Modeling He-rich subdwarfs through the Hot-flasher scenario

MILLER BERTOLAMI, M. M.; ALTHAUS, L. G.; UNGLAUB, K.; WEISS, A.

Barcelona, Espana

Workshop; 16th European White Dwarf Workshop; 2008

IEEC

Castellani & Castellani (1993) showed that, if enough mass is lost on the first red giant branch (RGB), low mass stars can depart from the RGB and undergo the helium core flash while they are entering in the white dwarf stage. These stars were termed Hot-Flashers by D’Cruz et al. (1996). Interestingly enough such high mass loss rates have been very recently inferred in some old and metal rich globular clusters (Kalirai et al. 2008) and play an important role in one of the most successful hypothesis for explaining the UV-upturn of elliptical galaxies (Yi & Yoon 2004). Sweigart (1997) showed that, under certain circumstances, the H-rich envelope can be engulfed during a Hot-Flash event by the convective zones developed during the primary He-core flash. In those cases H is burnt and a He-enriched surface if formed. He-enrichment can also be attained through dilution of the H-rich envelope in the He-rich interior of the star as showed by Lanz et al. (2004). Consequently, the Hot-Flasher scenario has been proposed together with the merger of two He-core white dwarfs (Saio & Jeffery 2000) to explain the observed properties field He-sdO stars (Stroeer et al. 2007) and to explain the extreme horizontal branch of some open and globular clusters (D’Cruz et al. 2000, Moehler et al. 2002, 2004, 2007).