The born again phenomenon: Details, cause, and implications for post-AGB stellar evolution

Details The born again phenomenon: Details, cause, and implications for post-AGB stellar evolution The born again phenomenon: Details, cause, and implications for post-AGB stellar evolution

Mar 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt.........1l&link_type=abstract

Thesis (PhD). UNIVERSITY OF DELAWARE, Source DAI-B 62/09, p. 4050, Mar 2002, 133 pages.

Astronomy and Astrophysics

Astrophysics

Scientific paper

Only a handful of very late helium flash, or Born Again (BA) objects have been observed. Among these are Sakurai's object, FG Sge, and V605 Aql. Each of these objects have been found to increase in luminosity by a few orders of magnitude, decrease in temperature and experience broad changes in chemical composition in a remarkably brief time. Our evolutionary models used to study this phenomena, have been fully evolved from pre- main sequence though the white dwarf phase and include the effects of combined hydrogen and helium burning. All models have a progenitor mass of 1 solar mass and we vary metallicity between Z = 0.001 and Z = 0.020. We alter mass loss beginning at the peak of the final asymptotic giant branch (AGB) helium pulse; in this way, we produce a sequence of models with different helium shell masses upon departure from the AGB (as well as different remnant masses). We achieve this by varying the parameter which appears in the Reimer's mass loss equation, the mass loss scheme that we use for cool stars, such as red giants and asymptotic giant branch stars. Using this grid of evolutionary models, we uncover details of this unusual phenomena and determine the criterion for its occurrence. Moreover, we determine a general global criterion for predicted behavior during post-AGB evolution. In doing so, we predict the percent of stars (of solar progenitor mass) that we expect to undergo the BA helium flash. Theoretical models are further used to predict the future behavior of the three observed BA objects. We introduce here the very late giant branch (VLGB) and discuss a secondary phenomena known as an AGB final thermal pulse (AFTP). It has been suggested in numerous papers that this phenomena may be linked to a variety of astrophysical objects: RCrb type stars, Wolf-Rayet central stars [WC], hydrogen deficient CSPN, PG 1159 type stars, and non-DA white dwarfs. We provide supporting evidence for each being linked to the BA helium flash through direct theoretical-observational comparisons. For this we rely on surface chemical composition. In particular, we find that only models that undergo this phenomena leave the AGB deficient in hydrogen and enriched in helium at the surface.

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