COMMON-ENVELOPE MERGERS: A POSSIBLE CHANNEL FOR FORMING SINGLE sdB STARS

We quantify an evolutionary channel for single sdB stars based on mergers of binaries containing a red giant star and a lower mass main-sequence or brown dwarf companion in our Galaxy. Population synthesis calculations that follow mergers during the common-envelope phase of evolution of such systems reveal a population of rapidly rotating horizontal branch stars with a distribution of core masses between 0.32 and 0.7 M(circle dot) that is strongly peaked between 0.47 and 0.54 M(circle dot). The high rotation rates in these stars are a natural consequence of the orbital angular momentum deposition during the merger and the subsequent stellar contraction of the merged object from the tip of the red giant branch. We suggest that centrifugally enhanced mass loss facilitated by the rapid rotation of these stars may lead to the formation of single sdB stars for some of these objects.