Diffusion and metal abundances in hot white dwarfs

2008 ◽  
pp. 176-187 ◽  
Author(s):  
Gérard Vauclair
Keyword(s):  
White Dwarfs ◽  
Metal Abundances

The Effect of CNO Metal Abundances on the Soft X-Ray Emission from He Rich White Dwarfs

1989 ◽  
Vol 114 ◽  
pp. 202-205
Author(s):  
M.A. Barstow
Keyword(s):  
Effective Temperature ◽  
Planetary Nebula ◽  
White Dwarfs ◽  
X Ray ◽  
Metal Abundances

AbstractPredicted soft X-ray fluxes for model atmospheres containing varying concentrations of CNO metals are compared with those observed by EXOSAT for the planetary nebula nucleus K1-16. An effective temperature in the range ≈ 125000 − 180000K is determined for K1-16 and a limit on the concentration of CNO in the atmosphere (between 0.02 and 20 ×solar relative to He) obtained. Some comments on the application of the models to the apparently metal rich star H1504+65 are included.

Metal abundances in the hot DA white dwarfs Wolf 1346 and Feige 24

1984 ◽  
Vol 287 ◽  
pp. 868 ◽  
Author(s):  
F. Wesemael ◽  
R. B. C. Henry ◽  
H. L. Shipman
Keyword(s):  
White Dwarfs ◽  
Metal Abundances

scholarly journals 18. Model Atmospheres for Hydrogen-Deficient White Dwarfs

1971 ◽  
Vol 42 ◽  
pp. 125-129
Author(s):  
I. Bues
Keyword(s):  
Effective Temperature ◽  
White Dwarfs ◽  
Atmospheric Parameters ◽  
Metal Abundances ◽  
Different Parts

The determination of atmospheric parameters for non-DA white dwarfs is investigated with the computed helium-rich model atmospheres by Bues (1970). Only poor predictions are possible from UBV colors alone for DB and DC stars. From uvby colors a determination of effective temperature is possible within 1000 K. Profiles of lines in different parts of the spectrum are necessary for better results.A deficiency of metal abundances for the cooler non-DA stars is obtained.

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 Are exoplanetesimals differentiated?

2020 ◽  
Vol 492 (2) ◽  
pp. 2683-2697 ◽  
Author(s):  
Amy Bonsor ◽  
Philip J Carter ◽  
Mark Hollands ◽  
Boris T Gänsicke ◽  
Zoë Leinhardt ◽  
...  
Keyword(s):  
White Dwarfs ◽  
Planetary Systems ◽  
Mantle Material ◽  
Planetary Body ◽  
Single Body ◽  
Exoplanetary Systems ◽  
Large Numbers ◽  
Metal Abundances ◽  
Larger Sample ◽  
Collisional Evolution

ABSTRACT Metals observed in the atmospheres of white dwarfs suggest that many have recently accreted planetary bodies. In some cases, the compositions observed suggest the accretion of material dominantly from the core (or the mantle) of a differentiated planetary body. Collisions between differentiated exoplanetesimalrrs produce such fragments. In this work, we take advantage of the large numbers of white dwarfs where at least one siderophile (core-loving) and one lithophile (rock-loving) species have been detected to assess how commonly exoplanetesimals differentiate. We utilize N-body simulations that track the fate of core and mantle material during the collisional evolution of planetary systems to show that most remnants of differentiated planetesimals retain core fractions similar to their parents, while some are extremely core rich or mantle rich. Comparison with the white dwarf data for calcium and iron indicates that the data are consistent with a model in which $66^{+4}_{-6}{{\ \rm per\ cent}}$ have accreted the remnants of differentiated planetesimals, while $31^{+5}_{-5}{{\ \rm per\ cent}}$ have Ca/Fe abundances altered by the effects of heating (although the former can be as high as $100{{\ \rm per\ cent}}$, if heating is ignored). These conclusions assume pollution by a single body and that collisional evolution retains similar features across diverse planetary systems. These results imply that both collisions and differentiation are key processes in exoplanetary systems. We highlight the need for a larger sample of polluted white dwarfs with precisely determined metal abundances to better understand the process of differentiation in exoplanetary systems.

scholarly journals Metal Abundances in Hot White Dwarfs: Predictions of the Diffusion Theory

1979 ◽  
Vol 53 ◽  
pp. 61-65
Author(s):  
Gérard Vauclair ◽  
Sylvie Vauclair
Keyword(s):  
White Dwarfs ◽  
Diffusion Processes ◽  
Diffusion Theory ◽  
Short Note ◽  
Theoretical Predictions ◽  
Metal Abundances ◽  
Radiative Acceleration

In the course of an exploration of the diffusion processes in white dwarfs, the role of the radiative acceleration has been investigated in detail and found to be quite important in hot white dwarfs (Vauclair, Vauclair and Greenstein, 1979; hereafter referred to as V2G). In view of the increasing interest accorded to hot white dwarfs which become accessible to observations in the ultraviolet owing to satellites like IUE, it appeared quite relevant to obtain theoretical predictions of the effect of the diffusion of metals in hot white dwarfs. This short note is a summary of the results presented in more detail elsewhere (V2G).

scholarly journals 16. The Line Spectra of White Dwarfs

1971 ◽  
Vol 42 ◽  
pp. 116-123
Author(s):  
P. A. Strittmatter ◽  
D. T. Wickramasinghe
Keyword(s):  
White Dwarfs ◽  
Temperature Range ◽  
Metal Abundances

Model atmospheres with temperatures in the range 10000 ≤ Teff ≤ 25000 K and with gravities 6 ≤ log(g) ≤ 9 have been constructed for various helium abundances with a view to understanding the spectra of the hotter white dwarfs. It is shown that the DB stars are confined to the temperature range 15000 ≤ Teff ≤ 18000 K in which convection also becomes important in the outer layers of helium rich stars. The DA stars seem to avoid this temperature range. The hydrogen and metal abundances in DB atmospheres are shown to be reduced by factors 105 and 103 respectively compared to solar values. Possible explanations of DB and DC spectra are discussed.

scholarly journals Metal Abundances in Cool White Dwarfs and the Diffusion Theory

1979 ◽  
Vol 53 ◽  
pp. 165-178
Author(s):  
Gérard Vauclair
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Convection Zone ◽  
Diffusion Theory ◽  
Short Review ◽  
Convective Mixing ◽  
Substantial Progress ◽  
Metal Abundances ◽  
Cool White Dwarfs ◽  
Made In

In this theoretical review about cool white dwarfs, I will restrict myself to the problem of the metallic content in white dwarf outer layers. The first section will be a short review of what we know about the metal abundances. The hottest presently known white dwarf showing metal in its spectrum is the DB GD 40 (Te = 15 000 K). This temperature will be considered here as the hot boundary of the “cool” white dwarfs. Many efforts have been recently devoted to the understanding of these metal abundances. Section 2 will be a summary of recent calculations of diffusion time scales in both hydrogen and helium white dwarfs. It will be seen that diffusion is so efficient in white dwarf conditions that the convection zone which develops in the envelope as the effective temperature decreases along the cooling sequence is never deep enough to bring back to the surface the metals which had previously diffused downwards. A discussion of the carbon white dwarfs, also called λ 4670 stars, will be presented in section 3. Recent calculations show that the convective mixing between a helium envelope and a carbon core would produce λ 4670 composition for only very special conditions and for this reason we believe that this is an improbable explanation for this type of white dwarfs. We clearly need another physical mechanism to compete with diffusion and to maintain an observable amount of metals in some cool white dwarf atmospheres. We discuss in section 4 the competition between diffusion and accretion. This seems a very promising mechanism in spite of the fact that considerable improvements are still needed in the theory of accretion. Substantial progress has to be made in this direction. A few problems related to this model are invoked in the conclusion.

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