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E.M. Green (Steward Obs., Univ. of Arizona), B.C. Chaboyer (Dartmouth College)
Subdwarf B (sdB) stars are believed to be core helium burning stars that have lost nearly all of their hydrogen envelopes. They are commonly found in the field of our Galaxy, are kinematically associated with the old disk, and have recently been identified on the extreme hot end of the horizontal branch in two metal-rich old open clusters. The energy distributions and lifetimes of sdB stars indicate that they are one of the most important contributors to the upturn in the far-ultraviolet flux exhibited by many elliptical galaxies and the bulge of M31. Several recent population synthesis studies have attempted to model their effect on the integrated light of old stellar systems by assuming that sdB's result from standard single-star evolution in sufficiently metal and helium enriched low mass stars.
There is now clear evidence that most sdB's occur in binaries, both in the field and in open clusters. Binary mass transfer and common envelope ejection offer mechanisms for the extreme mass loss necessary to create these stars, yet the details of such evolution in low density, metal-rich populations are very poorly understood. NGC\,6791 and NGC\,188, the only two open clusters known to have sdB stars, also contain large numbers of blue stragglers, another group of poorly understood stars thought to be the result of binary processes. We have identified a new sequence of giants in both clusters, parallel to the normal red giant branch but displaced 0.1 to 0.2 magnitudes blueward in B-V, which appears to be the missing link between the binary blue stragglers and binary sdB's. Specifically, we propose that metal-rich sdB stars evolve from anomalously blue giants, which in turn evolve from mass transfer blue stragglers. We summarize the evidence in favor of our hypothesis, list some of the implications, and outline our future research.