Red Supergiants in the Andromeda Galaxy (M31)

Philip Massey$^1$,
David R. Silva$^2$,
Emily M. Levesque$^3$,
Bertrand Plez$^4$,
Knut A. G. Olsen$^2$,
Geoffrey C. Clayton$^5$,
George Meynet$^6$,
and Andre Maeder $^6$

1--Lowell Observatory;
2--National Optical Astronomy Observatory;
3--Institute for Hawaii, University of Hawaii;
4--GRAAL, Universite Montpellier II;
5--Dept of Physics and Astronomy, Louisiana State University;
6---Geneva University, Geneva Observatory

Red supergiants (RSGs) are a short-lived stage in the evolution of moderately massive
stars (10-25$M_odot$), and as such their location in the H-R diagram provides
an exacting test of stellar evolutionary models. Since massive star evolution is
strongly affected by the amount of mass-loss a star suffers, and since the mass-loss
rates depend upon metallicity, it is highly desirable to study the physical properties
of these stars in galaxies of various metallicities. Here
we identify a sample of red supergiants in M31, the most metal-rich of the Local Group
galaxies. We determine the physical
properties of these stars using both moderate resolution spectroscopy and broad-band $V-K$
photometry. We find that on average the RSGs are our sample are variable in $V$
by 0.5~mag, smaller but comparable to the 0.9~mag found for Magellanic Cloud (MC) RSGs.
No such variability is seen at $K$, also in accord with what we know of Galactic and
MC RSGs. We find that there is a saturation effect in the model TiO band strengths
with metallicities higher than solar.
The physical properties we derive for the RSGs from our analysis with stellar atmosphere
models agree well with the current evolutionary tracks, a truly remarkable achievement given
the complex physics involved in each.
We do not confirm an earlier result that
the upper luminosities of RSGs depends upon metallicity; instead, the most luminous
RSGs have $log L/L_odot sim$5.2-5.3. We find that, on average,
the RSGs are considerably more reddened than O and B stars, suggesting that
circumstellar dust is adding a significant amount of extra
extinction, $sim$0.5~mag, on average. This is in accord with our earlier findings
on Milky Way and Magellanic Cloud stars. Finally, we call attention to a peculiar star whose
spectrum appears to be heavily veiled, possibly due to scattering by an expanding dust

Reference: ApJ, in press
Status: Manuscript has been accepted