The Yellow and Red Supergiants of M33

TL;DR

This study analyzes yellow and red supergiants in M33, using spectroscopy and improved models to resolve previous discrepancies in their observed properties and evolutionary predictions.

Contribution

The paper extends previous research by accurately identifying supergiants in M33 and demonstrating that latest Geneva models align well with observations, resolving past inconsistencies.

Findings

Good agreement between Geneva models and observed supergiant locations.

Accurate identification of 121 yellow and 189 red supergiants in M33.

Revised rotation curve of M33 based on supergiant data.

Abstract

Yellow and red supergiants are evolved massive stars whose numbers and locations on the HR diagram can provide a stringent test for models of massive star evolution. Previous studies have found large discrepancies between the relative 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 supergiant 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 $\lambda$7774 triplet. We examine ~1300 spectra in total and identify 121 yellow supergiants (a sample which is unbiased in luminosity above log(L/L\odot) ~ 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 agreement 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.