The temperatures of red supergiants in low metallicity environments

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 ($T_{\text{eff}}$) of each star by fitting their spectral energy distributions, as observed by VLT+SHOOTER, with MARCS model atmospheres. We find average temperatures of $T_{{\text{eff}}_{\text{WLM}}}=4400\pm 202$ K, $T_{{\text{eff}}_{\text{SMC}}}=4130\pm 103$ K, and $T_{{\text{eff}}_{\text{LMC}}}=4140\pm 148$ 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.

Accepted for publication in MNRAS

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• Investigating episodic mass loss in evolved massive stars: II. Physical properties of red supergiants at subsolar metallicity
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• Red supergiant stars in IC 1613 and metallicity-dependent mixing length in the evolutionary model
• Red Supergiants in the Milky Way and Nearby Galaxies
• A Low Metallicity Massive Contact Binary Star System Candidate in WLM identified by Hubble and James Webb Space Telescope imaging