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E.M. Green, J. Liebert (Steward Observatory, U of Arizona), R.A. Saffer (Dept of Astr & Astrophys, Villanova)
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 binary sdB stars 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. A larger number with periods on the order of hours or a few days have essentially invisible companions.
We suggest that the long period sdB binaries must have been produced by Roche lobe overflow/mass transfer from low mass, metal-rich giants near the first red giant branch tip, without forming a common envelope. 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. We present new evidence that most short period sdB's result from a common envelope following subsequent Roche lobe overflow of the initial secondary onto one of these white dwarfs. Rare post-common envelope sdB + main sequence (MS) binaries also exist, but available data suggest that most such systems involving lower MS companions probably merge. The two known MS survivors in short period sdB binaries have small and nearly identical masses, implying that the MS secondary must have lost a large fraction of its initial mass along with the envelope of the red giant sdB progenitor. This work was supported by NSF grant AST97-31655.