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M.S. O'Brien (Grinnell College, Grinnell, Iowa)
The initial stages of white dwarf evolution are characterized by high luminosity, high effective temperature, and increasingly high surface gravity, making it difficult to constrain stellar properties through traditional spectroscopic observations. We are aided, however, by the fact that many pre-white dwarfs (PWDs) are multiperiodic g-mode pulsators. These stars fall into two classes, the variable planetary nebula nuclei (PNNV) and the ``naked'' GW~Vir stars. Pulsations in PWDs provide a unique opportunity to probe their interiors, which are otherwise inaccesible to direct observation. Until now, however, the nature of the pulsation mechanism, the precise boundaries of the instability strip, and the mass distribution of the PWDs were complete mysteries. These problems must be addressed before we can apply knowledge of pulsating PWDs to improve understanding of white dwarf formation.
This thesis lays the groundwork for future theoretical investigations of these stars. We first use Whole Earth Telescope observations to determine the mass and luminosity of the majority of the GW~Vir pulsators. We find that pulsators of low mass have higher luminosity, suggesting the range of instability is highly mass-dependent. The observed trend of decreasing periods with decreasing luminosity matches a decrease in the maximum theoretical g-mode period accross the instability strip. We then show that the red edge can be caused by the lengthening of the driving timescale beyond the maximum sustainable period. This result is general for ionization-based driving mechanisms and explains the mass-dependence of the red edge. The form of the mass-dependence provides a vital starting point for future theoretical investigations of the driving mechanism. The blue edge probably remains undetected because of selection effects arising out of rapid evolution. Finally, we show that the observed rate of period change in cool GW~Vir pulsators will constrain neutrino emission in their cores, and we identify appropriate targets for future observation.
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The author(s) of this abstract have provided an email address for comments about the abstract: obrien@ac.grin.edu