The Red Supergiant Content of M31
  We investigate the red supergiant (RSG) population of M31, obtaining radial
velocities of 255 stars. These data substantiate membership of our
photometrically-selected sample, demonstrating that Galactic foreground stars
and extragalactic RSGs can be distinguished on the basis of B-V, V-R two-color
diagrams. In addition, we use these spectra to measure effective temperatures
and assign spectral types, deriving physical properties for 192 RSGs.
Comparison with the solar-metallicity Geneva evolutionary tracks indicates
astonishingly good agreement. The most luminous RSGs in M31 are likely evolved
from 25-30 Mo stars, while the vast majority evolved from stars with initial
masses of 20 Mo or less. There is an interesting bifurcation in the
distribution of RSGs with effective temperatures that increases with higher
luminosities, with one sequence consisting of early K-type supergiants, and
with the other consisting of M-type supergiants that become later (cooler) with
increasing luminosities. This separation is only partially reflected in the
evolutionary tracks, although that might be due to the mis-match in
metallicities between the solar Geneva models and the higher-than-solar
metallicity of M31. As the luminosities increase the median spectral type also
increases; i.e., the higher mass RSGs spend more time at cooler temperatures
than do those of lower luminosities, a result which is new to this study.
Finally we discuss what would be needed observationally to successfully build a
luminosity function that could be used to constrain the mass-loss rates of RSGs
as our Geneva colleagues have suggested.
