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Session 96 - Cepheids and Supergiant Variable Stars.
Display session, Thursday, January 16
Metropolitan Ballroom,
Previous studies have identified red stars across the face of NGC 6822, M31, and M33, but such samples are highly contaminated by foreground galactic M dwarfs. Using BVR CCD photometry of selected regions in these Local Group galaxies, plus neighboring control fields, I have developed a photometric criterion which sucessfully distinguishes foreground stars from bona fide Red Supergiants (RSGs). Follow-up spectroscopy with the Kitt Peak 4-m and WIYN telescopes are used to confirm the method, based both on the strengths of the Ca triplet and the radial velocities. We find that the most luminous RSGs are completely lacking in M31, in sharp contrast to the lower-metallicity systems NGC 6822 and M33. This finding is consistent with the predictions of the ``Conti scenario" (Conti 1976 Mem.\ Soc.\ Sci.\ Leige, 6^e Ser..\ 9, 193; Maeder amp; Conti 1994 ARAA, 32, 227) which suggests that in a higher metallicity system stars that might have become RSGs are more likely to become Wolf-Rayet stars due to increased mass-loss rates.
