Armagh Observatory Preprint Series No. xxx
Helium white dwarf mergers as progeny for extreme helium stars
Authors: C.S.Jeffery, H. SaioJournal: Stellar Collisions and Mergers, ASP Conf. Ser. ed. M.Shara , 2000 (in press)
Abstract. The origin of extreme helium stars poses a significant puzzle for stellar astrophysics. This paper summarizes the observational data concernin g extreme helium stars in general, and one, V652\,Her, in particular. One proposal involves the merger of two white dwarfs in a binary. Here, the rapid accretion of helium onto a helium white dwarf is used to simulate the merger of two helium white dwarfs. Helium-shell ignition occurs at the boundary between degenerate and non-degenerate helium aft er sufficient helium has been accreted, causing the star to become a yellow giant. Thereafter, the helium-burning shell burns in to the core, and the star contract s towards the helium main sequence where it will appear as a subdwarf B star. About halfway through this contraction, the model dimensions compare favourably with observations of mass, luminosity, effective temperature, surface compositio n, pulsation period and pulsation period change in V652\,Her. Competing models, such as the `late thermal pulse' model for example, fail to achieve such agreeme nt. The merged binary white dwarf hypothesis emerges, therefore, as the preferred explanation for the origin of at least some extreme helium stars.
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