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Description
We have surveyed M31 for its most luminous red stars; 300 candidates were found distributed across the face of the galaxy. JHK infrared photometry was observed for 57 of the candidates, and far-red (6800-8800A) spectra were also obtained for 45 stars. We have used the strength of the Ca II triplet (~8500A) and the J-H and H-K colors as luminosity discriminants to separate the supergiants from the foreground dwarfs. With these criteria, 23 probable late-type supergiants were identified. We discuss their distribution and relative numbers of the red supergiants in comparison with the known WR stars in M31. Accurate extinction and infrared (K=2.2{mu}m) and bolometric luminosities are also derived from the infrared photometry. We find that although visually bright red supergiants (M_V_=-8mag) exist in M31, the most luminous (M_bol_) red supergiants are not found in the relative numbers we would expect in a galaxy of its size and mass. It is probable that the rate of massive-star formation is less in M31, an Sb type spiral, than has been observed in the Sc spirals and irregulars.
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