Luminous and Variable Stars in M31 and M33. I. The Warm Hypergiants and Post-Red Supergiant Evolution

TL;DR

This paper identifies and characterizes a small group of luminous, variable stars in M31 and M33 that are likely post-red supergiants, shedding light on their evolution, properties, and potential as supernova progenitors.

Contribution

It reports the discovery and detailed analysis of warm hypergiants in M31 and M33, proposing their evolutionary link to LBVs and supernova progenitors, which was previously poorly understood.

Findings

Stars have A-F spectral types with strong hydrogen emission.

They exhibit infrared excess due to dust and free-free emission.

Mass loss rates range from 10^-6 to 10^-4 solar masses per year.

Abstract

The progenitors of Type IIP supernovae have an apparent upper limit to their initial masses of about 20 solar masses, suggesting that the most massive red supergiants evolve to warmer temperatures before their terminal explosion. But very few post-red supergiants are known. We have identified a small group of luminous stars in M31 and M33 that are candidates for post-red supergiant evolution. These stars have A -- F-type supergiant absorption line spectra and strong hydrogen emission. Their spectra are also distinguished by the Ca II triplet and [Ca II] doublet in emission formed in a low density circumstellar environment. They all have significant near- and mid-infrared excess radiation due to free-free emission and thermal emission from dust. We estimate the amount of mass they have shed and discuss their wind parameters and mass loss rates which range from a few times$ 10^-6 to 10^-4 solar masses/yr. On an HR Diagram, these stars will overlap the region of the LBVs at maximum light, however the warm hypergiants are not LBVs. Their non-spherical winds are not optically thick and they have not exhibited any significant variability. We suggest, however, that the warm hypergiants may be the progenitors of the "less luminous" LBVs such as R71 and even SN1987A.