On the Origin of Subdwarf B Stars and Related Metal-Rich Binaries

Abstract

Mounting evidence from subdwarf B (sdB) stars in the galactic field and their recently discovered counterparts in old open clusters indicates that at least two thirds of local disk sdB stars are binaries. Our recent radial velocity survey showed that sdB binaries naturally divide into two groups with contrasting spectroscopic and kinematic properties. Those with detectable spectral lines from a cooler companion invariably have periods longer than a year, while very short period sdB's have essentially invisible companions. We derive typical orbital separations for the components of the composite spectrum sdB's from their velocities. The current systems must have been produced by Roche lobe overflow/mass transfer from low mass, metal-rich giants near the first red giant branch tip, without undergoing common envelope envolution. The same process should also occur at slightly lower red giant luminosities, producing a wide binary with a helium white dwarf instead of an sdB star. Most short period sdB's probably result from a common envelope following Roche lobe overflow of the initial secondary onto the white dwarf. Rare post-common envelope sdB + main sequence (MS) binaries also exist, but available data suggest that most such systems involving lower MS companions end up merging. The small and nearly identical masses of the two known MS survivors in short period sdB binaries imply that both components must have lost a large fraction of their initial mass in the common envelope process.