Luminous and Variable Stars in M31 and M33. III. The Yellow and Red Supergiants and Post-Red Supergiant Evolution

TL;DR

This study investigates yellow and red supergiants in M31 and M33, revealing that a significant fraction are in a post-red supergiant phase with evidence of mass loss and circumstellar dust, advancing understanding of massive star evolution.

Contribution

The paper provides a detailed census and spectral analysis of evolved supergiants in M31 and M33, identifying post-red supergiant candidates and new hypergiants, thus offering insights into late-stage stellar evolution.

Findings

30-40% of yellow supergiants are post-RSGs with circumstellar dust

More than half of red supergiants show dusty ejecta

Two new warm hypergiants identified in M31

Abstract

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.