The Yellow and Red Supergiants of M33

Maria R. Drout (1, 2), Philip Massey (2), and Georges Meynet (3)

1 -- Center for Astrophysics, Harvard University; 2--Lowell Observatory; 3--Geneva University

Yellow and red supergiants are evolved massive stars whose numbers and lo- cations on the HR diagram can provide a stringent test for models of massive star evolution. Previous studies have found large discrepancies between the rel- ative number of yellow supergiants observed as a function of mass and those predicted by evolutionary models, while a disagreement between the predicted and observed locations of red supergiants on the HR diagram was only recently resolved. Here we extend these studies by examining the yellow and red super- giant populations of M33. Unfortunately, identifying these stars is difficult as this portion of the color-magnitude diagram is heavily contaminated by foreground dwarfs. We identify the red supergiants through a combination of radial velocities and a two-color surface gravity discriminant and, after re-characterizing the rotation curve of M33 with our newly selected red supergiants, we identify the yellow supergiants through a combination of radial velocities and the strength of the OI 7774 triplet. We examine about 1300 spectra in total and identify 121 yellow supergiants (a sample which is unbiased in luminosity above log(L/Lsun) = 4.8) and 189 red supergiants. After placing these objects on the HR diagram, we find that the latest generation of Geneva evolutionary tracks show excellent agree- ment with the observed locations of our red and yellow supergiants, the observed relative number of yellow supergiants with mass and the observed red supergiant upper mass limit. These models therefore represent a drastic improvement over previous generations.

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