Luminous and Variable Stars in M31 and M33. III. The Yellow and Red
  Supergiants and Post-Red Supergiant Evolution
  Recent supernova and transient surveys have revealed an increasing number of
non-terminal stellar eruptions. Though the progenitor class of these eruptions
includes the most luminous stars, little is known of the pre-supernova
mechanics of massive stars in their most evolved state, thus motivating a
census of possible progenitors. From surveys of evolved and unstable luminous
star populations in nearby galaxies, we select a sample of yellow and red
supergiant candidates in M31 and M33 for review of their spectral
characteristics and spectral energy distributions. Since the position of
intermediate and late-type supergiants on the color-magnitude diagram can be
heavily contaminated by foreground dwarfs, we employ spectral classification
and multi-band photometry from optical and near-infrared surveys to confirm
membership. Based on spectroscopic evidence for mass loss and the presence of
circumstellar dust in their SEDs, we find that $30-40\%$ of the yellow
supergiants are likely in a post-red supergiant state. Comparison with
evolutionary tracks shows that these mass-losing, post-RSGs have initial masses
between $20-40\,M_{\odot}$. More than half of the observed red supergiants in
M31 and M33 are producing dusty circumstellar ejecta. We also identify two new
warm hypergiants in M31, J004621.05+421308.06 and J004051.59+403303.00, both of
which are likely in a post-RSG state.
