scholarly journals Diffusion Coefficients in Dense Plasmas

1979 ◽  
Vol 53 ◽  
pp. 192-196
Author(s):  
G. Fontaine ◽  
G. Michaud
Keyword(s):  
Diffusion Model ◽  
Time Scales ◽  
White Dwarf ◽  
Diffusion Coefficients ◽  
White Dwarfs ◽  
Diffusion Time ◽  
Heavy Elements ◽  
Spectral Evolution ◽  
Dense Plasmas ◽  
Quantitative Estimates

It has been known for some time (Schatzman 1958) that diffusion could play an important role in the spectral evolution of white dwarfs. However, it is only recently that quantitative estimates have become available, making use of detailed envelope models (Fontaine and Michaud 1979, hereafter referred to as FM; Vauclair, Vauclair, and Greenstein 1979; Alcock and Illarionov 1979). These studies suggest that the observed monoelemental character of white dwarf spectra can qualitatively be explained in terms of the diffusion model. Moreover, it appears that the diffusion time scales are so short compared to evolutionary times that competing mechanisms, such as accretion, must be invoked to explain the small but measurable abundances of heavy elements in the spectra of cooler white dwarfs.

Observation of a Variable, ZZ Ceti White Dwarf: GD154

1993 ◽  
Vol 134 ◽  
pp. 201-204
Author(s):  
B. Pfeiffer ◽  
G. Vauclair ◽  
N. Dolez ◽  
M. Chevreton ◽  
J. R. Fremy ◽  
...  
Keyword(s):  
Time Scales ◽  
Effective Temperature ◽  
White Dwarf ◽  
White Dwarfs ◽  
Nonlinear Phenomena ◽  
Instability Strip ◽  
Temperature Range

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds.A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena.

scholarly journals Evidence of spectral evolution on the white dwarf sample from the Gaia mission

2020 ◽  
Vol 492 (4) ◽  
pp. 5003-5010 ◽  
Author(s):  
G Ourique ◽  
S O Kepler ◽  
A D Romero ◽  
T S Klippel ◽  
D Koester
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Mass Function ◽  
Initial Mass Function ◽  
Star Formation History ◽  
Initial Mass ◽  
Spectral Evolution ◽  
Formation History ◽  
Magnitude Diagram ◽  
Double Population

ABSTRACT Since the Gaia data release 2, several works have been published describing a bifurcation in the observed white dwarf colour−magnitude diagram for ${G_{\mathrm{BP}}}{}-{G_{\mathrm{RP}}}{} \gt 0$. Some possible explanations in the literature include the existence of a double population with different initial mass functions or two distinct populations, one formed by hydrogen-envelope and one formed by helium-envelope white dwarfs. We propose instead spectral evolution to explain the bifurcation. From a population synthesis approach, we find that spectral evolution occurs for effective temperatures below ${\simeq }11\, 000\, \mathrm{K}$ and masses mainly between $0.64\, \mathrm{M}_\odot$ and $0.74\, \mathrm{M}_\odot$, which correspond to around 16 per cent of all DA white dwarfs. We also find that the Gaia white dwarf colour–magnitude diagram indicates a star formation history that decreases abruptly for objects younger than $1.4\, \mathrm{Gyr}$ and a top-heavy initial mass function for the white dwarf progenitors.

scholarly journals Near-infrared variability in dusty white dwarfs: tracing the accretion of planetary material

2020 ◽  
Vol 494 (2) ◽  
pp. 2861-2874 ◽  
Author(s):  
Laura K Rogers ◽  
Siyi Xu (许偲艺) ◽  
Amy Bonsor ◽  
Simon Hodgkin ◽  
Kate Y L Su ◽  
...  
Keyword(s):  
Time Scales ◽  
White Dwarf ◽  
White Dwarfs ◽  
Near Infrared ◽  
Dust Emission ◽  
Infrared Emission ◽  
Wide Field ◽  
Infrared Telescope ◽  
Infrared Monitoring ◽  
K Band

ABSTRACT The inwards scattering of planetesimals towards white dwarfs is expected to be a stochastic process with variability on human time-scales. The planetesimals tidally disrupt at the Roche radius, producing dusty debris detectable as excess infrared emission. When sufficiently close to the white dwarf, this debris sublimates and accretes on to the white dwarf and pollutes its atmosphere. Studying this infrared emission around polluted white dwarfs can reveal how this planetary material arrives in their atmospheres. We report a near-infrared monitoring campaign of 34 white dwarfs with infrared excesses with the aim to search for variability in the dust emission. Time series photometry of these white dwarfs from the United Kingdom Infrared Telescope (Wide Field Camera) in the J-, H-, and K-bands was obtained over baselines of up to 3 yr. We find no statistically significant variation in the dust emission in all three near-infrared bands. Specifically, we can rule out variability at ∼1.3 per cent for the 13 white dwarfs brighter than 16th mag in K-band, and at ∼10 per cent for the 32 white dwarfs brighter than 18th mag over time-scales of 3 yr. Although to date two white dwarfs, SDSS J095904.69−020047.6 and WD 1226+110, have shown K-band variability, in our sample we see no evidence of new K-band variability at these levels. One interpretation is that the tidal disruption events that lead to large variabilities are rare occur on short time-scales, and after a few years the white dwarfs return to being stable in the near-infrared.

scholarly journals Diffusion and Metal Abundances in Hot White Dwarfs Gerard Vauclair

1989 ◽  
Vol 114 ◽  
pp. 176-187 ◽  
Author(s):  
Gérard Vauclair
Keyword(s):  
Time Scales ◽  
White Dwarf ◽  
White Dwarfs ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
Metal Abundances ◽  
Cool White Dwarfs ◽  
First Time ◽  
Radiative Acceleration

While the efficiency of gravitational settling to produce chemically pure atmospheres in white dwarf stars was outlined for the first time 30 years ago (Schatzman 1958), the competing role of the radiation flux in the hot white dwarfs was considered only 10 years ago (Fontaine and Michaud 1979; Vauclair, Vauclair and Greenstein 1979). At that time, there was more motivation to understand how metals could reappear in the long lived cool non DA white dwarfs, where diffusion time scales are shorter by orders of magnitude than evolutionary time scales. Various processes were invoked to help restore some metal content in the white dwarf atmospheres: convection mixing and dredge up, accretion of interstellar matter. In cool white dwarfs, the radiative acceleration is negligeable in the diffusion process; this is not the case at the hot end of the sequence where radiation may balance gravity. The short lived hot white dwarfs just started to become exciting with the contemporary discoveries that i) some show metallic lines in their spectra, both hydrogen rich and hydrogen poor; ii) some of these are pulsating. In the following years, the number of hot white dwarfs revealing trace abundance of metals has increased, mainly owing to IUE observations.

scholarly journals The Composition and Structure of White Dwarf Atmospheres Revealed by Extreme Ultraviolet Spectroscopy

1996 ◽  
Vol 152 ◽  
pp. 203-210
Author(s):  
Martin A. Barstow ◽  
Ivan Hubeny ◽  
Thierry Lanz ◽  
Jay B. Holberg ◽  
Edward M. Sion
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Extreme Ultraviolet ◽  
Important Change ◽  
Heavy Elements ◽  
Composition And Structure ◽  
Crucial Test ◽  
Central Stars ◽  
Quantitative Consideration

The ROSAT and EUVE all-sky surveys have resulted in an important change in our understanding of the general composition of hydrogen-rich DA white dwarf atmospheres, with the photospheric opacity dominated by heavy elements rather than helium in the hottest stars (T > 40, 000 K). Most stars cooler than 40,000 K have more or less pure H atmospheres. However, one question, which has not been resolved, concerned the specific nature of the heavy elements and the role of helium in the hottest white dwarfs. One view of white dwarf evolution requires that H-rich DA stars form by gravitational settling of He from either DAO or He-rich central stars of planetary nebulae. In this case, the youngest (hottest) DA white dwarfs may still contain visible traces of He. Spectroscopic observations now available with EUVE provide a crucial test of these ideas. Analysis of data from the EUVE Guest Observer programme and EUVE public archive allows quantitative consideration of the sources of EUV opacity and places limits on the abundance of He which may be present.

Diffusion time scales in white dwarfs

1979 ◽  
Vol 231 ◽  
pp. 826 ◽  
Author(s):  
G. Fontaine ◽  
G. Michaud
Keyword(s):  
Time Scales ◽  
White Dwarfs ◽  
Diffusion Time

scholarly journals Finite Element Analysis of Diffusion Processes in White Dwarfs

1989 ◽  
Vol 114 ◽  
pp. 249-252
Author(s):  
C. Pelletier ◽  
G. Fontaine ◽  
F. Wesemael
Keyword(s):  
Finite Element ◽  
White Dwarf ◽  
White Dwarfs ◽  
Diffusion Processes ◽  
Numerical Technique ◽  
Time Integration ◽  
Spectral Evolution ◽  
Element Analysis ◽  
Theoretical Understanding ◽  
Diffusion Problems

The spectral evolution of white dwarfs is governed by diffusion processes which enter into competition with mechanisms such as mass loss, convective mixing, and accretion from the interstellar medium in various phases of the evolution. Until recently, our theoretical understanding of the chemical evolution of these stars has been limited by the very severe numerical difficulties which plague a time-dependent description of the problem. Indeed, diffusion problems in white dwarf interiors and envelopes are particularly demanding from a computational standpoint: they involve relative chemical abundances spanning many orders of magnitude, time integration length of a few billion years, and many physical processes operating with greatly different time constants. We have already introduced in the field a robust numerical technique based on an implicit finite difference scheme designed for nonlinear two-point boundary value problems (Pelletier 1986). This method has been used to investigate a number of problems related to the spectral evolution of white dwarfs (Pelletier 1986; Pelletier et al. 1986; Dupuis et al. 1987). As requirements for further progress in the field become more exacting and in the interest of improving the efficiency, we have sought to develop even more powerful numerical techniques. We briefly introduce here an efficient computational approach to diffusion problems in white dwarfs based on a Galerkin finite element method to solve the convective-diffusion equation in an evolving white dwarf model. As an illustrative example, we discuss some sample results of a detailed investigation of the problem of chemical sedimentation (H, He, and C) in the envelopes of hot white dwarfs and the formation of DA stars.

scholarly journals EUVEandORFEUSObservations of the Cool DO White Dwarf HD 149499 B

1996 ◽  
Vol 152 ◽  
pp. 241-246
Author(s):  
R. Napiwotzki ◽  
S. Jordan ◽  
S. Bowyer ◽  
M. Hurwitz ◽  
D. Koester ◽  
...  
Keyword(s):  
White Dwarf ◽  
Spectroscopic Investigation ◽  
White Dwarfs ◽  
Space Experiment ◽  
Spectral Evolution ◽  
Basic Parameters

We present the results of a recent spectroscopic investigation of the cool DO white dwarf HD 149499 B in the EUV and FUV ranges. Observations were performed with the spectrograph of theEUVEsatellite and the Berkeley EUV/FUV spectrometer of theORFEUSspace experiment. The analysis of theORFEUSspectrum, performed with a grid of LTE model atmospheres, yielded the basic parametersTeff= 49500 ± 500 K and logg= 7.97 ± 0.08. This result is confirmed by theEUVEspectra. The photospheric hydrogen Lyman lines in the FUV spectrum indicate the presence of hydrogen: lognH/nHe= −0.65 ± 0.12. The implications of this finding for the spectral evolution of white dwarfs are discussed. A check of the LTE assumption was performed by a comparison with NLTE atmospheres calculated for appropriate parameters. The interstellar hydrogen column towards the HD 149499 system amounts toNH= (7 ± 2) · 1018cm−2.

scholarly journals Time-Dependent Studies of the Settling of Heavy Elements in the Envelopes of Cool White Dwarfs

1989 ◽  
Vol 114 ◽  
pp. 359-362
Author(s):  
J. Dupuis ◽  
C. Pelletier ◽  
G. Fontaine ◽  
F. Wesemael
Keyword(s):  
Time Scales ◽  
White Dwarfs ◽  
Time Dependent ◽  
Heavy Elements ◽  
Atmospheric Composition ◽  
Element Abundance ◽  
Gravitational Settling ◽  
Observational Fact ◽  
Qualitative Conclusion ◽  
Cool White Dwarfs

Gravitational settling is widely accepted as being a fundamental physical process acting upon superficial layers of white dwarfs and resulting in an important alteration of their atmospheric composition. Several investigators have been interested by the problem of gravitational settling in white dwarfs (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979; Alcock and Illarianov 1980; Muchmore 1984; Paquette et al. 1986). As pointed out in Paquette et al. 1986, they all reached the same qualitative conclusion: the gravitational settling time scales of metals in cool white dwarfs are small compared to their evolutionary time scales. These stars should therefore have their photospheres depleted of metals if there is no extrinsic source such as accretion for example. This is consistent with the observational fact that most of the cool white dwarfs spectra just show hydrogen and helium lines while the absence of metallic lines indicates a strong depletion of metals. Although the qualitative agreement between theory and observations is satisfactory, only time-dependent calculations can lead to a thorough understanding of the heavy element abundance patterns in cool white dwarfs. In particular, the predicted abundance of an element within the framework of the accretion-diffusion model does depend explicitly on the results of such calculations. We have already presented some preliminary results of numerical simulation of accretion episodes of heavy elements into white dwarfs (Dupuis et al. 1987). As part of an ongoing detailed investigation of these processes, we focus here exclusively on the mechanism of gravitational settling in white dwarfs in order to clear some confusion which has appeared in the literature.

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