Red Supergiants in M31: The Humphreys-Davidson limit at high metallicity

TL;DR

This study measures the luminosity of Red Supergiants in M31 and compares it with those in the LMC and SMC, finding no evidence that metallicity influences the Humphreys-Davidson limit, challenging existing theories about stellar mass loss.

Contribution

It provides observational evidence that the Humphreys-Davidson limit does not depend on metallicity, contradicting the line-driven wind hypothesis for RSG luminosity limits.

Findings

The maximum RSG luminosity in M31 is consistent with that in the LMC and SMC.

No significant metallicity dependence observed in the RSG luminosity function.

Line-driven winds are unlikely to be the cause of the Humphreys-Davidson limit.

Abstract

The empirical upper limit to Red Supergiant (RSG) luminosity, known as the Humphreys-Davidson (HD) limit, has been commonly explained as being caused by the stripping of stellar envelopes by metallicity-dependent, line-driven winds. As such, the theoretical expectation is that the HD limit should be higher at lower metallicity, where weaker mass-loss rates mean that higher initial masses are required for an envelope to be stripped. In this paper, we test this prediction by measuring the luminosity function of RSGs in M31 and comparing to those in the LMC and SMC. We find that $\log (L_{\rm max}/L_{\odot}) = 5.53 \pm 0.03$ in M31 (Z $\gtrsim$ Z$_{\odot}$), consistent with the limit found for both the LMC (Z $\sim$ 0.5 Z$_{\odot}$) and SMC (Z $\sim$ 0.25 Z$_{\odot}$), while the RSG luminosity distributions in these 3 galaxies are consistent to within 1$\sigma$. We therefore find no evidence for a metallicity dependence on both the HD limit and the RSG luminosity function, and conclude that line-driven winds on the main sequence are not the cause of the HD limit.