The temperatures of red supergiants in low metallicity environments

TL;DR

This study measures the effective temperatures of red supergiants in low and higher metallicity environments, finding that lower metallicity RSGs are warmer, with implications for stellar evolution models.

Contribution

It provides empirical temperature measurements of RSGs across different metallicities and compares them with theoretical models, highlighting discrepancies.

Findings

Lower metallicity RSGs are warmer than higher metallicity ones.

Geneva models reproduce the trend but predict systematically cooler RSGs.

Standard solar mixing length may not be applicable to RSGs.

Abstract

The temperatures of red supergiants (RSGs) are expected to depend on metallicity (Z) in such a way that lower-Z RSGs are warmer. In this work, we investigate the Z-dependence of the Hayashi limit by analysing RSGs in the low-Z galaxy Wolf-Lundmark-Mellote (WLM), and compare with the RSGs in the higher-Z environments of the Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC). We determine the effective temperature ($T_{\textrm{eff}}$) of each star by fitting their spectral energy distributions, as observed by VLT+SHOOTER, with MARCS model atmospheres. We find average temperatures of $T_{\textrm{eff}_{\textrm{WLM}}}=4400\pm202$ K, $T_{\textrm{eff}_{\textrm{SMC}}}=4130\pm103$ K, and $T_{\textrm{eff}_{\textrm{LMC}}}=4140\pm148$ K. From population synthesis analysis, we find that although the Geneva evolutionary models reproduce this trend qualitatively, the RSGs in these models are systematically too cool. We speculate that our results can be explained by the inapplicability of the standard solar mixing length to RSGs.