AAS 201st Meeting, January, 2003
Session 17. Stellar Evolution and Metal-Poor Stars
Poster, Monday, January 6, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[17.08] An HST Search for White Dwarf Companions to 'Blue' Giants in NGC 188 and NGC 6791

B. Chaboyer, E. Dambach (Dartmouth College), E.M. Green (Steward Observatory), W.B. Landsman (GSFC)

Subdwarf B (sdB) stars are evolved hot stars (24000 to 35000K) that are commonly found in the field of our Galaxy and have recently been identified in old disk, metal-rich clusters. They are likely to be one of the most important contributors to the upturn in the far-ultraviolet (FUV) flux exhibited by many elliptical galaxies and the bulge of M31. It is believed that sdB stars are core helium burning stars with very thin hydrogen envelopes. The mechanism which stripped the hydrogen envelope is not currently known. sdB stars could be formed from binary interactions. In particular, they may have evolved from anomalously blue giants, which in turn have evolved from blue stragglers. These `blue' giants have been identified by us in NGC 188 and 6791, and are 0.1 - 0.2 mag bluer in B-V than the normal red giant stars in each cluster. If the binary formation theory is correct, a significant fraction of these giants will have white dwarf companions that might be detected in the FUV. We have tested this scenario in a snapshot program to obtain STIS FUV images of a six of the bluer giants in both clusters. No FUV flux was detected in the NGC 6791 images, while 2 stars were detected in the NGC 188 sample. The non-detection in the NGC 6791 sample may simply be due to the large distance to this cluster and the relatively faint luminosity of white dwarfs. The FUV flux detected in the two NGC 188 stars could be due to a white dwarf companion, or it could be FUV flux from the hot chromosphere surrounding the giant star.

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Bulletin of the American Astronomical Society, 34, #4
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