AAS 195th Meeting, January 2000
Session 44. Late Stages of Stellar Evolution
Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall

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[44.06] The Born Again Phenomenon: Exploratory Evolutionary Models including Mass Loss

T.M. Lawlor, J. MacDonald (Univ. of Delaware)

The Born Again Phenomenon, also called a late helium flash, occurs in approximately 10 percent of all white dwarfs. A handful of such objects has been detected observationally, most notably Sakuraiís Object. We present and examine a grid of evolutionary models, evolved from the late AGB to the white dwarf cooling track, each calculated with a different mass loss rate. The progenitor main sequence stars are 1 solar mass and have a range of metallicities between 0.001 and 0.02. We begin our models at the peak of the last helium burning pulse on the AGB and determine a range of mass loss in which models experience a very late helium flash. We search for differences between models of the same metallicity that undergo the very late flash and those that do not. Such differences include helium mass and helium, hydrogen and CNO surface abundance at the time of and after the final AGB helium pulse. The affects of mass gain, such as is possible in binary systems, on white dwarf evolution are also discussed as a work in progress.

Our reasonably sized grid of evolved models also allows us to address other important questions concerning white dwarf evolution: In each case, we have found no models that can be considered to be hydrogen deficient; a condition observed in non-DA white dwarfs and PG1159 type stars. This includes models that experience a very late flash. From our models, we also determine how much helium is left late into the white dwarf cooling track.

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