Subluminous B Stars and Progenitors of Helium Core White Dwarfs

Abstract

It is now generally accepted that the subluminous B (sdB) stars can be identified with models for Extreme Horizontal Branch (EHB) stars burning He in their core, but with a very tiny inert hydrogen envelope (Heber, 1986). An EHB star bears great resemblance to a helium main-sequence star of half a solar mass and its further evolution should proceed similarly (i.e. directly to the white dwarf graveyard) as confirmed by evolutionary calculations (Dorman et al. 1993). How stars evolve to the EHB configuration is controversial. The problem is how the mass loss mechanism in the progenitor manages to remove all but a tiny fraction of the hydrogen envelope at precisely the same time as the He core has attained the minimum mass (≈ 0.5M_⊙) required for the He flash.