THEORETICAL ASTEROSEISMOLOGY OF WHITE DWARF STARS: THE ENCYCLOPEDIA SEISMOLOGICA

Abstract

ABSTRACT White dwarfs represent the most common fate suffered by stars as they exhaust their supply of nuclear fuel and cool to oblivion. According to conventional wisdom, these objects have degenerate carbon/oxygen cores overlain by a pure helium mantle (DB white dwarfs); in many cases, a thin veneer of hydrogen lies on the surface (DA white dwarfs). The PG 1159 stars are pre-white dwarfs that hold clues to their previous evolution through the planetary nebula stage; about half of these stars pulsate. My goal is to understand the internal structure of these stars by applying the tools of asteroseismology. I present parametric grids of evolutionary pre-white dwarf, DB, and DA models and develop the theoretical tools necessary to determine the internal structure of these stars. I use the nearly continuous photometric data provided by the Whole Earth Telescope on the pre-white dwarf PG 1159--035, and the DBV white dwarfs GD 358 and PG 1115+158 to determine their internal structure in detail. The mass of PG 1159-035 is 0.59 ± 0.01 solar mass, GD 358 has a mass of 0.61 ± 0.03 solar mass, and the mass of PG 1115+158 is 0.62 ± 0.06 solar mass. All three masses are similar to the mean masses of white dwarfs in general. The He/C/O surface layer mass of PG 1159-035 is 3.9 X M*3, which can be duplicated by stellar evolution models, although they cannot duplicate the observed surface abundance, suggesting that our knowledge of mass loss is incomplete. The helium layer mass of GD 358 is 2.0 X M*6 and is < M*4 for PG 1115+158. These helium layers are vastly thinner than suggested by stellar evolution theory and present problems for current evolutionary models of DB white dwarfs. If these helium layer masses are correct, then we must revise our thinking on how convective dredge up works in the presence of diffusion. I also present preliminary assessments of the structure of several pulsating DA white dwarf (DAV) stars. In spite of the relatively small number of observable modes in the DAV white dwarfs, several of them appear to have thick hydrogen layer masses (~ M*(5-6)), based on existing stellar mass estimates and assumptions about the identity of the observed modes. Finally, I describe how my structure determinations and recent spectroscopic observations may help to clarify our picture of white dwarf evolution.