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 Driving g-Mode Pulsation in γ Doradus Variables

2002 ◽  
Vol 185 ◽  
pp. 506-507
Author(s):  
Yanqin Wu
Keyword(s):  
White Dwarfs ◽  
Driving Mechanism ◽  
Convective Flux ◽  
Temperature Range ◽  
Surface Convection

AbstractSurface convection zones in γ Doradus variables (early F-type g-mode pulsators) can drive pulsation via the ‘convective driving’ mechanism. This mechanism is first proposed and studied in the context of ZZ Cetis (hydrogen-envelope white dwarfs with g-mode pulsations) by Brickhill (1991) and Goldreich & Wu (1999). We implement and develop the convective driving theory to suit the environments on γ Dors, in particular, stronger super-adiabatic gradient and weaker convective flux than those in white dwarfs. This results in a spectrum of gravity-modes being excited in the observed temperature range. However, there are more remaining problems than those that are solved.

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

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 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 Convective overshoot and macroscopic diffusion in pure-hydrogen-atmosphere white dwarfs

2019 ◽  
Vol 488 (2) ◽  
pp. 2503-2522 ◽  
Author(s):  
Tim Cunningham ◽  
Pier-Emmanuel Tremblay ◽  
Bernd Freytag ◽  
Hans-Günter Ludwig ◽  
Detlev Koester
Keyword(s):  
Diffusion Coefficients ◽  
White Dwarfs ◽  
Hydrogen Atmosphere ◽  
Theoretical Description ◽  
Time Variability ◽  
Pure Hydrogen ◽  
Temperature Range ◽  
Tracer Particles ◽  
Order Of Magnitude ◽  
Convective Overshoot

Abstract We present a theoretical description of macroscopic diffusion caused by convective overshoot in pure-hydrogen DA white dwarfs using 3D, closed-bottom, radiation hydrodynamics co5bold simulations. We rely on a new grid of deep 3D white dwarf models in the temperature range $11\, 400 \le T_{\mathrm{eff}} \le 18\, 000$ K where tracer particles and a tracer density are used to derive macroscopic diffusion coefficients driven by convective overshoot. These diffusion coefficients are compared to microscopic diffusion coefficients from 1D structures. We find that the mass of the fully mixed region is likely to increase by up to 2.5 orders of magnitude while inferred accretion rates increase by a more moderate order of magnitude. We present evidence that an increase in settling time of up to 2 orders of magnitude is to be expected, which is of significance for time-variability studies of polluted white dwarfs. Our grid also provides the most robust constraint on the onset of convective instabilities in DA white dwarfs to be in the effective temperature range from 18 000 to 18 250 K.

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.

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