scholarly journals Pulsations of White Dwarf Stars with Thick Hydrogen or Helium Surface Layers

1988 ◽  
Vol 123 ◽  
pp. 333-337
Author(s):  
Arthur N. Cox ◽  
Sumner G. Starrfield ◽  
Russell B. Kidman ◽  
W. Dean Pesnell
Keyword(s):  
Stellar Evolution ◽  
White Dwarf ◽  
Convection Zone ◽  
Helium Surface ◽  
Surface Layers ◽  
Dwarf Stars ◽  
Surface Mass ◽  
White Dwarf Stars ◽  
Surface Convection ◽  
Nonradial Pulsation

In order to see if there could be agreement between results of stellar evolution theory and those of nonradial pulsation theory, calculations of white dwarf models have been made for hydrogen surface masses of 10−4M⊙. Earlier results by Winget et al. (1982) indicated that surface masses greater than 10−8M⊙ would not allow nonradial pulsations, even though all the driving and damping is in surface layers only 10−12 of the mass thick. We show that the surface mass of hydrogen in the pulsating white dwarfs (ZZ Ceti variables) can be any value as long as it is thick enough to contain the surface convection zone.

scholarly journals Pulsations of White Dwarf Stars with Thick Hydrogen or Helium Surface Layers

1988 ◽  
pp. 333-337
Author(s):  
Arthur N. Cox ◽  
Sumner G. Starrfield ◽  
Russell B. Kidman ◽  
W. Dean Pesnell
Keyword(s):  
White Dwarf ◽  
Helium Surface ◽  
Surface Layers ◽  
Dwarf Stars ◽  
White Dwarf Stars

scholarly journals Pulsations of white dwarf stars with thick hydrogen or helium surface layers

1987 ◽  
Vol 317 ◽  
pp. 303 ◽  
Author(s):  
Arthur N. Cox ◽  
Russell B. Kidman ◽  
Sumner G. Starrfield ◽  
W. Dean Pesnell
Keyword(s):  
White Dwarf ◽  
Helium Surface ◽  
Surface Layers ◽  
Dwarf Stars ◽  
White Dwarf Stars

scholarly journals An Example Demonstrating the Potential for Asteroseismology of DB White Dwarf Stars

1993 ◽  
Vol 139 ◽  
pp. 116-116
Author(s):  
P.A. Bradley ◽  
M.A. Wood
Keyword(s):  
Internal Structure ◽  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Galactic Disk ◽  
Helium Surface ◽  
Dwarf Stars ◽  
University Of Texas ◽  
White Dwarf Stars ◽  
History Of

AbstractWe present the results of a parametric survey of evolutionary models of compositionally stratified white dwarfs with helium surface layers (DB white dwarfs). Because white dwarfs are the most common final end state of stellar evolution, determining their internal structure will offer us many clues about stellar evolution, the physics of matter under extreme conditions, plus the history of star formation and age of the local Galactic disk. As a first step towards determining the internal structure of DB white dwarf stars, we provide a comprehensive set of theoretical g-mode pulsation periods for comparison to observations.Because DB white dwarfs have a layered structure consisting of a helium layer overlying the carbon/oxygen core, some modes will have the same wavelength as the thickness of the helium layer, allowing a resonance to form. This resonance is called mode trapping (see Brassard et al. 1992 and references therein) and has directly observable consequences, because modes at or near the resonance have eigenfunctions and pulsation periods that are similar to each other. This results in much smaller period spacings between consecutive overtone modes of the same spherical harmonic index than the uniform period spacings seen between non-trapped modes. We demonstrate with an example how one can use the distribution of pulsation periods to determine the total stellar mass, the mass of the helium surface layer, and the extent of the helium/carbon and carbon/oxygen transition zones. With these tools, we have the prospect of being able to determine the structure of the observed DBV white dwarfs, once the requisite observations become available.We are grateful to C.J. Hansen, S.D. Kawaler, R.E. Nather, and D.E. Winget for their encouragement and many discussions. This research was supported by the National Science Foundation under grants 85-52457 and 90-14655 through the University of Texas and McDonald Observatory.

scholarly journals Pulsations of white dwarf stars with thick hydrogen or helium surface layers

2008 ◽  
pp. 358-362
Author(s):  
Arthur N. Cox ◽  
Sumner G. Starrfield ◽  
Russell B. Kidman ◽  
W. Dean Pesnell
Keyword(s):  
White Dwarf ◽  
Helium Surface ◽  
Surface Layers ◽  
Dwarf Stars ◽  
White Dwarf Stars

scholarly journals The High Resolution Spectrum of the Pulsating, Pre-White Dwarf Star PG 1159-035 (GW VIR)

1989 ◽  
Vol 114 ◽  
pp. 384-387
Author(s):  
James Liebert ◽  
F. Wesemael ◽  
D. Husfeld ◽  
R. Wehrse ◽  
S. G. Starrfield ◽  
...  
Keyword(s):  
High Resolution ◽  
Effective Temperature ◽  
White Dwarf ◽  
White Dwarfs ◽  
High Resolution Spectrum ◽  
Dwarf Star ◽  
Dwarf Stars ◽  
New Class ◽  
White Dwarf Stars ◽  
Nonradial Pulsation

First reported at the IAU Colloquium No. 53 on White Dwarfs (McGraw et al. 1979), PG 1159-035 (GW Vir) is the prototype of a new class of very hot, pulsating, pre-white dwarf stars. It shows complicated, nonradial pulsation modes which have been studied exhaustively, both observationally and theoretically. The effective temperature has been crudely estimated as 100,000 K with log g ~ 7 (Wesemael, Green and Liebert 1985, hereafter WGL).

(Pre)-white dwarf stars as measuring tools for yields of AGB nucleosynthesis

2019 ◽  
Vol 15 (S357) ◽  
pp. 158-161
Author(s):  
Lisa Löbling
Keyword(s):  
White Dwarf ◽  
Neutron Capture ◽  
Convection Zone ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Heavy Elements ◽  
Agb Stars ◽  
Subsequent Evolution ◽  
Dwarf Stars ◽  
White Dwarf Stars

AbstractIn the helium-rich intershell region of asymptotic giant branch (AGB) stars, slow neutron-capture nucleosynthesis produces heavy elements beyond iron. If the stars experience a final-flash of the He-burning shell, a pulse-driven convection zone establishes, the stars become hydrogen-deficient and exhibit former intershell material at their surfaces. In their subsequent evolution towards the white-dwarf cooling sequence, but still at constant luminosity, a strong stellar wind prevents diffusion to wipe out the information about AGB yields. We present and interpret the analysis results of hydrogen-rich and -deficient post-AGB stars, discuss difficulties in their analysis and review the implications on the understanding of post-AGB evolution.

scholarly journals The Effects of Time-Dependent Convection on White Dwarf Radial Pulsations

1989 ◽  
Vol 111 ◽  
pp. 259-259
Author(s):  
Arthur N. Cox ◽  
Sumner G. Starrfield
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Convection Zone ◽  
Time Dependent ◽  
Radial Pulsation ◽  
Helium Surface ◽  
Surface Layers ◽  
Solar Mass ◽  
Convection Model ◽  
Radial Pulsations

AbstractAfter the discovery of pulsations in white dwarfs, predictions were made that these DA and the hotter DB stars should be pulsating in radial modes with periods of a few seconds or less. The mechanisms are the normal kappa and gamma effects that periodically block the flow of radiative luminosity and the blocking effect of the frozen-in convection at the bottom of the convection zone. Blue edges of the instability strips are between 12,000K and 13,000K for the DA and between 32,000K and 33,000K for the DB variables. Extensive observations, however, have shown that these stars pulsate only in the few-hundred-second nonradial modes and not in any few-second radial modes. We have added the time dependent convection model of Cox, Brownlee, and Eilers (1966) to our pulsation analyses to further investigate the white dwarf radial modes. Since the time scale of the convection is usually short compared to the radial pulsation periods, convection is able to carry luminosity rapidly enough to nullify the kappa and gamma effects periodic radiation blocking. We find that most, and maybe all, radial pulsations for 0.6 solar mass carbon-oxygen white dwarfs with thin hydrogen or helium surface layers are stabilized for both these DA and DB classes, now finally in agreement with observations.

scholarly journals Forever young white dwarfs: when stellar ageing stops

2020 ◽  
Author(s):  
Maria Camisassa ◽  
Leandro Althaus ◽  
Santiago Torres ◽  
Alejandro Córsico ◽  
Sihao Cheng ◽  
...  
Keyword(s):  
Energy Source ◽  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Space Mission ◽  
Dwarf Stars ◽  
End Point ◽  
White Dwarf Stars ◽  
Single Standard ◽  
Stellar Mergers

Abstract White dwarf stars are the most common end point of stellar evolution. In particular, ultra-massive white dwarfs are expected to harbour oxygen-neon (ONe) cores as a result of single standard stellar evolution. However, a fraction of them could have carbon-oxygen (CO) cores and be born as a result of merger events. Recent observations provided by Gaia space mission, indicate that a fraction of the ultra-massive white dwarfs experience a strong delay in their cooling, which cannot be attributed only to the occurrence of crystallization, thus requiring an unknown energy source able to prolong their life for long periods of time. Here, we show that the energy released by 22Ne sedimentation in ultra-massive white dwarfs with CO cores is at the root of the long cooling delay of these stars. Our results provide solid sustain to the existence of CO-core ultra-massive white dwarfs and the occurrence of stellar mergers.

Variable white dwarfs

2019 ◽  
Vol 15 (S357) ◽  
pp. 107-109
Author(s):  
H. L. Shipman
Keyword(s):  
Present State ◽  
White Dwarf ◽  
White Dwarfs ◽  
Long Term Evolution ◽  
Surface Layers ◽  
Dwarf Stars ◽  
Comprehensive View ◽  
White Dwarf Stars ◽  
Term Evolution

AbstractAsteroseismology of white dwarf stars has led to a number of interesting results pertaining to the long term evolution and present state of white dwarf interiors. I will review recent results and will give a not necessarily comprehensive view of the prospects for further progress in this area. Two – but only two white dwarf stars - have shown the expected cooling as they age. Careful observations of a few white dwarfs with rich pulsational properties reveal interior compositions as well as the thickness of their surface layers. A few very well observed stars have revealed changes in their pulsational spectra which we don’t understand yet.

scholarly journals The Internal Structure of White Dwarf Stars Using the Whole Earth Telescope

1993 ◽  
Vol 139 ◽  
pp. 117-119
Author(s):  
P.A. Bradley
Keyword(s):  
Internal Structure ◽  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
History Of ◽  
The Way ◽  
Whole Earth

AbstractWhite dwarfs are the final end state for the majority of stars, and hold clues to help solve many current pressing astrophysical problems. We can perform asteroseismology on the pulsating white dwarfs to better understand their internal structure and input physics, paving the way to a better understanding of astrophysics, stellar evolution, and the history of our Galaxy. I describe briefly the potential of asteroseismology by using it to infer the internal structure of PG1159-035.

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