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Title:
The Evolution and Pulsation of Crystallizing White Dwarf Stars
Authors:
Montgomery, Michael Houston
Affiliation:
AA(THE UNIVERSITY OF TEXAS AT AUSTIN)
Publication:
Thesis (PHD). THE UNIVERSITY OF TEXAS AT AUSTIN , Source DAI-B 60/07, p. 3327, Jan 2000, 197 pages.
Publication Date:
00/1998
Category:
Physics: Astronomy and Astrophysics
Origin:
UMI
Comment:
Publication Number: AAT 9937100; Advisor: WINGET, D. E.
Bibliographic Code:
1998PhDT........21M

Abstract

This thesis addresses the physics relevant in crystallizing white dwarf stars. This problem is not merely of academic interest, since white dwarf stars provide us with one of the best methods for estimating for the age of the local Galactic disk. In addition, understanding these stars allows us to probe the physics of matter at temperatures and densities otherwise inaccessible in present-day laboratories. In the first part of my thesis, I explore the effect which phase separation of Carbon and Oxygen can have on the ages of white dwarf stars. I find that this additional energy source can lengthen white dwarf ages by at most 1.5 Gyr, with more likely values being in the range 0.4-0.6 Gyr. 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. These relatively small age delays are consistent with recent results that the oldest globular clusters may only be 2 Gyr older than the local Galactic disk. In the second part of my thesis, I consider the effect which a crystalline core has on the pulsations of white dwarf stars. From global calculations of g-mode eigenfunctions which include the response of the crystalline core, I find that the amplitudes of the g-modes are greatly reduced in the solid region. As a result, the g-mode oscillations can be accurately modeled by a modified boundary condition in which the g-modes are excluded from the solid core. As the white dwarf models become more crystallized, the mean period spacing and the periods themselves are lengthened, and can increase by as much as 30% for a model which is 90% crystallized by mass. I also show how mode trapping information can be used to disentangle the effects due to the hydrogen layer mass from those due to crystallization. If we are able to obtain mode identifications for enough modes in the DAV BPM 37093, then we may be able to ``empirically'' measure the degree of crystallization which is present in this object, and thereby test the theory of crystallization itself, now more than 30 years old. The simplicity of white dwarf stars makes them ideal targets of study, since we believe we can adequately model the physical processes occurring inside them. In addition, 98% of all stars are believed to end their lives as white dwarf stars. If we can understand these stars, then we can provide final boundary conditions for evolution of post-main sequence models. In this sense, they are fundamental objects.
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