Pre-white dwarf pulsation: variations on a theme

Date
1998
Authors
O'Brien, Michael
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Physics and Astronomy
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One of the least understood aspects of white dwarf evolution is the process by which they are formed. 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 their 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 use Whole Earth Telescope observations to determine the mass and luminosity of the majority of the GW Vir pulsators. With these observations, we identify the common properties and trends PWDs exhibit as a class;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 across the instability strip. We 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 it explains the mass-dependence of the red edge. The exact observed form of the mass-dependence provides a vital starting point for future theoretical investigations of the driving mechanism. We also show that 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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Physics and astronomy, Astrophysics
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