HST imaging shows is associated with a resolved dM companion. Why there should be a compact nebula around the cool star is puzzling; we speculate that it could be an accretion disk of material captured from the outflow that produced the surrounding faint PN. In a recent development, Spitzer has detected a 24 ?m excess from EGB 6, indicating a dust component at a temperature of ?260 K; the location of this dust relative to the hot and cool stars and the compact nebula remains uncertain at present.

  1. Hubble Space Telescope observations of white dwarfs in detached binaries

    Energy Technology Data Exchange (ETDEWEB)

    Bond, Howard E, E-mail: bond@stsci.ed [Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218 (United States)

    2009-06-01

    I describe three programs of Hubble Space Telescope (HST) observations of white dwarfs (WDs) in detached binaries. (1) I summarize the HST UV spectroscopy of the Hyades eclipsing binary V471 Tauri, containing a dK main-sequence star and a hot WD. By phasing time-resolved STIS spectra on the 9.25-min rotation period of the WD, we have shown that there are two opposite spots on the WD where there is magnetic accretion from the K star's wind. (2) Several programs of direct imaging and FGS astrometry are yielding high-precision visual orbits and dynamical masses, including the famous systems of Sirius and Procyon, as well as three fainter binaries in which both components are DC WDs. (3) Finally I discuss the bizarre nucleus of the low-surface-brightness planetary nebula EGB 6. The central star is associated with a compact emission-line nebula, which HST imaging shows is associated with a resolved dM companion. Why there should be a compact nebula around the cool star is puzzling; we speculate that it could be an accretion disk of material captured from the outflow that produced the surrounding faint PN. In a recent development, Spitzer has detected a 24 mum excess from EGB 6, indicating a dust component at a temperature of approx260 K; the location of this dust relative to the hot and cool stars and the compact nebula remains uncertain at present.

  2. 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.

  3. White dwarfs as the maximal soft x-ray scatterers

    Energy Technology Data Exchange (ETDEWEB)

    Akbari-Moghanjoughi, M. [Department of Physics, Faculty of Sciences, Azarbaijan Shahid Madani University, 51745-406 Tabriz (Iran, Islamic Republic of); International Centre for Advanced Studies in Physical Sciences and Institute for Theoretical Physics, Ruhr University Bochum, D-44780 Bochum (Germany)

    2013-09-15

    In this paper, we explore the effect of density on the structure formation and the electromagnetic wave (EMw) elastic scattering on quantum plasmas, using the generalized quantum hydrodynamic model valid for a wide range of the plasma density and relativistic degeneracy. It is found that the electron quantum diffraction effect caused by the Bohm potential has a fundamental effect on the ion correlations in a degenerate electron fluid and crystallization in quantum plasmas in the solid-density regime and beyond. The ion correlations and structure formation are shown to be fundamentally affected by the plasma density and the relativistic degeneracy parameters. Moreover, distinct behavior is shown to exist between the non-relativistic and relativistic matter density regimes, regarding the normalized EMw elastic scattering cross-sections. It is theoretically discovered that the maximal Thomson scattering coincides with the average density of a typical white dwarf corresponding to the soft X-ray wavelength regime. Current research can be very useful in plasma optical diagnostic methods for a wide range of electron number-density from warm dense matter and inertial confinement fusion to the astrophysical compact objects.

  4. MEASURING THE ROTATIONAL PERIODS OF ISOLATED MAGNETIC WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Brinkworth, Carolyn S. [Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125 (United States); Burleigh, Matthew R.; Lawrie, Katherine [Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH (United Kingdom); Marsh, Thomas R. [Department of Physics and Astronomy, University of Warwick, Coventry CV4 7AL (United Kingdom); Knigge, Christian [School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ (United Kingdom)

    2013-08-10

    We present time-series photometry of 30 isolated magnetic white dwarfs, surveyed with the Jacobus Kapteyn Telescope between 2002 August and 2003 May. We find that 9 were untestable due to varying comparison stars, but of the remaining 21, 5 (24%) are variable with reliably derived periods, while a further 9 (43%) are seen to vary during our study, but we were unable to derive the period. We interpret the variability to be the result of rotation of the objects. We find no correlation between rotation period and mass, temperature, magnetic field, or age. We have found variability in 9 targets with low magnetic field strengths and temperatures low enough for partially convective atmospheres, which we highlight as candidates for polarimetry to search for starspots. Most interestingly, we have found variability in one target, PG1658+441, which has a fully radiative atmosphere in which conventional starspots cannot form, but a magnetic field strength that is too low to cause magnetic dichroism. The source of variability in this target remains a mystery.

  5. A Spectroscopic Analysis of White Dwarfs in the Kiso Survey

    CERN Document Server

    Limoges, M -M

    2010-01-01

    We present a spectroscopic analysis of white dwarfs found in the Kiso survey. Spectroscopic observations at high signal-to-noise ratio have been obtained for all DA and DB stars in the Kiso Schmidt ultraviolet excess survey (KUV stars). These observations led to the reclassification of several KUV objects, including the discovery of three unresolved DA+DB double degenerate binaries. The atmospheric parameters (Teff and log g) are obtained from detailed model atmosphere fits to optical spectroscopic data. The mass distribution of our sample is characterized by a mean value of 0.606 Msun and a dispersion of 0.135 Msun for DA stars, and 0.758 Msun and a dispersion of 0.192 Msun for DB stars. Absolute visual magnitudes obtained from our spectroscopic fits allow us to derive an improved luminosity function for the DA and DB stars identified in the Kiso survey. Our luminosity function is found to be significantly different from earlier estimates based on empirical photometric calibrations of Mv for the same sample....

  6. Linear tides in inspiraling white dwarf binaries: resonance locks

    CERN Document Server

    Burkart, Joshua; Arras, Phil; Weinberg, Nevin N

    2012-01-01

    We calculate the tidal response of helium and carbon/oxygen (C/O) white dwarf (WD) binaries inspiraling due to gravitational wave emission. We show that resonance locks, previously considered in binaries with an early-type star, occur universally in WD binaries. In a resonance lock, the orbital and spin frequencies evolve in lockstep, so that the tidal forcing frequency is approximately constant and a particular normal mode remains resonant, producing efficient tidal dissipation and nearly synchronous rotation. Resonance locks in WDs can occur not only with global standing modes, but even when damping is so efficient that the resonant tidal response is a traveling wave. We derive simple analytic formulas for the tidal quality factor Q and tidal heating rate during a g-mode resonance lock, and verify our results numerically. We find that Q ~ 3 * 10^6 for orbital periods ~ 1 - 2 hr in C/O WDs, and Q ~ 3 * 10^8 for P_orb ~ 3 - 10 hr in helium WDs. Typically tidal heating occurs sufficiently close to the surface ...

  7. Measuring White Dwarf Accretion Rates via their Effective Temperatures

    CERN Document Server

    Townsley, D M; Townsley, Dean M.; Bildsten, Lars

    2003-01-01

    Our previous theoretical study of the impact of an accreting envelope on the thermal state of an underlying white dwarf (WD) has yielded equilibrium core temperatures, classical nova ignition masses and thermal luminosities for WDs accreting at time averaged rates of = 10^-11 - 10^-8 Msun/yr. These 's are appropriate to WDs in cataclysmic variables (CVs) of P_orb predicted for CV's as they transit the period gap. For DN below the period gap, we show that if is that given by gravitational radiation losses alone, then the WD masses are > 0.8 Msun. An alternative conclusion is that the masses are closer to 0.6 Msun and is 3-4 times larger than that expected from gravitational radiation losses. In either case, it is very plausible that a subset of CVs with P_orb < 2 hours will have T_eff's low enough for them to become non-radial pulsators, as discovered by van Zyl and collaborators in GW Lib.

  8. The cool magnetic DAZ white dwarf NLTT 10480

    CERN Document Server

    Kawka, Adela

    2011-01-01

    We have identified a new, cool magnetic white dwarf in the New Luyten Two-Tenths (NLTT) catalogue. The high proper-motion star NLTT 10480 (mu=0.5"/year) shows weak Zeeman-split lines of calcium, as well as characteristic Halpha and beta Zeeman triplets. Using VLT X-shooter spectra we measured a surface-averaged magnetic field B_S ~ 0.5 MG. The relative intensity of the pi and sigma components of the calcium and hydrogen lines imply a high inclination (i > 60 deg). The optical-to-infrared V-J colour index and the CaI/CaII ionization balance indicate a temperature between 4900 and 5200 K, while the Balmer line profiles favour a higher temperature of 5400 K. The discrepancy is potentially resolved by increasing the metallicity to 0.03x solar, hence increasing the electron pressure. However, the measured calcium abundance and abundance upper limits for other elements (Na, Al, Si. and Fe) imply a low photospheric metallicity < 10^{-4} solar. Assuming diffusion steady-state, a calcium accretion rate of log(dM/dt...

  9. SW Ursae Majoris, CU Velorum and AH Mensae: three more accreting white dwarfs unveiled?

    Science.gov (United States)

    Gänsicke, B. T.; Koester, D.

    1999-06-01

    We present IUE spectroscopy of the three cataclysmic variables SW UMa, CU Vel, and AH Men. In all three systems, a broad Lyalpha absorption profile is observed. In the two short-period dwarf novae SW UMa and CU Vel, the SWP flux is interpreted as photospheric emission from white dwarfs with effective temperatures of 16 000 K and 18 500 K, respectively, similar to the well-studied systems WZ Sge and VW Hyi. From the white dwarf temperatures derived in short period dwarf novae to date, there is no significant difference between the supposedly old, low-mass-transfer TOADs and the ``normal'' SU UMa dwarf novae. In the case of the novalike variable AH Men, the interpretation of the IUE data is not unambiguous. The SWP spectrum closely resembles that of a 19 000 K white dwarf, but this interpretation is in conflict with the distance dga150 pc which we derive from the published optical spectrum. The SWP spectrum can alternatively be explained by emission from an accretion disc with a moderate mass transfer rate ( ~ 10(-9.5) Msun yr(-1) ) seen at a low inclination ( ~ 20(deg) ). The possible contribution of the white dwarf to the observed UV flux cannot be determined quantitatively from the poor IUE spectrum, but may be as high as 50 %. Based on observations made with the International Ultraviolet Explorer, retrieved from the IUE\\,FA.

  10. White dwarf-main sequence binaries from LAMOST: the DR1 catalogue

    CERN Document Server

    Ren, Juanjuan; Luo, Ali; Zhao, Yongheng; Xiang, Maosheng; Liu, Xiaowei; Zhao, Gang; Jin, Ge; Zhang, Yong

    2014-01-01

    Context. White dwarf-main sequence (WDMS) binaries are used to study several different important open problems in modern astrophysics. Aims. The Sloan Digital Sky Survey (SDSS) identified the largest catalogue of WDMS binaries currently known. However, this sample is seriously affected by selection effects and the population of systems containing cool white dwarfs and early-type companions is under-represented.Here we search for WDMS binaries within the spectroscopic data release 1 of the LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) survey. LAMOST and SDSS follow different target selection algorithms. Hence, LAMOST WDMS binaries may be drawn from a different parent population and thus help in overcoming the selection effects incorporated by SDSS on the current observed population. Methods. We develop a fast and efficient routine based on the wavelet transform to identify LAMOST WDMS binaries containing a DA white dwarf and a M dwarf companion, and apply a decomposition/fitting routine to...

  11. Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey

    CERN Document Server

    Leggett, S K; Tremblay, P -E; Bergeron, P; Nitta, A

    2011-01-01

    We present the results of a search for cool white dwarfs in the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky Survey (SDSS) to identify cool hydrogen-rich white dwarf candidates by their neutral optical colors and blue near-infrared colors, as well as faint Reduced Proper Motion magnitudes. Optical spectroscopy was obtained at Gemini Observatory, and showed the majority of the candidates to be newly identified cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants. Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with effective temperature T_eff ~ 6000 K. The current followup of 1400 deg2 of sky has produced thirteen new white dwarfs. Model fits to the photometry show that seven of the newly identified white dwarfs have 4120 K <= T_eff <= 4480 K, and cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km/s <= v_tan <= 85 km/s and a...

  12. White dwarf atmosphere models with Ly-$\\alpha$ opacity in the analysis of the white dwarf cooling sequence of NGC 6397

    CERN Document Server

    Kowalski, Piotr M

    2007-01-01

    We discuss the importance of pure hydrogen white dwarf atmosphere models with Ly-$\\rm \\alpha$ far red wing opacity in the analysis of the white dwarf cooling sequence of the globular cluster NGC 6397. Our recently improved atmosphere models account for the previously missing opacity from the Ly-$\\rm \\alpha$ hydrogen line broadened by collisions of the absorbing hydrogen atoms with molecular and atomic hydrogen. These models are the first that well reproduce the UV colors and spectral energy distributions of cool white dwarfs with $T_{\\rm eff}<6000 \\rm K$ observed in the Galactic Disk. Fitting the observed $F814W$ magnitude and $F606W-F814W$ color we obtained a value for the true distance modulus, $\\mu=12.00 \\pm 0.02$, that is in agreement with recent analyses. We show that the stars at the end of the cooling sequence appear to be $\\rm \\sim 160 K$ cooler when models that account for Ly-$\\rm \\alpha$ opacity are used. This indicates that the age of NGC 6397 derived from the white dwarf cooling sequence using ...

  13. Evidence for Temperature Change and Oblique Pulsation from Light Curve Fits of the Pulsating White Dwarf GD 358

    OpenAIRE

    Montgomery, M. H.; Provencal, J. L.; Kanaan, A.; Mukadam, Anjum S.; Thompson, S. E.; Dalessio, J.; Shipman, H. L.; Winget, D. E.; Kepler, S. O.; Koester, D.

    2010-01-01

    Convective driving, the mechanism originally proposed by Brickhill (1991, 1983) for pulsating white dwarf stars, has gained general acceptance as the generic linear instability mechanism in DAV and DBV white dwarfs. This physical mechanism naturally leads to a nonlinear formulation, reproducing the observed light curves of many pulsating white dwarfs. This numerical model can also provide information on the average depth of a star's convection zone and the inclination angle ...

  14. An empirical initial-final mass relation from hot, massive white dwarfs in NGC 2168 (M35)

    OpenAIRE

    Williams, Kurtis A.; Bolte, M.; Koester, Detlev

    2004-01-01

    The relation between the zero-age main sequence mass of a star and its white-dwarf remnant (the initial-final mass relation) is a powerful tool for exploration of mass loss processes during stellar evolution. We present an empirical derivation of the initial-final mass relation based on spectroscopic analysis of seven massive white dwarfs in NGC 2168 (M35). Using an internally consistent data set, we show that the resultant white dwarf mass increases monotonically with proge...

  15. IUE observation of UV absorption in the spectrum of the C2 White Dwarf L1363-3

    Science.gov (United States)

    Vauclair, G.; Weidemann, V.; Koester, D.

    1982-05-01

    Strong ultraviolet absorption features have been observed in a short wavelength IUE spectrum of the weak carbon white dwarf L1363-3. The spectrum is quite similar to those obtained for the weak C2 white dwarf L145-141 and the DC white dwarf G33-49 and shows strong absorption at 1920 A and between 1520 A and 1680A. Identification with carbon absorption features is discussed.

  16. IUE observation of UV absorption in the spectrum of the C2 white dwarf L1363-3

    International Nuclear Information System (INIS)

    Strong ultraviolet absorption features have been observed in a short wavelength IUE spectrum of the weak carbon white dwarf L 1363-3. The spectrum is quite similar to those obtained for the weak C2 white dwarf L 145-141 and the DC white dwarf G 33-49 and shows strong absorption at lambda1920 A and between lambda1520 A and lambda1680 A. Identification with carbon absorption features is discussed. (orig.)

  17. A photometric selection of White Dwarf candidates in SDSS DR10

    CERN Document Server

    Fusillo, Nicola Pietro Gentile; Greiss, Sandra

    2015-01-01

    We present a method which uses cuts in colour-colour and reduced proper motion-colour space to select white dwarfs without the recourse to spectroscopy while allowing an adjustable compromise between completeness and efficiency. Rather than just producing a list of white dwarf candidates, our method calculates a probability of being a white dwarf (Pwd) for any object with available multi band photometry and proper motion. We applied this method to all objects in the SDSS DR10 photometric footprint and to a few selected sources in DR7 which did not have reliable photometry in DR9 or DR10. This application results in a sample of 61969 DR10 and 3799 DR7 photometric sources with calculated Pwd from which it is possible to select a sample of ~23000 high-fidelity white dwarf candidates with Teff >~ 7000 K and <= 19. This sample contains over 14000 high confidence white dwarfs candidates which have not yet received spectroscopic follow-up. These numbers show that, to date, the spectroscopic coverage of white dwar...

  18. TOWARD A SPECTROSCOPIC CENSUS OF WHITE DWARFS WITHIN 40 pc OF THE SUN

    International Nuclear Information System (INIS)

    We present the preliminary results of a survey aimed at significantly increasing the range and completeness of the local census of spectroscopically confirmed white dwarfs. The current census of nearby white dwarfs is reasonably complete only to about 20 pc of the Sun, a volume that includes around 130 white dwarfs, a sample too small for detailed statistical analyses. This census is largely based on follow-up investigations of stars with very large proper motions. We describe here the basis of a method that will lead to a catalog of white dwarfs within 40 pc of the Sun and north of the celestial equator, thus increasing by a factor of eight the extent of the northern sky census. White dwarf candidates are identified from the SUPERBLINK proper motion database, allowing us to investigate stars down to a proper motion limit ? > 40 mas yr–1, while minimizing the kinematic bias for nearby objects. The selection criteria and distance estimates are based on a combination of color-magnitude and reduced proper motion diagrams. Our follow-up spectroscopic observation campaign has so far uncovered 193 new white dwarfs, among which we identify 127 DA (including 9 DA+dM and 4 magnetic), 1 DB, 56 DC, 3 DQ, and 6 DZ stars. We perform a spectroscopic analysis on a subsample of 84 DAs, and provide their atmospheric parameters. In particular, we identify 11 new white dwarfs with spectroscopic distances within 25 pc of the Sun, including five candidates to the D < 20 pc sding five candidates to the D < 20 pc subset.

  19. Visitors from the Halo: 11 Gyr old White Dwarfs in the Solar Neighborhood

    CERN Document Server

    Kilic, Mukremin; Williams, Kurtis A; Kowalski, P M; von Hippel, Ted; Harris, Hugh C; Jeffery, Elizabeth J; DeGennaro, Steven; Brown, Warren R; McLeod, B

    2010-01-01

    We report the discovery of three nearby old halo white dwarf candidates in the Sloan Digital Sky Survey (SDSS), including two stars in a common proper motion binary system. These candidates are selected from our 2800 square degree proper motion survey on the Bok and U.S. Naval Observatory Flagstaff Station 1.3m telescopes, and they display proper motions of 0.4-0.5 arcsec/yr. Follow-up MMT spectroscopy and near-infrared photometry demonstrate that all three objects are hydrogen-dominated atmosphere white dwarfs with Teff = 3700 - 4100 K. For average mass white dwarfs, these temperature estimates correspond to cooling ages of 9-10 Gyr, distances of 70-80 pc, and tangential velocities of 140-200 km/s. Based on the UVW space velocities, we conclude that they most likely belong to the halo. Furthermore, the combined main-sequence and white dwarf cooling ages are 10-11 Gyr. Along with SDSS J1102+4113, they are the oldest field white dwarfs currently known. These three stars represent only a small fraction of the h...

  20. White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up

    CERN Document Server

    Althaus, Leandro G; Bertolami, Marcelo M Miller; Córsico, Alejandro H; García-Berro, Enrique

    2015-01-01

    We present new white dwarf evolutionary sequences for low-metallicity progenitors. White dwarf sequences have been derived from full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally-pulsing and the post-asymptotic giant branch phases. We show that for progenitor metallicities in the range 0.00003--0.001, and in the absence of carbon enrichment due to the occurrence of a third dredge-up episode, the resulting H envelope of the low-mass white dwarfs is thick enough to make stable H burning the most important energy source even at low luminosities. This has a significant impact on white dwarf cooling times. This result is independent of the adopted mass-loss rate during the thermally-pulsing and post-AGB phases, and the planetary nebulae stage. We conclude that in the absence of third dredge-up episodes, a significant part of the evolution of low-mass white dwarfs resulting from low-metallicity progenitors is dominated by stable H burning. Our study...

  1. A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356

    Science.gov (United States)

    Weisskopf, Martin C.; Wu, Kinwah; Trimble, Virginia; ODell, Stephen L.; Elsner, Ronald F.; Zavlin, Vyacheslav E.; Kouveliotou, Chryssa

    2006-01-01

    We report observations with the Chandra X-ray Observatory of the single, cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray observations of single white dwarfs, we also re-analyzed archival ROSAT data for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ 339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD 358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting the most restrictive upper limit to the X-ray luminosity from any cool white dwarf - Lx less than 6.0 x 10(exp 25) erg s(sup -1), at 99.7% confidence, for a 1- keV thermal-bremsstrahlung spectrum. The corresponding limit to the electron density is no less than 4.4x10(exp 11) per cubic centimeter. Our re-analysis of the archival data confirmed the non-detections reported by the original investigators. We discuss the implications of our and prior observations on models for coronal emission from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent constraints imposed by cyclotron radiation. In addition, we describe (in an appendix) a statistical methodology for detecting a source and for constraining the strength of a source, which applies even when the number of source or background events is small.

  2. The Solar Neighborhood. XX. Discovery and Characterization of 21 New Nearby White Dwarf Systems

    CERN Document Server

    Subasavage, John P; Bergeron, P; Dufour, P; Hambly, Nigel C

    2008-01-01

    We present medium resolution spectroscopy and multi-epoch VRI photometry for 21 new nearby (< 50 pc) white dwarf systems brighter than V ~ 17. Of the new systems, ten are DA (including a wide double degenerate system with two DA components), eight are DC, two are DZ, and one is DB. In addition, we include multi-epoch VRI photometry for eleven known white dwarf systems that do not have trigonometric parallax determinations. Using model atmospheres relevant for various types of white dwarfs (depending on spectral signatures), we perform spectral energy distribution modeling by combining the optical photometry with the near-infrared JHK from the Two Micron All-Sky Survey to derive physical parameters (i.e., effective temperature and distance estimates). We find that twelve new and six known white dwarf systems are estimated to be within the NStars and Catalog of Nearby Stars horizons of 25 pc. Coupled with identical analyses of the 56 white dwarf systems presented in Paper XIX of this series, a total of 20 ne...

  3. WHITE-DWARF-MAIN-SEQUENCE BINARIES IDENTIFIED FROM THE LAMOST PILOT SURVEY

    Energy Technology Data Exchange (ETDEWEB)

    Ren Juanjuan; Luo Ali; Li Yinbi; Wei Peng; Zhao Jingkun; Zhao Yongheng; Song Yihan; Zhao Gang, E-mail: jjren@nao.cas.cn, E-mail: lal@nao.cas.cn [Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 (China)

    2013-10-01

    We present a set of white-dwarf-main-sequence (WDMS) binaries identified spectroscopically from the Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST, also called the Guo Shou Jing Telescope) pilot survey. We develop a color selection criteria based on what is so far the largest and most complete Sloan Digital Sky Survey (SDSS) DR7 WDMS binary catalog and identify 28 WDMS binaries within the LAMOST pilot survey. The primaries in our binary sample are mostly DA white dwarfs except for one DB white dwarf. We derive the stellar atmospheric parameters, masses, and radii for the two components of 10 of our binaries. We also provide cooling ages for the white dwarf primaries as well as the spectral types for the companion stars of these 10 WDMS binaries. These binaries tend to contain hot white dwarfs and early-type companions. Through cross-identification, we note that nine binaries in our sample have been published in the SDSS DR7 WDMS binary catalog. Nineteen spectroscopic WDMS binaries identified by the LAMOST pilot survey are new. Using the 3{sigma} radial velocity variation as a criterion, we find two post-common-envelope binary candidates from our WDMS binary sample.

  4. WHITE-DWARF-MAIN-SEQUENCE BINARIES IDENTIFIED FROM THE LAMOST PILOT SURVEY

    International Nuclear Information System (INIS)

    We present a set of white-dwarf-main-sequence (WDMS) binaries identified spectroscopically from the Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST, also called the Guo Shou Jing Telescope) pilot survey. We develop a color selection criteria based on what is so far the largest and most complete Sloan Digital Sky Survey (SDSS) DR7 WDMS binary catalog and identify 28 WDMS binaries within the LAMOST pilot survey. The primaries in our binary sample are mostly DA white dwarfs except for one DB white dwarf. We derive the stellar atmospheric parameters, masses, and radii for the two components of 10 of our binaries. We also provide cooling ages for the white dwarf primaries as well as the spectral types for the companion stars of these 10 WDMS binaries. These binaries tend to contain hot white dwarfs and early-type companions. Through cross-identification, we note that nine binaries in our sample have been published in the SDSS DR7 WDMS binary catalog. Nineteen spectroscopic WDMS binaries identified by the LAMOST pilot survey are new. Using the 3? radial velocity variation as a criterion, we find two post-common-envelope binary candidates from our WDMS binary sample

  5. Additional Ultracool White Dwarfs Found in the Sloan Digital Sky Survey

    CERN Document Server

    Harris, H C; Gyuk, G; Subba-Rao, M; Anderson, S F; Hall, P B; Munn, J A; Liebert, J; Knapp, G R; Bizyaev, D; Malanushenko, E; Malanushenko, V; Pan, K; Schneider, D P; Smith, J A

    2008-01-01

    We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system -- its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310-00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk.

  6. DISCOVERY OF A BRIGHT, EXTREMELY LOW MASS WHITE DWARF IN A CLOSE DOUBLE DEGENERATE SYSTEM

    International Nuclear Information System (INIS)

    We report the discovery of a bright (V ? 13.7), extremely low mass white dwarf in a close double degenerate system. We originally selected GALEX J171708.5+675712 for spectroscopic follow-up among a group of white dwarf candidates in an ultraviolet-optical reduced proper-motion diagram. The new white dwarf has a mass of 0.18 Msun and is the primary component of a close double degenerate system (P = 0.246137 days, K1 = 288 km s-1) comprising a fainter white dwarf secondary with M2 ? 0.9 Msun. Light curves phased with the orbital ephemeris show evidence of relativistic beaming and weaker ellipsoidal variations. The light curves also reveal secondary eclipses (depth ?8 mmag) while the primary eclipses appear partially compensated by the secondary gravitational deflection and are below detection limits. Photospheric abundance measurements show a nearly solar composition of Si, Ca, and Fe (0.1-1 sun), while the normal kinematics suggest a relatively recent formation history. Close binary evolutionary scenarios suggest that extremely low mass white dwarfs form via a common-envelope phase and possible Roche lobe overflow.

  7. Toward a Spectroscopic Census of White Dwarfs Within 40 parsecs of the Sun

    CERN Document Server

    Limoges, M -M; Bergeron, P

    2013-01-01

    We present the preliminary results of a survey aimed at significantly increasing the range and completeness of the local census of spectroscopically confirmed white dwarfs. The current census of nearby white dwarfs is reasonably complete only to about 20 parsecs of the Sun, a volume that includes around 130 white dwarfs, a sample too small for detailed statistical analyses. This census is largely based on follow-up investigations of stars with very large proper motions. We describe here the basis of a method that will lead to a catalog of white dwarfs within 40 parsecs of the Sun and north of the celestial equator, thus increasing by a factor of 8 the extent of the northern sky census. White dwarf candidates are identified from the SUPERBLINK proper motion database, allowing us to investigate stars down to a proper motion limit mu>40 mas yr-1, while minimizing the kinematic bias for nearby objects. The selection criteria and distance estimates are based on a combination of color-magnitude and reduced proper m...

  8. Observations of ultraviolet carbon lines in the spectra of three DC white dwarfs

    International Nuclear Information System (INIS)

    Ultraviolet spectra of three DC white dwarfs have been obtained using the International Ultraviolet Explorer satellite (IUE). These objects are LDS 275A ( = EG 66 = 0935-37), G187--15 ( = LTT 16151 = EG 262 = 2059+31), and L791-40 ( = LTT 9491 = EG 264 = 2317-17). All three show the strong ultraviolet lines of neutral carbon recently found in the spectra of a number of other DC stars. Published observations of LDS 275B and G187--15 establish that they have continuous visual spectra, while recent scans of L791-40 and G187--15 made at Kitt Peak with the 2.1 m telescope and Image Intensified Dissector Scanner (IIDS) show weak lambdalambda4472 and 5876 lines of neutral helium and possibly the (0,0) Swan band of C2, respectively. These new objects enlarge the sample of degenerates already known to have atmospheres containing carbon, either from visual or ultraviolet observations. When this type of white dwarf is plotted along with the other spectral classes in the Stroemgren two-color diagram, it appears that white dwarfs observed having carbon occupy a restricted region, and no C2 white dwarfs occur that are cooler than T/sub eff/ roughly-equal6800 K. This is not consistent with the assumption that these white dwarfs are cooling at constant atmospheric carbon content and implies that a mechanism is present that decreases their relative atmospheric carbon abundances

  9. The first binary star evolution model producing a Chandrasekhar mass white dwarf

    CERN Document Server

    Yoon, S C

    2003-01-01

    Today, Type Ia supernovae are essential tools for cosmology, and recognized as major contributors to the chemical evolution of galaxies. The construction of detailed supernova progenitor models, however, was so far prevented by various physical and numerical difficulties in simulating binary systems with an accreting white dwarf component, e.g., unstable helium shell burning which may cause significant expansion and mass loss. Here, we present the first binary evolution calculation which models both stellar components and the binary interaction simultaneously, and where the white dwarf mass grows up to the Chandrasekhar limit by mass accretion. Our model starts with a 1.6 Msun helium star and a 1.0 Msun CO white dwarf in a 0.124 day orbit. Thermally unstable mass transfer starts when the CO core of the helium star reaches 0.53 Msun, with mass transfer rates of 1...8 times 10^{-6} Msun/yr. The white dwarf burns the accreted helium steadily until the white dwarf mass has reached ~ 1.3 Msun and weak thermal puls...

  10. New UltraCool and Halo White Dwarf Candidates in SDSS Stripe 82

    CERN Document Server

    Vidrih, S; Hewett, P C; Evans, N W; Gilmore, G; Hodgkin, S; Smith, M; Wyrzykowski, L; Belokurov, V; Fellhauer, M; Irwin, M J; McMahon, R G; Zucker, D; Munn, J A; Lin, H; Miknaitis, G; Harris, H C; Lupton, R H; Schneider, D P

    2007-01-01

    A 2.5 x 100 degree region along the celestial equator (Stripe 82) has been imaged repeatedly from 1998 to 2005 by the Sloan Digital Sky Survey. A new catalogue of ~4 million light-motion curves, together with over 200 derived statistical quantities, for objects in Stripe 82 brighter than r~21.5 has been constructed by combining these data by Bramich et al. (2007). This catalogue is at present the deepest catalogue of its kind. Extracting the ~130000 objects with highest signal-to-noise ratio proper motions, we build a reduced proper motion diagram to illustrate the scientific promise of the catalogue. In this diagram disk and halo subdwarfs are well-separated from the cool white dwarf sequence. Our sample of 1049 cool white dwarf candidates includes at least 8 and possibly 21 new ultracool DA type white dwarfs (T_eff < 4000K) and one new ultracool DB type white dwarf candidate identified from their SDSS optical and UKIDSS infrared photometry. At least 10 new halo white dwarfs are also identified from their...

  11. Discovery of Two New Thermally Bloated Low-Mass White Dwarfs Among the Kepler Binaries

    CERN Document Server

    Rappaport, S; Levine, A; Sanchis-Ojeda, R; Gandolfi, D; Nowak, G; Palle, E; Prsa, A

    2015-01-01

    We report the discovery of two new low-mass, thermally bloated, hot white dwarfs among the Kepler sample of eclipsing binaries. These are KIC 9164561 and KIC 10727668 with orbital periods of 1.2670 and 2.3058 days, respectively. The current primary in both systems is an A star of about 2 Msun. This brings the number of similar binaries among the Kepler sample to six, and the two new systems have the shortest orbital periods among them. The white dwarf in KIC 9164561 has the largest thermal bloating, compared to its cold degenerate radius, of about a factor of 14. We utilize RV measurements of the A star in KIC 9164561 to determine the white dwarf mass rather accurately: 0.197 +/- 0.005 Msun. The mass of the white dwarf in KIC 10727668 is based on the Doppler boosting signal in the Kepler photometry, and is less accurately determined to be 0.266 +/- 0.035 Msun. Based on the inferred radii and effective temperatures of these two white dwarfs we are able to make an independent theoretical estimate of their masse...

  12. High-dispersion observations of H-alpha in the suspected brown dwarf, white dwarf binary system G29-38

    International Nuclear Information System (INIS)

    High-dispersion spectroscopy of the H-alpha absorption line of the cool DA white dwarf G29-38 is reported. This is the star for which a recently detected IR excess has been suggested to be due to a possible brown dwarf companion. Three echelle spectra show no evidence for radial-velocity variations larger than about 1.1 + or - 8.7 km/s and are used to derive a weighted heliocentric radial velocity of 33.7 + or - 4.3 kms/s for the white dwarf. The observations of a sharp absorption-line core restricts the possible rotation of the white dwarf to 40 km/s or less and ensures that any surface magnetic field has a strength of 100,000 G or less. These results make it unlikely that the DA white dwarf has previously been in a cataclysmic variable accretion phase. 18 references

  13. White-dwarf + main-sequence binaries identified from the ninth data release of the Sloan Digital Sky Survey

    Science.gov (United States)

    Li, Lifang; Zhang, Fenghui; Han, Quanwang; Kong, Xiaoyang; Gong, Xiaobo

    2014-12-01

    We have identified 227 new spectroscopic white-dwarf + main-sequence (WDMS) binaries from the ninth data release (DR9) of the Sloan Digital Sky Survey (SDSS). The SDSS spectra of the newly found WDMS binaries with a DA white dwarf and an M-dwarf are analysed based on a spectral decomposition/fitting method. We obtain the effective temperatures, surface gravities and masses of the white dwarf, together with the spectral types and metallicities of the secondary star. Two independent distance estimates are derived from the flux-scaling factors between the WDMS SDSS spectra and the white dwarf and M-dwarf model spectra. We find that about 25 per cent of the newly found WDMS binaries show a significant discrepancy between the two distance estimates. This might be caused by the effects of M-dwarf stellar activity or irradiation of the M-dwarf companions by the white dwarf. The stellar parameter distributions are used to investigate the global properties of the newly found WDMS binaries. We find that the WDMS binaries with a low signal-to-noise ratio (S/N ? 4.0) usually have a massive DA white dwarf component with a higher surface gravity, and the metallicity is usually significantly different from solar. This suggests that the observational selection effects and the spectral S/N of the WDMS binaries have a significant influence on the determinations of these physical parameters of WDMS binaries.

  14. The role of HeH+ in cool helium rich white dwarfs

    CERN Document Server

    Harris, G J; Miller, S; Tennyson, J

    2004-01-01

    HeH$^+$ is found to be the dominant positive ion over a wide range of temperatures and densities relevant to helium rich white dwarfs. The inclusion of HeH$^+$ in ionization equilibrium computations increases the abundance of free electrons by a significant factor. For temperatures below 8000 K, He$^-$ free-free absorption is increased by up to a factor of 5, by the inclusion of HeH$^+$. Illustrative model atmospheres and spectral energy distributions are computed, which show that HeH$^+$ has a strong effect upon the density and pressure structure of helium rich white dwarfs with teff < 8000 K. The inclusion of HeH$^+$ significantly reddens spectral energy distributions and broad band color indices for models with Teff < 5500 K. This has serious implications for existing model atmospheres, synthetic spectra and cooling curves for helium rich white dwarfs.

  15. A Spectroscopic Survey & Analysis of Bright, Hydrogen-Rich White Dwarfs

    CERN Document Server

    Gianninas, Alexandros; Ruiz, Maria Teresa

    2011-01-01

    We have conducted a spectroscopic survey of over 1300 bright (V < 17.5), hydrogen-rich white dwarfs based largely on the last published version of the McCook & Sion catalog. The complete results from our survey, including the spectroscopic analysis of over 1100 DA white dwarfs, are presented. High signal-to-noise ratio optical spectra were obtained for each star and were subsequently analyzed using our standard spectroscopic technique where the observed Balmer line profiles are compared to synthetic spectra computed from the latest generation of model atmospheres appropriate for these stars. First, we present the spectroscopic content of our sample, which includes many misclassifications as well as several DAB, DAZ, and magnetic white dwarfs. Next, we look at how the new Stark broadening profiles affect the determination of the atmospheric parameters. When necessary, specific models and analysis techniques are used to derive the most accurate atmospheric parameters possible. In particular, we employ M ...

  16. The Future Evolution of White Dwarf Stars Through Baryon Decay and Time Varying Gravitational Constant

    CERN Document Server

    Ketchum, Jacob A

    2008-01-01

    Motivated by the possibility that the fundamental ``constants'' of nature could vary with time, this paper considers the long term evolution of white dwarf stars under the combined action of proton decay and variations in the gravitational constant. White dwarfs are thus used as a theoretical laboratory to study the effects of possible time variations, especially their implications for the future history of the universe. More specifically, we consider the gravitational constant $G$ to vary according to the parametric relation $G = G_0 (1 + t/t_\\ast)^{-p}$, where the time scale $t_\\ast$ is the same order as the proton lifetime. We then study the long term fate and evolution of white dwarf stars. This treatment begins when proton decay dominates the stellar luminosity, and ends when the star becomes optically thin to its internal radiation.

  17. Population boundaries for Neutron star-White Dwarf binaries in LISA's amplitude-frequency domain

    CERN Document Server

    Kopparapu, Ravi kumar

    2008-01-01

    In an earlier investigation, we proposed population boundaries for both inspiralling and mass-transferring double white dwarf (DWD) systems in the distance independent ``absolute'' amplitude-frequency domain of the proposed space-based gravitational-wave (GW) detector, {\\it LISA}. The mass-radius relationship of individual white dwarf stars, in combination with the constraints imposed by Roche geometries, permits us to identify five key population boundaries for DWD systems in various phases of evolution. Here we extend these boundaries to both inspiralling and mass-transferring neutron star-white dwarf (NSWD) binary systems, which occupy distinct sub-domains than DWDs, in the ``absolute'' amplitude-frequency space. Assuming that the currently known ultra-compact x-ray binaries (UCXBs) are NSWD systems in conservative mass transfer (CMT)phase, we assess the limits and applicability of our theoretical population boundaries with respect to observations. The fairly known issue of the apparent clustering of UCXBs...

  18. On the Dissociation Equilibrium of H2 in Very Cool, Helium-Rich White Dwarf Atmospheres

    CERN Document Server

    Kowalski, P M

    2006-01-01

    We investigate the dissociation equilibrium of $\\rm H_2$ in very cool, helium-rich white dwarf atmospheres. We present the solution of the non-ideal chemical equilibrium for the dissociation of molecular hydrogen in a medium of dense helium. We find that at the photosphere of cool white dwarfs of $T_{\\rm eff}\\rm=4000 K$, the non-ideality results in an increase of the mole fraction of molecular hydrogen by up to a factor of $\\sim 10$, compared to the equilibrium value for the ideal gas. This increases the $\\rm H_{2}-He$ CIA opacity by an order of magnitude and will affect the determination of the abundance of hydrogen in very cool, helium-rich white dwarfs.

  19. Detection of trace helium in G104 - 27, a 26,000 K DA white dwarf

    Energy Technology Data Exchange (ETDEWEB)

    Holberg, J.B.; Kidder, K.M.; Wesemael, F. (Arizona Univ., Tucson (USA) Montreal Universite, Montreal (Canada))

    1990-12-01

    The detection of the He I 4471 A feature in the moderately hot DA white dwarf G104 - 27 places this star within the rather sparse class of DAB white dwarfs. It does not appear possible, on the basis of the present data, to distinguish between a stratified and a homogeneously mixed atmosphere. While a stratified structure would be natural for a DA white dwarf, G104 - 27 resides near the red edge of the DB gap, in a region where DA stars could transform into DB stars through convective mixing in the underlying He envelope and thereby destroy envelope stratification. Both atmospheres are consistent with the existing soft X-ray flux limit for this star. 37 refs.

  20. The Initiation and Propagation of Helium Detonations in White Dwarf Envelopes

    CERN Document Server

    Shen, Ken J

    2014-01-01

    Detonations in helium-rich envelopes surrounding white dwarfs have garnered attention as triggers of faint thermonuclear ".Ia" supernovae and double detonation Type Ia supernovae. However, recent studies have found that the minimum size of a hotspot that can lead to a helium detonation is comparable to, or even larger than, the white dwarf's pressure scale height, casting doubt on the successful ignition of helium detonations in these systems. In this paper, we examine the previously neglected effects of C/O pollution and a full nuclear reaction network, and we consider hotspots with spatially constant pressure in addition to constant density hotspots. We find that the inclusion of these effects significantly decreases the minimum hotspot size for helium-rich detonation ignition, making detonations far more plausible during turbulent shell convection or during double white dwarf mergers. The increase in burning rate also decreases the minimum shell mass in which a helium detonation can successfully propagate ...

  1. Detection of trace helium in G104 - 27, a 26,000 K DA white dwarf

    International Nuclear Information System (INIS)

    The detection of the He I 4471 A feature in the moderately hot DA white dwarf G104 - 27 places this star within the rather sparse class of DAB white dwarfs. It does not appear possible, on the basis of the present data, to distinguish between a stratified and a homogeneously mixed atmosphere. While a stratified structure would be natural for a DA white dwarf, G104 - 27 resides near the red edge of the DB gap, in a region where DA stars could transform into DB stars through convective mixing in the underlying He envelope and thereby destroy envelope stratification. Both atmospheres are consistent with the existing soft X-ray flux limit for this star. 37 refs

  2. Excess Infrared Radiation from a Massive DAZ White Dwarf: GD362 - a Debris Disk?

    CERN Document Server

    Kilic, M; Leggett, S K; Winget, D E; Kilic, Mukremin; Hippel, Ted von

    2005-01-01

    We report the discovery of excess K-band radiation from a massive DAZ white dwarf star, GD362. Combining infrared photometric and spectroscopic observations, we show that the excess radiation cannot be explained by a stellar or substellar companion, and is likely to be caused by a debris disk. This would be only the second such system known, discovered 18 years after G29-38, the only single white dwarf currently known to be orbited by circumstellar dust. Both of these systems favor a model with accretion from a surrounding debris disk to explain the metal abundances observed in DAZ white dwarfs. Nevertheless, observations of more DAZs in the mid-infrared are required to test if this model can explain all DAZs.

  3. Accretion of a Terrestrial-Like Minor Planet by a White Dwarf

    CERN Document Server

    Melis, Carl; Dufour, P; Zuckerman, B; Burgasser, Adam J; Bergeron, P; Bochanski, J; Simcoe, R

    2011-01-01

    We present optical and infrared characterization of the polluted DAZ white dwarf GALEX J193156.8+011745. Imaging and spectroscopy from the ultraviolet to the thermal infrared indicates that the white dwarf hosts excess infrared emission consistent with the presence of an orbiting dusty debris disk. In addition to the five elements previously identified, our optical echelle spectroscopy reveals chromium and manganese and enables restrictive upper limits on several other elements. Synthesis of all detections and upper limits suggests that the white dwarf has accreted a differentiated parent body. We compare the inferred bulk elemental composition of the accreted parent body to expectations for the bulk composition of an Earth-like planet stripped of its crust and mantle and find relatively good agreement. At least two processes could be important in shaping the final bulk elemental composition of rocky bodies during the late phases of stellar evolution: irradiation and interaction with the dense stellar wind.

  4. Discovery of ZZ Cetis in detached white dwarf plus main-sequence binaries

    CERN Document Server

    Pyrzas, S; Hermes, J J; Copperwheat, C M; Rebassa-Mansergas, A; Dhillon, V S; Littlefair, S P; Marsh, T R; Parsons, S G; Savoury, C D J; Schreiber, M R; Barros, S C C; Bento, J; Breedt, E; Kerry, P

    2014-01-01

    We present the first results of a dedicated search for pulsating white dwarfs (WDs) in detached white dwarf plus main-sequence binaries. Candidate systems were selected from a catalogue of WD+MS binaries, based on the surface gravities and effective temperatures of the WDs. We observed a total of 26 systems using ULTRACAM mounted on ESO's 3.5m New Technology Telescope (NTT) at La Silla. Our photometric observations reveal pulsations in seven WDs of our sample, including the first pulsating white dwarf with a main-sequence companion in a post common envelope binary, SDSSJ1136+0409. Asteroseismology of these new pulsating systems will provide crucial insight into how binary interactions, particularly the common envelope phase, affect the internal structure and evolution of WDs. In addition, our observations have revealed the partially eclipsing nature of one of our targets, SDSSJ1223-0056.

  5. The frequency of occurrence of novae hosting an ONe white dwarf

    CERN Document Server

    Gil-Pons, P; José, J; Hernanz, M; Truran, J W

    2003-01-01

    In this paper, we revisit the problem of the determination of the frequency of occurrence of galactic nova outbursts which involve an oxygen-neon (ONe) white dwarf. The improvement with respect to previous work on the subject derives from the fact that we use the results that our evolutionary calculations provide for the final mass and for the chemical profiles of intermediate-to-massive primary components of close binary systems. In particular, the final evolutionary stages, such as the carbon burning phase, have been carefully followed for the whole range of masses of interest. The chemical profiles obtained with our evolutionary code are of interest in determining the chemical composition of the ejecta after being processed through the thermonuclear runaway, although such other factors as the efficiency of the mixing between the accreted material and that of the underlying white dwarf must also be considered. In our calculations of the frequency of occurrence of nova outbursts involving an ONe white dwarf,...

  6. 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.

  7. Connections between Tilted Accretion Disks around White Dwarfs and Substellar Companions

    CERN Document Server

    Montgomery, M M

    2011-01-01

    Accretion disks in white dwarf systems are believed to be tilted. In a recent publication, the lift force has been suggested to be a source to disk tilt, a source that is likely relevant to all accretion disk systems. Lift is generated by slightly different supersonic gas stream speeds flowing over and under the disk at the bright spot. In this conference proceeding, we focus on whether a brown dwarf donor star accreting onto a white dwarf primary has enough mass to contribute to disk tilt. We also would like to obtain whether a white dwarf - brown dwarf close binary system has enough mass to induce and maintain a disk tilt of four degrees. We adopt SDSS 103533.03+055158.4 as our model system which has a mass transfer rate of (10 \\pm 2) x 10-12 M* yr-1. We find that the brown dwarf in SDSS 1035 does not have enough mass to contribute to disk tilt. We find a gross magnitude of the minimum mass transfer rate to be - 10-10 M* yr-1 . We conclude that SDSS 1035 does not seem to have a high enough mass transfer rat...

  8. Connections Between Tilted Accretion Disks Around White Dwarfs and Substellar Companions

    CERN Document Server

    Montgomery, M M

    2010-01-01

    Accretion disks in white dwarf systems are believed to be tilted. In a recent publication, the lift force has been suggested to be a source to disk tilt, a source that is likely relevant to all accretion disk systems. Lift is generated by slightly different supersonic gas stream speeds flowing over and under the disk at the bright spot. In this conference proceeding, we focus on whether a brown dwarf donor star accreting onto a white dwarf primary has enough mass to contribute to disk tilt. We also would like to obtain whether a white dwarf - brown dwarf close binary system has enough mass to induce and maintain a disk tilt of four degrees. We adopt SDSS 103533.03+055158.4 as our model system which has a mass transfer rate of \\( (10\\pm2) \\times 10^{-12} \\) M$_{\\odot}$ yr$^{-1}$. We find that the brown dwarf in SDSS 1035 does not have enough mass to contribute to disk tilt. We find a gross magnitude of the minimum mass transfer rate to be $\\sim10^{-10}$M$_{\\odot}$yr$^{-1}$. We conclude that SDSS 1035 does not ...

  9. The Post-Merger Magnetized Evolution of White Dwarf Binaries: The Double-Degenerate Channel of Sub-Chandrasekhar Type Ia Supernovae and the Formation of Magnetized White Dwarfs

    CERN Document Server

    Ji, Suoqing; Garcia-Berro, Enrique; Tzeferacos, Petros; Jordan, George; Lee, Dongwook; Loren-Aguilar, Pablo; Cremer, Pascal; Behrends, Jan

    2013-01-01

    Type Ia supernovae (SNe Ia) play a crucial role as standardizable cosmological candles, though the nature of their progenitors is a subject of active investigation. Recent observational and theoretical work has pointed to merging white dwarf binaries, referred to as the double-degenerate channel, as the possible progenitor systems for some SNe Ia. Additionally, recent theoretical work suggests that mergers which fail to detonate may produce magnetized, rapidly-rotating white dwarfs. In this paper, we present the first multidimensional simulations of the post-merger evolution of white dwarf binaries to include the effect of the magnetic field. In these systems, the two white dwarfs complete a final merger on a dynamical timescale, and are tidally disrupted, producing a rapidly-rotating white dwarf merger surrounded by a hot corona and a thick, differentially-rotating disk. The disk is strongly susceptible to the magnetorotational instability (MRI), and we demonstrate that this leads to the rapid growth of an i...

  10. Trans-iron group elements in hot helium-rich white dwarfs

    Science.gov (United States)

    Werner, Klaus

    2014-10-01

    Our recent detection of twelve trans-iron group elements (Z=30-56) in the hot helium-rich white dwarf (i.e. spectral type DO) RE0503-289 is a unique discovery. This phenomenon was never observed before in any white dwarf. Abundance analyses hitherto performed for five species (Zn, Ge, Kr, Xe, Ba) reveal values between 155 and 23000 times solar. There are reasons to believe that these extreme overabundances are connected to the evolutionary history of the DO white dwarfs. They are the outcome of a late helium-shell flash that consumes all hydrogen in the stellar envelope and at the same time dredges up helium-rich intershell matter that is enriched with s-process elements. These elements were synthesised in the preceding AGB phase of the evolution. We argue that if RE0503-289 is a typical representative of the DO white dwarfs, then all DOs with similar effective temperature should also show these extraordinary chemical diversity and overabundances. In order to prove this hypothesis, we want to obtain FUV spectra of two other hot DO white dwarfs plus a representative of the PG1159 stars, which are thought to be immediate progenitors of DO white dwarfs. If all three targets show the same heavy-metal abundance pattern, then these objects hold the promise that their metal abundances can be used to constrain AGB star nucleosynthesis. If otherwise RE0503-289 remains a unique object, then an alternative evolutionary scenario (binary WD merger) is probably responsible for its extreme element abundances.

  11. Accretion of planetary debris onto the unique white dwarf GD394

    Science.gov (United States)

    Wilson, David

    2014-10-01

    It is now undisputed that metal-pollution of the H or He atmospheres observed in a substantial fraction of white dwarfs is due to accretion of planetary debris. While significant progress has been made in establishing the frequency of such evolved planetary systems, and statistics on the abundances of the debris material are beginning to emerge, very little is known regarding the delivery mechanism of the debris. Given that the red giant progenitors of the metal-polluted white dwarfs evaporated all planetary material within >1 AU, the debris found at many white dwarfs must originate originally from a substantial distance of the star. The canonical model is that remaining planets perturb the orbits of astroids, sending them onto a white dwarf grazing course, where the extreme gravity will result in their tidal disruption. The debris then must undergo complex dynamical evolution, and eventually fall into the white dwarf atmosphere. EUV light curves of GD394, an extremely metal-polluted white dwarf, suggest a strong variation of the metal abundances over the stellar surface. We propose to obtain spin-phase resolved high-resolution spectroscopy of GD394 over its 1.15d rotation period to resolve the abundance pattern on its surface from variations in the photospheric metal lines, and thereby probe the geometry of the circumstellar debris. Line profile variations can be used to reconstruct the surface distribution of the metals using Roche tomography. In addition, comparison to a single archive HST/GHRS observation from 1992 will place tight constraints on the long-term variation of the accretion rate, and the life time of the circumstellar debris.

  12. THE LONG-TERM EVOLUTION OF DOUBLE WHITE DWARF MERGERS

    Energy Technology Data Exchange (ETDEWEB)

    Shen, Ken J.; Kasen, Daniel [Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (United States); Bildsten, Lars [Kavli Institute for Theoretical Physics, Kohn Hall, University of California, Santa Barbara, CA 93106 (United States); Quataert, Eliot, E-mail: kenshen@astro.berkeley.edu [Department of Astronomy and Theoretical Astrophysics Center, University of California, Berkeley, CA 94720 (United States)

    2012-03-20

    In this paper, we present a model for the long-term evolution of the merger of two unequal mass C/O white dwarfs (WDs). After the dynamical phase of the merger, magnetic stresses rapidly redistribute angular momentum, leading to nearly solid-body rotation on a viscous timescale of 10{sup -4}-1 yr, long before significant cooling can occur. Due to heating during the dynamical and viscous phases, the less massive WD is transformed into a hot, slowly rotating, and radially extended envelope supported by thermal pressure. Following the viscous phase of evolution, the maximum temperature near the envelope base may already be high enough to begin off-center convective carbon burning. If not, Kelvin-Helmholtz contraction of the inner region of the envelope on a thermal timescale of 10{sup 3}-10{sup 4} yr compresses the base of the envelope, again yielding off-center burning. As a result, the long-term evolution of the merger remnant is similar to that seen in previous calculations: the burning shell diffuses inward over {approx}10{sup 4} yr, eventually yielding a high-mass O/Ne WD or a collapse to a neutron star, rather than a Type Ia supernova. During the cooling and shell-burning phases, the merger remnant radiates near the Eddington limit. Given the double WD merger rate of a few per 1000 yr, a few dozen of these {approx}10{sup 38} erg s{sup -1} sources should exist in a Milky Way type galaxy. While the end result is similar to that of previous studies, the physical picture and the dynamical state of the matter in our model differ from previous work. Furthermore, substantial remaining uncertainties related to the convective structure near the photosphere and mass loss during the thermal evolution may significantly affect our conclusions. Thus, future work within the context of the physical model presented here is required to better address the eventual fate of double WD mergers, including those for which one or both of the components is a He WD.

  13. Formation of high-field magnetic white dwarfs from common envelopes

    Science.gov (United States)

    Nordhaus, Jason; Wellons, Sarah; Spiegel, David S.; Metzger, Brian D.; Blackman, Eric G.

    2011-01-01

    The origin of highly magnetized white dwarfs has remained a mystery since their initial discovery. Recent observations indicate that the formation of high-field magnetic white dwarfs is intimately related to strong binary interactions during post-main-sequence phases of stellar evolution. If a low-mass companion, such as a planet, brown dwarf, or low-mass star, is engulfed by a post-main-sequence giant, gravitational torques in the envelope of the giant lead to a reduction of the companion’s orbit. Sufficiently low-mass companions in-spiral until they are shredded by the strong gravitational tides near the white dwarf core. Subsequent formation of a super-Eddington accretion disk from the disrupted companion inside a common envelope can dramatically amplify magnetic fields via a dynamo. Here, we show that these disk-generated fields are sufficiently strong to explain the observed range of magnetic field strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary analogue may also contribute to the origin of magnetar fields. PMID:21300910

  14. Observational Constraints on the Origin of Metals in Cool DA-type White Dwarfs

    CERN Document Server

    Chary, R R; Becklin, E E

    1998-01-01

    We present ISOCAM 7 micron and 15 micron observations of 12 nearby white dwarfs, 6 of which have been found to have metals such as Ca, Mg and Fe in their photospheres. Our purpose was to search for an excess of infrared emission above the stellar photospheres. We find that none of the white dwarfs other than G29-38 shows a detectable infrared excess and this places strong constraints on the existence of a dusty disk around these stars. We conclude that ongoing accretion of the interstellar medium seems an unlikely explanation for the existence of metals in the photospheres of cool hydrogen atmosphere white dwarfs. The excess associated with G29-38 is 3.8+/-1.0 mJy and 2.9+/-0.6 mJy at 7 micron and 15 micron respectively. The broadband spectrum of this star strengthens the hypothesis that the infrared excess arises from a disk of particulate matter surrounding the white dwarf rather than from a cool brown dwarf companion.

  15. Formation of high-field magnetic white dwarfs from common envelopes.

    Science.gov (United States)

    Nordhaus, Jason; Wellons, Sarah; Spiegel, David S; Metzger, Brian D; Blackman, Eric G

    2011-02-22

    The origin of highly magnetized white dwarfs has remained a mystery since their initial discovery. Recent observations indicate that the formation of high-field magnetic white dwarfs is intimately related to strong binary interactions during post-main-sequence phases of stellar evolution. If a low-mass companion, such as a planet, brown dwarf, or low-mass star, is engulfed by a post-main-sequence giant, gravitational torques in the envelope of the giant lead to a reduction of the companion's orbit. Sufficiently low-mass companions in-spiral until they are shredded by the strong gravitational tides near the white dwarf core. Subsequent formation of a super-Eddington accretion disk from the disrupted companion inside a common envelope can dramatically amplify magnetic fields via a dynamo. Here, we show that these disk-generated fields are sufficiently strong to explain the observed range of magnetic field strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary analogue may also contribute to the origin of magnetar fields. PMID:21300910

  16. The formation of high-field magnetic white dwarfs from common envelopes

    CERN Document Server

    Nordhaus, J; Spiegel, D S; Metzger, B D; Blackman, E G

    2010-01-01

    The origin of highly-magnetized white dwarfs has remained a mystery since their initial discovery. Recent observations indicate that the formation of high-field magnetic white dwarfs is intimately related to strong binary interactions during post-main-sequence phases of stellar evolution. If a low-mass companion, such as a planet, brown dwarf, or low-mass star is engulfed by a post-main-sequence giant, the hydrodynamic drag in the envelope of the giant leads to a reduction of the companion's orbit. Sufficiently low-mass companions in-spiral until they are shredded by the strong gravitational tides near the white dwarf core. Subsequent formation of a super-Eddington accretion disk from the disrupted companion inside a common envelope can dramatically amplify magnetic fields via a dynamo. Here, we show that these disk-generated fields are sufficiently strong to explain the observed range of magnetic field strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary analogue may also contribute...

  17. Near-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG1234+482

    CERN Document Server

    Steele, P R; Dobbie, P D; Barstow, M A

    2007-01-01

    We present a near-infrared spectrum of the hot ($T_{\\rm eff}$ $\\approx$ 55,000 K) DA white dwarf PG 1234+482. We confirm that a very low mass companion is responsible for the previously recognised infrared photometric excess. We compare spectra of M and L dwarfs, combined with an appropriate white dwarf model, to the data to constrain the spectral type of the secondary. We find that uncertainties in the 2MASS $HK$ photometry of the white dwarf prevent us from distinguishing whether the secondary is stellar or substellar, and assign a spectral type of L0$\\pm$1 (M9-L1).Therefore, this is the hottest and youngest ($\\approx 10^6$ yr) DA white dwarf with a possible brown dwarf companion.

  18. KOI 1224, a Fourth Bloated Hot White Dwarf Companion Found With Kepler

    OpenAIRE

    Breton, Rene P.; Rappaport, Saul A.; Kerkwijk, Marten H.; Carter, Josh A.

    2011-01-01

    We present an analysis and interpretation of the Kepler binary system KOI 1224. This is the fourth binary found with Kepler that consists of a thermally bloated, hot white dwarf in a close orbit with a more or less normal star of spectral class A or F. As we show, KOI 1224 contains a white dwarf with Teff = 14400 +/- 1100 K, mass = 0.20 +/- 0.02 Msun, and radius = 0.103 +/- 0.004 Rsun, and an F-star companion of mass = 1.59 +/- 0.07 Msun that is somewhat beyond its terminal-...

  19. Constraints on 12C(alpha,gamma)16O from White Dwarf Seismology

    CERN Document Server

    Metcalfe, T S; Winget, D E

    2002-01-01

    For many years, astronomers have promised that the study of pulsating white dwarfs would ultimately lead to useful information about the physics of matter under extreme conditions of temperature and pressure. We can now make good on that promise. Using observational data from the Whole Earth Telescope and a new analysis method employing a genetic algorithm, we empirically determine the central oxygen abundance of the helium-atmosphere variable white dwarf GD 358. We use this value, combined with detailed evolutionary calculations of the internal chemical profiles to place constraints on the 12C(alpha,gamma)16O nuclear reaction cross-section.

  20. Creating White Dwarf Photospheres in the Laboratory: Strategy for Astrophysics Applications

    OpenAIRE

    Falcon, Ross E.; Rochau, G. A.; Bailey, J. E.; Ellis, J. L.; Carlson, A. L.; Gomez, T. A.; Montgomery, M. H.; Winget, D. E.; Chen, E. Y.; Gomez, M. R.; Nash, T. J.; Pille, T. M.

    2012-01-01

    Astrophysics experiments by Falcon et al. to create white dwarf photospheres in the laboratory are currently underway. The experimental platform measures Balmer line profiles of a radiation-driven, pure hydrogen plasma in emission and in absorption for conditions at T_e ~ 1 eV, n_e ~ 10^17 cm^-3. These will be used to compare and test line broadening theories used in white dwarf atmosphere models. The flexibility of the platform allows us to expand the direction of our exper...

  1. Systematic relativistic quantum effects on screening of fusion rates in white dwarfs

    CERN Document Server

    Chitanvis, S M

    2006-01-01

    Relativistic electron degeneracy effects are dominant in ultra-dense plasmas (UDP), such as those found in white dwarfs. These effects can be treated systematically by obtaining an expansion of the screening length in inverse powers of $\\hbar^{2}$. In general, our theory leads to an ${\\cal O}(10)$ effect on the enhancement of fusion rates in white dwarfs. Further, it is shown analytically for these stellar conditions that Bose statistics of nuclei have a negligible effect on the screening length, in consonance with Monte Carlo simulations found in literature.

  2. Systematic quantum effects on screening of fusion rates in white dwarfs

    CERN Document Server

    Chitanvis, S M

    2006-01-01

    Electron degeneracy effects are dominant in ultra-dense plasmas (UDP), such as those found in white dwarfs. These effects can be treated systematically by obtaining an expansion of the screening length in inverse powers of $\\hbar^{2}$. The theory exhibits Thomas-Fermi-like screening in an appropriate regime. In general, our theory leads to an ${\\cal O}(1)$ effect on the enhancement of fusion rates in white dwarfs. Further, it is shown analytically for these stellar conditions that Bose statistics of nuclei have a negligible effect on the screening length, in consonance with Monte Carlo simulations found in literature.

  3. Spectropolarimetry and Infrared Photometry of Magnetic White Dwarfs: Vacuum Polarization Effect or Magnetic CIA?

    CERN Document Server

    Gnedin, Y N; Larionov, V M; Naidenov, I D; Natsvlishvili, T M; Piotrovich, M Y; Gnedin, Yu. N.

    2004-01-01

    We present brief review of two probable physical mechanisms that can explain the results of photometric and spectropolarimetric observations of magnetic white dwarfs: vacuum polarization effect into a strong magnetic field and, so-called, magnetic collision induced absorption (magnetic CIA). Both mechanisms provide observed rotation of polarization ellipse and suppression of spectral energy distributions. The results of spectropolarimetric observations of magnetic white dwarfs made at Russian BTA-6m and the results of the near infrared photometric observations with Russian-Italian AZT-24 telescope located at Campo Imperatore are also presented.

  4. Photospheric phosphorus in the FUSE spectra of GD71 and two similar DA white dwarfs

    OpenAIRE

    Dobbie, Paul; Barstow, Martin; Hubeny, Ivan; Holberg, Jay; Burleigh, Matt; Forbes, Anne

    2005-01-01

    We report the detection, from FUSE data, of phosphorus in the atmospheres of GD71 and two similar DA white dwarfs. This is the first detection of a trace metal in the photosphere of the spectrophotometric standard star GD71. Collectively, these objects represent the coolest DA white dwarfs in which photospheric phosphorus has been observed. We use a grid of homogeneous non-LTE synthetic spectra to measure abundances of[P/H]=-8.57 +0.09 -0.13, -8.70 +0.23 -0.37 and -8.36 +0.1...

  5. A spectrophotometric analysis of the hot helium-rich white dwarf HD 149499 B

    Science.gov (United States)

    Sion, E. M.; Guinan, E. F.; Wesemael, F.

    1982-01-01

    A comprehensive analysis of the hot helium-rich white dwarf HD 149499 B is presented based on International Ultraviolet Explorer ultraviolet spectra along with available optical spectra and photometry. Line strengths, line profiles, and continuum fluxes are analyzed in terms of a new grid of hot, high-gravity stellar atmospheres of mixed composition. The line strengths furnish the strongest temperature and gravity diagnostics because they are not affected by the contamination of the white dwarf optical spectrum and photometric properties by the K0 V companion, 2.35 arcsec away. Wegner's (1978) classification of HD 149499 B as a DO star is confirmed.

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

    International Nuclear Information System (INIS)

    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.

  7. 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.

  8. Faint Blue Objects in the Hubble Deep Field South Revealed: White Dwarfs, Subdwarfs, and Quasars

    OpenAIRE

    Kilic, Mukremin; Mendez, R. A.; Von Hippel, Ted; Winget, D. E.

    2005-01-01

    We explore the nature of the faint blue objects in the Hubble Deep Field South. We have derived proper motions for the point sources in the Hubble Deep Field South using a 3 year baseline. Combining our proper motion measurements with spectral energy distribution fitting enabled us to identify 4 quasars and 42 stars, including 3 white dwarf candidates. Two of these white dwarf candidates, HDFS 1444 and 895, are found to display significant proper motion, 21.1 $\\pm$ 7.9 mas/y...

  9. Density Effects in Sodium and ionized Calcium perturbed by helium in Cool White Dwarf Stars

    Science.gov (United States)

    Allard, N. F.

    2013-12-01

    Traces of heavy metal in cool DZ white dwarfs may be attributed to the accretion of circumstellar dust. This dust is thought to originate from the tidal disruption of some rocky material, they provide a unique opportunity to study the composition of extra-solar planetary systems. The determinations of precise atmospheric parameters and abundances require accurate description of the line profiles of the identified features. We present absorption spectra of Na-He and Ca+-He for the conditions prevailing in cool white dwarfs.

  10. Rotation velocities of white dwarfs determined from the CaII K line

    OpenAIRE

    Berger, L.; Koester, D.; Napiwotzki, R.; Reid, I. N.; Zuckerman, B.

    2005-01-01

    We determine projected rotation velocities v sini in DAZ white dwarfs, for the first time using the rotational broadening of the CaII K line. The results confirm previous findings that white dwarfs are very slow rotators, and set even more stringent upper limits of typically less than 10 km/s. The few exceptions include 3 stars known or suspected to be variable ZZ Ceti stars, where the line broadening is very likely not due to rotation. The results demonstrate that the angul...

  11. Comment on "Strong constraints on magnetized white dwarfs surpassing the Chandrasekhar mass limit"

    CERN Document Server

    Das, Upasana

    2014-01-01

    We show that the upper bound for the central magnetic field of a super-Chandrasekhar white dwarf calculated by Nityananda and Konar [Phys. Rev. D 89, 103017 (2014)] is completely erroneous. This in turn strengthens the argument in favor of the stability of the recently proposed magnetized super-Chandrasekhar white dwarfs. We also point out several other numerical errors in their work. Overall we conclude, based on our calculations, that the arguments put forth by Nityananda and Konar are fallacious and misleading.

  12. Mass of highly magnetized white dwarfs exceeding the Chandrasekhar limit: An analytical view

    OpenAIRE

    Kundu, Aritra; Mukhopadhyay, Banibrata

    2012-01-01

    In recent years a number of white dwarfs has been observed with very high surface magnetic fields. We can expect that the magnetic field in the core of these stars would be much higher (~ 10^{14} G). In this paper, we analytically study the effect of high magnetic field on relativistic cold electron, and hence its effect on the stability and the mass-radius relation of a magnetic white dwarf. In strong magnetic fields, the equation of state of the Fermi gas is modified and L...

  13. Constraining the Initial-Final Mass Relation with Wide Double White Dwarfs

    Science.gov (United States)

    Andrews, Jeffrey; Agueros, Marcel A.; Gianninas, Alex; Kilic, Mukremin; Dhital, Saurav; Anderson, Scott F.

    2015-01-01

    Population synthesis models predict the existence of large numbers of wide binaries for which the end state is a wide double white dwarf (DWD) system. I discuss the methods I used to find these systems in the Sloan Digital Sky Survey, which ultimately increased the known number of candidate binaries from ~45 to 112. I then describe an observational campaign to obtain spectra of the bright (g final mass relation, which relates a given main-sequence star to the mass of the white dwarf it will produce. Our analysis avoids some of the difficult-to-model systematics introduced by other methods, generating independent and complementary constraints.

  14. HST+COS spectra of the double white dwarf CSS 41177 place the secondary inside the pulsational instability strip

    Science.gov (United States)

    Bours, M. C. P.; Marsh, T. R.; Gänsicke, B. T.; Parsons, S. G.

    2015-03-01

    We present Hubble Space Telescope + Cosmic Origins Spectrograph (HST+COS) data of the eclipsing double white dwarf binary CSS 41177. Due to the temperature difference between the two white dwarfs, the HST+COS far-ultraviolet data are dominated by the hot, primary white dwarf and allow us to precisely measure its temperature (T1). Using eclipse observations, we also tightly constrain the temperature of the cooler secondary white dwarf (T2). Our results, where T1 = 22 439 ± 59 K and T2 = 10 876 ± 32 K, with the uncertainties being purely statistical, place the secondary inside and close to the blue edge of the empirical instability strip for low-temperature hydrogen-atmosphere white dwarfs. Dedicated high-speed photometry is encouraged to probe for the presence of pulsations, which will constrain the border of the instability strip as well as probe a new region of low gravity within the strip.

  15. A SPECTROSCOPIC SURVEY AND ANALYSIS OF BRIGHT, HYDROGEN-RICH WHITE DWARFS

    International Nuclear Information System (INIS)

    We have conducted a spectroscopic survey of over 1300 bright (V ? 17.5), hydrogen-rich white dwarfs based largely on the last published version of the McCook and Sion catalog. The complete results from our survey, including the spectroscopic analysis of over 1100 DA white dwarfs, are presented. High signal-to-noise ratio optical spectra were obtained for each star and were subsequently analyzed using our standard spectroscopic technique where the observed Balmer line profiles are compared to synthetic spectra computed from the latest generation of model atmospheres appropriate for these stars. First, we present the spectroscopic content of our sample, which includes many misclassifications as well as several DAB, DAZ, and magnetic white dwarfs. Next, we look at how the new Stark broadening profiles affect the determination of the atmospheric parameters. When necessary, specific models and analysis techniques are used to derive the most accurate atmospheric parameters possible. In particular, we employ M dwarf templates to obtain better estimates of the atmospheric parameters for those white dwarfs that are in DA+dM binary systems. Certain unique white dwarfs and double-degenerate binary systems are also analyzed in greater detail. We then examine the global properties of our sample including the mass distribution and their distribution as a function of temperature. We then proceed to test the accuracy and robustness of our method by comparing our results to those ofhod by comparing our results to those of other surveys such as SPY and Sloan Digital Sky Survey. Finally, we revisit the ZZ Ceti instability strip and examine how the determination of its empirical boundaries is affected by the latest line profile calculations.

  16. COOL WHITE DWARFS FOUND IN THE UKIRT INFRARED DEEP SKY SURVEY

    International Nuclear Information System (INIS)

    We present the results of a search for cool white dwarfs in the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky Survey to identify cool hydrogen-rich white dwarf candidates by their neutral optical colors and blue near-infrared colors, as well as faint reduced proper motion magnitudes. Optical spectroscopy was obtained at Gemini Observatory and showed the majority of the candidates to be newly identified cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants. Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with effective temperature Teff ? 6000 K. The current follow-up of 1400 deg2 of sky has produced 13 new white dwarfs. Model fits to the photometry show that seven of the newly identified white dwarfs have 4120 K ?Teff ? 4480 K, and cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km s-1 ? vtan ? 85 km s-1 and are likely to be thick disk 10-11 Gyr-old objects. The other half of the sample has 4610 K ?Teff ? 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr, and 60 km s-1 ? vtan ? 100 km s-1. These are either thin disk remnants with unusually high velocities, or lower-mass remnants of thick disk or halo late-F or G stars.

  17. A SPECTROSCOPIC SURVEY AND ANALYSIS OF BRIGHT, HYDROGEN-RICH WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Gianninas, A.; Bergeron, P. [Departement de Physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada); Ruiz, M. T., E-mail: gianninas@astro.umontreal.ca, E-mail: bergeron@astro.umontreal.ca, E-mail: mtruiz@das.uchile.cl [Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago (Chile)

    2011-12-20

    We have conducted a spectroscopic survey of over 1300 bright (V {<=} 17.5), hydrogen-rich white dwarfs based largely on the last published version of the McCook and Sion catalog. The complete results from our survey, including the spectroscopic analysis of over 1100 DA white dwarfs, are presented. High signal-to-noise ratio optical spectra were obtained for each star and were subsequently analyzed using our standard spectroscopic technique where the observed Balmer line profiles are compared to synthetic spectra computed from the latest generation of model atmospheres appropriate for these stars. First, we present the spectroscopic content of our sample, which includes many misclassifications as well as several DAB, DAZ, and magnetic white dwarfs. Next, we look at how the new Stark broadening profiles affect the determination of the atmospheric parameters. When necessary, specific models and analysis techniques are used to derive the most accurate atmospheric parameters possible. In particular, we employ M dwarf templates to obtain better estimates of the atmospheric parameters for those white dwarfs that are in DA+dM binary systems. Certain unique white dwarfs and double-degenerate binary systems are also analyzed in greater detail. We then examine the global properties of our sample including the mass distribution and their distribution as a function of temperature. We then proceed to test the accuracy and robustness of our method by comparing our results to those of other surveys such as SPY and Sloan Digital Sky Survey. Finally, we revisit the ZZ Ceti instability strip and examine how the determination of its empirical boundaries is affected by the latest line profile calculations.

  18. Constraints on the pairing properties of main sequence stars from observations of white dwarfs in binary systems

    CERN Document Server

    Ferrario, Lilia

    2012-01-01

    Observations of main sequence stars conducted over the last several decades have clearly shown that something like 50 per cent of stars of spectral types G and F occur in multiple systems. For earlier spectral types, the incidence of multiplicity is even higher. Thus, a volume limited sample of white dwarfs should reflect the percentage of binarity observed in stars of F to late B spectral types, which are their Main Sequence progenitors. However, a study of the local volume limited sample of white dwarfs (20 pc from the Sun) conducted by Holberg has shown that a white dwarf has a probability of only $\\sim 32$ per cent of occurring in a binary system, in stark contrast to the observations of multiplicity of Main Sequence stars. Others studies have also led to the same conclusion. In this paper, we argue that the "hidden" white dwarfs are either in double white dwarf systems or in Sirius-like systems. We also show that the white dwarf progenitors of the SDSS white dwarf - M dwarf wide binaries are distributed ...

  19. SPECTROSCOPIC CONFIRMATION OF UV-BRIGHT WHITE DWARFS FROM THE SANDAGE TWO-COLOR SURVEY OF THE GALACTIC PLANE

    International Nuclear Information System (INIS)

    We present spectroscopic observations confirming the identification of hot white dwarfs among UV-bright sources from the Sandage Two-color Survey of the Galactic Plane and listed in the Lanning (Lan) catalog of such sources. A subsample of 213 UV-bright Lan sources have been identified as candidate white dwarfs based on the detection of a significant proper motion. Spectroscopic observations of 46 candidates with the KPNO 2.1 m telescope confirm 30 sources to be hydrogen white dwarfs with subtypes in the DA1-DA6 range, and with one of the stars (Lan 161) having an unresolved M dwarf as a companion. Five more sources are confirmed to be helium white dwarfs, with subtypes from DB3 to DB6. One source (Lan 364) is identified as a DZ 3 white dwarf, with strong lines of calcium. Three more stars are found to have featureless spectra (to within detection limits) and are thus classified as DC white dwarfs. In addition, three sources are found to be hot subdwarfs: Lan 20 and Lan 480 are classified as sdOB, and Lan 432 is classified sdB. The remaining four objects are found to be field F star interlopers. Physical parameters of the DA and DB white dwarfs are derived from model fits.

  20. Habitable planets around white and brown dwarfs: the perils of a cooling primary.

    Science.gov (United States)

    Barnes, Rory; Heller, René

    2013-03-01

    White and brown dwarfs are astrophysical objects that are bright enough to support an insolation habitable zone (IHZ). Unlike hydrogen-burning stars, they cool and become less luminous with time; hence their IHZ moves in with time. The inner edge of the IHZ is defined as the orbital radius at which a planet may enter a moist or runaway greenhouse, phenomena that can remove a planet's surface water forever. Thus, as the IHZ moves in, planets that enter it may no longer have any water and are still uninhabitable. Additionally, the close proximity of the IHZ to the primary leads to concern that tidal heating may also be strong enough to trigger a runaway greenhouse, even for orbital eccentricities as small as 10(-6). Water loss occurs due to photolyzation by UV photons in the planetary stratosphere, followed by hydrogen escape. Young white dwarfs emit a large amount of these photons, as their surface temperatures are over 10(4) K. The situation is less clear for brown dwarfs, as observational data do not constrain their early activity and UV emission very well. Nonetheless, both types of planets are at risk of never achieving habitable conditions, but planets orbiting white dwarfs may be less likely to sustain life than those orbiting brown dwarfs. We consider the future habitability of the planet candidates KOI 55.01 and 55.02 in these terms and find they are unlikely to become habitable. PMID:23537137