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

CERN Document Server

We present a detailed spectroscopic analysis of the hot DA white dwarf G191-B2B, using the best signal to noise, high resolution near and far UV spectrum obtained to date. This is constructed from co-added \\textit{HST} STIS E140H, E230H, and \\textit{FUSE} observations, covering the spectral ranges of 1150-3145\\AA\\, and 910-1185\\AA\\, respectively. With the aid of recently published atomic data, we have been able to identify previously undetected absorption features down to equivalent widths of only a few m\\AA. In total, 976 absorption features have been detected to $3\\sigma$ confidence or greater, with 947 of these lines now possessing an identification, the majority of which are attributed to Fe and Ni transitions. In our survey, we have also potentially identified an additional source of circumstellar material originating from Si {\\sc iii}. While we confirm the presence of Ge detected by \\citet{vennes05a}, we do not detect any other species. Furthermore, we have calculated updated abundances for C, N, O, Si,...

Preval, Simon P; Holberg, Jay B; Dickinson, Nathan J

2013-01-01

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

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...

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

2014-01-01

3

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

Science.gov (United States)

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

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

2014-01-01

4

Stellar laboratories III. New Ba V, Ba VI, and Ba VII oscillator strengths and the barium abundance in the hot white dwarfs G191-B2B and RE0503-289  

OpenAIRE

For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Reliable Ba V - VII oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and to determine their p...

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

2014-01-01

5

Stellar laboratories III. New Ba V, Ba VI, and Ba VII oscillator strengths and the barium abundance in the hot white dwarfs G191-B2B and RE0503-289  

CERN Document Server

For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Reliable Ba V - VII oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and to determine their photospheric Ba abundances. We newly calculated Ba V - VII oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE0503-289. For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba VI and Ba VII lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE0503-289. The Ba VI / Ba VII ionization equil...

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

2014-01-01

6

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

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...

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

2015-01-01

7

Evidence for the stratification of Fe in the photosphere of G191-B2B  

CERN Document Server

The presence of heavy elements in the atmospheres of the hottest H-rich DA white dwarfs has been the subject of considerable interest. While theoretical calculations can demonstrate that radiative forces, counteracting the effects of gravitational settling, can explain the detections of individual species, the predicted abundances do not accord well with observation. However, accurate abundance measurements can only be based on a thorough understanding of the physical structure of the white dwarf photospheres, which has proved elusive. Recently, the availability of new non-LTE model atmospheres with improved atomic data has allowed self-consistent analysis of the EUV, far UV and optical spectra of the prototypical object G191$-$B2B. Even so, the predicted and observed stellar fluxes remain in serious disagreement at the shortest wavelengths (below $\\approx 190$Å), while the inferred abundances remain largely unaltered. We show here that the complete spectrum of G191$-$B2B can be explained by a model atmosphe...

Barstow, M A; Holberg, J B

1999-01-01

8

High-Resolution Spectroscopy of G191-B2B in the Extreme Ultraviolet  

CERN Document Server

We report a high-resolution (R=3000-4000) spectroscopic observation of the DA white dwarf G191-B2B in the extreme ultraviolet band 220-245 A. A low- density ionised He component is clearly present along the line-of-sight, which if completely interstellar implies a He ionisation fraction considerably higher than is typical of the local interstellar medium. However, some of this material may be associated with circumstellar gas, which has been detected by analysis of the C IV absorption line doublet in an HST STIS spectrum. A stellar atmosphere model assuming a uniform element distribution yields a best fit to the data which includes a significant abundance of photospheric He. The 99-percent confidence contour for the fit parameters excludes solutions in which photospheric He is absent, but this result needs to be tested using models allowing abundance gradients.

Cruddace, R G; Yentis, D J; Brown, C M; Gursky, H; Barstow, M A; Bannister, N P; Fraser, G W; Spragg, J E; Lapington, J S; Tandy, J A; Sanderson, B; Culhane, J L; Barbee, T W; Kordas, J F; Goldstein, W H; Fritz, G G

2001-01-01

9

ROSAT EUV and soft X-ray studies of atmospheric composition and structure in G191-B2B  

Science.gov (United States)

Previous studies of the hot DA white dwarf GI91-B2B have been unable to determine whether the observed soft X-ray and EUV opacity arises from a stratified hydrogen and helium atmosphere or from the presence of trace metals in the photosphere. New EUV and soft X-ray photometry of this star, made with the ROSAT observatory, when analyzed in conjunction with the earlier data, shows that the stratified models cannot account for the observed fluxes. Consequently, we conclude that trace metals must be a substantial source of opacity in the photosphere of G191-B2B.

Barstow, M. A.; Fleming, T. A.; Finley, D. S.; Koester, D.; Diamond, C. J.

1993-01-01

10

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

CERN Document Server

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 ...

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

11

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

CERN Document Server

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 ...

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

12

Circumstellar features in hot DA white dwarfs  

CERN Document Server

We present a phenomenological study of highly ionised, non-photospheric absorption features in high spectral resolution vacuum ultraviolet spectra of 23 hot DA white dwarfs. Prior to this study, four of the survey objects (Feige 24, REJ 0457-281, G191-B2B and REJ 1614-085) were known to possess these features. We find four new objects with multiple components in one or more of the principal resonance lines: REJ 1738+665, Ton 021, REJ 0558-373 and WD 2218+706. A fifth object, REJ 2156-546 also shows some evidence of multiple components, though further observations are required to confirm the detection. We discuss possible origins for these features including ionisation of the local interstellar environment, the presence of material inside the gravitational well of the white dwarf, mass loss in a stellar wind, and the existence of material in an ancient planetary nebula around the star. We propose ionisation of the local interstellar medium as the origin of these features in G191-B2B and REJ 1738+665, and demon...

Bannister, N P; Holberg, J B; Bruhweiler, F C

2003-01-01

13

High-Resolution EUV Spectroscopy of White Dwarfs  

Science.gov (United States)

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

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

2014-01-01

14

Heavy element abundances in DA white dwarfs  

CERN Document Server

We present a series of systematic abundance measurements for a group of hot DA white dwarfs in the temperature range 20,000-75,000K, based on far-UV spectroscopy with STIS on HST, IUE and FUSE. Using our latest heavy element blanketed non-LTE stellar atmosphere calculations we have addressed the heavy element abundance patterns for the hottest stars for the first time, showing that they are similar to objects like G191-B2B. The abundances observed in the cooler (<50,000K) white dwarfs are something of a mystery. Some of the patterns (e.g. REJ1032) can be explained by self-consistent levitation-diffusion calculations but there is then a serious difficulty in understanding the appearance of the apparently pure H atmospheres. We also report the detection of photospheric HeII in the atmosphere of WD2218+706.

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

2000-01-01

15

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

OpenAIRE

We propose a new probe of the dependence of the fine structure constant, alpha, 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 on the fractional variation of alpha of (Delta alpha/alpha)=(4.2 +- 1.6)x10^(-5) and (-6.1 +- 5.8)x10^(-5) from Fe V and Ni V spectra, res...

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

2013-01-01

16

Albus 1: A very bright white dwarf candidate  

CERN Document Server

We have serendipitously discovered a previously-unknown, bright source (B_T = 11.75+/-0.07 mag) with a very blue V_T-K_s color, to which we have named Albus 1. A photometric and astrometric study using Virtual Observatory tools has shown that it possesses an appreciable proper motion and magnitudes and colors very similar to those of the well known white dwarf G 191-B2B. We consider Albus 1 as a DA-type white dwarf located at about 40 pc. If confirmed its nature, Albus 1 would be the sixth brightest isolated white dwarf in the sky, which would make it an excellent spectrophotometric standard.

Solano, Jose Antonio Caballero Enrique

2007-01-01

17

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

Science.gov (United States)

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

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

18

Limits on variations of the fine-structure constant with gravitational potential from white-dwarf spectra  

CERN Document Server

We propose a new probe of the variation of the fine structure constant, alpha, in 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 on the fractional variation of alpha of (Delta alpha/alpha)=(4.2 +- 1.6)x10^(-5) and (-6.1 +- 5.8)x10^(-5) from Fe V and Ni V spectra, respectively, at a dimensionless gravitational potential relative to Earth of (Delta phi) ~ 5x10^(-5). With better determinations of the laboratory wavelengths of the lines employed these results could be improved by up to two orders of magnitude.

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

2013-01-01

19

The Ionization of the Local Interstellar Medium, as Revealed by FUSE Observations of N, O and Ar toward White Dwarf Stars  

CERN Document Server

FUSE spectra of the white dwarf stars G191-B2B, GD 394, WD 2211-495 and WD 2331-475 cover the absorption features out of the ground electronic states of N I, N II, N III, O I and Ar I in the far ultraviolet, providing new insights on the origin of the partial ionization of the Local Interstellar Medium (LISM), and for the case of G191-B2B, the interstellar cloud that immediately surrounds the solar system. Toward these targets the interstellar abundances of Ar I, and sometimes N I, are significantly below their cosmic abundances relative to H I. In the diffuse interstellar medium, these elements are not likely to be depleted onto dust grains. Generally, we expect that Ar should be more strongly ionized than H (and also O and N whose ionizations are coupled to that of H via charge exchange reactions) because the cross section for the photoionization of Ar I is very high. Our finding that Ar I/H I is low may help to explain the surprisingly high ionization of He in the LISM found by other investigators. Our res...

Jenkins, E B; Gry, C; Vallerga, J V; Sembach, K R; Shelton, R L; Ferlet, R; Vidal-Madjar, A; York, D G; Linsky, J L; Roth, K C; Dupree, A K; Edelstein, J D

2000-01-01

20

Measuring chemical evolution and gravitational dependence of \\alpha using ultraviolet Fe V and Ni V transitions in white-dwarf spectra  

CERN Document Server

In this paper, we present the details of the ab initio high-precision configuration interaction and many-body perturbation theory calculations that were used in a previous work to place limits on the dependence of the fine-structure constant, alpha, on the gravitational field of the white-dwarf star G191-B2B. These calculations were combined with laboratory wavelengths and spectra from the Hubble Space Telescope Imaging Spectrograph to obtain limits on the gravitational alpha-dependence using Fe V and Ni V transitions. The uncertainty in these results are dominated by the uncertainty in the laboratory wavelengths. In this work we also present ab initio calculations of the isotopic shifts of the Fe V transitions. We show that improved laboratory spectra will enable determination of the relative isotope abundances in Fe V to an accuracy ~20%. Therefore this work provides a strong motivation for new laboratory measurements.

Ong, A; Flambaum, V V

2013-01-01

21

Hot White Dwarfs  

CERN Document Server

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, the DO white dwarfs, the DB white dwarfs, the DBA white dwarfs, and the Hot DQ white dwarfs (an entirely new class). The most recent observational and theoretical advances are brought to bear on the topic.

Sion, Edward M

2011-01-01

22

An interpretation of the spectral properties of hot hydrogen-rich white dwarfs with stratified H/He model atmospheres  

Science.gov (United States)

A grid of stratified H/He model atmospheres applicable to the interpretation of the spectral properties of hot H-rich white dwarfs (WDs) is computed. Samples of hot DA WDs observed with Exosat and Einstein are analyzed using the models. Six out of six objects with T(eff) = 35,000 K or less do not show a EUV/soft X-ray flux deficiency and therefore can be understood solely in terms of pure hydrogen atmospheres. A majority of DA WDs hotter than this value do show a flux deficiency and thus require the presence of some absorbers in their atmospheres. It is shown that the Exosat broadband photometry of Feige 24 and G191 B2B cannot be explained in terms of stratified atmospheres. Absorption by heavy elements is certainly responsible for the required EUV/soft X-ray opacity source in these cases. However, the Exosat data are consistent with the hypothesis of stratified atmospheres in the four remaining objects.

Vennes, Stephane; Fontaine, Gilles

1992-01-01

23

White dwarf planets  

OpenAIRE

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?...

Bonsor Amy; Veras Dimitri; Villaver Eva; Mustill Alexander J.; Wyatt Mark C.

2013-01-01

24

Gaia photometry for white dwarfs  

OpenAIRE

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 wh...

Carrasco Marti?nez, Jose? Manuel; Catala?n, S.; Jordi I Nebot, Carme; Tremblay, P. -e; Napiwotzki, R.; Luri Carrascoso, Xavier; Robin, A. C.; Kowalski, P. M.

2014-01-01

25

White Dwarf Stars  

Science.gov (United States)

This NASA site provides information about white dwarfs, produced when stars like our Sun exhaust their nuclear fuel and blow off much of their mass. The site contains an explanation of their properties and composition. Additional links include an introductory article, online quiz, cool facts, FAQ, and other resources.

2007-01-26

26

White Dwarf Stars  

Science.gov (United States)

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.

Kepler, S. O.

2014-10-01

27

White dwarf planets  

Directory of Open Access Journals (Sweden)

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.

Bonsor Amy

2013-04-01

28

Spectra of southern white dwarfs  

International Nuclear Information System (INIS)

Spectra of 47 white dwarf suspects south of declination -450 are discussed. The sample consists of 35 DA's, 4 DB's, 3 DC's, 4 sdGp's and 1 sdKp. Contrary to previous claims, of the six stars redder than B-V = 0.5, only one is a classical white dwarf. (author)

29

ASTRO-H White Paper - White Dwarf  

OpenAIRE

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...

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

30

Equations of State for White Dwarfs  

OpenAIRE

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...

Heikkila?, Elena

2009-01-01

31

Asteroseismology of white dwarf stars  

CERN Document Server

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...

Córsico, A H

2014-01-01

32

Gaia photometry for white dwarfs  

Science.gov (United States)

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

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

2014-05-01

33

White Dwarf Kinematics vs Mass  

OpenAIRE

We have investigated the relationship between the kinematics and mass of young (<3x10^8 years) white dwarfs using proper motions. Our sample is taken from the colour selected catalogues of SDSS (Eisenstein et al. 2006) and the Palomar-Green Survey (Liebert, Bergeron & Holberg 2005), both of which have spectroscopic temperature and gravity determinations. We find that the dispersion decreases with increasing white dwarf mass. This can be explained as a result of less scatteri...

Wegg, Christopher; Phinney, E. Sterl

2012-01-01

34

White Dwarf Planets from GAIA  

OpenAIRE

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.

Silvotti, Roberto; Sozzetti, Alessandro; Lattanzi, Mario

2010-01-01

35

Gaia photometry for white dwarfs  

CERN Document Server

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...

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

2014-01-01

36

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.

37

Accretion belts on white dwarfs  

International Nuclear Information System (INIS)

Matter which falls onto a slowly rotating white dwarf from an accretion disk forms a rapidly rotating belt, whose chemical composition and angular momentum are mixed with the surrounding white dwarf material by shear instabilities. The structure and evolution of accretion belts is computed in a simplified model . After a sufficient amount of material is accreted, hydrogen is ignited at the bottom of the belt. The model presented here differs drastically from the models of spherically symmetric accretion, on which current Nova theories are based. (orig.)

38

Gravitational Radiation from Strongly Magnetized White Dwarfs  

OpenAIRE

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.

Heyl, Jeremy S.

2000-01-01

39

The evolution of iron white dwarf stars  

Directory of Open Access Journals (Sweden)

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.

J. A. Panei

2001-01-01

40

White dwarf research with Gaia  

CERN Document Server

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.

Jordan, Stefan

2014-01-01

41

Rotation of White Dwarf Stars  

CERN Document Server

I discuss and consider the status of observational determinations of the rotation velocities of white dwarf stars via asteroseismology and spectroscopy. While these observations have important implications on our understanding of the angular momentum evolution of stars in their late stages of evolution, more direct methods are sorely needed to disentangle ambiguities.

Kawaler, Steven D

2014-01-01

42

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

43

G-dwarfs, white dwarfs and the local dark matter  

International Nuclear Information System (INIS)

The observed paucity of low metalicity stars (the G-dwarf problem), the observed paucity of low luminosity white dwarfs and the apparent overabundance of matter in the solar neighborhood are discussed in a single context. In particular, bimodal models of star formation with enhanced cooling of low luminosity white dwarfs may be able to simply solve the G-dwarf problem as well as the local dark matter problem without the overproduction of observable white dwarfs. Constraints from galactic chemical evolution are also discussed. 55 refs., 5 figs

44

White dwarfs in the WTS: Eclipsing binaries  

Directory of Open Access Journals (Sweden)

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.

Burleigh M.R.

2013-04-01

45

Diffusion coefficients in white dwarfs  

CERN Document Server

Models of diffusion in white dwarfs universally rely on the coefficients calculated by Paquette et al. (1986). We present new calculations of diffusion coefficients based on an advanced microscopic theory of dense plasmas and a numerical simulation approach that intrinsically accounts for multiple collisions. Our method is validated against a state-of-the-art method and we present results for the diffusion of carbon ions in a helium plasma.

Saumon, D; Daligault, J

2014-01-01

46

Photometric Identification of Cool White Dwarfs  

OpenAIRE

We investigate the use of a narrow-band DDO51 filter for photometric identification of cool white dwarfs. We report photometric observations of 30 known cool white dwarfs with temperatures ranging from 10,000 K down to very cool temperatures (<3500 K). Follow-up spectroscopic observations of a sample of objects selected using this filter and our photometric observations show that DDO51 filter photometry can help select cool white dwarf candidates for follow-up multi--object ...

Kilic, M.; Winget, D. E.; Von Hippel, Ted; Claver, C. F.

2004-01-01

47

Merging White Dwarfs and Thermonuclear Supernovae  

OpenAIRE

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...

Kerkwijk, Marten H.

2012-01-01

48

Photometric Variability of Magnetic White Dwarfs  

Science.gov (United States)

We present our survey results searching for photometric variability and rotation periods of 77 magnetic white dwarfs (MWDs). The sample consists of 24 bright (V MWD. Where periods of variability have been determined, the magnetic field strength, temperature, mass and age of the white dwarf have been compared to determine whether any physically interesting correlations emerge.

Lawrie, K. A.; Burleigh, M. R.; Brinkworth, C. S.; Marsh, T. R.; Hodgkin, S. T.; Baker, D. E. A.; Cossins, P.; Littlejohns, O. M.; Scott, A. E.; Steele, P. R.

2013-01-01

49

PREFACE: 16th European White Dwarfs Workshop  

Science.gov (United States)

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

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

2009-07-01

50

An overview of white dwarf stars  

Directory of Open Access Journals (Sweden)

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.

Charpinet S.

2013-03-01

51

SDSS DR7 White Dwarf Catalog  

CERN Document Server

We present a new catalog of spectroscopically-confirmed white dwarf stars from the Sloan Digital Sky Survey 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 a more than factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalog 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...

Kleinman, S J; Koester, D; Pelisoli, Ingrid; Peçanha, Viviane; Nitta, A; Costa, J E S; Krzesinski, J; Dufour, P; Lachapelle, F -R; Bergeron, P; Yip, Ching-Wa; Harris, Hugh C; Eisenstein, Daniel J; Althaus, L; Córsico, A

2012-01-01

52

Keck Telescope Observations of Externally-Polluted White Dwarfs  

Science.gov (United States)

Beginning in the late 1990s the Keck telescope and HIRES echelle spectrometer have contributed mightily to investigations of white dwarf photospheres that contain elements heavier than helium that have been accreted from surrounding planetary systems. Today we report new Keck measurements of helium atmosphere (DB and DZ) white dwarfs, of Hyades white dwarfs, and of white dwarfs in binary systems.

Zuckerman, Ben M.; NASA, Research was Supported in Part by

2013-01-01

53

On the origin of high-field magnetic white dwarfs  

OpenAIRE

Every two years, white dwarf researchers and enthusiasts meet to exchange their knowledge and discuss recent developments in white dwarf theory and observations. These proceedings are from the 18th European White Dwarf Workshop held at Pedagogical University of Cracow (Krakow, Poland) on 13th - 17th August, 2012. The scientific topics discussed in this Workshop included: pulsating white dwarfs; luminosity function, mass distribution, and populations; white dwarf structure and evolution;...

Garc??a-berro Montilla, Enrique; Torres Gil, Santiago; Loren Aguilar, Pablo; Aznar Siguan, Gabriela; Camacho D??az, Judit; Kulebi, Baybar; Isern Vilaboy, Jordi; Althaus, Leandro Gabriel; Corsico, Alejandro H.

2013-01-01

54

White Dwarfs, Neutron Stars and Black Holes  

Science.gov (United States)

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)

Szekeres, P.

1977-01-01

55

New white dwarf candidates in the Hyades  

CERN Document Server

On the basis of the PPMXL catalogue we search for white dwarfs as candidates for members of the Hyades in a region up to 40 pc from the cluster centre.We use the proper motions from PPMXL in the convergent point method to determine probable kinematic members. We cross-match the kinematic candidates with catalogues containing white dwarfs and, finally, check the kinematic with the photometric distances for consistency. We find the 10 classical white dwarfs in the Hyades and determine their individual kinematic distances. Additionally, we identified 17 new probable (former) Hyades white dwarfs, i.e. white dwarfs co-moving with the bulk space motion of the Hyades cluster. At present, none of them can be excluded from membership on the basis of the measured radial velocities. For another 10 objects, the kinematic and the photometric distances are discordant which rates them as probable non-members. Among the probable members, five white dwarfs are in binary systems, three are known, two are new. There is good ind...

Schilbach, Elena

2011-01-01

56

Thirteenth Marcel Grossmann Meeting, Summary of the session, White Dwarf Pulsars and Rotating White Dwarf Theory  

CERN Document Server

This is the summary of the parallel session entitled "White Dwarf Pulsars and Rotating White Dwarf Theory", chaired by Yukikatsu Terada in Thirteenth Marcel Grossmann Meeting. The origin of cosmic rays remains a mystery, even over 100 years since their discovery. Neutron stars (NSs) are considered textbook cases of particle acceleration sites in our Galaxy, but many unresolved numerical problems remain. Searches for new acceleration sites are crucial for astrophysics. The magnetized white dwarfs (MWDs) have the same kind of rotating magnetosphere as NSs, and may be the source of up to 10% of galactic cosmic ray electrons. In the parallel session of the "white dwarf pulsars and rotating white dwarf theory", we focus on the current observational results on white dwarf pulsars, related theories of the radiation process both in white dwarfs and neutron stars, and the origin and rule of white dwarf pulsars, as well as surveying on the current theories of the internal structure and the equation of state of white dw...

Terada, Yukikatsu

2013-01-01

57

Six detached white-dwarf close binaries  

OpenAIRE

We determine the orbits of four double degenerate systems (DDs), composed of two white dwarfs, and of two white dwarf -- M dwarf binaries. The four DDs, WD1022+050, WD1428+373, WD1824+040, and WD2032+188, show orbital periods of 1.157155(5) d, 1.15674(2) d, 6.26602(6) d and 5.0846(3) d respectively. These periods combined with estimates for the masses of the brighter component, based on their effective temperatures, allow us to constrain the masses of the unseen companions. ...

Morales-rueda, L.; Marsh, T. R.; Maxted, P. F. L.; Nelemans, G.; Karl, C.; Napiwotzki, R.; Moran, C. K. J.

2005-01-01

58

Six detached white-dwarf close binaries  

CERN Document Server

We determine the orbits of four double degenerate systems (DDs), composed of two white dwarfs, and of two white dwarf -- M dwarf binaries. The four DDs, WD1022+050, WD1428+373, WD1824+040, and WD2032+188, show orbital periods of 1.157155(5) d, 1.15674(2) d, 6.26602(6) d and 5.0846(3) d respectively. These periods combined with estimates for the masses of the brighter component, based on their effective temperatures, allow us to constrain the masses of the unseen companions. We estimate that the upper limit for the contribution of the unseen companions to the total luminosity in the four DDs ranges between 10 and 20 per cent. In the case of the two white dwarf - M dwarf binaries, WD1042-690 and WD2009+622, we calculate the orbital parameters by fitting simultaneously the absorption line from the white dwarf and the emission core from the M-dwarf. Their orbital periods are 0.337083(1) d and 0.741226(2) d respectively. We find signatures of irradiation on the inner face of WD2009+622's companion. We calculate th...

Morales-Rueda, L; Maxted, P F L; Nelemans, G; Karl, C; Napiwotzki, R; Moran, C K J

2005-01-01

59

Supernovae from White Dwarfs Near Black Holes  

Science.gov (United States)

We describe a new thermonuclear explosion model for Type I (or Type II) supernovae whereby relativistic terms enhance the self gravity of a carbon-oxygen white dwarf (or red-giant core) as it passes or orbits near a black hole. This relativistic compression can cause the central density to exceed the threshold for pycnonuclear or thermonuclear reactions so that an explosion ensues. We have considered three possible environments: 1) white dwarfs orbiting a low-mass (˜ 10 20 M?) black hole; 2) white dwarfs encountering a massive (˜ 1 3 × 103 M?) black hole in a dense globular cluster; and 3) white dwarfs passing a supermassive (˜ 106 109 M?) black hole in a dense galactic core. We estimate the rate at which such events could occur to be significantly less than the rate of normal Type Ia supernovae for all three classes. Nevertheless, they should be frequent enough to warrant a search for this new class of supernova. We show results of three-dimensional thermonuclear burn calculations of white dwarfs or red-giant cores ignited near a supermassive black hole. Such an event might have produced the observed "mixed-morphology" Sgr A East supernova remnant (SNR) in the Galactic core.

Mathews, Grant J.; Wilson, James R.; Dearborn, David S. P.

2005-07-01

60

Supernovae from White Dwarfs Near Black Holes  

International Nuclear Information System (INIS)

We describe a new thermonuclear explosion model for Type I (or Type II) supernovae whereby relativistic terms enhance the self gravity of a carbon-oxygen white dwarf (or red-giant core) as it passes or orbits near a black hole. This relativistic compression can cause the central density to exceed the threshold for pycnonuclear or thermonuclear reactions so that an explosion ensues. We have considered three possible environments: 1) white dwarfs orbiting a low-mass (? 10 - 20 Msun ) black hole; 2) white dwarfs encountering a massive (? 1 - 3 x 103 Msun ) black hole in a dense globular cluster; and 3) white dwarfs passing a supermassive (? 106 - 109 Msun ) black hole in a dense galactic core. We estimate the rate at which such events could occur to be significantly less than the rate of normal Type Ia supernovae for all three classes. Nevertheless, they should be frequent enough to warrant a search for this new class of supernova. We show results of three-dimensional thermonuclear burn calculations of white dwarfs or red-giant cores ignited near a supermassive black hole. Such an event might have produced the observed 'mixed-morphology' Sgr A East supernova remnant (SNR) in the Galactic core

61

White dwarf stars with carbon atmospheres.  

Science.gov (United States)

White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 and 8-10, where 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 approximately 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, although these objects might be the cooler counterpart of the unique and extensively studied PG 1159 star H1504+65 (refs 4-7). These stars, together with H1504+65, might accordingly form a new evolutionary sequence that follows the asymptotic giant branch. PMID:18033290

Dufour, P; Liebert, J; Fontaine, G; Behara, N

2007-11-22

62

THE MASSES OF POPULATION II WHITE DWARFS  

International Nuclear Information System (INIS)

Globular star clusters are among the first stellar populations to have formed in the Milky Way, and thus only a small sliver of their initial spectrum of stellar types are still burning hydrogen on the main sequence today. Almost all of the stars born with more mass than 0.8 Msun have evolved to form the white dwarf cooling sequence of these systems, and the distribution and properties of these remnants uniquely holds clues related to the nature of the now evolved progenitor stars. With ultra-deep Hubble Space Telescope imaging observations, rich white dwarf populations of four nearby Milky Way globular clusters have recently been uncovered, and are found to extend impressive 5-8 mag in the faint-blue region of the Hertzsprung-Russell diagram. In this paper, we characterize the properties of these population II remnants by presenting the first direct mass measurements of individual white dwarfs near the tip of the cooling sequence in the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS multiobject spectroscopic observations, our results indicate that 0.8 Msun population II main-sequence stars evolving today form 0.53 ± 0.01 Msun white dwarfs. We discuss the implications of this result as it relates to our understanding of stellar structure and evolution of population II stars and for the age of the Galactic halo, as measured with white dwarf cooling theory.

63

Understanding white dwarf binary evolution with white dwarf/main sequence binaries: first results from SEGUE  

CERN Document Server

Close white dwarf binaries make up a wide variety of objects such as double white dwarf binaries, which are possible SN Ia progenitors, cataclysmic variables, super soft sources, or AM CVn stars. The evolution and formation of close white dwarf binaries crucially depends on the rate at which angular momentum is extracted from the binary orbit. The two most important sources of angular momentum loss are the common envelope phase and magnetic braking. Both processes are so far poorly understood. Observational population studies of white dwarf/main sequence binaries provide the potential to significantly progress with this situation and to clearly constrain magnetic braking and the CE-phase. However, the current population of white dwarf/main sequence binaries is highly incomplete and heavily biased towards young systems containing hot white dwarfs. The SDSSII/SEGUE collaboration awarded us with 5 fibers per plate pair in order to fill this gap and to identify the required unbiased sample of old white dwarf/main...

Schreiber, M R; Schwope, A D

2006-01-01

64

THE LINK BETWEEN PLANETARY SYSTEMS, DUSTY WHITE DWARFS, AND METAL-POLLUTED WHITE DWARFS  

Energy Technology Data Exchange (ETDEWEB)

It has long been suspected that metal-polluted white dwarfs (types DAZ, DBZ, and DZ) and white dwarfs with dusty disks possess planetary systems, but a specific physical mechanism by which planetesimals are perturbed close to a white dwarf has not yet been fully posited. In this paper, we demonstrate that mass loss from a central star during post-main-sequence evolution can sweep planetesimals into interior mean motion resonances with a single giant planet. These planetesimals are slowly removed through chaotic excursions of eccentricity that in time create radial orbits capable of tidally disrupting the planetesimal. Numerical N-body simulations of the solar system show that a sufficient number of planetesimals are perturbed to explain white dwarfs with both dust and metal pollution, provided other white dwarfs have more massive relic asteroid belts. Our scenario requires only one Jupiter-sized planet and a sufficient number of asteroids near its 2:1 interior mean motion resonance. Finally, we show that once a planetesimal is perturbed into a tidal crossing orbit, it will become disrupted after the first pass of the white dwarf, where a highly eccentric stream of debris forms the main reservoir for dust-producing collisions. These simulations, in concert with observations of white dwarfs, place interesting limits on the frequency of planetary systems around main-sequence stars, the frequency of planetesimal belts, and the probability that dust may obscure future terrestrial planet finding missions.

Debes, John H. [Space Telescope Science Institute, Baltimore, MD 21218 (United States); Walsh, Kevin J. [Southwest Research Institute, Boulder, CO (United States); Stark, Christopher [Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 (United States)

2012-03-10

65

3D Model Atmospheres of White Dwarfs  

Science.gov (United States)

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.

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

2014-01-01

66

The most massive pulsating white dwarf stars  

Science.gov (United States)

Massive pulsating white dwarf stars are extremely rare, because of their small size and because they are the final product of high-mass stars, which are less common. Because of their intrinsic smaller size, they are fainter than the normal size white dwarf stars. The motivation to look for this type of stars is to be able to study in detail their internal structure and also derive generic properties for the sub-class of variables, the massive ZZ Ceti stars. Our goal is to investigate whether the internal structures of these stars differ from the lower-mass ones, which in turn could have been resultant from the previous evolutionary stages. In this paper, we present the ensemble seismological analysis of the known massive pulsating white dwarf stars. Some of these pulsating stars might have substantial crystallized cores, which would allow us to probe solid physics in extreme conditions.

Castanheira, Barbara G.; Kepler, S. O.

2014-02-01

67

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

68

HST spectrophotometry of accreting white dwarf pulsators  

Science.gov (United States)

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.

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

2015-01-01

69

Unlocking the secrets of white dwarf stars  

CERN Document Server

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

Van Horn, Hugh M

2015-01-01

70

White dwarfs: the connection with the parent's masses  

International Nuclear Information System (INIS)

The dependence between the mass of white dwarfs and that of their main sequence parent stars is determined. The birth-rates of white dwarfs with different masses and the death-rate of the main sequence stars are interconsistent

71

On Thermohaline Mixing in Accreting White Dwarfs  

CERN Document Server

We discuss the recent claim that the thermohaline ("fingering") instability is important in accreting white dwarfs, increasing the derived accretion fluxes potentially by orders of magnitude. We present an alternative view and conclude that at least in the steady state this is not the case and the current method of estimating accretion fluxes is correct.

Koester, Detlev

2014-01-01

72

White dwarf evolution - Cradle-to-grave constraints via pulsation  

Science.gov (United States)

White dwarf evolution, particularly in the early phases, is not very strongly constrained by observation. Fortunately, white dwarfs undergo nonradial pulsation in three distinct regions of the H-R diagram. These pulsations provide accurate masses, surface compositional structure and rotation velocities, and help constrain other important physical properties. We demonstrate the application of the tools of stellar seismology to white dwarf evolution using the hot white dwarf star PG 1159-035 and the cool DAV (or ZZ Ceti) stars as examples. From pulsation studies, significant challenges to the theory of white dwarf evolution emerge.

Kawaler, Steven D.

1990-01-01

73

The Prototype Dwarf Nova SS Cygni: The White Dwarf Exposed  

Science.gov (United States)

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.

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

2010-01-01

74

Circumstellar Disks at White Dwarfs: Observations  

CERN Document Server

I present a thorough review of existing observational studies of circumstellar disks at white dwarf stars. This is done from a historical perspective beginning just prior to the ground-based discovery of dust at G29-38 nearly 25 years ago, and following the many scientific breakthroughs made with the Spitzer Space Telescope. The chapter is ideal for new or advanced students of this field and includes significant material on the observational properties of metal-polluted white dwarf atmospheres, which are physically connected to their circumstellar environments. Included are all published observations circa mid-2010, with an emphasis on the nascent field of exoterrestrial planetary (bulk) chemistry via disk-polluted atmospheric spectroscopy.

Farihi, J

2011-01-01

75

The SDSS White Dwarf - M Star Library  

CERN Document Server

The Sloan Digital Sky Survey (SDSS), originally targeted at quasi-stellar objects, has provided us with a wealth of astronomical byproducts through the last decade. Since then, the number of white dwarfs (WDs) with physically bound main-sequence star companions (mostly dM stars) has increased radically, allowing for fundamentally new insights into stellar physics. Different methods for the retrieval and follow-up analysis of SDSS WD-dM binaries have been applied in the literature, leading to a rising number of WD-dM catalogs. Here we present a detailed literature search, coupled with our own hunting for SDSS WD-dMs by color selection, the outcome being named the "SDSS White Dwarf - M Star Library". We also explain improvements of our automated spectral analysis method.

Heller, René; Østensen, Roy H

2011-01-01

76

Watch This Space: Observing Merging White Dwarfs  

CERN Document Server

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.

Webbink, Ronald F

2010-01-01

77

CSS 41177: an eclipsing double white dwarf binary  

Science.gov (United States)

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.

Bours, Madelon

2013-10-01

78

White dwarfs in Be star binary systems  

Science.gov (United States)

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.

Apparao, K. M. V.

1991-01-01

79

Black holes, white dwarfs and neutron stars  

International Nuclear Information System (INIS)

Basic principles of the physics of compact objects - white dwarfs, neutron stars and black holes, are stated. State equations and models of superdnse objects are discussed with provision for effects of the general relativity theory. For compact objects physical properties in the ground state are analyzed. The structure of stars is studied, when analyzing effect of differnt disturbances on these objects: rotations, magnetic fields, thermal fluxes accretion etc

80

Asteroseismology of DAV White Dwarf Stars  

Energy Technology Data Exchange (ETDEWEB)

The author reviews the seismological structural determinations of ZZ Ceti stars done to date, and supplement these with additional preliminary determinations of his own. He compares the constraints on the hydrogen layer mass to see what trends emerge and also determines if the observed hydrogen layer masses are consistent with proposed theories. He then looks ahead to the prospects of further DAV white dwarf seismology.

Bradley, Paul A.

1997-12-31

81

Six White Dwarfs with Circumstellar Silicates  

CERN Document Server

Spitzer Space Telescope spectra reveal 10 micron silicate emission from circumstellar dust orbiting six externally-polluted white dwarfs. Micron-size glasses with an olivine stoichiometry can account for the distinctively broad wings that extend to 12 microns; these particles likely are produced by tidal-disruption of asteroids. The absence of infrared PAH features is consistent with a scenario where extrasolar rocky planets are assembled from carbon-poor solids.

Jura, M; Zuckerman, B

2008-01-01

82

Axions and the white dwarf luminosity function  

Energy Technology Data Exchange (ETDEWEB)

The evolution of white dwarfs can be described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. Since the shape of the bright branch of this function is only sensitive to the average cooling rate, we use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. We show here that the inclusion of the axion emissivity in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function, thus providing the first positive indication that axions could exist. Our results indicate that the best fit is obtained for m{sub a}cos{sup 2}beta approx = 2-6 meV, where m{sub a} is the mass of the axion and cos{sup 2}beta is a free parameter, and that values larger than 10 meV are clearly excluded.

Isern, J; Catalan, S [Institut de Ciencies de l' Espai (CSIC), Facultat de Ciencies, Campus UAB, 08193 Bellaterra (Spain); GarcIa-Berro, E; Torres, S, E-mail: isern@ieec.ca, E-mail: catalan@ieec.ca, E-mail: garcia@fa.upc.ed, E-mail: santi@fa.upc.ed [Institut d' Estudis Espacials de Catalunya (IEEC), c/Gran Capita 2-4, 08034 Barcelona (Spain)

2009-06-01

83

Axions and the white dwarf luminosity function  

International Nuclear Information System (INIS)

The evolution of white dwarfs can be described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. Since the shape of the bright branch of this function is only sensitive to the average cooling rate, we use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. We show here that the inclusion of the axion emissivity in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function, thus providing the first positive indication that axions could exist. Our results indicate that the best fit is obtained for macos2? ? 2-6 meV, where ma is the mass of the axion and cos2? is a free parameter, and that values larger than 10 meV are clearly excluded.

84

Axions and the white dwarf luminosity function  

CERN Document Server

The evolution of white dwarfs can be described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. Since the shape of the bright branch of this function is only sensitive to the average cooling rate, we use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. We show here that the inclusion of the axion emissivity in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function, thus providing the first positive indication that axions could exist. Our results indicate that the best fit is obtained for m_a cos^2beta ~ 2-6 meV, where m_a is the mass of the axion and cos^2beta is a free parameter, and that values larger than 10 meV are clearly excluded.

Isern, J; García-Berro, E; Torres, S

2008-01-01

85

Detonations in white dwarf dynamical interactions  

CERN Document Server

In old, dense stellar systems collisions of white dwarfs are a rather frequent phenomenon. Here we present the results of a comprehensive set of Smoothed Particle Hydrodynamics simulations of close encounters of white dwarfs aimed to explore the outcome of the interaction and the nature of the final remnants for different initial conditions. Depending on the initial conditions and the white dwarf masses, three different outcomes are possible. Specifically, the outcome of the interaction can be either a direct or a lateral collision or the interaction can result in the formation of an eccentric binary system. In those cases in which a collision occurs, the infalling material is compressed and heated such that the physical conditions for a detonation may be reached during the most violent phases of the merger. While we find that detonations occur in a significant number of our simulations, in some of them the temperature increase in the shocked region rapidly lifts degeneracy, leading to the quenching of the bu...

Aznar-Siguán, Gabriela; Lorén-Aguilar, Pablo; José, Jordi; Isern, Jordi

2013-01-01

86

White Dwarf Mass Distribution in the SDSS  

CERN Document Server

We determined masses for the 7167 DA and 507 DB white dwarf stars classified as single and non-magnetic in data release four of the Sloan Digital Sky Survey (SDSS). We obtained revised Teff and log g determinations for the most massive stars by fitting the SDSS optical spectra with a synthetic spectra grid derived from model atmospheres extending to log g=10.0. We also calculate radii from evolutionary models and create volume-corrected mass distributions for our DA and DB samples. The mean mass for the DA stars brighter than g=19 and hotter than Teff=12000K is M(DA)= 0.593+/-0.016M(Sun). For the 150 DBs brighter than g=19 and hotter than Teff=16000K, we find M(DB)=0.711+/-0.009 M(Sun). It appears the mean mass for DB white dwarf stars may be significantly larger than that for DAs. We also report the highest mass white dwarf stars ever found, up to 1.33 M(Sun).

Kepler, S O; Nitta, A; Köster, D; Castanheira, B G; Giovannini, O; Costa, A F M; Althaus, L

2006-01-01

87

The Masses of Population II White Dwarfs  

CERN Document Server

Globular star clusters are among the first stellar populations to have formed in the Milky Way, and thus only a small sliver of their initial spectrum of stellar types are still burning hydrogen on the main-sequence today. Almost all of the stars born with more mass than 0.8 M_sun have evolved to form the white dwarf cooling sequence of these systems, and the distribution and properties of these remnants uniquely holds clues related to the nature of the now evolved progenitor stars. With ultra-deep HST imaging observations, rich white dwarf populations of four nearby Milky Way globular clusters have recently been uncovered, and are found to extend an impressive 5 - 8 magnitudes in the faint-blue region of the H-R diagram. In this paper, we characterize the properties of these population II remnants by presenting the first direct mass measurements of individual white dwarfs near the tip of the cooling sequence in the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS multiobject spectro...

Kalirai, Jason S; Richer, Harvey B; Bergeron, P; Catelan, Marcio; Hansen, Brad M S; Rich, R Michael

2009-01-01

88

Magnetic Field Evolution in Accreting White Dwarfs  

CERN Document Server

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 ...

Cumming, A

2002-01-01

89

The White Dwarf Mass in Interacting Binaries  

Science.gov (United States)

We are undertaking a comprehensive study of accreting white dwarfs in two broad types of interacting binaries, cataclysmic variables (CVs) and symbiotic stars, using X-ray and multi-wavelength data. Our goal is to understand the detailed accretion and X-ray emission processes in these binaries, and therefore determine what information can be extracted from X-ray observations of these systems. In paritular, we must measure the current masses of white dwarfs in CVs and symbiotic stars and understand if they gain or lose mass over time. We believe that these are all worthy objectives by themselves, with added interest in the context of Type Ia supernova progenitor models and the apparently diffuse Galactic ridge and bulge X-ray emission. For both these issues, we need surveys with well-understood selection effects to measure the space density of CVs and symbiotic stars, and X-ray surveys will likely play a key role. With these long-term goals in mind, we are undertaking several interlinked projects with overlapping sets of objectives and collaborators. In this proposal, we seek support for a subset of our overall research program, thematically linked to one of the most important parameters in any CVs and symbiotic stars: the white dwarf mass (Mwd). The depth of the gravitational potential of the white dwarf sets the maximum temperature that the accreting plasma can reach; therefore, by measuring the maximum temperature in the X-ray spectra of CVs and symbiotic stars, one can infer Mwd. This method has long been applied to magnetic CVs; we believe that it is also applicable to non-magnetic cases. We propose an empirical confirmation of this method for quiescent dwarf novae, and investigate any systematic uncertainties that may be inherent in this method. We already know that CVs and symbiotic stars with strong hard (>10 keV) X-ray emission harbor massive white dwarfs, and have used this fact to study the population such systems detected in INTEGRAL and Swift BAT surveys. We have begun a comparison study of a medium energy (2-10 keV) X-ray selected sample, which will most likely include many systems with lower-mass white dwarfs. Finally, we have been conducting a survey of hard and medium energy X-ray emitting symbiotic stars, including some individual cases of special interest. We include a particular case which may allow us to measure the birth mass of a white dwarf in a symbiotic binary. These projects make use of data to be obtained through 2 approved Suzaku AO-7 proposals led by the PI, as well as archival XMM-Newton and Swift data that are already in the public domain.

Mukai, Koji

90

Spectroscopic observations of high proper motion DA white dwarfs  

CERN Document Server

We used the revised New Luyten Two-Tenths (rNLTT) catalog to select high proper motion white dwarf candidates. We studied the spectra of 70 hydrogen-rich (DA) white dwarfs, which were obtained at the Cerro Tololo Inter-American Observatory (CTIO) and extracted from the Sloan Digital Sky Survey (SDSS). We determined their effective temperature and surface gravity by fitting their Balmer line profiles to model white dwarf spectra. Using evolutionary mass-radius relations we determined their mass and cooling age. We also conducted a kinematical study of the white dwarf sample and found that most belong to the thin disk population. We have identified three magnetic white dwarfs and estimated their surface magnetic field. Finally, we have identified 6 white dwarfs that lie within 20 pc from the Sun.

Arazimova, E; Vennes, S

2009-01-01

91

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

Energy Technology Data Exchange (ETDEWEB)

Laser Interferometer Space Antenna (LISA) is a proposed mission to detect and study gravitational radiation in the frequency range from 10{sup -4} to 10{sup -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 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.

Edlund, Jeffrey A [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States); Tinto, Massimo [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States); Krolak, Andrzej [Max-Planck-Institute for Gravitational Physics, Albert Einstein Institute, D-14476 Golm (Germany); Nelemans, Gijs [Department of Astrophysics, IMAPP, Radboud University Nijmegen (Netherlands)

2005-09-21

92

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

93

White dwarf-main sequence binaries from SDSS DR8: unveiling the cool white dwarf population  

CERN Document Server

The spectroscopic catalogue of white dwarf-main sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS) is the largest and most homogeneous sample of compact binary stars currently known. However, because of selection effects, the current sample is strongly biased against systems containing cool white dwarfs and/or early type companions, which are predicted to dominate the intrinsic population. In this study we present colour selection criteria that combines optical (ugriz DR8 SDSS) plus infrared (yjhk DR9 UKIRT Infrared Sky Survey (UKIDSS), JHK Two Micron All Sky Survey (2MASS) and/or w1w2 Wide-Field Infrared Survey Explorer (WISE)) magnitudes to select 3419 photometric candidates of harbouring cool white dwarfs and/or dominant (M dwarf) companions. We demonstrate that 84 per cent of our selected candidates are very likely genuine WDMS binaries, and that the white dwarf effective temperatures and secondary star spectral types of 71 per cent of our selected sources are expected to be below <~1000...

Rebassa-Mansergas, A; Schreiber, M R; Gaensicke, B T; Koester, D

2013-01-01

94

Comparison of theoretical white dwarf cooling timescales (Research Note)  

OpenAIRE

Context. An accurate assessment of white dwarf cooling times is paramount so that white dwarf cosmochronology of Galactic populations can be put 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 use white dwarfs as probes of Galactic evolution and test-beds of fundamental physics. Aims. We estimate for the first time the consistency of resu...

Salaris, Maurizio; Althaus, Leandro Gabriel; Garc??a-berro Montilla, Enrique

2013-01-01

95

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

OpenAIRE

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...

Das, Upasana; Mukhopadhyay, Banibrata

2014-01-01

96

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

CERN Document Server

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.

Das, Upasana

2014-01-01

97

Search for Higgs shifts in white dwarfs  

CERN Document Server

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.

Onofrio, Roberto

2014-01-01

98

Search for Higgs Shifts in White Dwarfs  

Science.gov (United States)

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.

Onofrio, Roberto; Wegner, Gary A.

2014-08-01

99

The Masses of Population II White Dwarfs  

OpenAIRE

Globular star clusters are among the first stellar populations to have formed in the Milky Way, and thus only a small sliver of their initial spectrum of stellar types are still burning hydrogen on the main-sequence today. Almost all of the stars born with more mass than 0.8 M_sun have evolved to form the white dwarf cooling sequence of these systems, and the distribution and properties of these remnants uniquely holds clues related to the nature of the now evolved progenito...

Kalirai, Jason S.; Davis, D. Saul; Richer, Harvey B.; Bergeron, P.; Catelan, Marcio; Hansen, Brad M. S.; Rich, R. Michael

2009-01-01

100

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

101

Accretion on to Magnetic White Dwarfs  

Directory of Open Access Journals (Sweden)

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.

Wickramasinghe Dayal

2014-01-01

102

Simulating black hole white dwarf encounters  

Science.gov (United States)

The existence of supermassive black holes lurking in the centers of galaxies and of stellar binary systems containing a black hole with a few solar masses has been established beyond reasonable doubt. The idea that black holes of intermediate masses (˜1000M) may exist in globular star clusters has gained credence over recent years but no conclusive evidence has been established yet. An attractive feature of this hypothesis is the potential to not only disrupt solar-type stars but also compact white dwarf stars. In close encounters the white dwarfs can be sufficiently compressed to thermonuclearly explode. The detection of an underluminous thermonuclear explosion accompanied by a soft, transient X-ray signal would be compelling evidence for the presence of intermediate mass black holes in stellar clusters. In this paper we focus on the numerical techniques used to simulate the entire disruption process from the initial parabolic orbit, over the nuclear energy release during tidal compression, the subsequent ejection of freshly synthesized material and the formation process of an accretion disk around the black hole.

Rosswog, Stephan; Ramirez-Ruiz, Enrico; Hix, W. Raphael; Dan, M.

2008-07-01

103

White dwarfs in the Capodimonte deep field  

CERN Document Server

In this article we describe the search for white dwarfs (WDs) in the multi-band photometric data of the Capodimonte deep field survey. The WD candidates were selected through the V-R_C vs B-V color-color diagram. For two bright objects, the WD nature has been confirmed spectroscopically, and the atmospheric parameters (Teff and logg) have been determined. We have computed synthetic stellar population models for the observed field and the expected number of white dwarfs agrees with the observations. The possible contamination by turn-off and horizontal branch halo stars has been estimated. The quasar (QSO) contamination has been determined by comparing the number of WD candidates in different color bins with state-of-the-art models and previous observations. The WD space density is measured at different distances from the Sun. The total contamination (non-degenerate stars + QSOs) in our sample is estimated to be around 30%. This work should be considered a small experiment in view of more ambitious projects to...

Silvotti, R; Cignoni, M; Alcalá, J M; Capaccioli, M; Grado, A; Pannella, M

2008-01-01

104

Low Frequency Gravitational Waves from White Dwarf MACHO Binaries  

CERN Document Server

The possibility that Galactic halo MACHOs are white dwarfs has recently attracted much attention. Using the known properties of white dwarf binaries in the Galactic disk as a model, we estimate the possible contribution of halo white dwarf binaries to the low-frequency (10^{-5} Hz} < f < 10^{-1}Hz) gravitational wave background. Assuming the fraction of white dwarfs in binaries is the same in the halo as in the disk, we find the confusion background from halo white dwarf binaries could be five times stronger than the expected contribution from Galactic disk binaries, dominating the response of the proposed space based interferometer LISA. Low-frequency gravitational wave observations will be the key to discovering the nature of the dark MACHO binary population.

Hiscock, W A; Routzahn, J R; Kulick, B; Hiscock, William A.; Larson, Shane L.; Routzahn, Joshua R.; Kulick, Ben

2000-01-01

105

Evolutionary and pulsational properties of white dwarf stars  

CERN Document Server

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 ...

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

2010-01-01

106

Element abundances in cool white dwarfs. II. Ultraviolet observations of DZ white dwarfs  

CERN Document Server

We present a small data base of homogeneously derived photospheric element abundances of DZ white dwarfs and related objects. Our previous investigations are supplemented with the analysis of ultraviolet spectra for nine white dwarfs. Of particular interest is the detection of Lalpha absorption in van Maanen 2 and a determination of the effective temperature of this star. The new value is about 1000K lower than previous results due to the strong ultraviolet absorption by metals which has to be considered consistently. The metal abundances of our sample stars are compatible with the predictions from the two-phase accretion model of Dupuis et al. (1992, 1993). Small deviations can be observed for the abundance ratios in some objects. This could indicate non-solar metal-to-metal ratios in the accreted material. Hydrogen can be detected in virtually all of our objects. However, its average accretion rate must be at least two orders of magnitude lower than the metal accretion rate.

Wolff, B; Liebert, J

2002-01-01

107

White Dwarf - Red Dwarf Systems Resolved with the Hubble Space Telescope: I. First Results  

CERN Document Server

First results are presented for a Hubble Space Telescope Advanced Camera for Surveys snapshot study of white dwarfs with likely red dwarf companions. Of 48 targets observed and analyzed so far, 27 are totally or partially resolved into two or more components, while an additional 15 systems are almost certainly unresolved binaries. These results provide the first direct empirical evidence for a bimodal distribution of orbital separations among binary systems containing at least one white dwarf.

Farihi, J; Wachter, S

2006-01-01

108

Simulation of the White Dwarf -- White Dwarf galactic background in the LISA data  

CERN Document Server

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. 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. Since the galactic white-dwarfs background will be observed by LISA not as a stationary but rather as a cyc...

Edlund, J A; Królak, A; Nelemans, G; Edlund, Jeffrey A.; Tinto, Massimo; Kr\\'olak, Andrzej; Nelemans, Gijs

2005-01-01

109

The White Dwarf -- White Dwarf galactic background in the LISA data  

CERN Document Server

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. 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. Since the galactic white-dwarfs background will be observed by LISA not as a stationary but rather as a cyc...

Edlund, J A; Królak, A; Nelemans, G; Edlund, Jeffrey A.; Tinto, Massimo; Krolak, Andrzej; Nelemans, Gijs

2005-01-01

110

Gravitational Settling of 22Ne and White Dwarf Evolution  

Science.gov (United States)

We study the effects of the sedimentation of the trace element 22Ne in the cooling of white dwarfs. In contrast with previous studies-which adopted a simplified treatment of the effects of 22Ne sedimentation-this is done self-consistently for the first time, using an up-to-date stellar evolutionary code in which the diffusion equation is coupled with the full set of equations of stellar evolution. Due the large neutron excess of 22Ne, this isotope rapidly sediments in the interior of the white dwarf. Although we explore a wide range of parameters, we find that when using the most reasonable assumptions concerning the diffusion coefficient and the physical state of the white dwarf interior, the delay introduced by the ensuing chemical differentiation is minor for a typical 0.6 Msolar white dwarf. For more massive white dwarfs, say MWD~1.0 Msolar, the delay turns out to be considerably larger. These results are in qualitatively good accord with those obtained in previous studies, but we find that the magnitude of the delay introduced by 22Ne sedimentation was underestimated by a factor of ~2. We also perform a preliminary study of the impact of 22Ne sedimentation on the white dwarf luminosity function. Finally, we hypothesize on the possibility of detecting the sedimentation of 22Ne using pulsating white dwarfs in the appropriate effective temperature range with accurately determined rates of change of the observed periods.

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

2008-04-01

111

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.

112

Search for the coolest white dwarfs in the Galaxy  

CERN Document Server

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...

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

2010-01-01

113

The white dwarf companions of 56 Per and HR 3643  

CERN Document Server

We have obtained low-dispersion IUE spectra of the stars 56 Persei (F4 V) and HR 3643 (F7 II), as part of a survey of late-type stars with a 1565 A flux excess in the TD-1 ultraviolet sky survey. The IUE spectrum of each star reveals the presence of a hot white dwarf companion. We fit the Ly alpha profile and ultraviolet continuum using pure hydrogen models, but the distance of the primary star is also needed to uniquely constrain the white dwarf parameters. We derive Teff = 16,420 +- 420 K, logg = 8.46 +- 0.2 for the white dwarf companion to 56 Per, using the photometric distance of 30.1 +- 2.8 pc. The implied white dwarf mass is 0.90 +- 0.12 Msun, considerably above the median mass (~0.6 Msun) of single white dwarfs. The parameters of the white dwarf in HR 3643 are not well constrained, mainly due to a large uncertainty in the distance. By assuming a reasonable range of gravity for the white dwarf (7.3 < log g < 9.0), we derive -1.4 < Mv < 0.6 for the F7 II star, and 28,970 < Teff < 35, 99...

Landsman, W B; Bergeron, P; Landsman, Wayne; Simon, Theodore; Bergeron, Pierre

1995-01-01

114

Binary white dwarfs in the halo of the Milky Way  

CERN Document Server

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...

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

2014-01-01

115

HST Spectroscopy of the Hottest White Dwarfs  

CERN Document Server

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.

Rauch, Thomas

2007-01-01

116

HST Spectroscopy of the Hottest White Dwarfs  

Science.gov (United States)

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 (T eff). Since stars with T eff as high as 100,000 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.

Rauch, Thomas; Werner, Klaus

117

New mass limit of white dwarfs  

CERN Document Server

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.

Das, Upasana

2013-01-01

118

New Mass Limit of White Dwarfs  

Science.gov (United States)

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.

Das, Upasana; Mukhopadhyay, Banibrata

2013-09-01

119

Maximum mass of magnetic white dwarfs  

CERN Document Server

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...

Paret, D Manreza; Horvath, J E

2015-01-01

120

Magnetic field evolution in white dwarfs  

International Nuclear Information System (INIS)

The evolution of magnetic fields in a star experiencing radial contraction and cooling into a 0.6 solar mass white dwarf phase is examined analytically. An MHD equation is defined for a radially contracting star with a weak poloidal field that does not disturb the structure or evolution of the star. The distribution of conductivity within the star is approximated. A Newton-Raphson finite difference code is used to track the evolution of the star by iterative solution of the radial equation. A deconvolution algorithm is defined to track the decay eigenmodes. The central conductivity of a star increases rapidly, decreasing the decay time scale beyond the age of the star. Dominance of successively lower eigenmodes can cause multiple reversals in the surface field. 43 references

121

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

122

The Torino Observatory Parallax Program: White Dwarf Candidates  

CERN Document Server

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.

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

2003-01-01

123

RX J2130.6+4710 - an eclipsing white dwarf - M-dwarf binary star  

CERN Document Server

(Abridged) We report the detection of eclipses in the close white-dwarf - M-dwarf binary star RXJ2130.6+4710. We estimate that the spectral type of the M-dwarf is M3.5Ve or M4Ve. We estimate that the effective temperature of the white dwarf is Teff = 18000K +- 1000K. We have used the width of the primary eclipse and duration of totality measured precisely from Ultracam u' data combined with the amplitude of the ellipsoidal effect in the I band and the semi-amplitudes of the spectroscopic orbits to derive masses and radii for the M-dwarf and white dwarf. The M-dwarf has a mass of 0.555 +- 0.023 solar masses and a radius of 0.534 +- 0.053 solar radii, which is a typical radius for stars of this mass. The mass of the white dwarf is 0.554 +- 0.017 solar masses and its radius is 0.0137 +- 0.0014 solar radii, which is the radius expected for a carbon-oxygen white dwarf of this mass and effective temperature RXJ2130.6+4710 is a rare example of a pre-cataclysmic variable star which will start mass transfer at a perio...

Maxted, P F L; Morales-Rueda, L; Barstow, M A; Dobbie, P D; Schreiber, M; Dhillon, V S; Brinkworth, C S

2004-01-01

124

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.

125

Hard X-ray Emission from White Dwarfs  

Science.gov (United States)

Hot white dwarfs may exhibit photospheric emission at X-ray wavelengths, but their X- ray emission should be soft, mutch less than 0.5 keV. Hard X-ray emission, at approx. 1 keV, is not expected from white dwarfs, unless they are in binary systems and the hard X-ray emission is produced by a late-type companion's coronal activity or by accretion of a companion's material onto the surface of the white dwarf. We proposed to use the ROSAT archive to search for hard X-ray emission from white dwarfs in order to determine whether hard X-ray emission may provide a sensitive diagnostic for the existence of a binary companion.

Chu, You-Hua; Gruendl, Robert

2004-01-01

126

General relativistic white dwarfs and their astrophysical implications  

Science.gov (United States)

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.

Boshkayev, Kuantay; Rueda, Jorge A.; Ruffini, Remo; Siutsou, Ivan

2014-09-01

127

Three eclipsing white dwarf plus main sequence binaries from SDSS  

Science.gov (United States)

We identify SDSS 0110+1326, SDSS 0303+0054 and SDSS 1435+3733 as three eclipsing white dwarf plus main sequence binaries from the Sloan Digital Sky Survey, and report on their follow-up observations. Orbital periods for the three systems are established through multi-season photometry. Time-resolved spectroscopic observations lead to the determination of the radial velocities of the secondary stars. A decomposition technique of the SDSS spectra is used to estimate the surface gravities and effective temperatures of the white dwarfs, as well as the spectral types of the secondaries. By combining the constraints from the spectral decomposition, the radial velocity data and the modeling of the systems' light curves, we determine the physical parameters of the stellar components. Two of the white dwarfs are of low mass (Mwd ~ 0.4 Modot), while the third white dwarf is unusually massive (MWD ~ 0.8-0.9 Modot) for a post-common envelope system.

Pyrzas, S.; Gänsicke, B. T.; Marsh, T. R.; Aungwerojwit, A.; Rebassa-Mansergas, A.; Southworth, J.; Rodríguez-Gil, P.; Schreiber, M. R.; Koester, D.

2009-06-01

128

The Local Population of White Dwarfs within 25 pc  

Science.gov (United States)

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

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

2015-01-01

129

General Relativistic White Dwarfs and Their Astrophysical Implications  

CERN Document Server

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.

Boshkayev, Kuantay; Ruffini, Remo; Siutsou, Ivan

2014-01-01

130

Simplified Hydrostatic Carbon Burning in White Dwarf Interiors  

OpenAIRE

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 ...

Fo?rster, F.; Lesaffre, P.; Podsiadlowski, P.

2010-01-01

131

The White Dwarf Cooling Sequence of NGC6397  

OpenAIRE

We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC6397, focussing attention on the cluster's white dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in depth, with an apparent cutoff at magnitude F814W=27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf l...

Hansen, Brad M. S.; Anderson, Jay; Brewer, James; Dotter, Aaron; Fahlman, Greg G.; Hurley, Jarrod; Kalirai, Jason; King, Ivan; Reitzel, David; Richer, Harvey B.; Rich, R. Michael; Shara, Michael M.; Stetson, Peter B.

2007-01-01

132

On the stability of ultra-magnetized white dwarfs  

Science.gov (United States)

Ultra-magnetized white dwarfs with interior fields up to 1018 G have recently been proposed to have a critical mass limit M max ? 2.58 M ?, which surpasses the traditional Chandrasekhar limit M ch ? 1.44 M ?. We show that several stability criteria and fundamental physical aspects that take place when huge magnetic fields and high densities are present have been neglected in the determination of such a new mass limit for white dwarfs, invalidating that result.

Cáceres, Diego L.; Rueda, Jorge A.; Ruffini, Remo

2014-09-01

133

QUIESCENT NUCLEAR BURNING IN LOW-METALLICITY WHITE DWARFS  

Energy Technology Data Exchange (ETDEWEB)

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.

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

134

Binary white dwarfs in the halo of the Milky Way  

Science.gov (United States)

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

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

2014-09-01

135

Dark halo baryons not in ancient halo white dwarfs  

OpenAIRE

Having ruled out the possibility that stellar objects are the main contributor of the dark matter embedding galaxies, microlensing experiments cannot exclude the hypothesis that a significant fraction of the Milky Way dark halo might be made of MACHOs with masses in the range $0.5-0.8 \\msun$. Ancient white dwarfs are generally considered the most plausible candidates for such MACHOs. We report the results of a search for such white dwarfs in a proper motion survey covering a...

Cre?ze?, M.; Mohan, V.; Robin, A. C.; Reyle?, C.; Mc Cracken, H.; Cuillandre, J. -c; Le Fe?vre, O.; Mellier, Y.

2004-01-01

136

A Critical Examination of Halo White Dwarf Candidates  

OpenAIRE

A detailed analysis of halo white dwarf candidates is presented, which is based on model atmosphere fits to observed energy distributions built from photoelectric or photographic magnitudes. Most of the candidates identified in reduced proper motion diagrams are shown to be too warm (Teff > 5000 K) and most likely too young to be members of the galactic halo, while the tangential velocities of the cooler and thus older white dwarfs are shown to be entirely consistent with th...

Bergeron, P.

2002-01-01

137

White dwarfs, black holes and neutron stars in close binaries  

Science.gov (United States)

In this thesis some aspects of the formation and evolution of binaries containing white dwarfs, black holes and neutron stars are investigated. In the first part the formation of observed single undermassive white dwarfs and double helium white dwarfs is studied. I conclude that the formation of single undermassive white dwarfs can be explained by the evolution of `binaries' consisting of a solar-like star and a massive planet or brown dwarf. A tidal instability causes the low-mass companion to be drawn into the solar-like star when it evolves up the giant branch, expelling the giants envelope, leaving a single undermassive white dwarf. I reconstructed the evolution of three observed double helium white dwarfs, using the unique core mass -- radius relation for giants with degenerate cores, to find the pre-mass-transfer orbital separations and came to the following conclusions: (i) The last mass-transfer phase can be described with the spiral-in formalism with high common-envelope efficiency. (ii) The first mass-transfer phase cannot be described by a spiral-in nor by stable mass transfer but can be described very well with a formalism based on the angular momentum balance, with one free parameter which for the three observed systems has a very similar value. Further, I calculated the current population of close double white dwarfs and interacting white dwarfs (AM CVn stars) and concluded: (i) The recently proposed cooling curves for helium white dwarfs overestimate the luminosity for the lowest mass helium white dwarfs (ii) The fraction of double white dwarfs among all white dwarfs can only be brought into agreement with observations if the initial binary fraction is not above 50 % (iii) The model with an exponentially decaying star formation rate gives a slightly better fit to the observed period distribution for double white dwarfs than a constant star formation rate. For the AM CVn stars I conclude that in order to distinguish between different models and formation channels both the theory of helium accretion disks and the homogeneity and completeness of the observations (particularly regarding the distances to the AM CVn stars) need to be improved. I already started this by reducing and analyzing high-speed spectroscopic data of AM CVn itself and found, for the first time, a clear direct signature of the binary nature of AM CVn in its spectrum. The study of black hole binaries led to two conclusions. The first is that the observed space velocities of black hole binaries imply that in the supernova in which the black hole was produced, some 30 -- 50% of the mass of the exploding helium star was ejected from the system if the explosion was symmetric. The second is that the mass-loss rates for for Wolf-Rayet stars currently used in stellar evolution calculations still overestimate the mass loss, yielding very low masses for massive stars when they explode. A mass-loss law more in agreement with the observed values significantly increases these final masses, improving the possibility for the formation of black holes in binaries. Finally I describe the population of binaries consisting of two compact objects, either white dwarfs, neutron stars or black holes and use these to calculate the unresolved noise background produced by double white dwarfs and calculate the population of resolved binaries and binaries with signals sufficiently strong that they may be detected above the noise level for the low-frequency gravitational wave detector in space (LISA).

Nelemans, G.

2001-03-01

138

Discovery of a widely separated ultracool dwarf-white dwarf binary  

Science.gov (United States)

We present the discovery of the widest known ultracool dwarf-white dwarf binary. This binary is the first spectroscopically confirmed widely separated system from our target sample. We have used the Two-Micron All-Sky Survey (2MASS) and SuperCOSMOS archives in the southern hemisphere, searching for very widely separated ultracool dwarf-white dwarf binaries, and find one common proper motion system, with a separation of 3650-5250au at an estimated distance of 41-59pc, making it the widest known system of this type. Spectroscopy reveals 2MASS J0030-3740 is a DA white dwarf with Teff = 7600 +/- 100K, log(g) = 7.79-8.09 and MWD = 0.48-0.65Msolar. We spectroscopically type the ultracool dwarf companion (2MASS J0030-3739) as M9+/-1 and estimate a mass of 0.07-0.08Msolar,Teff = 2000-2400K and log(g) = 5.30-5.35, placing it near the mass limit for brown dwarfs. We estimate the age of the system to be >1.94Gyr (from the white dwarf cooling age and the likely length of the main-sequence lifetime of the progenitor) and suggest that this system and other such wide binaries can be used as benchmark ultracool dwarfs.

Day-Jones, A. C.; Pinfield, D. J.; Napiwotzki, R.; Burningham, B.; Jenkins, J. S.; Jones, H. R. A.; Folkes, S. L.; Weights, D. J.; Clarke, J. R. A.

2008-08-01

139

NLTT 5306: the shortest period detached white dwarf+brown dwarf binary  

Science.gov (United States)

We have spectroscopically confirmed a brown dwarf mass companion to the hydrogen atmosphere white dwarf NLTT 5306. The white dwarf's atmospheric parameters were measured using the Sloan Digital Sky Survey and X-shooter spectroscopy as Teff = 7756 ± 35 K and log(g) = 7.68 ± 0.08, giving a mass for the primary of MWD = 0.44 ± 0.04 M? at a distance of 71 ± 4 pc with a cooling age of 710 ± 50 Myr. The existence of the brown dwarf secondary was confirmed through the near-infrared arm of the X-shooter data and a spectral type of dL4-dL7 was estimated using standard spectral indices. Combined radial velocity measurements from the Sloan Digital Sky Survey, X-shooter and the Hobby-Eberly Telescope's High Resolution Spectrograph of the white dwarf give a minimum mass of 56 ± 3 MJup for the secondary, confirming the substellar nature. The period of the binary was measured as 101.88 ± 0.02 min using both the radial velocity data and i'-band variability detected with the Isaac Newton Telescope. This variability indicates `day' side heating of the brown dwarf companion. We also observe H? emission in our higher resolution data in phase with the white dwarf radial velocity, indicating that this system is in a low level of accretion, most likely via a stellar wind. This system represents the shortest period white dwarf+brown dwarf binary and the secondary has survived a stage of common envelope evolution, much like its longer period counterpart, WD 0137-349. Both systems likely represent bona fide progenitors of cataclysmic variables with a low-mass white dwarf and a brown dwarf donor.

Steele, P. R.; Saglia, R. P.; Burleigh, M. R.; Marsh, T. R.; Gänsicke, B. T.; Lawrie, K.; Cappetta, M.; Girven, J.; Napiwotzki, R.

2013-03-01

140

Hard X-ray Emission Associated with White Dwarfs II  

CERN Document Server

We have previously conducted a search for X-ray sources coincident with white dwarfs using the white dwarf catalog compiled by McCook & Sion (1999} and the ROSAT sources in the WGACAT (Paper I). To include the white dwarfs discovered since 1999 and to include the X-ray sources detected in ROSAT Position Sensitive Proportional Counter (PSPC) observations made with a boron filter, we have carried out another search using an updated list of white dwarfs and the final catalogs of the ROSAT PSPC observations with and without a boron filter. Forty-seven new X-ray sources convincingly coincident with white dwarfs are found and reported in this paper. Among these, only 5 show hard X-ray emission: three possess confirmed or suggested late-type companions, one is apparently single, and the other is likely a misclassified BL Lac object. The apparently single white dwarf with hard X-ray emission, KPD 0005+5106, was discussed extensively in Paper I. Photospheric origin for the hard X-ray emission from hot DO and DQZO ...

Chu, Y H; Gruendl, R A; Webbink, R F

2004-01-01

141

The White Dwarf in EM Cygni: Beyond The Veil  

CERN Document Server

We present a spectral analysis of the FUSE spectra of EM Cygni, a Z Cam DN system. The FUSE spectrum, obtained in quiescence, consists of 4 individual exposures (orbits): two exposures, at orbital phases phi ~ 0.65 and phi ~ 0.90, have a lower flux; and two exposures, at orbital phases phi =0.15 and 0.45, have a relatively higher flux. The change of flux level as a function of the orbital phase is consistent with the stream material (flowing over and below the disk from the hot spot region to smaller radii) partially masking the white dwarf. We carry out a spectral analysis of the FUSE data, obtained at phase 0.45 (when the flux is maximual, using the codes TLUSTY and SYNSPEC. Using a single white dwarf spectral component, we obtain a white dwarf temperature of 40,000K, rotating at 100km/s. The white dwarf, or conceivably, the material overflowing the disk rim, shows suprasolar abundances of silicon, sulphur and possibly nitrogen. Using a white dwarf+disk composite model, we obtain that the white dwarf temper...

Godon, P; Barrett, P E; Linnell, A P

2009-01-01

142

White dwarf models of supernovae and cataclysmic variables  

International Nuclear Information System (INIS)

If the accreting white dwarf increases its mass to the Chandrasekhar mass, it will either explode as a Type I supernova or collapse to form a neutron star. In fact, there is a good agreement between the exploding white dwarf model for Type I supernovae and observations. We describe various types of evolution of accreting white dwarfs as a function of binary parameters (i.e,. composition, mass, and age of the white dwarf, its companion star, and mass accretion rate), and discuss the conditions for the precursors of exploding or collapsing white dwarfs, and their relevance to cataclysmic variables. Particular attention is given to helium star cataclysmics which might be the precursors of some Type I supernovae or ultrashort period x-ray binaries. Finally we present new evolutionary calculations using the updated nuclear reaction rates for the formation of O+Ne+Mg white dwarfs, and discuss the composition structure and their relevance to the model for neon novae. 61 refs., 14 figs

143

Presupernova evolution of accreting white dwarfs with rotation  

CERN Document Server

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....

Yoon, S C

2004-01-01

144

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.

145

Evolution of Close White Dwarf Binaries  

CERN Document Server

We describe the evolution of double degenerate binary systems, consisting of components obeying the zero temperature mass radius relationship for white dwarf stars, from the onset of mass transfer to one of several possible outcomes including merger, tidal disruption of the donor, or survival as a semi-detached AM CVn system. We use a combination of analytic solutions and numerical integrations of the standard orbit-averaged first-order evolution equations, including direct impact accretion and the evolution of the components due to mass exchange. We include also the effects of mass-loss during super-critical (super-Eddington) mass transfer and the tidal and advective exchanges of angular momentum between the binary components. We cover much the same ground as Marsh et al.(2004) with the additional effects of the advective or consequential angular momentum from the donor and its tidal coupling to the orbit which is expected to be stronger than that of the accretor. With the caveat that our formalism does not ...

Gokhale, V; Frank, J; Gokhale, Vayujeet; Peng, Xiao Meng; Frank, Juhan

2006-01-01

146

Spin and Magnetism of White Dwarfs  

CERN Document Server

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...

Kissin, Yevgeni

2015-01-01

147

Discovery of an ultramassive pulsating white dwarf  

CERN Document Server

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 ...

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

2013-01-01

148

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.

149

The Binary White Dwarf LHS 3236  

CERN Document Server

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....

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

150

White dwarf cooling and large extra dimensions  

International Nuclear Information System (INIS)

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

151

Magnetically powered outbursts from white dwarf mergers  

CERN Document Server

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...

Beloborodov, Andrei M

2013-01-01

152

Magnetically powered outbursts from white dwarf mergers  

Science.gov (United States)

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 109 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 ˜ 1041-1042 erg s-1 and a characteristic peak duration comparable to 1 d may be expected from the merger. In contrast to classical novae or supernovae, the transient does not involve nuclear energy. The decay after its peak reflects the damping of differential rotation in the merger remnant. Such outbursts may be detected in the local Universe with current and upcoming optical surveys.

Beloborodov, Andrei M.

2014-02-01

153

The shortest period detached white dwarf + main-sequence binary  

CERN Document Server

We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently discovered 94 minute orbital period eclipsing white dwarf / main-sequence binary SDSS J085746.18+034255.3 (CSS 03170) and use these observations to measure the system parameters. We detect a shallow secondary eclipse and hence are able to determine an orbital inclination of 85.5 +/- 0.2 deg. The white dwarf has a mass of 0.51 +/- 0.05 Msun and a radius of 0.0247 +/- 0.0008 Rsun. With a temperature of 35,300 +/- 400K the white dwarf is highly over-inflated if it has a carbon-oxygen core, however if it has a helium core then its mass and radius are consistent with evolutionary models. Therefore, the white dwarf in SDSS J085746.18+034255.3 is most likely a helium core white dwarf with a mass close to the upper limit expected from evolution. The main-sequence star is an M8 dwarf with a mass of 0.09 +/- 0.01 Msun and a radius of 0.110 +/- 0.004 Rsun placing it close to the hydrogen burning limit. The system emerged from a...

Parsons, S G; Gänsicke, B T; Dhillon, V S; Copperwheat, C M; Littlefair, S P; Pyrzas, S; Drake, A J; Koester, D; Schreiber, M R; Rebassa-Mansergas, A

2011-01-01

154

The shortest period detached white dwarf + main-sequence binary  

Science.gov (United States)

We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently discovered 94-min orbital period eclipsing white dwarf/main-sequence binary SDSS J085746.18+034255.3 (CSS 03170) and use these observations to measure the system parameters. We detect a shallow secondary eclipse and hence are able to determine an orbital inclination of i = 85?5 ± 0?2. The white dwarf has a mass of 0.51 ± 0.05 M? and a radius of 0.0247 ± 0.0008 R?. With a temperature of 35 300 ± 400 K the white dwarf is highly overinflated if it has a carbon-oxygen core; however, if it has a helium core then its mass and radius are consistent with evolutionary models. Therefore, the white dwarf in SDSS J085746.18+034255.3 is most likely a helium core white dwarf with a mass close to the upper limit expected from evolution. The main-sequence star is an M8 dwarf with a mass of 0.09 ± 0.01 M? and a radius of 0.110 ± 0.004 R? placing it close to the hydrogen burning limit. The system emerged from a common envelope ˜20 million years ago and will reach a semidetached configuration in ˜400 million years, becoming a cataclysmic variable with a period of 66 min, below the period minimum.

Parsons, S. G.; Marsh, T. R.; Gänsicke, B. T.; Dhillon, V. S.; Copperwheat, C. M.; Littlefair, S. P.; Pyrzas, S.; Drake, A. J.; Koester, D.; Schreiber, M. R.; Rebassa-Mansergas, A.

2012-01-01

155

White Dwarf - Red Dwarf Systems Resolved with the Hubble Space Telescope. II. Full Snapshot Survey Results  

CERN Document Server

{Abrigded} Results are presented for a Hubble Space Telescope Advanced Camera for Surveys high-resolution imaging campaign of 90 white dwarfs with known or suspected low mass stellar and substellar companions. Of the 72 targets which remain candidate and confirmed white dwarfs with near-infrared excess, 43 are spatially resolved into two or more components, and a total of 12 systems are potentially triples. There is a possible, slight deficit of earlier spectral types (bluer colors) among the spatially unresolved companions, exactly the opposite of expectations if significant mass is transferred to the companion during the common envelope phase. Using the best available distance estimates, the low mass companions to white dwarfs exhibit a bimodal distribution in projected separation. This result supports the hypothesis that during the giant phases of the white dwarf progenitor, any unevolved companions either migrate inward to short periods of hours to days, or outward to periods of hundreds to thousands of y...

Farihi, J; Wachter, S

2010-01-01

156

New white dwarf stars in the Sloan Digital Sky Survey Data Release 10  

Science.gov (United States)

We report the discovery of 9088 new spectroscopically confirmed white dwarfs and subdwarfs in the Sloan Digital Sky Survey Data Release 10. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), and estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon-dominated spectra DQs. We found 1 central star of a planetary nebula, 2 new oxygen spectra on helium atmosphere white dwarfs, 71 DQs, 42 hot DO/PG1159s, 171 white dwarf+main-sequence star binaries, 206 magnetic DAHs, 327 continuum-dominated DCs, 397 metal-polluted white dwarfs, 450 helium-dominated white dwarfs, 647 subdwarfs and 6887 new hydrogen-dominated white dwarf stars.

Kepler, S. O.; Pelisoli, I.; Koester, D.; Ourique, G.; Kleinman, S. J.; Romero, A. D.; Nitta, A.; Eisenstein, D. J.; Costa, J. E. S.; Külebi, B.; Jordan, S.; Dufour, P.; Giommi, Paolo; Rebassa-Mansergas, Alberto

2015-02-01

157

New White Dwarf Stars in the Sloan Digital Sky Survey Data Release 10  

CERN Document Server

We report the discovery of 9 088 new spectroscopically confirmed white dwarfs and subdwarfs in the Sloan Digital Sky Survey Data Release 10. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), and estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon dominated spectra DQs. We found 1 central star of a planetary nebula, 2 new oxygen spectra on helium atmosphere white dwarfs, 71 DQs, 42 hot DO/PG1159s, 171 white dwarf+main sequence star binaries, 206 magnetic DAHs, 327 continuum dominated DCs, 397 metal polluted white dwarfs, 450 helium dominated white dwarfs, 647 subdwarfs and 6887 new hydrogen dominated white dwarf stars.

Kepler, S O; Koester, Detlev; Ourique, Gustavo; Kleinman, Scot J; Romero, Alejandra Daniela; Nitta, Atsuko; Eisenstein, Daniel J; Costa, José Eduardo da Silveira; Külebi, Baybars; Jordan, Stefan; Dufour, Patrick; Giommi, Paolo; Rebassa-Mansergas, Alberto

2014-01-01

158

Anomalous cooling of the massive white dwarf in U Geminorum following a narrow dwarf nova outburst  

CERN Document Server

We obtained Hubble GHRS medium resolution (G160M grating) phase-resolved spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf nova quiescence, 13 days and 61 days following the end of a narrow outburst. The spectral wavelength ranges were centered upon three different line regions: N V (1238Å, 1242Å), Si III (1300Å) and He II (1640Å). All of the quiescent spectra at both epochs are dominated by absorption lines and show no emission features. The Si III and He II absorption line velocities versus orbital phase trace the orbital motion of the white dwarf but the N~V absorption velocities appear to deviate from the white dwarf motion. We confirm our previously reported low white dwarf rotational velocity, V sin i= 100 km/s. We obtain a white dwarf orbital velocity semi-amplitude K1=107 km/s. Using the gamma-velocity of Wade (1981) we obtain an Einstein redshift of 80.4 km/s and hence a carbon core white dwarf mass of ~1.1 Msun. We report the first subsolar chemical abundances of C ...

Sion, E M; Szkody, P; Sparks, W B; Gänsicke, B T; Huang, M; Mattei, J A; Sion, Edward M.; Szkody, Paula; Sparks, Warren; Gaensicke, Boris; Huang, Min; Mattei, Janet

1997-01-01

159

Dissecting accretion and outflows in accreting white dwarf binaries  

CERN Document Server

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of accreting white dwarfs. For a summary, we refer to the paper.

de Martino, D; Balman, S; Bernardini, F; Bianchini, A; Bode, M; Bonnet-Bidaud, J -M; Falanga, M; Greiner, J; Groot, P; Hernanz, M; Israel, G; Jose, J; Motch, C; Mouchet, M; Norton, A J; Nucita, A; Orio, M; Osborne, J; Ramsay, G; Rodriguez-Gil, P; Scaringi, S; Schwope, A; Traulsen, I; Tamburini, F

2015-01-01

160

Element abundances in cool white dwarfs. II. Ultraviolet observations of DZ white dwarfs  

Science.gov (United States)

We present a small data base of homogeneously derived photospheric element abundances of DZ white dwarfs and related objects. Our previous investigations are supplemented with the analysis of ultraviolet spectra for nine white dwarfs. Of particular interest is the detection of Lalpha absorption in van Maanen 2 and a determination of the effective temperature of this star. The new value is about 1000 K lower than previous results due to the strong ultraviolet absorption by metals which has to be considered consistently. The metal abundances of our sample stars are compatible with the predictions from the two-phase accretion model of Dupuis et al. (\\cite{Dupuis92}, \\cite{Dupuis93a}, \\cite{Dupuis93b}). Small deviations can be observed for the abundance ratios in some objects. This could indicate non-solar metal-to-metal ratios in the accreted material. Hydrogen can be detected in virtually all of our objects. However, its average accretion rate must be at least two orders of magnitude lower than the metal accretion rate. Based partly on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. StScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. DK was Visiting Astronomer at the German-Spanish Astronomical Centre, Calar Alto, operated by the Max-Planck-Institute for Astronomy, Heidelberg, jointly with the Spanish National Commission for Astronomy.

Wolff, B.; Koester, D.; Liebert, J.

2002-04-01

161

The mass and temperature distribution of accreting white dwarfs  

Science.gov (United States)

2011 was an extraordinary year for astronomy, with the Nobel Prize for Physics being awarded for the discovery of dark enery. Yet, we are unable to explain the precise nature of type Ia supernovae, the very tool that led to that discovery. We propose an ambitious ultaviolet survey of a large sample of accreting white dwarfs that will improve by an order of magnitude our insight into the evolution of compact binaries and the effect that the accretion of mass and angular momentum has on the structure of the white dwarf. This COS survey will double the number of accreting white dwarfs with accurate effective temperatures, thereby measuring their mean secular accretion rates. We will investigate the spin rates of accreting white dwarfs, which have recently been subject of discussion in the context of super-Chandrasehkar SNIa. Measuring the abundances of the accretion flow onto the white dwarfs, we will precisely determine the frequency of systems that underwent thermal-time scale mass transfer, one of the canonical pathways thought to lead to type Ia explosions. Finally, using the synergy of GAIA parallaxes with the analysis of our COS data, we will quadruple the number of accreting white dwarfs with accurate mass measurements, and probe in exquisite detail the possibility that white dwarfs, in particular those that underwent thermal-time scale mass transfer, grow in mass. Confirming mass growth in these systems would provide an alternative channel for long-delay time SNIa, solving the discrepancy between the observed rates and the currently favored double-degenerate model.

Gaensicke, Boris

2012-10-01

162

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

CERN Document Server

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...

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

163

Signs of a faint disc population at polluted white dwarfs  

CERN Document Server

Observations of atmospheric metals and dust discs around white dwarfs provide important clues to the fate of terrestrial planetary systems around intermediate mass stars. We present Spitzer IRAC observations of 15 metal polluted white dwarfs to investigate the occurrence and physical properties of circumstellar dust created by the disruption of planetary bodies. We find subtle infrared excess emission consistent with warm dust around KUV 15519+1730 and HS 2132+0941, and weaker excess around the DZ white dwarf G245-58, which, if real, makes it the coolest white dwarf known to exhibit a 3.6 micron excess and the first DZ star with a bright disc. All together our data corroborate a picture where 1) discs at metal-enriched white dwarfs are commonplace and most escape detection in the infrared (possibly as narrow rings), 2) the discs are long lived, having lifetimes on the order of 10^6 yr or longer, and 3) the frequency of bright, infrared detectable discs decreases with age, on a timescale of roughly 500 Myr, su...

Bergfors, Carolina; Dufour, Patrick; Rocchetto, Marco

2014-01-01

164

Signs of a faint disc population at polluted white dwarfs  

Science.gov (United States)

Observations of atmospheric metals and dust discs around white dwarfs provide important clues to the fate of terrestrial planetary systems around intermediate-mass stars. We present Spitzer Infrared Array Camera observations of 15 metal polluted white dwarfs to investigate the occurrence and physical properties of circumstellar dust created by the disruption of planetary bodies. We find subtle infrared excess emission consistent with warm dust around KUV 15519+1730 and HS 2132+0941, and weaker excess around the DZ white dwarf G245-58, which, if real, makes it the coolest white dwarf known to exhibit a 3.6 ?m excess and the first DZ star with a bright disc. All together our data corroborate a picture where (1) discs at metal-enriched white dwarfs are commonplace and most escape detection in the infrared (possibly as narrow rings), (2) the discs are long lived, having lifetimes on the order of 106 yr or longer and (3) the frequency of bright, infrared detectable discs decreases with age, on a time-scale of roughly 500 Myr, suggesting large planetesimal disruptions decline on this same time-scale.

Bergfors, C.; Farihi, J.; Dufour, P.; Rocchetto, M.

2014-11-01

165

Evolutionary calculations of phase separation in crystallizing white dwarf stars  

CERN Document Server

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...

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

1999-01-01

166

The White Dwarf Cooling Sequence of NGC6397  

CERN Document Server

We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC6397, focussing attention on the cluster's white dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in depth, with an apparent cutoff at magnitude F814W=27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf luminosity function in this cluster. We perform a detailed comparison between cooling models and the observed distribution of white dwarfs in colour and magnitude, taking into account uncertainties in distance, extinction, white dwarf mass, progenitor lifetimes, binarity and cooling model uncertainties. After marginalising over these variables, we obtain values for the cluster distance modulus and age of \\mu_0 = 12.02 \\pm 0.06 and T_c = 11.47 \\pm 0.47Gyr (95% confidence limits). Our inferred distance and white dwarf initial-final mass relat...

Hansen, B M S; Brewer, J; Dotter, A; Fahlman, G G; Hurley, J; King, I; Reitzel, David B; Richer, H B; Rich, R M; Shara, M M; Stetson, P B; Hansen, Brad M. S.; Anderson, Jay; Brewer, James; Dotter, Aaron; Fahlman, Greg. G.; Hurley, Jarrod; King, Ivan; Reitzel, David; Richer, Harvey B.; Shara, Michael M.; Stetson, Peter B.

2007-01-01

167

Do all barium stars have a white dwarf companion?  

Science.gov (United States)

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

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

1983-01-01

168

Monte Carlo simulations of the halo white dwarf population  

CERN Document Server

The interpretation of microlensing results towards the Large Magellanic Cloud (LMC) still remains controversial. Whereas white dwarfs have been proposed to explain these results and, hence, to contribute significantly to the mass budget of our Galaxy, there are as well several constraints on the role played by white dwarfs. In this paper we analyze self-consistently and simultaneously four different results, namely, the local halo white dwarf luminosity function, the microlensing results reported by the MACHO team towards the LMC, the results of Hubble Deep Field (HDF) and the results of the EROS experiment, for several initial mass functions and halo ages. We find that the proposed log-normal initial mass functions do not contribute to solve the problem posed by the observed microlensing events and, moreover, they overproduce white dwarfs when compared to the results of the HDF and of the EROS survey. We also find that the contribution of hydrogen-rich white dwarfs to the dynamical mass of the halo of the Ga...

García-Berro, E; Isern, J; Burkert, A

2004-01-01

169

A Multi-Survey Approach to White Dwarf Discovery  

CERN Document Server

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 ...

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

2012-01-01

170

Quiescent nuclear burning in low-metallicity white dwarfs  

CERN Document Server

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 $0.53\\, M_{\\sun}$ to $0.83\\, M_{\\sun}$. 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 ...

Bertolami, Marcelo M Miller; Garcia-Berro, Enrique

2013-01-01

171

Detection of a white dwarf in a visual binary system  

Science.gov (United States)

The F6 giant HD 160365 was detected to have a white dwarf companion about 8 arcsec south of the star. The UV energy distribution observed with IUE shows that the white dwarf has an effective temperature of 23,000 +/- 2000 K. If log g = 8 the Lya profile indicates an effective temperature around 24,500 K. Using the theoretical models by Wesemael et al. (1980) one finds a visual magnitude of m(V) about 16.5. For T(eff) = 24,500 K one expects for a white dwarf a luminosity of log L/L(solar) about 1.3 and M(V) about 10.67. This gives a distance modulus for the system of m(V) - M(V) = 5.83 and an absolute magnitude M(V)= 0.3 for the giant.

Boehm-Vitense, Erika

1992-01-01

172

Understanding the Cool DA White Dwarf, G29-38  

CERN Document Server

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...

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

173

Tidal Resonance in White Dwarf-Black Hole Binaries  

Science.gov (United States)

Recent X-ray evidence supports the existence of intermediate mass ( ~ 103--105 Msun) black holes in galaxies, and white dwarfs are known to be common endpoints of stellar evolution. Thus, in general, encounters between intermediate mass black holes and white dwarfs seem unavoidable, and binaries might be formed through gravitational bremsstrahlung or tidal capture. We consider the subsequent orbital evolution in the Newtonian approximation before tidal disruption. The orbit will evolve rapidly due to gravitational radiation and there will be resonances with internal normal modes of the white dwarf. The modes will in turn affect the orbital evolution through back-reaction. The mode amplitudes may become nonlinear and heat the star through damping or breaking. This could possibly result in a type Ia supernova explosion in a relativistically deep gravitational well. Possible implications of this scenario will be suggested.

Rathore, Y.; Blandford, R. D.

2000-12-01

174

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

175

The formation of DA white dwarfs with thin hydrogen envelopes  

CERN Document Server

We study the formation and evolution of DA white dwarfs, the progenitors of which have experienced a late thermal pulse (LTP) shortly after the departure from the thermally pulsing AGB. To this end, we compute the complete evolution of an initially 2.7 Mo star all the way from the zero-age main sequence to the white dwarf stage. We find that most of the original H-rich material of the post-AGB remnant is burnt during the post-LTP evolution, with the result that, at entering its white dwarf cooling track, the remaining H envelope becomes 10^-6 Mo in agreement with asteroseismological inferences for some ZZ Ceti stars.

Althaus, L G; Corsico, A H; García-Berro, E; Gil-Pons, P

2005-01-01

176

Asteroseismology of pulsating DA white dwarfs with fully evolutionary models  

Directory of Open Access Journals (Sweden)

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.

Althaus L.G.

2013-03-01

177

Globular cluster interstellar media: ionized and ejected by white dwarfs  

Science.gov (United States)

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.

McDonald, I.; Zijlstra, A. A.

2015-01-01

178

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

Directory of Open Access Journals (Sweden)

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.

Pinfield D.J.

2013-04-01

179

Do AM Hercules white dwarfs have toroidal internal fields?  

International Nuclear Information System (INIS)

A previous author has determined the orientations of the magnetic poles of eight AM Hercules white dwarfs. These data imply a quasi-static aligning torque, which I compare to previous theories. The fact of alignment, but not the position angles, is consistent with our earlier magnetostatic model. The angles may be explained if the dominant torque is gravitational and the white dwarfs are dynamically prolate. I construct an explicit but simplified internal model of such a star in which its figure is distorted from sphericity by a toroidal internal magnetic field, and discuss some implications. (author)

180

Axions and the pulsation periods of variable white dwarfs revisited  

OpenAIRE

Axions are the natural consequence of the introduction of the Peccei-Quinn symmetry to solve the strong CP problem. All the efforts to detect such elusive particles have failed up to now. Nevertheless, it has been recently shown that the luminosity function of white dwarfs is best fitted if axions with a mass of a few meV are included in the evolutionary calculations. Our aim is to show that variable white dwarfs can provide additional and independent evidence about the exis...

Isern, Jordi; Garci?a-berro, Enrique; Althaus, Leandro G.; Co?rsico, Alejandro H.

2010-01-01

181

Testing energy non-additivity in white dwarfs  

CERN Document Server

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.

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

2013-01-01

182

Binary white dwarfs in the halo of the Milky Way  

OpenAIRE

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 presen...

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

2014-01-01

183

The White Dwarf Cooling Sequence of the Globular Cluster Messier 4  

OpenAIRE

We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with limiting magnitude V = 30, I = 28. The white dwarf luminosity function rises sharply for I >25.5, consistent with the behaviour expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 magnitudes fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age diffe...

Hansen, Brad M. S.; Brewer, James; Fahlman, Greg G.; Gibson, Brad K.; Ibata, Rodrigo; Limongi, Marco; Rich, R. Michael; Richer, Harvey B.; Shara, Michael M.; Stetson, Peter B.

2002-01-01

184

HST Observations of the White Dwarf Cooling Sequence of M4  

OpenAIRE

We investigate in detail the white dwarf cooling sequence of the globular cluster Messier 4. In particular we study the influence of various systematic uncertainties, both observational and theoretical, on the determination of the cluster age from the white dwarf cooling sequence. These include uncertainties in the distance to the cluster and the extinction along the line of sight, as well as the white dwarf mass, envelope and core compositions and the white dwarf --main seq...

Hansen, Brad; Richer, Harvey; Fahlman, Greg; Stetson, Peter; Brewer, James; Currie, Thayne; Gibson, Brad; Ibata, Rodrigo; Rich, R. Michael; Shara, Michael

2004-01-01

185

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

Science.gov (United States)

This paper presents a sensitive and comprehensive IRAC 3-8 ?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 descendents of intermediate-mass stars with M>~3 Msolar 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: nine open cluster white dwarfs, 22 high-mass field white dwarfs, and 17 metal-rich field white dwarfs. In particular, these targets include: seven Hyades and four field white dwarfs of similar age, one Pleiades and 19 field white dwarfs of similar age, and van Maanen 2 and 16 similarly metal-rich white dwarfs with ages between 1 and 7 Gyr. No substellar companion candidates were identified at any star. By demanding a 15% minimum photometric excess at 4.5 ?m to indicate a companion detection, upper limits in the planetary-mass regime are established at 34 of the sample white dwarfs, 20 of which have limits below 10 MJ according to substellar cooling models. Specifically, limits below the minimum mass for deuterium burning are established at all Pleiades and Hyades white dwarfs, as well as similarly young field white dwarfs, half a dozen of which receive limits at or below 5 MJ. Two IRAC epochs at vMa 2 rule out T>~200 K proper-motion companions within 1200 AU.

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

2008-07-01

186

Evolution of crystallizing pure 12C white dwarfs  

International Nuclear Information System (INIS)

We describe the first results of a quantitative exploration of white dwarf evolution for models incorporating an accurate dense plasma equation of state and a full treatment of partial ionization and convection in the envelope. We discuss in detail the results for a 1 M/sub sun/, pure 12C star. The cooing curve and luminosity function deviate appreciably from the behavior predicted by Mestel's cooling theory above log (L/L/sub sun/) approx. =-0.5 due to neutrino energy losses and below log (L/L/sub sun/) approx. =-3.5 due to Debye cooling. Crystallization occurs at GAMMAapprox. =160, and the effects produced by the release of latent heat are evident. The combined effect of increased heat capacity due to Coulomb interactions and release of latent heat during crystallization increases the stellar lifetime by nearly a factor of 3 in the phases preceding Debye cooling, as expected. Deep convective cooling appears less important than previously thought. Agreement between the theoretical luminosity function and the observational functions derived by Weidemann from the data of Luyten and of Eggen and Greenstein is generally good, and white dwarf lifetimes determined from the cooling curve are consistent with the ages of white dwarfs in clusters. Resolution of differences between the theoretical discovery function and the number of known white dwarfs at very high and low luminosities may provide new information concerning the distribution and origin of the white dwhe distribution and origin of the white dwarfs. The direct effects of crystallization are too small to permit observational detection at present. Debye cooling may offer an indirect test, however, because its effect is dramatic and roughly composition independent. For this, unfortunately, an accurate observational luminosity function down to at least log (L/L/sub sun/) approx. =-5 appears needed

187

ULTRACAM observations of two accreting white dwarf pulsators  

CERN Document Server

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 ...

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

2008-01-01

188

The brightest pure-H ultracool white dwarf  

CERN Document Server

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...

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

189

The White Dwarf in AE Aqr Brakes Harder  

CERN Document Server

Taking advantage of the very precise de Jager et al. optical white dwarf orbit and spin ephemerides; ASCA, XMMN, and Chandra X-ray observations spread over 10 yrs; and a cumulative 27 yr baseline, we have found that in recent years the white dwarf in AE Aqr is spinning down at a rate that is slightly faster than predicted by the de Jager et al. spin ephemeris. At the present time, the observed period evolution is consistent with either a cubic term in the spin ephemeris with Pdouble_dot = 3.46(56)E-19 per d, which is inconsistent in sign and magnitude with magnetic-dipole radiation losses, or an additional quadratic term with Pdot = 2.0(1.0)E-15 d/d, which is consistent with a modest increase in the accretion torques spinning down the white dwarf. Regular monitoring, in the optical, ultraviolet, and/or X-rays, is required to track the evolution of the spin period of the white dwarf in AE Aqr.

Mauche, C W

2006-01-01

190

The Frequency of Debris Disks at White Dwarfs  

CERN Document Server

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 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 1-7Msol 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 white dwarf cooling ages; white dwarfs accrete debris from several generations of disks for ~10Myr. The average total mass accreted by these stars ranges from that of 200km asteroids to Ceres-sized objects, indicating that white dwarfs accrete moons and d...

Barber, Sara D; Kilic, Mukremin; Leggett, S K; Dufour, P; Bloom, J S; Starr, D L

2012-01-01

191

The frequency of planetary debris around young white dwarfs  

CERN Document Server

(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.

Koester, Detlev; Farihi, Jay

2014-01-01

192

The Ages of Very Cool Hydrogen-rich White Dwarfs  

Science.gov (United States)

The evolution of white dwarfs is essentially a cooling process that depends primarily on the energy stored in their degenerate cores and on the transparency of their envelopes. In this paper we compute accurate cooling sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres for the full range of masses of interest. For this purpose we use the most accurate available physical inputs for both the equation of state and opacities of the envelope and for the thermodynamic quantities of the degenerate core. We also investigate the role of the latent heat in the computed cooling sequences. We present separately cooling sequences in which the effects of phase separation of the carbon-oxygen binary mixture upon crystallization have been neglected, and the delay introduced in the cooling times when this mechanism is properly taken into account, in order to compare our results with other published cooling sequences which do not include a treatment of this phenomenon. We find that the cooling ages of very cool white dwarfs with pure hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr at log(L/Lsolar)=-4.5 for an otherwise typical ~0.6 Msolar white dwarf, when phase separation is neglected. If phase separation of the binary mixture is included, then the cooling ages are further increased by roughly 10%. Cooling tracks and cooling isochrones in several color-magnitude diagrams are presented as well.

Salaris, M.; García-Berro, E.; Hernanz, M.; Isern, J.; Saumon, D.

2000-12-01

193

Axions and the cooling of white dwarf stars  

CERN Document Server

White dwarfs are the end-product of the lifes of intermediate- and low-mass stars and their evolution is described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. We show here that the shape of the bright branch of this function is only sensitive to the averaged cooling rate of white dwarfs and we propose to use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. Our results indicate that the inclusion of the emission of axions in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function. The best fit is obtained for m_a cos^2 \\beta ~ 5 meV, where m_a is the mass of the axion and cos^2 \\beta is a free parameter. We also show that values larger than 10 meV are clearly excluded. The existin...

Isern, J; Torres, S; Catalan, S

2008-01-01

194

Lithium production in the merging of white dwarf stars  

CERN Document Server

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...

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

2012-01-01

195

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

CERN Document Server

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.

De, Sanchari

2014-01-01

196

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

197

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

198

Population synthesis for symbiotic stars with white dwarf accretors  

CERN Document Server

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...

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

2006-01-01

199

Central stars of planetary nebulae: The white dwarf connection  

CERN Document Server

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.

Werner, K

2011-01-01

200

Chandra grating spectroscopy of three hot white dwarfs  

Science.gov (United States)

Context. 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 PG 1159 (PG 1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD 246). 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. 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 PG 1159 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 LB 1919. 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 GD 246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG 1520+525 constrains the effective temperature to Teff = 150 000 ± 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GW Vir class (PG 1159 - 035) defines the location of the blue edge of the GW Vir 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 PG 1159-type stars are reliable and reproduce well the observed spectra from soft X-ray to optical wavelengths.

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

2012-10-01

201

The frequency of planetary debris around young white dwarfs  

Science.gov (United States)

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 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

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

2014-06-01

202

Chandra Grating Spectroscopy of Three Hot White Dwarfs  

Science.gov (United States)

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.

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

2013-01-01

203

The Dynamics of White Dwarfs, Black Holes and Stellar Cusps  

Science.gov (United States)

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

Wegg, Christopher

2013-03-01

204

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

CERN Document Server

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.

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

205

Detached white-dwarf close-binary stars CV's extended family  

CERN Document Server

I review detached binaries consisting of white dwarfs with either other white dwarfs or low mass main-sequence stars in tight orbits around them. Orbital periods have been measured for 15 white dwarf/white dwarf systems and 22 white dwarf/M dwarf systems. While small compared to the number of periods known for CVs (>300), I argue that each variety of detached system has a space density an order of magnitude higher that of CVs. While theory matches the observed distribution of orbital periods of the white dwarf/white dwarf binaries, it predicts white dwarfs of much lower mass than observed. Amongst both types of binary are clear examples of helium core white dwarfs, as opposed to the usual CO composition; similar systems must exist amongst the CVs. White dwarf/M dwarf binaries suffer from selection effects which diminish the numbers seen at long and short periods. They are useful for the study of irradiation; I discuss evidence to suggest that Balmer emission is broadened by optical depth effects to an extent ...

Marsh, T R

1999-01-01

206

Dynamical Tides in Compact White Dwarf Binaries: Helium Core White Dwarfs, Tidal Heating, and Observational Signatures  

CERN Document Server

Tidal dissipation in compact white dwarf (WD) binary systems significantly influences the physical conditions (such as surface temperature and rotation rate) of the WDs prior to mass transfer or merger. In these systems, the dominant tidal effects involve the excitation of gravity waves and their dissipation in the outer envelope of the star. We calculate the amplitude of tidally excited gravity waves in low-mass (0.3M_\\odot) helium-core (He) WDs as a function of the tidal forcing frequency \\omega. Like carbon-oxygen (CO) WDs studied in our previous paper, we find that the dimensionless tidal torque F(\\omega) (inversely proportional to the effective tidal quality factor) has an erratic dependence on \\omega. On average, F(\\omega) scales approximately as \\omega^6, and is several orders of magnitude smaller for He WDs than for CO WDs. We find that tidal torques can begin to synchronize the WD rotation when the orbital period is less than about a hour, although a nearly constant asynchronization is maintained eve...

Fuller, Jim

2012-01-01

207

Sirius B - A still mysterious white dwarf  

Science.gov (United States)

Observations and knowledge of Sirius B, the companion star to Sirius A are reviewed, noting the solar mass and terrestrial radius of the dwarf. The system is 2.65 pc distant, with separation between the stars oscillating from 8-32 AU. Spectral observations are best obtained when the stars are at maximum distance, and redshifts which affirm the theory of relativity have been detected. Copernicus satellite observations have also revealed the emissions of UV and X rays from the dwarf. It is noted that Grecian records over 2000 yr old cite Sirius as a red star, which implies that the dwarf was then a red star since Sirius A is a main sequence star, an implication which does not correspond with the known evolution of stars. Another mystery is cited, that of the tribal records of the Sudanese Dogon, which maintains an ancient legend of Sirius A having an invisible companion called Digitaria, composed of a substance called segala, which cannot be lifted by all the humans on earth combined.

Wesemael, F.; Fontaine, G.

1982-02-01

208

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

CERN Document Server

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...

Farihi, J; Zuckerman, B

2008-01-01

209

Black holes, white dwarfs and neutron stars  

International Nuclear Information System (INIS)

Basic principles of the physics of compact physical object - white drafts, neutron stars and black holes, are stated. Modern representations on pulsars and compact X-ray sources are considerd. Investigation results on compact object accretion, gravitational radiation and collapse at usual and supermassive stars are discussed

210

WHITE DWARF-RED DWARF SYSTEMS RESOLVED WITH THE HUBBLE SPACE TELESCOPE. II. FULL SNAPSHOT SURVEY RESULTS  

International Nuclear Information System (INIS)

Results are presented for a Hubble Space Telescope Advanced Camera for Surveys high-resolution imaging campaign of 90 white dwarfs with known or suspected low-mass stellar and substellar companions. Of the 72 targets that remain candidate and confirmed white dwarfs with near-infrared excess, 43 are spatially resolved into two or more components, and a total of 12 systems are potentially triples. For 68 systems where a comparison is possible, 50% have significant photometric distance mismatches between their white dwarf and M dwarf components, suggesting that white dwarf parameters derived spectroscopically are often biased due to the cool companion. Interestingly, 9 of the 30 binaries known to have emission lines are found to be visual pairs and hence widely separated, indicating an intrinsically active cool star and not irradiation from the white dwarf. There is a possible, slight deficit of earlier spectral types (bluer colors) among the spatially unresolved companions, exactly the opposite of expectations if significant mass is transferred to the companion during the common envelope phase. Using the best available distance estimates, the low-mass companions to white dwarfs exhibit a bimodal distribution in projected separation. This result supports the hypothesis that during the giant phases of the white dwarf progenitor, any unevolved companions either migrate inward to short periods of hours to days, or outward to periods of hundreds to thousands of years. No int of hundreds to thousands of years. No intermediate projected separations of a few to several AU are found among these pairs. However, a few double M dwarfs (within triples) are spatially resolved in this range, empirically demonstrating that such separations were readily detectable among the binaries with white dwarfs. A straightforward and testable prediction emerges: all spatially unresolved, low-mass stellar and substellar companions to white dwarfs should be in short-period orbits. This result has implications for substellar companion and planetary orbital evolution during the post-main-sequence lifetime of their stellar hosts.

211

A population synthesis study of the luminosity function of hot white dwarfs  

CERN Document Server

We present a coherent and detailed Monte Carlo simulation of the population of hot white dwarfs. We assess the statistical significance of the hot end of the white dwarf luminosity function and the role played by the bolometric corrections of hydrogen-rich white dwarfs at high effective temperatures. We use the most up-to-date stellar evolutionary models and implement a full description of the observational selection biases to obtain realistic simulations of the observed white dwarf population. Our theoretical results are compared with the luminosity function of hot white dwarfs obtained from the Sloan Digital Sky Survey (SDSS), for both DA and non-DA white dwarfs. We find that the theoretical results are in excellent agreement with the observational data for the population of white dwarfs with hydrogen deficient atmospheres (non-DA white dwarfs). For the population of white dwarfs with hydrogen-rich atmospheres (white dwarfs of the DA class), our simulations show some discrepancies with the observations for ...

Torres, S; Krzesinski, J; Kleinman, S J

2014-01-01

212

PG 1346+082 - An interacting binary white dwarf system  

Science.gov (United States)

PG 1346+082 is both a photometric and a spectroscopic variable, spanning the B-magnitude range 13.6-17.2. High-speed photometric data reveal rapid flickering in the low-state light curve. The system also shows spectroscopic variations, displaying broad shallow He I absorption lines at maximum light and a weak emission feature at He I (4471 A) at minimum light. Hydrogen lines are conspicuous by their absence. Is is concluded that PG 1346+082 is an interacting binary white dwarf system. Furthermore, because continuum fits to IUE high-state data suggest temperatures consistent with membership in the DB white dwarf instability strip, it is suggested that some of the photometric variations may arise from pulsations.

Wood, M. A.; Winget, D. E.; Nather, R. E.; Liebert, James; Wesemael, F.

1987-01-01

213

Spitzer Surveys of Infrared Excesses of White Dwarfs  

CERN Document Server

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 T_eff 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.

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

2010-01-01

214

Merging Binary Stars and the magnetic white dwarfs  

CERN Document Server

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 ...

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

2014-01-01

215

Quantum gases and white dwarfs with quantum gravity  

Science.gov (United States)

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.

Moussa, Mohamed

2014-11-01

216

Limb-Darkening Coefficients for Eclipsing White Dwarfs  

CERN Document Server

We present extensive calculations of linear and non-linear limb-darkening coefficients as well as complete intensity profiles appropriate for modeling the light-curves of eclipsing white dwarfs. We compute limb-darkening coefficients in the Johnson-Kron-Cousins UBVRI photometric system as well as the Large Synoptic Survey Telescope (LSST) ugrizy system using the most up-to-date model atmospheres available. In all, we provide the coefficients for seven different limb-darkening laws. We describe the variations of these coefficients as a function of the atmospheric parameters, including the effects of convection at low effective temperatures. Finally, we discuss the importance of having readily available limb-darkening coefficients in the context of present and future photometric surveys like the LSST, Palomar Transient Factory (PTF), and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The LSST, for example, may find ~10^5 eclipsing white dwarfs. The limb-darkening calculations presented h...

Gianninas, A; Kilic, Mukremin; Bergeron, P

2013-01-01

217

Evidence of Precession of the White Dwarf in Cataclysmic Variables  

CERN Document Server

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 spectroscopy. Here, we show that the precession of the fast rotating magnetically accreting white dwarf can successfully explain these phenomena. The theory of compact objects predicts certain relations between the spin and precession periods, and our finding provides a good test for the theory and establishes a qualitative model to be explored both theoretically and observationally. Detection of precession can become a powerful tool in searching for the internal properties of compact stars, which would be otherwise inaccessible for us.

Tovmassian, G; Neustroev, V

2006-01-01

218

Impact of Impurity Sedimentation on Cooling of White Dwarfs  

Science.gov (United States)

The endpoint of stellar evolution of 98% of stars is the white dwarf phase in which nuclear fuel no longer burns, residual energy radiates away, and the star slowly cools. The cooling process in white dwarfs has important astrophysical consequences and is interesting in its own right because it calls for detailed knowledge of thermodynamic and transport properties of dense, strongly-coupled plasmas. Under these extreme conditions, ions are strongly coupled and ionic quantum effects can be important. Indeed, white dwarfs are made of a dense core of fully ionized mixtures, mainly carbon and oxygen, sustained against gravitational collapse by the pressure of degenerate, relativistic electrons. Energy outflow from the core is regulated by thin surface layers of helium and hydrogen. The most important sources of energy within the core are associated with crystallization; however, it has been suggested(*) that the cooling could be strongly delayed by the gravitational settling of ^22Ne impurities in liquid white dwarfs interiors, a mechanism previously overlooked. Simple models for this process treat the sedimentation of ^22Ne as diffusion in a classical, strongly coupled ionic plasma immersed in a uniform neutralizing electron background. Such a model suffers from uncertainties in the diffusion coefficient, which cannot be obtained via standard plasma kinetic theory. We first assessed this uncertainty by computing the diffusion coefficient using particle-particle particle-mesh molecular dynamics over the entire strongly coupled fluid regime. An analytical model was developed to extend the results into the quantum regime. These results were extended to the more realistic situation of ionic mixtures. These findings and their impact on the cooling process will be discussed. (*) L. Bildsten and D.M. Hall, ApJ 549, L219 (2001)

Daligault, Jerome

2004-11-01

219

GD 154: White dwarf with multi- and monoperiodic pulsation  

Directory of Open Access Journals (Sweden)

Full Text Available We present the white dwarf GD 154 as an example where either monoperiodic or multiperiodic pulsation were found at different epochs. The mono-multi-monoperiodic stage seems to alternate. Many questions have been raised. Is this behaviour connected to the evolution of DAV stars? How often does it happen? Is there any regularity in this change of the pulsational behaviour or is it irregular?

Bognár Zs.

2013-03-01

220

A Progress Report on the Carbon Dominated Atmosphere White Dwarfs  

OpenAIRE

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.

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

2009-01-01

221

A progress report on the carbon dominated atmosphere white dwarfs  

International Nuclear Information System (INIS)

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.

222

A progress report on the carbon dominated atmosphere white dwarfs  

Energy Technology Data Exchange (ETDEWEB)

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.

Dufour, P; Liebert, J; Swift, B; Sukhbold, T [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Fontaine, G, E-mail: dufourpa@as.arizona.ed [Departement de Physique, Universite de Montreal, C.P.6128, Succ. Centre-Ville, Montreal, Quebec, H3C 3J7 (Canada)

2009-06-01

223

On the Critical Mass: the case of white dwarfs  

OpenAIRE

We recall the generalization of the Feynman-Metropolis-Teller approximation for a compressed atom using a relativistic Fermi-Thomas model. These results within a Wigner-Seitz approximation lead to a new equation of state for white dwarfs and to a new value of their critical mass, smaller than the one obtained by Chandrasekhar. The possible observations of these effects in binary neutron stars are outlined.

Ruffini, Remo J.

2000-01-01

224

The double-layered chemical structure in DB white dwarfs  

CERN Document Server

We study the structure and evolution of white dwarf stars with helium-rich atmospheres (DB) in a self-consistent way with the predictions of time-dependent element diffusion. Our treatment of diffusion includes gravitational settling and chemical and thermal diffusion. OPAL radiative opacities for arbitrary metallicity and carbon-and oxygen-rich compositions are employed. Emphasis is placed on the evolution of the diffusion-modeled double-layered chemical structure. This structure, which is characterized by a pure helium envelope atop an intermediate remnant shell rich in helium, carbon and oxygen, is expected for pulsating DB white dwarfs, assuming that they are descendants of hydrogen-deficient PG1159 post-AGB stars. We find that, depending on the stellar mass, if DB white dwarf progenitors are formed with a helium content smaller than \\approx 10^-3 M_*, a single-layered configuration is expected to emerge during the DB pulsation instability strip. We also explore the consequences of diffusively evolving ch...

Althaus, L G

2004-01-01

225

High-energy emission of fast rotating white dwarfs  

CERN Document Server

The process of energy release in the magnetosphere of a fast rotating, magnetized white dwarf can be explained in terms of the canonical spin-powered pulsar model. Applying this model to the white dwarf companion of the low mass close binary AE Aquarii leads us to the following conclusions. First, the system acts as an accelerator of charged particles whose energy is limited to E_p < 3 TeV and which are ejected from the magnetosphere of the primary with the rate L_kin < 10^{32} erg/s. Due to the curvature radiation of the accelerated primary electrons the system should appear as a source of soft gamma-rays (~ 100 keV) with the luminosity < 3x10^{27} erg/s. The TeV emission of the system is dominated by the inverse Compton scattering of optical photons on the ultrarelativistic electrons. The optical photons are mainly contributed by the normal companion and the stream of material flowing through the magnetosphere of the white dwarf. The luminosity of the TeV source depends on the state of the system (...

Ikhsanov, N R

2005-01-01

226

Towards an Understanding of the Atmospheres of Cool White Dwarfs  

CERN Document Server

Cool white dwarfs with Teff < 6000 K are the remnants of the oldest stars that existed in our Galaxy. Their atmospheres, when properly characterized, can provide valuable information on white dwarf evolution and ultimately star formation through the history of the Milky Way. Understanding the atmospheres of these stars requires joined observational effort and reliable atmosphere modeling. We discuss and analyze recent observations of the near-ultraviolet (UV) and near-infrared (IR) spectrum of several cool white dwarfs including DQ/DQp stars showing carbon in their spectra. We present fits to the entire spectral energy distribution (SED) of selected cool stars, showing that the current pure-hydrogen atmosphere models are quite reliable, especially in the near-UV spectral region. Recently, we also performed an analysis of the coolest known DQ/DQp stars investigating further the origin of the C2 Swan bands-like spectral features that characterize the DQp stars. We show that the carbon abundances derived for ...

Kowalski, Piotr M; Holberg, Jay; Leggett, Sandy

2012-01-01

227

White Dwarfs, Neutron Stars, Black Holes and the EUV  

Science.gov (United States)

White dwarfs are recognized as formally analogous to neutron stars being stellar configurations where the thermal contribution to support is secondary. Both stellar types exist with various intrinsic parameters (spin mass magnetic field temperature/age) and environmental parameters (accretion environment if any characterized by overall rate and flow geometry). Comparison of analogous systems using scalings of these parameters can be fruitful e.g. comparison of stars with similar magnetic moments. Source class characterization is mature enough that such analogies can be used to compare theoretical ideas across a wide dynamic range in parameters one example being theories of quasiperioic oscillations. However the white dwarf side of this program is limited by available high resolution spectroscopy and photometry in the EUV. EUV observation is challenging but this is where white dwarf spectral energy distributions often reach maximum values. (For neutron stars the same role is played by X-rays.) Two types of cataclysmic variable systems exemplified by AM Her and EX Hya are used to illustrate these ideas in detail. Dynamical timescales and envisioned spectroscopic performance parameters of next-generation EUV satellites make possible a new level of source modeling that blends timing and spectroscopic tests. Extensions to include black holes are also considered.

Wood, Kent S.

228

Hard X-ray Emission Associated with White Dwarfs  

CERN Document Server

We have used the WGACAT to search for hard X-ray sources associated with white dwarfs (WDs) from the catalog of McCook & Sion (1999). We find 17 X-ray sources coincident with WDs showing significant hard X-ray emission at energies >0.5 keV. Twelve of these WDs are in known binary systems, in two of which the accretion of the close companion's material onto the white dwarf produces the hard X-ray emission, and in the other ten of which the late-type companions' coronal activity emits hard X-rays. One WD is projected near an AGN which is responsible for the hard X-ray emission. The remaining four WDs and two additional white dwarfs with hard X-ray emission appear single. The lack of near-IR excess from the apparently single WDs suggests that either X-ray observations are more effective than near-IR photometry in diagnosing faint companions or a different emission mechanism is needed. It is intriguing that 50% of the six apparently single WDs with hard X-ray emission are among the hottest WDs. We have compar...

O'Dwyer, I J; Gruendl, R A; Guerrero, M A; Webbink, R F

2003-01-01

229

Describing SGRs/AXPs as fast and magnetized white dwarfs  

CERN Document Server

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 ...

Malheiro, M

2013-01-01

230

Hydrodynamic simulations of white-dwarf-main-sequence star collisions  

International Nuclear Information System (INIS)

Two-dimensional hydrodynamic simulations of head-on collisions between a one solar mass white dwarf and a 0.13 solar mass main-sequence star have been performed. The impact velocities at first contact were 2000 and 6000 km/s, in the two cases considered. These speeds corresponded to relative velocities of the stars, at infinity, of 300 and 5700 km/s, respectively. Temperatures of 3 to 5 x 108K are achieved by most of the main-sequence star mass, and nuclear reactions release over 1049 erg in approximately 1 hr. Accretion of main-sequence star mass by the white dwarf is followed by a dynamic ejection episode, which totally disrupts the non-degenerate star and leaves the white dwarf intact. The results demonstrate that the disruptive process considered here is as important as the amalgamative collisions considered previously. Such disruptive collisions should sharply modify the luminosity function, the luminosity, the energy balance and the evolution of dense galactic nuclei. (author)

231

White Dwarf based evaluation of the GALEX absolute calibration  

CERN Document Server

This paper describes a revised photometric calibration of the \\emph{Galaxy Evolution Explorer} magnitudes, based on measurements of DA white dwarfs. The photometric magnitudes of white dwarfs measured by \\emph{GALEX} are compared to predicted magnitudes based on independent spectroscopic data (108 stars) and alternately to \\emph{IUE} UV fluxes of the white dwarfs (218 stars). The results demonstrate a significant non-linear correlation and small offset between archived \\emph{GALEX} fluxes and observed and predicted UV fluxes for our sample. The primary source of non-linearity may be due to detector dead time corrections for brighter stars, but it should be noted that there was a predicted non-linearity in the fainter stars as well. Sample expressions are derived which 'correct' observed \\emph{GALEX} magnitudes to an absolute magnitude scale that is linear with respect, and directly related, to the \\emph{Hubble Space Telescope} photometric scale. These corrections should be valid for stars dimmer than magnitud...

Camarota, L

2013-01-01

232

Evidence for Terrestrial Planetary System Remnants at White Dwarfs  

CERN Document Server

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...

Farihi, J

2010-01-01

233

The Spectral Types of White Dwarfs in Messier 4  

CERN Document Server

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 due simply 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, ...

Davis, D Saul; Rich, R Michael; Reitzel, David R; Kalirai, Jason S

2009-01-01

234

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.

235

Atmospheric parameters and carbon abundance for hot DB white dwarfs  

CERN Document Server

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....

Koester, Detlev; Gänsicke, Boris T

2014-01-01

236

The evolution of helium white dwarfs; 2, Thermal instabilities  

CERN Document Server

We calculated a grid of evolutionary models for white dwarfs with helium cores (He-WDs) and investigated the occurrence of hydrogen-shell flashes due to unstable hydrogen burning via CNO cycling. Our calculations show that such thermal instabilities are restricted to a certain mass range (M=0.21...0.30Msun), consistent with earlier studies. Models within this mass range undergo the more hydrogen shell flashes the less massive they are. This is caused by the strong dependence of the envelope mass on the white dwarf core mass. The maximum luminosities from hydrogen burning during the flashes are of the order of 10^5 Lsun. Because of the development of a pulse-driven convection zone whose upper boundary temporarily reaches the surface layers, the envelope's hydrogen content decreases by Delta(X)=0.06 per flash. Our study further shows that an additional high mass-loss episode during a flash-driven Roche lobe overflow to the white dwarf's companion does not affect the final cooling behaviour of the models. Indepe...

Driebe, T; Schönberner, D; Herwig, F

1999-01-01

237

Thermally Stable Nuclear Burning on Accreting White Dwarfs  

CERN Document Server

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...

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

2007-01-01

238

COOL WHITE DWARFS IDENTIFIED IN THE SECOND DATA RELEASE OF THE UKIRT INFRARED DEEP SKY SURVEY  

International Nuclear Information System (INIS)

We have paired the second data release of the Large Area Survey of the UKIRT Infrared Deep Sky Survey with the fifth data release of the Sloan Digital Sky Survey to identify 10 cool white dwarf candidates, from their photometry and astrometry. Of these 10, one was previously known to be a very cool white dwarf. We have obtained optical spectroscopy for seven of the candidates using the GMOS-N spectrograph on Gemini North, and have confirmed all seven as white dwarfs. Our photometry and astrometry indicate that the remaining two objects are also white dwarfs. The model analysis of the photometry and available spectroscopy shows that the seven confirmed new white dwarfs, and the two new likely white dwarfs, have effective temperatures in the range of T eff = 5400-6600 K. Our analysis of the previously known white dwarf confirms that it is cool, with T eff = 3800 K. The cooling age for this dwarf is 8.7 Gyr, while that for the nine ? 6000 K white dwarfs is 1.8-3.6 Gyr. We are unable to determine the masses of the white dwarfs from the existing data, and therefore we cannot constrain the total ages of the white dwarfs. The large cooling age for the coolest white dwarf in the sample, combined with its low estimated tangential velocity, suggests that it is an old member of the thin disk, or a member of the thick disk of the Galaxy, with an age of 10-11 Gyr. The warmer white dwarfs appear to have velocities typical of the thick disk or even halo; thescal of the thick disk or even halo; these may be very old remnants of low-mass stars, or they may be relatively young thin-disk objects with unusually high space motion.

239

Peculiar variations of white dwarf pulsation frequencies and maestro  

Science.gov (United States)

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.

Dalessio, James Ruland

240

Detection of a white dwarf companion to the white dwarf binary SDSSJ125733.63+542850.5  

CERN Document Server

SDSSJ125733.63+542850.5 (hereafter SDSSJ1257+5428) is a white dwarf from the Sloan Digital Sky Survey recently shown to exhibit high-amplitude radial velocity variations on a period of 4.56 hours suggesting that it has either a neutron star or black-hole binary companion. At a distance of only 48 pc, this would make it the closest remnant of a supernova known and imply that such systems are common in our Galaxy and others. Here we present optical spectroscopy that shows that the companion star in SDSSJ1257+5428 is in fact another white dwarf. SDSSJ1257+5428's spectrum is thus a composite, with narrow line cores from a cool, low mass white dwarf (7000 K; 0.2 Msun), and broad wings from its hotter, high-mass companion (10,000 K; > 1 Msun). We present evidence that suggests that the high-mass star is rapidly rotating with v sin i = 500 to 1000 km/s. This suggests that the most recent phase of mass transfer was long-lasting and stable as against the usually-assumed common envelope phase. Within the constraints se...

Marsh, T R; Steeghs, D; Southworth, J; Koester, D; Harris, V; Merry, L

2010-01-01

241

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

OpenAIRE

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 branc...

Jeffery, C. Simon; Karakas, Amanda I.; Saio, Hideyuki

2011-01-01

242

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

CERN Document Server

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...

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

2013-01-01

243

HST Spectra of GW Librae: A Hot Pulsating White Dwarf in a Cataclysmic Variable  

OpenAIRE

We have obtained Hubble Space Telescope UV spectra of the white dwarf in GW Lib, the only known non-radially pulsating white dwarf in a cataclysmic variable, and the first known DAZQ variable. The UV light curve reveals large amplitude (10%) pulsationsin the UV with the same periods (646, 376 and 237 s) as those seen at optical wavelengths, but the mean spectrum fits with an average white dwarf temperature (14,700K for a 0.6M_{odot} white dwarf) that is too hot to be in the ...

Szkody, P.; Gansicke, B.; Howell, S. B.; Sion, E.

2002-01-01

244

High-speed Photometric Observations of ZZ Ceti White Dwarf Candidates  

CERN Document Server

We present high-speed photometric observations of ZZ Ceti white dwarf candidates drawn from the spectroscopic survey of bright DA stars from the Villanova White Dwarf Catalog by Gianninas et al., and from the recent spectroscopic survey of white dwarfs within 40 parsecs of the Sun by Limoges et al. We report the discovery of six new ZZ Ceti pulsators from these surveys, and several photometrically constant DA white dwarfs, which we then use to refine the location of the ZZ Ceti instability strip.

Green, E M; Gianninas, A; Bergeron, P; Fontaine, G; Dufour, P; O'Malley, C J; Guvenen, B; Biddle, L I; Pearson, K; Deyoe, T W; Bullivant, C W; Hermes, J J; Van Grootel, V; Grosjean, M

2015-01-01

245

The Accreting White Dwarf in SS Cygni Revealed  

CERN Document Server

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...

Sion, Edward; Myzcka, Janine; Blair, William

2010-01-01

246

Examining Flare Rates in Close M dwarf + White Dwarf binary pairs  

Science.gov (United States)

We present a preliminary study to examine the statistical flare rates for M dwarfs with close white dwarf companions (WD+dM; typical separations rates. Our analysis of M dwarfs with close WD companions demonstrated that the M dwarfs are more active than their field counterparts. One implication of having a close binary companion is presumed to be increased stellar rotation through disk-disruption, tidal effects, and/or angular momentum exchange; increased stellar activity has long been attributed to an increase in stellar rotation. We examine the difference between the flare rates observed in close WD+dM binary systems and field M dwarfs. Our sample consists of a subset of 202 (70 of which are magnetically active) close WD+dM pairs from Morgan et al. that were observed in the Sloan Digital Sky Survey Stripe 82, a transient observing mode where multi-epoch observations in the Sloan ugriz bands were obtained. We present results that will assist in identifying and categorizing transient phenomena and limiting expensive follow-up observations for future time-domain studies, such as LSST.

Morgan, Dylan P.; West, Andrew A.; Becker, Andrew C.

2015-01-01

247

Atmospheric parameters and carbon abundance for hot DB white dwarfs  

Science.gov (United States)

Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near effective temperatures of 25 000 K are extremely difficult to determine from optical spectroscopy. The neutral He lines reach a maximum in this range and change very little with effective temperature and surface gravity. Moreover, an often unknown amount of hydrogen contamination can change the resulting parameters significantly. 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 - whose spectra are 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 of white dwarfs of spectral classes DB and hot DQ. High-resolution UV spectra of five DBs were studied with model atmospheres. We determined stellar parameters and abundances or upper limits of C and Si. These objects were compared with cooler DBs below 20 000 K. We find photospheric C and no other heavy elements - with extremely high limits on the C/Si ratio - in two of the five hot DBs. We compare various explanations for this unusual composition that have been proposed in the literature: accretion of interstellar or circumstellar matter, radiative levitation, carbon dredge-up from the deeper interior below the helium layer, and a residual stellar wind. None of these explanations is completely satisfactory, and the problem of the origin of the hot DQ remains an open question.

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

2014-08-01

248

THE BINARY FRACTION OF LOW-MASS WHITE DWARFS  

International Nuclear Information System (INIS)

We describe spectroscopic observations of 21 low-mass (?0.45 Msun) white dwarfs (WDs) from the Palomar-Green survey obtained over four years. We use both radial velocities and infrared photometry to identify binary systems, and find that the fraction of single, low-mass WDs is ?30%. We discuss the potential formation channels for these single stars including binary mergers of lower-mass objects. However, binary mergers are not likely to explain the observed number of single low-mass WDs. Thus, additional formation channels, such as enhanced mass loss due to winds or interactions with substellar companions, are likely.

249

The movie about the magnetism in isolated white dwarfs  

Science.gov (United States)

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.

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

2004-10-01

250

White-Dwarf Kicks and Implications for Barium Stars  

OpenAIRE

The formation mechanism of the barium stars is thought to be well understood. Barium-rich material, lost in a stellar wind from a thermally-pulsing asymptotic-giant branch star in a binary system, is accreted by its companion main-sequence star. Now, many millions of years later, the primary is an unseen white dwarf and the secondary has itself evolved into a giant which displays absorption lines of barium in its spectrum and is what we call a barium star. A similar wind-acc...

Izzard, Robert G.; Dermine, Tyl; Church, Ross P.

2010-01-01

251

Gravitational waves from double white dwarfs and AM CVn binaries  

Energy Technology Data Exchange (ETDEWEB)

I give a brief overview of our model for the galactic population of compact binaries that is used to predict the low-frequency gravitational wave signal from the galaxy, and discuss recent observational developments that will enable us to test and improve this model. The SPY project will discover some 150 new close double white dwarfs and, recently, two ROSAT sources turned out to be new AM CVn candidates, one with an orbital period of only 5 min. I give an update on the expected binaries that will be resolved by LISA and discuss what we can learn about the galactic population of compact binaries once LISA gives her first results.

Nelemans, Gijs [Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA(United Kingdom)

2003-05-21

252

A SUBSTELLAR COMPANION TO THE WHITE DWARF-RED DWARF ECLIPSING BINARY NN Ser  

International Nuclear Information System (INIS)

NN Ser is a short-period (P = 3.12 hr) close binary containing a very hot white dwarf primary with a mass of 0.535 Msun and a fully convective secondary with a mass of 0.111 Msun. The changes in the orbital period of the eclipsing binary were analyzed based on our five newly determined eclipse times together with those compiled from the literature. A small-amplitude (0fd00031) cyclic period variation with a period of 7.56 years was discovered to be superimposed on a possible long-term decrease. The periodic change was plausibly explained as the light-travel time effect via the presence of a tertiary companion. The mass of the tertiary companion is determined to be M3sin i' = 0.0107(±0.0017) Msun when a total mass of 0.646 Msun for NN Ser is adopted. For orbital inclinations i' ? 49.056, the mass of the tertiary component was calculated to be M 3 ? 0.014 Msun; thus it would be an extrasolar planet. The third body is orbiting the white dwarf-red dwarf eclipsing binary at a distance shorter than 3.29 AU. Since the observed decrease rate of the orbital period is about two orders larger than that caused by gravitational radiation, it can be plausibly interpreted by magnetic braking of the fully convective component, which is driving this binary to evolve into a normal cataclysmic variable.

253

Stars with Unusual Compositions: Carbon and Oxygen in Cool White Dwarf Stars  

CERN Document Server

I present a broad historical review on cool white dwarf stars with carbon and/or oxygen detected at the photosphere. This book chapter covers the observational signatures, physical properties, and evolution of DQ, DBQ and Hot DQ white dwarfs, and also present an overview of the main challenges that future investigations of these types of object should try to address.

Dufour, P

2011-01-01

254

New population synthesis model: Preliminary results for close double white dwarf populations  

CERN Document Server

An update is presented to the software package SeBa for simulating single star and binary evolution in which new stellar evolution tracks have been implemented. SeBa is applied to study the population of close double white dwarf and the delay time distribution of double white dwarf mergers that may lead to Supernovae Type Ia.

Toonen, Silvia; Zwart, Simon Portegies

2011-01-01

255

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

Science.gov (United States)

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.

Wegner, G.

1983-01-01

256

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

Science.gov (United States)

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

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

257

Discovery of a luminous white dwarf in a young star cluster in the Large Magellanic Cloud  

OpenAIRE

We have identified a candidate 1-2 x 10^5 year old luminous white dwarf in NGC 1818, a young star cluster in the Large Magellanic Cloud. This discovery strongly constrains the boundary mass M_c at which stars stop forming neutron stars and start forming white dwarfs, to M_c > 7.6 Msun.

Elson, Rebecca A. W.; Sigurdsson, Steinn; Hurley, Jarrod; Davies, Melvyn B.; Gilmore, Gerard F.

1998-01-01

258

Results from recent time-series photometric studies of pulsating extremely low-mass white dwarfs.  

Science.gov (United States)

Extremely low-mass (ELM; pulsate in an extension of the DAV (hydrogen-atmosphere variable) instability strip as ELM white dwarf pulsators. We present a synopsis of all pulsating ELM white dwarfs known to date and highlight recent findings from our time-series analyses.

Bell, Keaton; Brown, Warren R.; Gianninas, Alex; Hermes, JJ; Kepler, S. O.; Kilic, Mukremin; Montgomery, Michael H.; Winget, Donald E.

2015-01-01

259

White dwarfs as a source of constraints on exotic physics  

CERN Document Server

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

Malec, B

2001-01-01

260

SIMPLIFIED HYDROSTATIC CARBON BURNING IN WHITE DWARF INTERIORS  

International Nuclear Information System (INIS)

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 approaching ignition in Type Ia supernova progenitors, including the effects of the main e --captures and ?-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 energy generation, and the neutrino losses between both simplified networks and the detailed reaction network in a fixed temperature and density parcel of gas.

261

Remnant evolution after a carbon-oxygen white dwarf merger  

CERN Document Server

We systematically explore the evolution of the merger of two carbon-oxygen (CO) white dwarfs. The dynamical evolution of a 0.9 Msun + 0.6 Msun CO white dwarf merger is followed by a three-dimensional SPH simulation. We use an elaborate prescription in which artificial viscosity is essentially absent, unless a shock is detected, and a much larger number of SPH particles than earlier calculations. Based on this simulation, we suggest that the central region of the merger remnant can, once it has reached quasi-static equilibrium, be approximated as a differentially rotating CO star, which consists of a slowly rotating cold core and a rapidly rotating hot envelope surrounded by a centrifugally supported disc. We construct a model of the CO remnant that mimics the results of the SPH simulation using a one-dimensional hydrodynamic stellar evolution code and then follow its secular evolution. The stellar evolution models indicate that the growth of the cold core is controlled by neutrino cooling at the interface bet...

Yoon, Sung-Chul; Rosswog, Stephan

2007-01-01

262

Near-UV absorption in very cool DA white dwarfs  

CERN Document Server

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...

Saumon, D; Kowalski, P M

2014-01-01

263

The progenitors of magnetic white dwarfs in open clusters  

CERN Document Server

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...

Külebi, B; Jordan, S; Euchner, F

2013-01-01

264

The ages of very cool hydrogen-rich white dwarfs  

CERN Document Server

The evolution of white dwarfs is essentially a cooling process that depends primarily on the energy stored in their degenerate cores and on the transparency of their envelopes. In this paper we compute accurate cooling sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres for the full range of masses of interest. For this purpose we use the most accurate available physical inputs for both the equation of state and opacities of the envelope and for the thermodynamic quantities of the degenerate core. We also investigate the role of the latent heat in the computed cooling sequences. We present separately cooling sequences in which the effects of phase separation of the carbon-oxygen binary mixture upon crystallization have been neglected, and the delay introduced in the cooling times when this mechanism is properly taken into account, in order to compare our results with other published cooling sequences which do not include a treatment of this phenomenon. We find that the cooling ages of...

Salaris, M; Hernanz, M; Isern, J; Saumon, D S

2000-01-01

265

Merging binary stars and the magnetic white dwarfs  

Science.gov (United States)

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.

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

2015-02-01

266

LIMB-DARKENING COEFFICIENTS FOR ECLIPSING WHITE DWARFS  

Energy Technology Data Exchange (ETDEWEB)

We present extensive calculations of linear and nonlinear limb-darkening coefficients as well as complete intensity profiles appropriate for modeling the light-curves of eclipsing white dwarfs. We compute limb-darkening coefficients in the Johnson-Kron-Cousins UBVRI photometric system as well as the Large Synoptic Survey Telescope (LSST) ugrizy system using the most up to date model atmospheres available. In all, we provide the coefficients for seven different limb-darkening laws. We describe the variations of these coefficients as a function of the atmospheric parameters, including the effects of convection at low effective temperatures. Finally, we discuss the importance of having readily available limb-darkening coefficients in the context of present and future photometric surveys like the LSST, Palomar Transient Factory, and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The LSST, for example, may find {approx}10{sup 5} eclipsing white dwarfs. The limb-darkening calculations presented here will be an essential part of the detailed analysis of all of these systems.

Gianninas, A.; Strickland, B. D.; Kilic, Mukremin [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019 (United States); Bergeron, P., E-mail: alexg@nhn.ou.edu, E-mail: benstrickland@ou.edu, E-mail: kilic@ou.edu, E-mail: bergeron@astro.umontreal.ca [Departement de Physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada)

2013-03-20

267

LIMB-DARKENING COEFFICIENTS FOR ECLIPSING WHITE DWARFS  

International Nuclear Information System (INIS)

We present extensive calculations of linear and nonlinear limb-darkening coefficients as well as complete intensity profiles appropriate for modeling the light-curves of eclipsing white dwarfs. We compute limb-darkening coefficients in the Johnson-Kron-Cousins UBVRI photometric system as well as the Large Synoptic Survey Telescope (LSST) ugrizy system using the most up to date model atmospheres available. In all, we provide the coefficients for seven different limb-darkening laws. We describe the variations of these coefficients as a function of the atmospheric parameters, including the effects of convection at low effective temperatures. Finally, we discuss the importance of having readily available limb-darkening coefficients in the context of present and future photometric surveys like the LSST, Palomar Transient Factory, and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The LSST, for example, may find ?105 eclipsing white dwarfs. The limb-darkening calculations presented here will be an essential part of the detailed analysis of all of these systems.

268

Limb-darkening Coefficients for Eclipsing White Dwarfs  

Science.gov (United States)

We present extensive calculations of linear and nonlinear limb-darkening coefficients as well as complete intensity profiles appropriate for modeling the light-curves of eclipsing white dwarfs. We compute limb-darkening coefficients in the Johnson-Kron-Cousins UBVRI photometric system as well as the Large Synoptic Survey Telescope (LSST) ugrizy system using the most up to date model atmospheres available. In all, we provide the coefficients for seven different limb-darkening laws. We describe the variations of these coefficients as a function of the atmospheric parameters, including the effects of convection at low effective temperatures. Finally, we discuss the importance of having readily available limb-darkening coefficients in the context of present and future photometric surveys like the LSST, Palomar Transient Factory, and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The LSST, for example, may find ~105 eclipsing white dwarfs. The limb-darkening calculations presented here will be an essential part of the detailed analysis of all of these systems.

Gianninas, A.; Strickland, B. D.; Kilic, Mukremin; Bergeron, P.

2013-03-01

269

DA WHITE DWARFS OBSERVED IN THE LAMOST PILOT SURVEY  

Energy Technology Data Exchange (ETDEWEB)

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.

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

270

Heavy element abundance patterns in hot DA white dwarfs  

CERN Document Server

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 abundance measurements, which can be related to the formal upper limits in those stars where particular elements are not detected. We find that the presence or absence of heavy elements in the hot DA white dwarfs largely reflects what would be expected if radiative levitation is the supporting mechanism, although the measured abundances do not match the predicted values very well, as reported by other authors in the past. Almost all stars hotter than ~50000K contain heavy elements. For most of these the spread in element abundances is quite narrow...

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

2003-01-01

271

Simplified Hydrostatic Carbon Burning in White Dwarf Interiors  

CERN Document Server

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...

Förster, Francisco; Podsiadlowski, Philipp

2010-01-01

272

Lyman-alpha wing absorption in cool white dwarf stars  

CERN Document Server

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 ...

Rohrmann, R D; Kepler, S O

2010-01-01

273

ST/STIS Spectroscopy of the White Dwarfs in the Short-Period Dwarf Novae LL And and EF Peg  

OpenAIRE

We present new HST/STIS observations of the short-period dwarf novae LL And and EF Peg during deep quiescence. We fit stellar models to the UV spectra and use optical and IR observations to determine the physical parameters of the whitedwarfs in the systems, the distances to the binaries, and the properties of thesecondary stars. Both white dwarfs are relatively cool, having T_{eff} near 15000K, and consistent with a mass of 0.6 M-sun. The white dwarf in LL And appears to be...

Howell, S.; Gansicke, B.; Szkody, P.; Sion, E.

2002-01-01

274

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

CERN Document Server

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.

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

275

The White Dwarf Cooling Sequence of the Globular Cluster Messier 4  

CERN Document Server

We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with limiting magnitude V = 30, I = 28. The white dwarf luminosity function rises sharply for I >25.5, consistent with the behaviour expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 magnitudes fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models (Hansen 1999) to fit each luminosity function yields ages of 7.3 +/- 1.5 Gyr for the disk and 12.7 +/- 0.7 Gyr for M4 (2-sigma statistical errors).

Hansen, B M S; Fahlman, G G; Gibson, B K; Ibata, R; Limongi, M; Rich, R M; Richer, H B; Shara, M M; Stetson, P B; Hansen, Brad M. S.; Brewer, James; Fahlman, Greg G.; Gibson, Brad K.; Ibata, Rodrigo; Limongi, Marco; Richer, Harvey B.; Shara, Michael M.; Stetson, Peter B.

2002-01-01

276

Faint Blue Objects in the Hubble Deep Field South Revealed: White Dwarfs, Subdwarfs, and Quasars  

CERN Document Server

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/yr and 34.9 $\\pm$ 8.0 mas/yr, and are consistent with being thick disk or halo white dwarfs located at ~2 kpc. The other faint blue objects analyzed by Mendez & Minniti do not show any significant proper motion and are inconsistent with being halo white dwarfs; they do not contribute to the Galactic dark matter. The observed population of stars and white dwarfs is consistent with standard Galactic models.

Kilic, M; Von Hippel, T; Winget, D E; Kilic, Mukremin; Hippel, Ted von

2005-01-01

277

Search for white dwarf companions of cool stars with peculiar element abundances  

International Nuclear Information System (INIS)

A search for a white dwarf companions of cool stars with peculiar element abundances was undertaken. One additional star the xi Cet, was found with a white dwarf companion. It was found that HR 1016, 56Uma, 16 Ser, have high excitation emission lines which indicate a high temperature object in the system. It is suggested that since these indications for high temperature companions were seen for all nearby Ba stars, it is highly probable that all Ba stars have white dwarf companions, and that the peculiar element abundances seen in the Ba stars are due to mass transfer. Observations, arguments and conclusions are presented. White dwarf companions were not found. Together with the Li and Be abundances and the chromospheric emission line spectra in these stars were studied. No white dwarf companions were seen for subgiant CH stars

278

SWIFT J1644+57: A WHITE DWARF TIDALLY DISRUPTED BY A 104 M? BLACK HOLE?  

International Nuclear Information System (INIS)

We propose that the remarkable object Swift J1644+57, in which multiple recurring hard X-ray flares were seen over a span of several days, is a system in which a white dwarf was tidally disrupted by an intermediate-mass black hole. Disruption of a white dwarf rather than a main-sequence star offers a number of advantages in understanding the multiple, and short, timescales seen in the light curve of this system. In particular, the short internal dynamical timescale of a white dwarf offers a more natural way of understanding the short rise times (?100 s) observed. The relatively long intervals between flares (?5 × 104 s) may also be readily understood as the period between successive pericenter passages of the remnant white dwarf. In addition, the expected jet power is larger when a white dwarf is disrupted. If this model is correct, the black hole responsible must have a mass ?5 M?.

279

Swift J1644+57: A White Dwarf Tidally Disrupted by a 104 M ? Black Hole?  

Science.gov (United States)

We propose that the remarkable object Swift J1644+57, in which multiple recurring hard X-ray flares were seen over a span of several days, is a system in which a white dwarf was tidally disrupted by an intermediate-mass black hole. Disruption of a white dwarf rather than a main-sequence star offers a number of advantages in understanding the multiple, and short, timescales seen in the light curve of this system. In particular, the short internal dynamical timescale of a white dwarf offers a more natural way of understanding the short rise times (~100 s) observed. The relatively long intervals between flares (~5 × 104 s) may also be readily understood as the period between successive pericenter passages of the remnant white dwarf. In addition, the expected jet power is larger when a white dwarf is disrupted. If this model is correct, the black hole responsible must have a mass <~ 105 M ?.

Krolik, Julian H.; Piran, Tsvi

2011-12-01

280

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

CERN Document Server

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....

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

2013-01-01

281

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

CERN Document Server

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 ...

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

2005-01-01

282

Stabilization of helium shell burning by rotation in accreting white dwarfs  

CERN Document Server

The currently favored scenario for the progenitor evolution of Type Ia supernovae (SNe Ia) presumes that white dwarfs in close binary systems grow to the Chandrasekhar limit via mass accretion from their non-degenerate companions. However, the accreted hydrogen and/or helium usually participate thermally unstable or even violent nuclear reactions in a geometrically confined region, due to the compactness of the white dwarf. Since shell flashes induced by the thermal instability might induce significant loss of mass, efficient mass increase of white dwarfs by hydrogen and/or helium accretion has been seriously questioned. A good understanding of the stability of thermonuclear shell sources is therefore crucial in order to investigate the evolution of accreting white dwarfs as SNe Ia progenitors. Here, we present a quantitative criterion for the thermal stability of thermonuclear shell sources, and discuss the effects of rotation on the stability of helium shell burning in helium accreting CO white dwarfs with ...

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

2005-01-01

283

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

CERN Document Server

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...

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

1998-01-01

284

Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements  

CERN Document Server

A photometric and spectroscopic analysis of 152 cool white dwarf stars is presented. The discovery of 7 new DA white dwarfs, 2 new DQ white dwarfs, 1 new magnetic white dwarf, and 3 weak magnetic white dwarf candidates, is reported, as well as 19 known or suspected double degenerates. The photometric energy distributions, the Halpha line profiles, and the trigonometric parallax measurements are combined and compared to model atmosphere calculations to determine the effective temperature and the radius of each object, and also to constrain the atmospheric composition. New evolutionary sequences with C/O cores with thin and thick hydrogen layers are used to derive masses and ages. We confirm the existence of a range in Teff between 5000 and 6000K where almost all white dwarfs have H-rich atmospheres. There is little evidence for mixed H/He dwarfs, with the exception of 2 He-rich DA stars, and 5 C2H white dwarfs which possibly have mixed H/He/C atmospheres. The DQ sequence terminates near 6500K, below which they...

Bergeron, P; Ruiz, M T; Ruiz, Maria Teresa

2002-01-01

285

Three New Eclipsing White-dwarf - M-dwarf Binaries Discovered in a Search for Transiting Planets Around M-dwarfs  

CERN Document Server

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 8X 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 solar radi...

Law, Nicholas M; Street, Rachel; Fulton, Benjamin J; Hillenbrand, Lynne A; Shporer, Avi; Lister, Tim; Baranec, Christoph; Bloom, Joshua S; Bui, Khanh; Burse, Mahesh P; Cenko, S Bradley; Das, H K; Davis, Jack T C; Dekany, Richard G; Filippenko, Alexei V; Kasliwal, Mansi M; Kulkarni, S R; Nugent, Peter; Ofek, Eran O; Poznanski, Dovi; Quimby, Robert M; Ramaprakash, A N; Riddle, Reed; Silverman, Jeffrey M; Sivanandam, Suresh; Tendulkar, Shriharsh

2011-01-01

286

Stellar evolution on the borderline of white dwarf and neutron star formation  

OpenAIRE

This thesis is about the evolution of stars, specifically about the final fate of stars at the borderline between the formation of white dwarfs and neutron stars. It is well known that the mass and the metallicity are the two determining factors in stellar evolution, and for a given initial chemical composition, the mass essentially defines the final fate: Lower mass stars produce white dwarfs, while higher mass stars produce neutron stars and supernovae. The transition region between white d...

Poelarends, A. J. T.

2007-01-01

287

The progenitors of magnetic white dwarfs in open clusters  

Science.gov (United States)

Context. White dwarfs are the final stages of stellar evolution for most stars in the galaxy and magnetic white dwarfs (MWDs) represent at least ten percent of the whole sample. 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 MWD sample without binary interaction or inclusion of a population of progenitor with unobservable small-scale fields. Aims: 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. This enables an independent way of determining the mass of their progenitors. The discovery of the magnetic WD 0836+201 in the Praesepe cluster prompted the question whether magnetic fields affect the IFMR. In this work we investigate this suggestion through investigations of all three known MWDs in open clusters. Methods: 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. Furthermore, we use mass-radius relations and evolutionary models to constrain the histories of the probable cluster members. Results: 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 NGC 6819-8, WD 0836+201 and estimated the mass, cooling age and the progenitor masses of the three probable MWD members of open clusters. According to our analysis, the newly identified cluster member SDSS J085523.87+164059.0 is an ultra-massive MWD of mass 1.12 ± 0.11 M?. Conclusions: We increase the sample of MWDs with known progenitor masses to ten, with the rest of the data coming from the common proper motion binaries. Our investigations show that, when effects of the magnetic fields are included in the diagnostics, the estimated properties of these cluster MWDs do not show evidence for deviations from the IFMR. Furthermore we estimate the precision of the magnetic diagnostics which would be necessary to determine quantitatively whether magnetism has any effect on the mass-loss.

Külebi, B.; Kalirai, J.; Jordan, S.; Euchner, F.

2013-06-01

288

Post-common envelope binaries from SDSS-X: The origin of low-mass white dwarfs  

CERN Document Server

We present the first white dwarf mass distributions of a large and homogeneous sample of post-common envelope binaries (PCEBs) and wide white dwarf-main sequence binaries (WDMS) directly obtained from observations. Both distributions are statistically independent, with PCEBs showing a clear concentration of systems towards the low-mass end of the distribution, and the white dwarf mass distribution of wide WDMS binaries being similar to those of single white dwarfs. Our results provide evidence that the majority of low-mass (Mwd < 0.5Msun) white dwarfs are formed in close binaries.

Rebassa-Mansergas, Alberto; Schreiber, Matthias; Girven, Jonathan; Gansicke, Boris

2010-01-01

289

Post-common envelope binaries from SDSS-X: the origin of low-mass white dwarfs  

Science.gov (United States)

We present the first white dwarf mass distributions of a large and homogeneous sample of post-common envelope binaries (PCEBs) and wide white dwarf main-sequence (WDMS) binaries directly obtained from observations. Both distributions are statistically independent, with PCEBs showing a clear concentration of systems towards the low-mass end of the distribution and the white dwarf mass distribution of wide WDMS binaries being similar to that of single white dwarfs. Our results provide evidence that the majority of low-mass (Mwd? 0.5 M?) white dwarfs are formed in close binaries.

Rebassa-Mansergas, A.; Nebot Gómez-Morán, A.; Schreiber, M. R.; Girven, J.; Gänsicke, B. T.

2011-05-01

290

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.

291

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

CERN Document Server

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 ...

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

2012-01-01

292

White dwarf masses and nova efficiency in magnetic cataclysmic variables  

International Nuclear Information System (INIS)

We show that current ideas for forming the period gap in cataclysmic variables (CVs) give a simple explanation for the spike in the observed distribution of magnetic CVs, provided that they have a bimodal distribution of white dwarf masses M1 at the start of their secular evolution. The majority should have M1 very close to a value of ? 0.6-0.7 Msolar masses and form at periods of ? 4-6 hr, with a much smaller group having larger M1 and forming at periods up to 10 hr. Nova explosions remove almost exactly the accreted mass for the low-mass group, so that M1 remains at the initial value of ? 0.6-0.7 Msolar masses while the high-mass group may lose small amounts of mass. These conclusions are supported by nova theory and offer the possibility of an observational test of gap forming hypotheses. (author)

293

THE ELM SURVEY. V. MERGING MASSIVE WHITE DWARF BINARIES  

Energy Technology Data Exchange (ETDEWEB)

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.

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

294

The Gravitational Wave Emission of White Dwarf Dynamical Interactions  

Science.gov (United States)

We compute the emission of gravitational waves of white dwarf dynamical interactions and close encounters in dense stellar environments and we compare it with the sensitivity curves of planned space-borne gravitational wave detectors, like eLISA and ALIA. We find that for the three possible outcomes of these interactions—which are the formation of an eccentric binary system, a lateral collision in which several mass transfer episodes occur, and a direct one in which just a single mass transfer episode takes place—only those in which an eccentric binary are formed are likely to be detected by the planned gravitational wave mission eLISA, while ALIA would be able to detect the gravitational wave signal emitted in lateral collisions.

Aznar-Siguán, Gabriela; García-Berro, Enrique; Lorén-Aguilar, Pablo

295

On the possible observational signatures of white dwarf dynamical interactions  

CERN Document Server

We compute the possible observational signatures of white dwarf dynamical interactions in dense stellar environments. Specifically, we compute the emission of gravitational waves, and we compare it with the sensitivity curves of planned space-borne gravitational wave detectors. We also compute the light curves for those interactions in which a detonation occurs, and one of the stars is destroyed, as well as the corresponding neutrino luminosities. We find that for the three possible outcomes of these interactions - which are the formation of an eccentric binary system, a lateral collision in which several mass transfer episodes occur, and a direct one in which just a single mass transfer episode takes place - only those in which an eccentric binary are formed are likely to be detected by the planned gravitational wave mission eLISA, while more sensitive detectors would be able to detect the signals emitted in lateral collisions. On the other hand, the light curves (and the thermal neutrino emission) of these ...

Aznar-Siguán, G; Magnien, M; Lorén-Aguilar, P

2014-01-01

296

Tidal disruption of white dwarfs by intermediate mass black holes  

Directory of Open Access Journals (Sweden)

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.

Bode T.

2012-12-01

297

Tidal disruption of white dwarfs by intermediate mass black holes  

Science.gov (United States)

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.

Laguna, P.; Haas, R.; Shcherbakov, R. V.; Bode, T.

2012-12-01

298

Cosmological physics with black holes (and possibly white dwarfs)  

Science.gov (United States)

The notion that microparsec-scale black holes can be used to probe gigaparsec-scale physics may seem counterintuitive, at first. Yet, the gravitational observatory LISA will detect cosmologically-distant coalescing pairs of massive black holes, accurately measure their luminosity distance and help identify an electromagnetic counterpart or a host galaxy. A wide variety of new black hole studies and a gravitational version of Hubble's diagram become possible, if host galaxies are successfully identified. Furthermore, if dark energy is a manifestation of large-scale modified gravity, deviations from general relativistic expectations could become apparent in a gravitational signal propagated over cosmological scales, especially when compared to the electromagnetic signal from a same source. Finally, since inspirals of white dwarfs into massive black holes at cosmological distances may permit pre-merger localizations, we suggest that careful monitoring of these events and any associated electromagnetic counterpart could lead to high-precision cosmological measurements with LISA.

Menou, Kristen; Haiman, Zoltan; Kocsis, Bence

2008-05-01

299

Deriving Precise Ages for Field White Dwarfs using Bayesian Techniques  

Science.gov (United States)

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 < 15% uncertainties, often with little sensitivity to our choice of modern initial-final mass relation. We additionally predict the capabilities of this technique to derive WD ages and atmospheric types in the GAIA era, when thousands of WDs will have distances accurate to <2%.

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

2015-01-01

300

Dynamical Tides in Compact White Dwarf Binaries: Influence of Rotation  

CERN Document Server

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...

Fuller, Jim

2014-01-01

301

Supersymmetric model for triggering supernova Ia in isolated white dwarfs  

International Nuclear Information System (INIS)

We propose a model for supernovae Ia explosions based on a phase transition to a supersymmetric state which becomes the active trigger for the deflagration starting the explosion in an isolated sub-Chandrasekhar white dwarf star. With two free parameters we fit the rate and several properties of type Ia supernovae and address the gap in the supermassive black hole mass distribution. One parameter is a critical density fit to about 3x107 g/cc while the other has the units of a space time volume and is found to be of order 0.05 Gyr RE3 where RE is the earth radius. The model involves a phase transition to an exact supersymmetry in a small core of a dense star.

302

Constraints on the gravitational constant from observations of white dwarfs  

International Nuclear Information System (INIS)

Recently some authors have questioned whether Newton's law of gravitation is actually true on scales less than 1 km. The available constraints on the gravitational constant show that its laboratory value G0 may differ from the value at infinity Gsub(infinity) by approximately 40%. Long (1976) reported experimental evidence for departures from Newton's law. In this note it is shown that the difference between G0 and Gsub(infinity) modifies the mass-radius relation of degenerate stars. The observations of white dwarfs are consistent with the theory of stellar evolution only if G0 differs from Gsub(infinity) by not more than approximately 10%. This estimate may be improved by a higher accuracy of observations. (Auth.)

303

Extremely Low-Mass White Dwarfs with Massive Companions  

Science.gov (United States)

After spending several dozen nights on the 6.5m MMT and the Kitt Peak 4m telescopes as part of the ELM Survey, we have identified 54 short period binary white dwarf (WD) systems, including half a dozen WDs with massive unseen companions. In cycle 14, we obtained Chandra observations of two of these targets and identified the first progenitors of underluminous supernovae .Ia. Here, we propose to obtain Chandra observations of the three remaining WD binary systems with massive companions. These systems have the best chance of containing neutron star companions among the 54 systems found in our survey, with a 42-65% likelihood of M>1.4 Msun companions. Our proposed three targets will provide an opportunity to study either new WD + NS systems or double WD progenitors of supernovae .Ia and Ia.

Kilic, Mukremin

2014-09-01

304

Detectability of substellar companions around white dwarfs with Gaia  

CERN Document Server

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.

Silvotti, Roberto; Lattanzi, Mario; Morbidelli, Roberto

2014-01-01

305

LOW MASS STELLAR AND SUBSTELLAR COMPANIONSHIP AMONG NEARBY WHITE DWARFS  

Directory of Open Access Journals (Sweden)

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.

M. Radiszcz

2009-01-01

306

Remnant evolution after a carbon-oxygen white dwarf merger  

Science.gov (United States)

We systematically explore the evolution of the merger of two carbon-oxygen (CO) white dwarfs. The dynamical evolution of a 0.9Msolar + 0.6Msolar CO white dwarf merger is followed by a 3D smoothed particle hydrodynamics (SPH) simulation. The calculation uses a state-of-the-art equation of state that is coupled to an efficient nuclear reaction network that accurately approximates all stages from helium burning up to nuclear statistical equilibrium. We use an elaborate prescription in which artificial viscosity is essentially absent, unless a shock is detected, and a much larger number of SPH particles than earlier calculations. Based on this simulation, we suggest that the central region of the merger remnant can, once it has reached quasi-static equilibrium, be approximated as a differentially rotating CO star, which consists of a slowly rotating cold core and a rapidly rotating hot envelope surrounded by a centrifugally supported disc. We construct a model of the CO remnant that mimics the results of the SPH simulation using a 1D hydrodynamic stellar evolution code and then follow its secular evolution, where we include the effects of rotation on the stellar structure and the transport of angular momentum. The influence of the Keplerian disc is implicitly treated by considering mass accretion from the disc on to the hot envelope. The stellar evolution models indicate that the growth of the cold core is controlled by neutrino cooling at the interface between the core and the hot envelope, and that carbon ignition in the envelope can be avoided despite high effective accretion rates. This result suggests that the assumption of forced accretion of cold matter that was adopted in previous studies of the evolution of double CO white dwarf merger remnants may not be appropriate. Specifically we find that off-centre carbon ignition, which would eventually lead to the collapse of the remnant to a neutron star, can be avoided if the following conditions are satisfied. (1) When the merger remnant reaches quasi-static equilibrium, the local maximum temperature at the interface between the core and the envelope must be lower than the critical limit for carbon ignition. (2) Angular momentum loss from the central merger remnant should not occur on a time-scale shorter than the local neutrino cooling time-scale at the interface. (3) The mass accretion rate from the centrifugally supported disc must be sufficiently low to 10-5Msolaryr-1). Our results imply that at least some products of double CO white dwarfs merger may be considered good candidates for the progenitors of Type Ia supernovae. In this case, the characteristic time delay between the initial dynamical merger and the eventual explosion would be ~105yr.

Yoon, S.-C.; Podsiadlowski, Ph.; Rosswog, S.

2007-09-01

307

Astrometric determination of white dwarf radial velocities with Gaia?  

CERN Document Server

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 measurements of Halpha and Hbeta NLTE cores exist, a purely astrometric determination would allow disentangling the gravitational redshift from the Doppler shift. The best instrument for measuring the tiny perspective acceleration is the Gaia satellite of the European Space Agency, aiming at absolute astrometric measurements of one billion stars down to 20th magnitude with unprecedented accuracy. At 15th magnitude, the predicted angular accuracy of Gaia is about 20 micro-arcseconds. In this article, we estimate whether it is possible t...

Jordan, Stefan

2012-01-01

308

THE FREQUENCY OF DEBRIS DISKS AT WHITE DWARFS  

Energy Technology Data Exchange (ETDEWEB)

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.

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

309

White Dwarfs in Globular Clusters HST Observations of M4  

CERN Document Server

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...

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

310

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.

311

The Contribution of Halo White Dwarf Binaries to the Laser Interferometer Space Antenna Signal  

Science.gov (United States)

Galactic double white dwarfs were postulated as a source of confusion limited noise for the Laser Interferometer Space Antenna (LISA), the future space-based gravitational wave observatory. Until very recently, the Galactic population consisted of a relatively well-studied disk population, a somewhat studied smaller bulge population and a mostly unknown, but potentially large halo population. It has been argued that the halo population may produce a signal that is much stronger (factor of ~5 in spectral amplitude) than the disk population. However, this surprising result was not based on an actual calculation of a halo white dwarf population, but was derived on (1) the assumption that one can extrapolate the halo population properties from those of the disk population and (2) the postulated (unrealistically) high number of white dwarfs in the halo. We perform the first calculation of a halo white dwarf population using population synthesis models. Our comparison with the signal arising from double white dwarfs in the Galactic disk+bulge clearly shows that it is impossible for the double white dwarf halo signal to exceed that of the rest of the Galaxy. Using microlensing results to give an upper limit on the content of white dwarfs in the halo (~30% baryonic mass in white dwarfs), our predicted halo signal is a factor of 10 lower than the disk+bulge signal. Even in the implausible case, where all of the baryonic halo mass is found in white dwarfs, the halo signal does not become comparable to that of the disk+bulge, and thus would still have a negligible effect on the detection of other LISA sources.

Ruiter, Ashley J.; Belczynski, Krzysztof; Benacquista, Matthew; Holley-Bockelmann, Kelly

2009-03-01

312

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

Directory of Open Access Journals (Sweden)

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

Córsico A.H.

2013-03-01

313

Recent Progress Characterizing Convection Using White Dwarf Light Curves from the Whole Earth Telescope  

Science.gov (United States)

Convection remains of the largest sources of theoretical uncertainties in our understanding of stars and other natural phenomena. Montgomery (2005b) shows how precise observations of white dwarf light curves are used to observationally determine the depth of the pulsator's convection zone and its sensitivity to changes in temperature. The Whole Earth Telescope (WET) and the Delaware Asteroseismic Research Center (DARC) are currently conducting a project to map convection across the white dwarf instability strips. We present preliminary results for the DA BPM 31594 and the DB EC 04207-4748 and show the current status of our project mapping convection across the hydrogen white dwarf instability strip.

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

2013-01-01

314

White dwarfs in the UKIDSS Large Area Survey: The Substellar Companion Fraction  

CERN Document Server

We present a near-infrared photometric search for unresolved substellar companions and debris disks around white dwarfs in the UKIRT Infrared Deep Sky Survey Large Area Survey. We cross-correlate the SDSS DR4 and McCook & Sion catalogues of white dwarfs with the UKIDSS DR8 producing 3109 and 163 unique matches respectively. Cooling models are fitted to the optical photometry of a subsample of DA white dwarfs and extended to the near-infrared. A comparison is then made with the observed photometry to identify those stars with a near-infrared excess consistent with the presence of a cool companion or debris disk. Where present, we have estimated the approximate spectral type of any putative companion, or an upper limit on the temperature of a debris disk. In total we identify 14-16 new candidate white dwarf + very low mass stellar systems, 9-11 candidate white dwarf + brown dwarf systems, and 3 candidate white dwarf + debris disks. We place lower limits on the unresolved (0.4+/-0.3%, f(WD+dT)>0.2% and f(WD+...

Steele, P R; Dobbie, P D; Jameson, R F; Barstow, M A; Satterthwaite, R P

2011-01-01

315

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.

316

HST/STIS spectroscopy of the exposed white dwarf in the short-period dwarf nova EK TrA  

CERN Document Server

We present high resolution Hubble Space Telescope ultraviolet spectroscopy of the dwarf nova EK TrA obtained in deep quiescence. The Space Telescope Imaging Spectrograph data reveal the broad Ly-alpha absorption profile typical of a moderately cool white dwarf, overlayed by numerous broad emission lines of He, C, N, and Si and by a number of narrow absorption lines, mainly of CI and SiII. Assuming a white dwarf mass in the range 0.3-1.4Msun we derive Teff=17500-23400K for the primary in EK TrA; Teff=18800K for a canonical mass of 0.6Msun. From the narrow photospheric absorption lines, we measure the white dwarf rotational velocity, v*sin i=200+-100kms. Even though the strong contamination of the photospheric white dwarf absorption spectrum by the emission lines prevents a detailed quantitative analysis of the chemical abundances of the atmosphere, the available data suggest slightly sub-solar abundances. The high time resolution of the STIS data allows us to associate the observed ultraviolet flickering with ...

Gänsicke, B T; Sion, E M; Hoard, D W; Howell, S; Cheng, F H; Hubeny, I

2001-01-01

317

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

318

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 white dwarf matter by a primordial black hole are considered. The possibility of detecting these neutrinos and their oscillations is discussed

319

Resonance model for high-frequency QPOs in white dwarfs, neutron stars and black holes  

Science.gov (United States)

High-frequency QPOs reflect non-linear, and likely resonant, oscillations in accretion disks. In black holes, and probably in neutron stars, but not in white dwarfs, strong gravity plays a crucial role in their formation.

Klu?niak, W?odzimierz

2008-05-01

320

ECHOES OF A DECAYING PLANETARY SYSTEM: THE GASEOUS AND DUSTY DISKS SURROUNDING THREE WHITE DWARFS  

International Nuclear Information System (INIS)

We have performed a comprehensive ground-based observational program aimed at characterizing the circumstellar material orbiting three single white dwarf stars previously known to possess gaseous disks. Near-infrared imaging unambiguously detects excess infrared emission toward Ton 345 and allows us to refine models for the circumstellar dust around two of the three white dwarf stars. We find that each white dwarf hosts gaseous and dusty disks that are roughly spatially coincident, a result that is consistent with a scenario in which dusty and gaseous material has its origin in remnant parent bodies of the white dwarfs' planetary systems. We briefly describe a new model for the gas disk heating mechanism in which the gaseous material behaves like a 'Z II' region. In this Z II region, gas primarily composed of metals is photoionized by ultraviolet light and cools through optically thick allowed Ca II-line emission.

321

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

CERN Document Server

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-atmosphere) white dwarfs. Our exciting main result is that, given the right location in parameter space, some carbon-atmosphere white dwarfs are predicted to show pulsational instability against gravity modes. We are eagerly waiting the results of observational searches for luminosity variations in these stars.

Fontaine, G; Dufour, P

2008-01-01

322

A New Very Cool White Dwarf Discovered by the Sloan Digital Sky Survey  

CERN Document Server

Early data taken during commissioning of the SDSS have resulted in the discovery of a very cool white dwarf. It appears to have stronger collision induced absorption from molecular hydrogen than any other known white dwarf, suggesting it has a cooler temperature than any other. While its distance is presently unknown, it has a surprisingly small proper motion, making it unlikely to be a halo star. An analysis of white dwarf cooling times suggests that this object may be a low-mass star with a helium core. The SDSS imaging and spectroscopy also recovered LHS 3250, the coolest previously known white dwarf, indicating that the SDSS will be an effective tool for identifying these extreme objects.

Harris, H C; Liebert, J; Vanden Berk, Daniel E; Anderson, S F; Knapp, G R; Fan, X; Margon, B; Munn, J A; Nichol, R C; Pier, J R; Schneider, D P; Smith, J A; Winget, D E; York, D G; Brinkmann, J; Burles, S M; Chen, B; Connolly, A J; Csabai, I; Frieman, Joshua A; Gunn, J E; Hennessy, G S; Hindsley, R B; Ivezic, Z; Kent, S; Lamb, D Q; Lupton, R H; Newberg, H J; Schlegel, D J; Smee, S; Strauss, M A; Thakar, A R; Uomoto, A; Yanny, B

2001-01-01

323

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

CERN Document Server

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.

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

2015-01-01

324

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

325

Non-thermal electromagnetic radiation from pulsating and collapsing magnetized white dwarfs  

Energy Technology Data Exchange (ETDEWEB)

The non-thermal electromagnetic radiation from pulsating and collapsing magnetized white dwarfs is considered. This radiation generates when the star magnetosphere compresses during collapse and its magnetic field greatly increases. The electric field produced involves acceleration of charged particles, which generates radiation when moving in the magnetic field. Thus the pulsating and collapsing magnetized white dwarfs can be powerful sources of electromagnetic pulses. These pulses can be observed by means of modern instruments (radio, X- and gamma- telescopes).

Kryvdyk, Volodymyr, E-mail: kryvdyk@univ.kiev.u [Department Astronomy, Kyiv National University, av. Glushkova 2/1, Kyiv 03022 (Ukraine)

2009-06-01

326

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

OpenAIRE

White dwarfs with hydrogen-rich atmospheres (DA) are the most abundant of all degenerate objects. In recent years work has been dedicated to increase the accuracy of their model atmospheres. Most notably, convective motions are now treated with 3D radiation-hydrodynamics instead of the standard mixing-length theory. We present and describe selected 3D model atmospheres close and within the instability strip of the pulsating ZZ Ceti white dwarfs. Our 3D simulations depend only weakly on numeri...

-e, Tremblay P.; -g, Ludwig H.

2013-01-01

327

Merging White Dwarf/Black Hole Binaries and Gamma-Ray Bursts  

OpenAIRE

The merger of compact binaries, especially black holes and neutron stars, is frequently invoked to explain gamma-ray bursts (GRB's). In this paper, we present three dimensional hydrodynamical simulations of the relatively neglected mergers of white dwarfs and black holes. During the merger, the white dwarf is tidally disrupted and sheared into an accretion disk. Nuclear reactions are followed and the energy release is negligible. Peak accretion rates are ~0.05 Msun/s (less f...

Fryer, C. L.; Woosley, S. E.; Herant, M.; Davies, Melvyn B.

1998-01-01

328

White dwarf mergers and the origin of R Coronae Borealis stars  

CERN Document Server

We present a nucleosynthesis study of the merger of a 0.4 solar masses helium white dwarf with a 0.8 solar masses carbon-oxygen white dwarf, coupling the thermodynamic history of Smoothed Particle Hydrodynamics particles with a post-processing code. The resulting chemical abundance pattern, particularly for oxygen and fluorine, is in qualitative agreement with the observed abundances in R Coronae Borealis stars.

Lorén-Aguilar, P; José, J; García-Berro, E; Althaus, L G; Isern, J

2011-01-01

329

Radiative activity of magnetic white dwarf undergoing Lorentz-force-driven torsional vibrations  

OpenAIRE

We study radiative activity of magnetic white dwarf undergoing torsional vibrations about axis of its own dipole magnetic moment under the action of Lorentz restoring force. It is shown that pulsating white dwarf can convert its vibration energy into the energy of magneto-dipole emission, oscillating with the frequency equal to the frequency of Alfv\\'en torsional vibrations, provided that internal magnetic field is decayed. The most conspicuous feature of the vibration energ...

Bastrukov, S. I.; Yu, J. W.; Xu, R. X.; Molodtsova, I. V.

2010-01-01

330

The accretion-diffusion scenario for metals in cool white dwarfs  

OpenAIRE

We calculated diffusion timescales for Ca, Mg, Fe in hydrogen atmosphere white dwarfs with temperatures between 5000 and 25000 K. With these timescales we determined accretion rates for a sample of 38 DAZ white dwarfs from the recent studies of Zuckerman et al. (2003) and Koester et al. (2005). Assuming that the accretion rates can be calculated with the Bondi-Hoyle formula for hydrodynamic accretion, we obtained estimates for the interstellar matter density around the accre...

Koester, D.; Wilken, D.

2006-01-01

331

IRTF Observations of White Dwarfs with Possible Near-Infrared Excess  

OpenAIRE

Near-infrared photometry and spectroscopy is obtained for a heterogeneous sample of nearby white dwarfs with possible excess flux as identified primarily in the Two Micron All Sky Survey. Among the sample of 43 stars are a number of white dwarfs that are either metal-rich, magnetic, or binary suspects. With a few notable exceptions in four (or possibly five) distinct categories, the newly obtained JHK photometric data fail to corroborate the putative excesses, with K_IRTF - ...

Farihi, J.

2009-01-01

332

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

CERN Document Server

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.

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

333

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

Science.gov (United States)

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.

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

334

The white dwarf cooling sequence of NGC 6791: an unique tool for stellar astrophysics  

CERN Document Server

NGC 6791 is a well-studied, metal-rich open cluster that is so close to us that can be imaged down to luminosities fainter than that of the termination of its white dwarf cooling sequence, thus allowing for an in-depth study of its white dwarf population. We use a Monte Carlo simulator that employs up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The cooling sequences for carbon-oxygen cores account for the delays introduced by both Ne^22 sedimentation in the liquid phase and by carbon-oxygen phase separation upon crystallization. We do not find evidence for a substantial fraction of helium-core white dwarfs, and hence our results support the suggestion that the origin of the bright peak of the white dwarf luminosity function can only be attributed to a population of unresolved binary white dwarfs. Moreover, our results indicate that the number distribution of secondary masses of the population of unresolved ...

García-Berro, E; Renedo, I; Camacho, J; Althaus, L G; Córsico, A H; Salaris, M; Isern, J

2011-01-01

335

The DB gap and a new class of pulsating white dwarfs  

Directory of Open Access Journals (Sweden)

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.

Shibahashi H.

2013-03-01

336

Merging White Dwarf/Black Hole Binaries and Gamma-Ray Bursts  

Science.gov (United States)

The merger of compact binaries, especially black holes and neutron stars, is frequently invoked to explain gamma-ray bursts (GRBs). In this paper, we present three-dimensional hydrodynamical simulations of the relatively neglected mergers of white dwarfs and black holes. During the merger, the white dwarf is tidally disrupted and sheared into an accretion disk. Nuclear reactions are followed, and the energy release is negligible. Peak accretion rates are ~0.05 Msolar s-1 (less for lower mass white dwarfs) and last for approximately a minute. Many of the disk parameters can be explained by a simple analytic model that we derive and compare to our simulations. This model can be used to predict accretion rates for white dwarf and black hole (or neutron star) masses that are not simulated here. Although the mergers studied here create disks with larger radii and longer accretion times than those from the merger of double neutron stars, a larger fraction of the white dwarf's mass becomes part of the disk. Thus the merger of a white dwarf and a black hole could produce a long-duration GRB. The event rate of these mergers may be as high as 10-6 yr-1 per galaxy.

Fryer, Chris L.; Woosley, S. E.; Herant, Marc; Davies, Melvyn B.

1999-08-01

337

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

CERN Document Server

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...

Jeffery, C Simon; Saio, Hideyuki

2011-01-01

338

On the DB gap of white dwarf evolution: effects of hydrogen mass fraction and convective overshooting  

International Nuclear Information System (INIS)

We investigate the spectral evolution of white dwarfs by considering the effects of hydrogen mass in the atmosphere and convective overshooting above the convection zone. Our numerical results show that white dwarfs with MH ? 10-16 Mo-dot show the DA spectral type between 46000 ?eff ?H ? 10-15 Mo-dot appear as DA stars until they cool to Teff ? 31000 K; from then on they will evolve into DB white dwarfs as a result of convective mixing. If MH in the white dwarfs is more than 10-14 Mo-dot, the convective mixing will not occur when Teff > 20000 K, thus these white dwarfs always appear as DA stars. White dwarfs within the temperature range 46000 ?eff ?< 31000 K always show the DA spectral type, which coincides with the DB gap. We notice the importance of the convective overshooting and suggest that the overshooting length should be proportional to the thickness of the convection zone to better fit the observations.

339

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

Science.gov (United States)

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.

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

2008-03-01

340

What is the origin of the hottest known white dwarf?  

Science.gov (United States)

KPD0005+5106 is the hottest known white dwarf {Teff=200,000 K}. It has a helium-dominated atmosphere with trace amounts of light metals. Our recent analysis places the star in a region of the HRD where very weak stellar winds prevent gravitational settling of heavy elements. Therefore, the surface composition allows to conclude on the evolutionary history. We speculate that KPD0005+5106 represents a third post-AGB evolutionary sequence, which is distinct from the canonical hydrogen-rich and hydrogen-deficient sequences. The star could be the long-searched progeny of the RCrB stars, which are believed to be the result of a double-degenerate merger. We found that the metal abundances in KPD0005+5106 are rather similar to those in RCrB stars, however, a claim on the evolutionary link is premature. The abundance determinations for KPD0005+5106 are afflicted with large errors because of the relatively large uncertainty in Teff. Our immediate aim is to use ionization balances of metals in order to determine Teff to high precision, which is a prerequisite to tightly constrain abundances. HST-UV spectroscopy is the only means to reach this goal.

Werner, Klaus

2011-10-01

341

Every interacting double white dwarf binary may merge  

CERN Document Server

Interacting double white dwarf binaries can give rise to a wide variety of astrophysical outcomes ranging from faint thermonuclear and Type Ia supernovae to the formation of neutron stars and stably accreting AM Canum Venaticorum systems. One key factor affecting the final outcome is whether mass transfer remains dynamically stable or instead diverges, leading to the tidal disruption of the donor and the merger of the binary. It is typically thought that for low ratios of the donor mass to the accretor mass, mass transfer remains stable, especially if accretion occurs via a disk. In this Letter, we examine disk-accreting binaries with extremely low mass ratios and find that the initial phase of hydrogen-rich mass transfer leads to a classical nova-like outburst on the accretor. Dynamical friction within the expanding nova shell shrinks the orbit and causes the mass transfer rate to increase dramatically above the accretor's Eddington limit, resulting in a binary merger. While further calculations are necessar...

Shen, Ken J

2015-01-01

342

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

343

The accretion and spreading of matter on white dwarfs  

CERN Document Server

For a slowly rotating non-magnetized white dwarf the accretion disk extends all the way to the star. Here the matter impacts and spreads towards the poles as new matter continuously piles up behind it. We have solved the 3d compressible Navier-Stokes equations on an axisymmetric grid to determine the structure of this boundary layer for different viscosities corresponding to different accretion rates. The high viscosity cases show a spreading BL which sets off a gravity wave in the surface matter. The accretion flow moves supersonically over the cusp making it susceptible to the rapid development of gravity wave and/or Kelvin-Helmholtz instabilities. This BL is optically thick and extends more than 30 degrees to either side of the disk plane after 3/4 of a Keplerian rotation period (t=19s). The low viscosity cases also show a spreading BL, but here the accretion flow does not set off gravity waves and it is optically thin.

Fisker, J L; Bürger, T; Fisker, Jacob Lund; Balsara, Dinshaw S.; Burger, Tom

2005-01-01

344

He-accreting white dwarfs: accretion regimes and final outcomes  

Science.gov (United States)

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.

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

2014-12-01

345

White dwarfs as the maximal soft x-ray scatterers  

Energy Technology Data Exchange (ETDEWEB)

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.

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

346

White-Dwarf Kicks and Implications for Barium Stars  

CERN Document Server

The formation mechanism of the barium stars is thought to be well understood. Barium-rich material, lost in a stellar wind from a thermally-pulsing asymptotic-giant branch star in a binary system, is accreted by its companion main-sequence star. Now, many millions of years later, the primary is an unseen white dwarf and the secondary has itself evolved into a giant which displays absorption lines of barium in its spectrum and is what we call a barium star. A similar wind-accretion mechanism is also thought to form the low-metallicity CH and carbon-enhanced metal-poor stars. Qualitatively the picture seems clear but quantitatively it is decidedly murky: several key outstanding problems remain which challenge our basic understanding of binary-star physics. Barium stars with orbital periods less than about 4,000 days should -- according to theory -- be in circular orbits because of tidal dissipation, yet they are often observed to be eccentric. Only one barium-star period longer than 10^4 days has been published...

Izzard, Robert G; Church, Ross P

2010-01-01

347

COSMOLOGICAL FAST RADIO BURSTS FROM BINARY WHITE DWARF MERGERS  

Energy Technology Data Exchange (ETDEWEB)

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 ?10{sup 4} sky{sup –1} day{sup –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.

Kashiyama, Kazumi; Mészáros, Peter [Department of Astronomy and Astrophysics, Department of Physics, Center for Particle and Gravitational Astrophysics, Pennsylvania State University, University Park, PA 16802 (United States); Ioka, Kunihito, E-mail: kzk15@psu.edu, E-mail: nnp@psu.edu, E-mail: kunihito.ioka@kek.jp [Theory Center, Institute of Particle and Nuclear Studies, KEK, Department of Particle and Nuclear Physics, the Graduate University for Advanced Studies (Sokendai), Tsukuba 305-0801 (Japan)

2013-10-20

348

A Spectroscopic Analysis of White Dwarfs in the Kiso Survey  

CERN Document Server

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....

Limoges, M -M

2010-01-01

349

Turbulent Mixing on Helium-Accreting White Dwarfs  

CERN Document Server

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...

Piro, Anthony L

2015-01-01

350

The Viscous Evolution of White Dwarf Merger Remnants  

CERN Document Server

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...

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

2012-01-01

351

The structure and fate of white dwarf merger remnants  

CERN Document Server

We present a large parameter study where we investigate the structure of white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD masses and compositions are explored and we also probe the effect of different initial conditions. We investigated the degree of mixing between the WDs, the conditions for detonations as well as the amount of gas ejected. We find that systems with lower mass ratios have more total angular momentum and as a result more mass is flung out in a tidal tail. Nuclear burning can affect the amount of mass ejected. Many WD binaries that contain a helium-rich WD achieve the conditions to trigger a detonation. In contrast, for carbon-oxygen transferring systems only the most massive mergers with a total mass above ~2.1 solar masses detonate. Even systems with lower mass may detonate long after the merger if the remnant remains above the Chandrasekhar mass and carbon is ignited at the centre. Finally, our findings are discussed in the context of several possible observed ...

Dan, M; Brueggen, M; Podsiadlowski, P

2013-01-01

352

Dynamical tides in compact white dwarf binaries: influence of rotation  

Science.gov (United States)

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.

Fuller, Jim; Lai, Dong

2014-11-01

353

A Search for Habitable Planets Around White Dwarfs  

Science.gov (United States)

A large fraction of white dwarfs (WDs) may host planets in their habitable zones. Here we propose to take advantage of the unique capability of DECam on the CTIO 4m to perform a pre-LSST transit survey that is capable of detecting the first planet in the habitable zone of a WD. We propose to obtain DECam time-series photometry of the CFHT Legacy Survey (CFHTLS) Wide2 field to search for planetary, substellar, and stellar companions to more than 500 WDs. Thanks to the 3 square degree field of view of DECam, for the first time in history, a large number of WDs can be observed at the same time, which is essential for discovering transits. We will use the original data taken 8 years ago for the CFHTLS and our stacked DECam images to identify the WDs based on their colors and proper motions. Given the observing window from the ground, if every WD has an Earth-like planet in its habitable zone, we expect to find up to 3 planets in this initial survey. In addition to planets, we will also constrain the frequency of eclipsing substellar and stellar companions to WDs, which has important implications for the LSST. Our stacked dataset will also provide a significantly improved WD luminosity function for the Galactic disk and halo.

Kilic, Mukremin; Barber, Sara; Januzzi, Buell; Dey, Arjun; Stetson, Peter

2014-02-01

354

The Long-Term Evolution of Double White Dwarf Mergers  

CERN Document Server

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...

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

2011-01-01

355

The Shortest Period Detached Binary White Dwarf System  

CERN Document Server

We identify SDSS J010657.39-100003.3 (hereafter J0106-1000) as the shortest period detached binary white dwarf (WD) system currently known. We targeted J0106-1000 as part of our radial velocity program to search for companions around known extremely low-mass (ELM, ~ 0.2 Msol) WDs using the 6.5m MMT. We detect peak-to-peak radial velocity variations of 740 km/s with an orbital period of 39.1 min. The mass function and optical photometry rule out a main-sequence star companion. Follow-up high-speed photometric observations obtained at the McDonald 2.1m telescope reveal ellipsoidal variations from the distorted primary but no eclipses. This is the first example of a tidally distorted WD. Modeling the lightcurve, we constrain the inclination angle of the system to be 67 +- 13 deg. J0106-1000 contains a pair of WDs (0.17 Msol primary + 0.43 Msol invisible secondary) at a separation of 0.32 Rsol. The two WDs will merge in 37 Myr and most likely form a core He-burning single subdwarf star. J0106-1000 is the shortest...

Kilic, Mukremin; Kenyon, S J; Prieto, Carlos Allende; Andrews, J; Kleinman, S J; Winget, K I; Winget, D E; Hermes, J J

2011-01-01

356

The ELM Survey. II. Twelve Binary White Dwarf Merger Systems  

CERN Document Server

We describe new radial velocity and X-ray observations of extremely low-mass white dwarfs (ELM WDs, ~0.2 Msol) in the Sloan Digital Sky Survey Data Release 4 and the MMT Hypervelocity Star survey. We identify four new short period binaries, including two merger systems. These observations bring the total number of short period binary systems identified in our survey to 20. No main-sequence or neutron star companions are visible in the available optical photometry, radio, and X-ray data. Thus, the companions are most likely WDs. Twelve of these systems will merge within a Hubble time due to gravitational wave radiation. We have now tripled the number of known merging WD systems. We discuss the characteristics of this merger sample and potential links to underluminous supernovae, extreme helium stars, AM CVn systems, and other merger products. We provide new observational tests of the WD mass-period distribution and cooling models for ELM WDs. We also find evidence for a new formation channel for single low-mas...

Kilic, Mukremin; Prieto, Carlos Allende; Agueros, M A; Heinke, Craig; Kenyon, S J

2010-01-01

357

Death by Dynamics: Planetoid-Induced Explosions on White Dwarfs  

CERN Document Server

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...

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

2015-01-01

358

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

Energy Technology Data Exchange (ETDEWEB)

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.

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

359

The Nature of Millisecond Pulsars with Helium White Dwarf Companions  

CERN Document Server

We examine the growing data set of binary millisecond pulsars that are thought to have a helium white dwarf companion. These systems are believed to form when a low- to intermediate-mass companion to a neutron star fills its Roche lobe between central hydrogen exhaustion and core helium ignition. We confirm that our own stellar models reproduce a well-defined period-companion mass relation irrespective of the details of the mass transfer process. With magnetic braking this relation extends to periods of less than 1d for a 1Msun giant donor. With this and the measured binary mass functions we calculate the orbital inclination of each system for a given pulsar mass. We expect these inclinations to be randomly oriented in space. If the masses of the pulsars were typically 1.35Msun then there would appear to be a distinct dearth of high-inclination systems. However if the pulsar masses are more typically 1.55 to 1.65Msun then the distribution of inclinations is indeed indistinguishable from random. If it were as ...

Smedley, Sarah L; Ferrario, Lilia; Wickramasinghe, Dayal T

2013-01-01

360

Dusty Disks around White Dwarfs I: Origin of Debris Disks  

CERN Document Server

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 ...

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

2010-01-01

361

On the white dwarf cooling sequence with extremely large telescopes  

CERN Document Server

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...

Bono, G; Gilmozzi, R

2012-01-01

362

Xuv spectroscopic observation of the white dwarf star HZ 43  

International Nuclear Information System (INIS)

A soft x-ray objective grating spectrometer utilizing grazing incidence optics was designed for astronomical observations, assembled at Stanford, calibrated, and flown in 1983. The target was the hot white dwarf star HZ 43, which is of particular interest because it is one of the two brightest nonsolar extreme ultraviolet sources in the sky. The emission is thought to be thermal in origin, with an effective temperature of 60,000 degrees. The basic experimental configuration involved a confocal, coaxial pail of nested Woelter-Schwarzschild Type I telescopes, an annulus of free standing gold 500 I/mm transmission gratings, and a microchannel plate detector in the focal plane with proximity focused resistive anode readout. The telescopes were manufactured using the single diamond point turning technique at Lawrence Livermore National Lab. The point response function was measured in the visible. The angular size (full width at half maximum) was 25 arc seconds with a sharp central core of less than 5 arc second angular extent. The transmission gratings were supplied by the co-investigators at the Astronomical Institute of the Netherlands. The instrument as a whole was calibrated in a vacuum facility at U. C. Berkeley

363

Measuring White Dwarf Accretion Rates via their Effective Temperatures  

CERN Document Server

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.

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

2003-01-01

364

On the possible observational signatures of white dwarf dynamical interactions  

Science.gov (United States)

We compute the possible observational signatures of white dwarf dynamical interactions in dense stellar environments. Specifically, we compute the emission of gravitational waves, and we compare it with the sensitivity curves of planned space-borne gravitational wave detectors. We also compute the light curves for those interactions in which a detonation occurs, and one of the stars is destroyed, as well as the corresponding neutrino luminosities. We find that for the three possible outcomes of these interactions - which are the formation of an eccentric binary system, a lateral collision in which several mass transfer episodes occur, and a direct one in which just a single mass transfer episode takes place - only those in which an eccentric binary are formed are likely to be detected by the planned gravitational wave mission eLISA, while more sensitive detectors would be able to detect the signals emitted in lateral collisions. On the other hand, the light curves (and the thermal neutrino emission) of these interactions are considerably different, producing both very powerful outbursts and low-luminosity events. Finally, we also calculate the X-ray signature produced in the aftermath of those interactions for which a merger occurs. We find that the temporal evolution follows a power law with the same exponent found in the case of the mergers of two neutron stars, although the total energy released is smaller.

Aznar-Siguán, G.; García-Berro, E.; Magnien, M.; Lorén-Aguilar, P.

2014-09-01

365

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

CERN Document Server

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...

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

2014-01-01

366

The Discovery of Binary White Dwarfs that will Merge within 500 Myr  

CERN Document Server

We present radial velocity observations of four extremely low-mass (0.2 Msol) white dwarfs. All four stars show peak-to-peak radial velocity variations of 540 - 710 km/s with 1.0 - 5.9 hr periods. The optical photometry rules out main-sequence companions. In addition, no milli-second pulsar companions are detected in radio observations. Thus the invisible companions are most likely white dwarfs. Due to the loss of angular momentum through gravitational radiation, three of the systems will merge within 500 Myr. The remaining system will merge within a Hubble time. The mass functions for three of the systems imply companions more massive than 0.44 Msol; thus those are carbon/oxygen core white dwarfs. However, the chance of a supernova Ia event is only 1% to 5%. These systems will most likely form single R Coronae Borealis stars, providing evidence for a white dwarf + white dwarf merger mechanism for these unusual objects. One of the systems, SDSS J105353.89+520031.0 has a 70% chance of having a low-mass white d...

Kilic, Mukremin; Prieto, Carlos Allende; Kenyon, S J

2009-01-01

367

A photometric selection of White Dwarf candidates in SDSS DR10  

CERN Document Server

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...

Fusillo, Nicola Pietro Gentile; Greiss, Sandra

2015-01-01

368

Post-common-envelope binaries from SDSS - V. Four eclipsing white dwarf main-sequence binaries  

Science.gov (United States)

We identify SDSS011009.09+132616.1, SDSS030308.35+005444.1, SDSS143547.87+ 373338.5 and SDSS154846.00+405728.8 as four eclipsing white dwarf plus main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS), and report on follow-up observations of these systems. SDSS0110+1326, SDSS1435+3733 and SDSS1548+4057 contain DA white dwarfs, while SDSS0303+0054 contains a cool DC white dwarf. Orbital periods and ephemerides have been established from multiseason photometry. SDSS1435+3733, with Porb = 3h has the shortest orbital period of all known eclipsing WDMS binaries. As for the other systems, SDSS0110+1326 has Porb = 8h, SDSS0303+0054 has Porb = 3.2h and SDSS1548+4057 has Porb = 4.4h. Time-resolved spectroscopic observations have been obtained and the H? and CaII??8498.02, 8542.09, 8662.14 triplet emission lines, as well as the NaI??8183.27, 8194.81 absorption doublet were used to measure the radial velocities of the secondary stars in all four systems. A spectral decomposition/fitting technique was then employed to isolate the contribution of each of the components to the total spectrum, and to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light-curve modelling code for close binary systems to fit the eclipse profiles and the ellipsoidal modulation/reflection effect in the light curves, to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of MWD ~ 0.4-0.6Msolar, in line with the expectations from close binary evolution. The DC white dwarf in SDSS0303+0054 has a mass of MWD >~ 0.85Msolar, making it unusually massive for a post-common-envelope system. The companion stars in all four systems are M dwarfs of spectral type M4 and later. Our new additions raise the number of known eclipsing WDMS binaries to 14, and we find that the average white dwarf mass in this sample is MWD> =0.57 +/- 0.16Msolar, only slightly lower than the average mass of single white dwarfs. The majority of all eclipsing WDMS binaries contain low-mass (<0.6Msolar) secondary stars, and will eventually provide valuable observational input for the calibration of the mass-radius relations of low-mass main-sequence stars and of white dwarfs.

Pyrzas, S.; Gänsicke, B. T.; Marsh, T. R.; Aungwerojwit, A.; Rebassa-Mansergas, A.; Rodríguez-Gil, P.; Southworth, J.; Schreiber, M. R.; Nebot Gomez-Moran, A.; Koester, D.

2009-04-01

369

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences. I. Adiabatic properties  

Science.gov (United States)

Context. Many low-mass white dwarfs with masses M?/M? ? 0.45, including the so-called extremely low-mass white dwarfs (M?/M? ? 0.20 - 0.25), have recently been discovered in the field of our Galaxy through dedicated photometric surveys. The subsequent discovery of pulsations in some of them has opened the unprecedented opportunity of probing the internal structure of these ancient stars. Aims: We present a detailed adiabatic pulsational study of these stars based on full evolutionary sequences derived from binary star evolution computations. The main aim of this study is to provide a detailed theoretical basis of reference for interpreting present and future observations of variable low-mass white dwarfs. Methods: Our pulsational analysis is based on a new set of He-core white-dwarf models with masses ranging from 0.1554 to 0.4352 M? derived by computing the non-conservative evolution of a binary system consisting of an initially 1 M? ZAMS star and a 1.4 M? neutron star. We computed adiabatic radial (? = 0) and non-radial (? = 1,2) p and g modes to assess the dependence of the pulsational properties of these objects on stellar parameters such as the stellar mass and the effective temperature, as well as the effects of element diffusion. Results: We found that for white dwarf models with masses below ~ 0.18 M?, g modes mainly probe the core regions and p modes the envelope, therefore pulsations offer the opportunity of constraining both the core and envelope chemical structure of these stars via asteroseismology. For models with M? ? 0.18 M?, on the other hand, g modes are very sensitive to the He/H compositional gradient and therefore can be used as a diagnostic tool for constraining the H envelope thickness. Because both types of objects have not only very distinct evolutionary histories (according to whether the progenitor stars have experienced CNO-flashes or not), but also have strongly different pulsation properties, we propose to define white dwarfs with masses below ~ 0.18 M? as ELM (extremely low-mass) white dwarfs, and white dwarfs with M? ? 0.18 M? as LM (low-mass) white dwarfs.

Córsico, A. H.; Althaus, L. G.

2014-09-01

370

Intermediate polars in the Swift/BAT survey: spectra and white dwarf masses  

Science.gov (United States)

Context: White dwarf masses in cataclysmic variables are difficult to determine accurately, but are fundamental for understanding binary system parameters, as well as binary evolution. Aims: We investigate the X-ray spectral properties of a sample of Intermediate Polars (IP) detected above 15 keV to derive the masses of their accreting white dwarfs. Methods: We use data from the Swift/BAT instrument which during the first 2.5 yrs of operation has detected 22 known intermediate polars. The X-ray spectra of these sources are used to estimate the mass of the white dwarfs. Results: We are able to produce a mass estimate for 22 out of 29 of the confirmed intermediate polars. Comparison with previous mass measurements shows good agreement. For GK Per, we were able to detect spectral changes due to the changes in the accretion rate. Conclusions: The Swift/BAT detector with its combination of sensitivity and all-sky coverage provides an ideal tool to determine accurate white dwarf masses in intermediate polars. This method should be applied to other magnetic white dwarf binaries.

Brunschweiger, J.; Greiner, J.; Ajello, M.; Osborne, J.

2009-03-01

371

WHITE-DWARF-MAIN-SEQUENCE BINARIES IDENTIFIED FROM THE LAMOST PILOT SURVEY  

Energy Technology Data Exchange (ETDEWEB)

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.

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

372

HST Observations of the White Dwarf Cooling Sequence of M4  

CERN Document Server

We investigate in detail the white dwarf cooling sequence of the globular cluster Messier 4. In particular we study the influence of various systematic uncertainties, both observational and theoretical, on the determination of the cluster age from the white dwarf cooling sequence. These include uncertainties in the distance to the cluster and the extinction along the line of sight, as well as the white dwarf mass, envelope and core compositions and the white dwarf --main sequence mass relation. We find that fitting to the full two-dimensional colour-magnitude diagram offers a more robust method for age determination than the traditional method of fitting the one-dimensional white dwarf luminosity function. After taking into account the various uncertainties, we find a best fit age of 12.1 Gyr, with a 95% lower limit of 10.3 Gyr. We also perform fits using two other sets of cooling models from the literature. The models of Chabrier et al (2000) yield an encouragingly similar result, although the models of Sala...

Hansen, B; Fahlman, G; Stetson, P; Brewer, J; Currie, T; Gibson, B; Ibata, R; Rich, R M; Shara, M M; Hansen, Brad; Richer, Harvey; Fahlman, Greg; Stetson, Peter; Brewer, James; Currie, Thayne; Gibson, Brad; Ibata, Rodrigo

2004-01-01

373

The masses of the millisecond pulsar J1012+5307 and its white-dwarf companion  

CERN Document Server

We report on spectroscopy of the white-dwarf companion of the millisecond radio pulsar PSR J1012+5307. We find strong Balmer absorption lines, as would be expected for a cool DA white dwarf. The profiles are much narrower than usual, however, and lines are seen up to H12 indicating that the companion has a low gravity and hence a low mass. This is consistent with the expectation---based on evolutionary considerations and on the mass function---that it is a low-mass white dwarf with a helium core. By comparing the spectra to model atmospheres, we derive an effective temperature T_{\\rm{}eff}=8550\\pm25 \\,K and a surface gravity \\log{}g=6.75\\pm0.07 (cgs units). Using the Hamada-Salpeter mass-radius relation for helium white dwarfs, with an approximate correction for finite-temperature effects, we infer a mass \\mwd=0.16\\pm0.02\\,\\msun. This is the lowest mass among all spectroscopically identified white dwarfs. We determine radial velocities from our spectra, and find a radial-velocity amplitude of 280\\pm15\\,\\kms. ...

Van Kerkwijk, M H; Kulkarni, S R

1996-01-01

374

A SPITZER SPACE TELESCOPE STUDY OF THE DEBRIS DISKS AROUND FOUR SDSS WHITE DWARFS  

International Nuclear Information System (INIS)

We present Spitzer Space Telescope data of four isolated white dwarfs that were previously known to harbor circumstellar gaseous disks. Infrared Array Camera photometry shows a significant infrared excess in all of the systems, SDSS0738+1835, SDSS0845+2257, SDSS1043+0855, and SDSS1617+1620, indicative of a dusty extension to those disks. The 4.5 ?m excesses seen in SDSS0738, SDSS0845, and SDSS1617 are 7.5, 5.7, and 4.5 times the white dwarf contribution, respectively. In contrast, in SDSS1043, the measured flux density at 4.5 ?m is only 1.7 times the white dwarf contribution. We compare the measured IR excesses in the systems to models of geometrically thin, optically thick disks, and find that we are able to match the measured spectral energy distributions to within 3? of the uncertainties, although disks with unfeasibly hot inner dust temperatures generally provide a better fit than those below the dust sublimation temperature. Possible explanations for the dearth of dust around SDSS1043+0855 are briefly discussed. Including our previous study of SDSS1228+1040, all five white dwarfs with gaseous debris disks have significant amounts of dust around them. It is evident that gas and dust can coexist around these relatively warm, relatively young white dwarfs.

375

The Solar Neighborhood. XX. Discovery and Characterization of 21 New Nearby White Dwarf Systems  

CERN Document Server

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...

Subasavage, John P; Bergeron, P; Dufour, P; Hambly, Nigel C

2008-01-01

376

THE WHITE DWARFS WITHIN 20 PARSECS OF THE SUN: KINEMATICS AND STATISTICS  

International Nuclear Information System (INIS)

We present the kinematical properties, distribution of spectroscopic subtypes, and stellar population subcomponents of the white dwarfs within 20 pc of the Sun. We find no convincing evidence of halo white dwarfs in the total 20 pc sample of 129 white dwarfs nor is there convincing evidence of genuine thick disk subcomponent members within 20 parsecs. Virtually, the entire 20 pc sample likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc sample is 1.6, a manifestation of deepening envelope convection which transforms DA stars with sufficiently thin H surface layers into non-DAs. The addition of five new stars to the 20 pc sample yields a revised local space density of white dwarfs of (4.9 ± 0.5) x 10-3 pc-3 and a corresponding mass density of (3.3 ± 0.3) x 10-3 M sun pc-3. We find that at least 15% of the white dwarfs within 20 parsecs of the Sun (the DAZ and DZ stars) have photospheric metals that possibly originate from accretion of circumstellar material (debris disks) around them. If this interpretation is correct, this suggests the possibility that the same percentage have planets or asteroid-like bodies orbiting them.

377

Visitors from the Halo: 11 Gyr old White Dwarfs in the Solar Neighborhood  

CERN Document Server

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...

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

378

Discovery of Two New Thermally Bloated Low-Mass White Dwarfs Among the Kepler Binaries  

CERN Document Server

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...

Rappaport, S; Levine, A; Sanchis-Ojeda, R; Gandolfi, D; Nowak, G; Palle, E; Prsa, A

2015-01-01

379

White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up  

CERN Document Server

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...

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

2015-01-01

380

On the possibility of using seismic probes to study the core composition in pulsating white dwarfs  

CERN Document Server

White dwarfs correspond to the final stages of stellar evolution of solar-type stars. In these objects, production of energy by nuclear burning has ended which means that a white dwarf simply cools down over the course of the next billion years. It is now known that white dwarfs spend some of their cooling history in an instability strip. The pulsating white dwarfs with an hydrogen atmosphere (called DAV or ZZ Ceti stars) show non-radial oscillation modes with periods in the range 100 - 1200s. In this work we try to illustrate how the oscillation p-mode frequencies of idealized white dwarf models change as the result of a different chemical composition in the core, with the ultimate goal of determining the chemical stratification from seismic observations. The presence of acoustic glitches in the internal structure results in a periodic signal in the frequencies. We find that this signal depends on the chemical stratification/composition of the core in a form that can be analytically modelled.

Faria, J P

2012-01-01

381

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.

382

The first binary star evolution model producing a Chandrasekhar mass white dwarf  

CERN Document Server

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...

Yoon, S C

2003-01-01

383

Detonations in Sub-Chandrasekhar Mass C+O White Dwarfs  

CERN Document Server

Explosions of sub-Chandrasekhar-mass white dwarfs are one alternative to the standard Chandrasekhar-mass model of Type Ia supernovae. They are interesting since binary systems with sub-Chandrasekhar-mass primary white dwarfs should be common and this scenario would suggest a simple physical parameter which determines the explosion brightness, namely the mass of the exploding white dwarf. Here we perform one-dimensional hydrodynamical simulations, associated post-processing nucleosynthesis and multi-wavelength radiation transport calculations for pure detonations of carbon-oxygen white dwarfs. The light curves and spectra we obtain from these simulations are in good agreement with observed properties of Type Ia supernovae. In particular, for white dwarf masses from 0.97 - 1.15 Msun we obtain 56Ni masses between 0.3 and 0.8 Msun, sufficient to capture almost the complete range of Type Ia supernova brightnesses. Our optical light curve rise times, peak colours and decline timescales display trends which are gene...

Sim, S A; Hillebrandt, W; Kromer, M; Pakmor, R; Fink, M; Ruiter, A J; Seitenzahl, I R

2010-01-01

384

The Future Evolution of White Dwarf Stars Through Baryon Decay and Time Varying Gravitational Constant  

CERN Document Server

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.

Ketchum, Jacob A

2008-01-01

385

Accretion of a Terrestrial-Like Minor Planet by a White Dwarf  

CERN Document Server

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.

Melis, Carl; Dufour, P; Zuckerman, B; Burgasser, Adam J; Bergeron, P; Bochanski, J; Simcoe, R

2011-01-01

386

Granulation in DA white dwarfs from CO5BOLD 3D model atmospheres  

Science.gov (United States)

Time-dependent 3D simulations of pure-hydrogen DA white dwarf atmospheres have been computed in recent years. Synthetic Balmer lines spectra drawn from these radiation-hydrodynamics (RHD) simulations have been shown to predict surface gravities significantly lower than the standard 1D models, in much better agreement with the expectation that white dwarfs cool at constant mass. We have now computed a grid of CO5BOLD pure-hydrogen 3D model atmospheres for surface gravities from log g = 7 to log g = 8.5 and effective temperatures from 6000 to 13,000 K. Over this range, we observe a significant variation of the intensity contrast of the surface granulation patterns, which indicates the strength of the 3D effects. Furthermore, the size and appearance of granules are also varying considerably. An explanation of these behaviours can lead to a better understanding of the physical processes responsible for the energy transfer in white dwarf atmospheres.

Tremblay, P.-E.; Ludwig, H.-G.; Freytag, B.; Steffen, M.

387

White-dwarf red-giant mergers, early-type R stars, J stars and lithium  

CERN Document Server

Early-type R stars and J stars are a special type of carbon star, having enhanced nitrogen ($\\rm [N/Fe]\\approx 0.5$), lithium, a low \\iso{12}{C}/\\iso{13}{C} ratio ($<15$) and no s-element enhancements. The merger of a helium white dwarf with a red giant is regarded to be a possible model for the origin of early-type R stars, but the details of nucleosynthesis are not clear. In this paper we investigate three possible channels for helium white-dwarf + red-giant mergers, and find that, amongst the three, only a high-mass helium white dwarf subducted into a low core-mass red giant can make an early-type R star. Nucleosynthesis of elements carbon, nitrogen, oxygen and lithium correspond well with the observations. Furthermore, we find that the J stars may represent a short and luminous stage in the evolution of an early-R star.

Zhang, Xianfei

2013-01-01

388

Population boundaries for Neutron star-White Dwarf binaries in LISA's amplitude-frequency domain  

CERN Document Server

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...

Kopparapu, Ravi kumar

2008-01-01

389

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

Science.gov (United States)

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

Di Stefano, Rosanne; Harris, R.

2010-02-01

390

White dwarfs stripped by massive black holes: sources of coincident gravitational and electromagnetic radiation  

Science.gov (United States)

White dwarfs inspiralling into black holes of mass MBH >~ 105 Msolar are detectable sources of gravitational waves in the Laser Interferometer Space Antenna (LISA) band. In many of these events, the white dwarf begins to lose mass during the main observational phase of the inspiral. The mass-loss starts gently and can last for thousands of orbits. The white dwarf matter overflows the Roche lobe through the L1 point at each pericentre passage and the mass-loss repeats periodically. The process occurs very close to the black hole and the released gas can accrete, creating a bright source of radiation with luminosity close to the Eddington limit, L ~ 1043ergs-1. This class of inspirals offers a promising scenario for dual detections of gravitational waves and electromagnetic radiation.

Zalamea, I.; Menou, K.; Beloborodov, A. M.

2010-11-01

391

White dwarfs stripped by massive black holes: EMRIs with coincident gravitational and electromagnetic radiation  

Science.gov (United States)

White dwarfs inspiraling into black holes of mass MBH ˜ 105 are bright sources of gravita-tional waves in the LISA band. In many of these events, the white dwarf begins to lose mass during the main observational phase of the inspiral. The mass loss starts gently and can last for thousands of orbits. The white dwarf matter overflows the Roche lobe through the L1 point at each pericenter passage and the mass loss repeats periodically. The process occurs very close to the black hole and the released gas can accrete, creating a bright source of radiation with luminosity close to the Eddington limit, N ˜ 1043 erg/s. This class of inspirals offers a promising scenario for dual detections of gravitational waves and electromagnetic radiation.

Zalamea, Ivan; Menou, Kristen; Beloborodov, Andrei

392

Nucleosynthesis during the Merger of White Dwarfs and the Origin of R Coronae Borealis Stars  

CERN Document Server

Many hydrogen deficient stars are characterised by surface abundance patterns that are hard to reconcile with conventional stellar evolution. Instead, it has been suggested that they may represent the result of a merger episode between a helium and a carbon-oxygen white dwarf. In this Letter, we present a nucleosynthesis study of the merger of a 0.4 M_sol helium white dwarf with a 0.8 M_sol carbon-oxygen white dwarf, by coupling the thermodynamic history of Smoothed Particle Hydrodynamics particles with a post-processing code. The resulting chemical abundance pattern, particularly for oxygen and fluorine, is in qualitative agreement with the observed abundances in R Coronae Borealis stars.

Longland, R; José, J; García-Berro, E; Althaus, L G; Isern, J

2011-01-01

393

Precise Atmospheric Parameters for the Shortest-period Binary White Dwarfs: Gravitational Waves, Metals, and Pulsations  

Science.gov (United States)

We present a detailed spectroscopic analysis of 61 low-mass white dwarfs and provide precise atmospheric parameters, masses, and updated binary system parameters based on our new model atmosphere grids and the most recent evolutionary model calculations. For the first time, we measure systematic abundances of He, Ca, and Mg for metal-rich, extremely low mass white dwarfs and examine the distribution of these abundances as a function of effective temperature and mass. Based on our preliminary results, we discuss the possibility that shell flashes may be responsible for the presence of the observed He and metals. We compare stellar radii derived from our spectroscopic analysis to model-independent measurements and find good agreement except for white dwarfs with T eff MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.

Gianninas, A.; Dufour, P.; Kilic, Mukremin; Brown, Warren R.; Bergeron, P.; Hermes, J. J.

2014-10-01

394

Pulsations of Pre-White Dwarfs with Hydrogen-dominated Atmospheres  

CERN Document Server

We carried out a fully non-adiabatic analysis for nonradial oscillations of pre-white dwarfs evolved from the post-Asymptotic Giant Branch (AGB) with hydrogen-dominated envelopes. It is shown that nuclear reactions in the hydrogen burning-shell excite low-degree g-modes in the period range of about 40-200 s for the pre-white dwarf models with Teff=40,000 K - 300,000 K. It is also shown that the amount of hydrogen {has} a significant influence on the instability domain of such pre-white dwarfs in the Hertzsprung-Russel (H-R) diagram. Thus, the thickness of hydrogen-dominated envelopes may be well constrained by observing the presence of the g-mode oscillations.

Maeda, Kazuhiro

2014-01-01

395

H? polarization and line profiles in white dwarfs with strong magnetic fields  

International Nuclear Information System (INIS)

H? circular polarization and intensity profiles in white dwarfs with strong magnetic fields are computed for dipole and dipole-like configurations of the magnetic field. The polar fields vary from 4 x 107 to 108 gauss. The profiles are severely distorted by the strong nonhomogeneous fields. They are strongly dependent on field strength and magnetic geometry. An interesting and potentially very useful feature of the profiles is the appearance of sharp sigma+ (red) components. The wavelength location of those components is fairly insensitive to field strength: either they are not there or they are at the correct wavelength. This makes them useful in identifying H? in magnetic white dwarfs. The intensity and polarization spectra of known magnetic white dwarfs are examined in the light of the models

396

Hard X-ray light-curves of accreting magnetized white dwarfs  

Energy Technology Data Exchange (ETDEWEB)

It is pointed out that the hard (>approx. 3 keV) X-ray light-curves of accreting magnetic white dwarfs can be understood as due solely to occultations of the emission region by the white-dwarf body. The prevalence of quasi-sinusoidal hard X-ray light-curves amongst the intermediate polars probably implies that the emission regions occupy a large fraction (2f >approx. 0.5) of the white-dwarf surface; for the AM Her stars it is tentatively concluded that f >approx. 10/sup -3/. The likely behaviour of the light-curve amplitude as the X-ray brightness changes, is discussed and it is shown that optical emission directly from the polecaps may dominate the pulsed light in some intermediate polars.

King, A.R.; Shaviv, G. (Leicester Univ. (UK). Dept. of Astronomy)

1984-12-15

397

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

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...

Ji, Suoqing; Garcia-Berro, Enrique; Tzeferacos, Petros; Jordan, George; Lee, Dongwook; Loren-Aguilar, Pablo; Cremer, Pascal; Behrends, Jan

2013-01-01

398

Accretion of planetary debris onto the unique white dwarf GD394  

Science.gov (United States)

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.

Wilson, David

2014-10-01

399

Accreting carbon-oxygen white dwarfs at explosive C ignition  

Science.gov (United States)

The conditions at which an accreting carbon-oxygen white dwarf (CO WD) ignites C explosively have been considered to be possibly responsible for the maximum luminosity of Type Ia supernovae (SNe Ia). However, the exact conditions for this explosion have been in controversy. Employing the latest powerful stellar evolution code, Modules for Experiments in Stellar Astrophysics, and the descriptions of the single-degenerate (SD) model for progenitors of SNe Ia, we studied the evolution of several Pop I CO WDs from the onset of mass accretion to the explosive C ignition, where the temperature increases sharply but the density keeps in constant at the maximum temperature point (Tmax) of the accreting CO WD. The accreted material is CO-rich and we include two sets of calculations, i.e. one is for a simple accretion like in Lesaffre et al. (2006) while the other artificially adds an energy generation rate (1 × 105 erg g-1 s-1) in the outermost 0.005 M? material during accretion to mimic the energy delivered inwards from H- and He-burning. Both simulations show that most CO WDs ignite C explosively (at the Tmax) in the centre, with very similar properties such as WD masses (˜1.387 M?), central densities (˜2.63 × 109 g cm-3), central temperatures(˜109 K) and binding energies (˜5.23 × 1050 erg). This means that most SNe Ia are ignited initially in the centre of CO WDs based on the SD model. However, the central density and WD mass have been influenced by the initial C/O ratio at the centre of CO WDs. A few initially very cold massive CO WDs ignite C at off-centre points but still in the deep interior of WDs. In these explosions, the offset distance to the centre, the WD mass and central density at the explosion clearly increase with the cooling age, while the inclusion of an extra energy source resists this increasing in part. The off-centre explosions can only occasionally happen in relatively old galaxies and could partly contribute to the diversity of SNe Ia.

Chen, Xuefei; Han, Zhanwen; Meng, Xiangcun

2014-03-01

400

Accretion onto the white dwarf and X-ray production in nonmagnetic cataclysmic variables  

International Nuclear Information System (INIS)

Cataclysmic variables (CVs) are short-period binary systems (periods -- a few hours) in which a white dwarf accretes from a close, low-mass companion star. For most of their lifetimes, they derive their luminosity from the gravitational energy released by the accreting matter. Nuclear burning, although rather more efficient (--6 x 10/sup 18/ erg g/sup -1/ compared with --2 x 10/sup 17/ erg g/sup -1/), is confined to bright (-- 10/sup 38/ erg s/sup -1/), but short (--10 yr) and infrequent (--10/sup 5/ yr) nova explosions. The author discusses X-ray observations of nonmagnetic CVs and relate them to current theories of the boundary layer. He argues that hard (>2 keV) emission probably arises in a hot corona surrounding the white dwarf (formed from the accretion flow because of thermal instabilities in the boundary layer region) and then show that there is considerable observational support for this idea. Because of conductive losses from the corona into the white dwarf body, hard X-ray production is likely to be quite inefficient (--1-10%), with most of the boundary layer accretion energy ultimately emerging as soft X-ray emission from the heated white dwarf surface. Finally, the picture offers a possible explanation of the quasi-periodic soft X-ray pulsations seen during the dwarf nova outbursts of some systems, especially in terms of the channeling of the conduction losses by transient, dynamo-generated surface magnetic fields

401

Near-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG1234+482  

CERN Document Server

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.

Steele, P R; Dobbie, P D; Barstow, M A

2007-01-01

402

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.

403

Effects of the magnetic fields on the helium white dwarfs structure  

CERN Document Server

In this paper the effect of the magnetic field on the form of the equation of state and helium white dwarfs structure are discussed. The influence of the temperature and magnetic field on white dwarfs parameters have been investigated. The mass-radius relations for different parameters were obtained. The occurrence of unstable branches in the mass-radius relation are presented for the temperature equals T=3 10^8 K and for different values of the strength of magnetic field. Theoretical model of the star with two Landau levels is obtained.

Manka, R; Brzezina, A; Bednarek, I

2001-01-01

404

Creating White Dwarf Photospheres in the Laboratory: Strategy for Astrophysics Applications  

OpenAIRE

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...

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

405

Systematic quantum effects on screening of fusion rates in white dwarfs  

CERN Document Server

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.

Chitanvis, S M

2006-01-01

406

Systematic relativistic quantum effects on screening of fusion rates in white dwarfs  

CERN Document Server

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.

Chitanvis, S M

2006-01-01

407

Theoretical and observational review of results on nova explosions occurring on ONeMg white dwarfs  

Energy Technology Data Exchange (ETDEWEB)

The nova outburst is the second most violent explosion that occurs in a galaxy. This review presents the recent observational and theoretical studies that have demonstrated that there exist two classes of nova outburst. One type of nova occurs on a CO white dwarf and the other type of nova occurs on an ONeMg white dwarf. The second class of outbursts are much more violent and occur much more frequently then the first class of outbursts. Hydrodynamic simulations of both kinds of outbursts are in excellent agreement with the observations. 51 refs.

Starrfield, S.

1986-07-07

408

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

OpenAIRE

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

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

2002-01-01

409

The spectrum of the recycled PSR J0437-4715 and its white dwarf companion  

OpenAIRE

We present extensive spectral and photometric observations of the recycled pulsar/white-dwarf binary containing PSR J0437-4715, which we analyzed together with archival X-ray and gamma-ray data, to obtain the complete mid-infrared to gamma-ray spectrum. We first fit each part of the spectrum separately, and then the whole multi-wavelength spectrum. We find that the optical-infrared part of the spectrum is well fit by a cool white dwarf atmosphere model with pure hydrogen com...

Durant, M.; Kargaltsev, O.; Pavlov, G. G.; Kowalski, P. M.; Posselt, B.; Kerkwijk, M. H.; Kaplan, D. L.

2011-01-01

410

The gamma-ray source Geminga - A white dwarf, rotating around a black hole?  

Science.gov (United States)

A 'kinematic' model of Geminga is proposed which explains the observed period and the rate of its increase. The model is based on the hypothesis that the observed period is connected with the orbital period of a close binary system consisting of (1) a white dwarf filling the Roche lobe with a mass of 0.6 solar mass and (2) a black hole with a mass of 4.4 solar masses. Two processes are noted to be governing in this close binary system: (1) intense mass transfer from the white dwarf to the black hole and (2) loss by the system of rotational motion due to the emission of gravitational radiation.

Bisnovatyi-Kogan, G. S.

1985-05-01

411

Radiative activity of magnetic white dwarf undergoing Lorentz-force-driven torsional vibrations  

CERN Document Server

We study radiative activity of magnetic white dwarf undergoing torsional vibrations about axis of its own dipole magnetic moment under the action of Lorentz restoring force. It is shown that pulsating white dwarf can convert its vibration energy into the energy of magneto-dipole emission, oscillating with the frequency equal to the frequency of Alfv\\'en torsional vibrations, provided that internal magnetic field is decayed. The most conspicuous feature of the vibration energy powered radiation in question is the lengthening of periods of oscillating emission; the rate of period elongation is determined by the rate magnetic field decay.

Bastrukov, S I; Xu, R X; Molodtsova, I V

2010-01-01

412

Creating White Dwarf Photospheres in the Laboratory: Strategy for Astrophysics Applications  

CERN Document Server

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 experiments using other compositions. We discuss future prospects such as exploring helium plasmas and carbon/oxygen plasmas relevant to the photospheres of DBs and hot DQs, respectively.

Falcon, Ross E; 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

413

HST+COS spectra of the double white dwarf CSS 41177 place the secondary inside the pulsational instability strip  

CERN Document Server

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 = 22439 +/- 59 K and T2 = 10876 +/- 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.

Bours, Madelon; Gaensicke, Boris; Parsons, Steven

2015-01-01

414

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.

415

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.

416

The substellar companion in the eclipsing white dwarf binary SDSS J141126.20+200911.1  

Science.gov (United States)

We present high time resolution SDSS-g' and SDSS-z' light curves of the primary eclipse in SDSS J141126.20+200911.1, together with time-resolved X-Shooter spectroscopy and near-infrared (NIR) JHKs photometry. Our observations confirm the substellar nature of the companion, making SDSS J141126.20+200911.1 the first eclipsing white dwarf/brown dwarf binary known. We measure a (white dwarf model dependent) mass and radius for the brown dwarf companion of M2 = 0.050 ± 0.002 M? and R2 = 0.072 ± 0.004 M?, respectively. The lack of a robust detection of the companion light in the z'-band eclipse constrains the spectral type of the companion to be later than L5. Comparing the NIR photometry to the expected white dwarf flux reveals a clear Ks-band excess, suggesting a spectral type in the range L7-T1. The radius measurement is consistent with the predictions of evolutionary models, and suggests a system age in excess of 3 Gyr. The low companion mass is inconsistent with the inferred spectral type of L7-T1, instead predicting a spectral type nearer T5. This indicates that irradiation of the companion in SDSS J141126.20+200911.1 could be causing a significant temperature increase, at least on one hemisphere.

Littlefair, S. P.; Casewell, S. L.; Parsons, S. G.; Dhillon, V. S.; Marsh, T. R.; Gänsicke, B. T.; Bloemen, S.; Catalan, S.; Irawati, P.; Hardy, L. K.; Mcallister, M.; Bours, M. C. P.; Richichi, Andrea; Burleigh, M. R.; Burningham, B.; Breedt, E.; Kerry, P.

2014-12-01

417

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

Science.gov (United States)

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 J121258.25-012310.1 and GK Vir. For both systems, we measure the gravitational redshift of the white dwarf (WD) and combine it with light-curve model fits to determine the inclinations, masses and radii. For SDSS J1212-0123, we find an inclination of i= 85?7 ± 0?5, masses of MWD= 0.439 ± 0.002 M? and Msec= 0.273 ± 0.002 M?, and radii RWD= 0.0168 ± 0.0003 R? and Rsec= 0.306 ± 0.007 R?. For GK Vir, we find an inclination of i= 89?5°± 0?6, masses of MWD= 0.564 ± 0.014 M? and Msec= 0.116 ± 0.003 M? and radii RWD= 0.0170 ± 0.0004 R? and Rsec= 0.155 ± 0.003 R?. The mass and radius of the WD in GK Vir are consistent with evolutionary models for a 50 000 K carbon-oxygen (CO) core WD. Although the mass and radius of the WD in SDSS J1212-0123 are consistent with CO core models, evolutionary models imply that a WD with such a low mass and in a short period binary must have a helium core. The mass and radius measurements are consistent with helium core models but only if the WD has a very thin hydrogen envelope (MH/MWD? 10-6). Such a thin envelope has not been predicted by any evolutionary models. The mass and radius of the secondary star in GK Vir are consistent with evolutionary models after correcting for the effects of irradiation by the WD. The secondary star in SDSS J1212-0123 has a radius ˜9 per cent larger than predicted.

Parsons, S. G.; Marsh, T. R.; 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.; Nebot Gómez-Morán, A.; Pyrzas, S.; Savoury, C. D. J.; Schreiber, M. R.; Schmidtobreick, L.; Schwope, A. D.; Steele, P. R.; Tappert, C.

2012-03-01

418

Intermediate polars in the Swift/BAT survey: Spectra and white dwarf masses  

CERN Document Server

White dwarf masses in cataclysmic variables are difficult to determine accurately, but are fundamental for understanding binary system parameters, as well as binary evolution. We investigate the X-ray spectral properties of a sample of Intermediate Polars detected above 15 keV to derive the masses of their accreting white dwarfs. We use data from the Swift/BAT instrument which during the first 2.5 yrs of operation has detected 22 known intermediate polars. The X-ray spectra of these sources are used to estimate the mass of the white dwarfs. We are able to produce a mass estimate for 22 out of 29 of the confirmed intermediate polars. Comparison with previous mass measurements shows good agreement. For GK Per, we were able to detect spectral changes due to the changes in the accretion rate. The Swift/BAT detector with its combination of sensitivity and all-sky coverage provides an ideal tool to determine accurate white dwarf masses in intermediate polars.

Brunschweiger, Jorg; Ajello, Marco; Osborne, Julian

2009-01-01

419

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: I. Adiabatic properties  

CERN Document Server

The discovery of pulsations in some low-mass white dwarfs, including the so-called extremely low-mass white dwarfs, has opened the unprecedented opportunity of probing the internal structure of these ancient stars. We present a detailed adiabatic pulsational study of these stars based on a new set of He-core white-dwarf models with masses ranging from $0.1554$ to $0.4352 M_{\\odot}$ derived by computing the non-conservative evolution of a binary system consisting of an initially $1 M_{\\odot}$ ZAMS star and a $1.4 M_{\\odot}$ neutron star. We computed adiabatic radial ($\\ell= 0$) and non-radial ($\\ell= 1, 2$) $p$ and $g$ modes to assess the dependence of the pulsational properties of these objects on stellar parameters such as the stellar mass and the effective temperature, as well as the effects of element diffusion. We found that for white dwarf models with masses below $\\sim 0.18 M_{\\odot}$, $g$ modes mainly probe the core regions and $p$ modes the envelope, therefore pulsations offer the opportunity of const...

Córsico, A H

2014-01-01

420

Asteroseismology of the \\emph{Kepler} V777 Her variable white dwarf with fully evolutionary models  

CERN Document Server

DBV stars are pulsating white dwarfs with atmospheres rich in He. Asteroseismology of DBV stars can provide valuable clues about the origin, structure and evolution of hydrogen-deficient white dwarfs, and may allow to study neutrino and axion physics. Recently, a new DBV star, KIC 8626021, has been discovered in the field of the \\emph{Kepler} spacecraft. It is expected that further monitoring of this star in the next years will enable astronomers to determine its detailed asteroseismic profile. We perform an asteroseismological analysis of KIC 8626021 on the basis of fully evolutionary DB white-dwarf models. We employ a complete set of evolutionary DB white-dwarf structures covering a wide range of effective temperatures and stellar masses. They have been obtained on the basis of a complete treatment of the evolutionary history of progenitors stars. We compute g-mode adiabatic pulsation periods for this set of models and compare them with the pulsation properties exhibited by KIC 8626021. On the basis of the ...

Córsico, Alejandro H; Bertolami, Marcelo M Miller; Bischoff-Kim, Agnès

2011-01-01

421

Ancient planetary systems are orbiting a large fraction of white dwarf stars  

CERN Document Server

Infrared studies have revealed debris likely related to planet formation in orbit around ~30% of youthful, intermediate mass, main sequence stars. We present evidence, based on atmospheric pollution by various elements heavier than helium, that a comparable fraction of the white dwarf descendants of such main sequence stars are orbited by planetary systems. These systems have survived, at least in part, through all stages of stellar evolution that precede the white dwarf. During the time interval (~200 million years) that a typical polluted white dwarf in our sample has been cooling it has accreted from its planetary system the mass of one of the largest asteroids in our solar system (e.g., Vesta or Ceres). Usually, this accreted mass will be only a fraction of the total mass of rocky material that orbits these white dwarfs; for plausible planetary system configurations we estimate that this total mass is likely to be at least equal to that of the Sun's asteroid belt, and perhaps much larger. We report abunda...

Zuckerman, B; Klein, B; Koester, D; Jura, M

2010-01-01

422

The masses of PSR J1911-5958A and its white dwarf companion  

CERN Document Server

We present spectroscopic and photometric observations of the optical counterpart to PSR J1911-5958A, a millisecond pulsar located towards the globular cluster NGC 6752. We measure radial velocities from the spectra and determine the systemic radial velocity of the binary and the radial-velocity amplitude of the white-dwarf orbit. Combined with the pulsar orbit obtained from radio timing, we infer a mass ratio of Mpsr/Mwd=7.36+-0.25. The spectrum of the counterpart is that of a hydrogen atmosphere, showing Balmer absorption lines upto H12, and we identify the counterpart as a helium-core white dwarf of spectral type DA5. Comparison of the spectra with hydrogen atmosphere models yield a temperature Teff=10090+-150 K and a surface gravity log g=6.44+-0.20 cm s^-2. Using mass-radius relations appropriate for low-mass helium-core white dwarfs, we infer the white-dwarf mass Mwd=0.18+-0.02 Msun and radius Rwd=0.043+-0.009 Rsun. Combined with the mass ratio, this constrains the pulsar mass to Mpsr=1.40^+0.16_-0.10 Ms...

Bassa, C G; Van Kerkwijk, M H; Verbunt, F

2006-01-01

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Smoothed Particle Hydrodynamics simulations of white dwarf collisions and close encounters  

CERN Document Server

The collision of two white dwarfs is a quite frequent event in dense stellar systems, like globular clusters and galactic nuclei. In this paper we present the results of a set of simulations of the close encounters and collisions of two white dwarfs. We use an up- to-date smoothed particle hydrodynamics code that incorporates very detailed input physics and an improved treatment of the artificial viscosity. Our simulations have been done using a large number of particles (~ 4 \\times 10^5) and covering a wide range of velocities and initial distances of the colliding white dwarfs. We discuss in detail when the initial eccentric binary white dwarf survives the closest approach, when a lateral collision in which several mass transfer episodes occur is the outcome of the newly formed binary system, and which range of input parameters leads to a direct collision, in which only one mass transfer episode occurs. We also discuss the characteristics of the final configuration and we assess the possible observational s...

Lorén-Aguilar, P; García-Berro, E

2010-01-01