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Session 67 - Stars: Evolution, Atmospheres, Intrinsic.
Display session, Thursday, June 11
Atlas Ballroom,
Stars bluer than the main sequence turnoff in old disk, near-solar metallicity populations have usually been considered relatively rare special cases and not a normal part of stellar evolution. Consequently, they have often been neglected in discussions of overall cluster properties. Attempts to understand the unexpected far-UV excess observed in external galaxies, plus the need to include all sources of blue light in population synthesis models, have led to many recent investigations of metal-rich blue stars, particularly subdwarf B stars. Our own investigations in open clusters and the field strongly suggest that most metal-rich BHB stars are in binaries. As a result, we speculate that metal-rich BHB stars may be related to blue stragglers, which are now also generally recognized to result from binary processes. We have recently identified a new sequence of stars that parallels the normal red giant branch but is displaced 0.1--0.2 mag blueward in B-V. We summarize the preliminary evidence in favor of binarism among this third group of stars, and suggest that they represent the giant branch evolution of blue stragglers, as well as possibly the progenitors of metal-rich BHB stars. In any case, the large relative numbers of blue stragglers, ''blue'' giant branch stars, and BHB stars in NGC 6791 and NGC 188 seem to require that we consider them a normal (if binary) result of stellar evolution.
