2021MNRAS.505.4422G

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, and compare with the RSGs in the higher Z environments of the Small Magellanic Cloud and Large Magellanic Cloud. We determine the effective temperature (T_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_eff_WLM_=4400±202 K, T_eff_SMC_=4130±103 K, and T_eff_LMC_=4140±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.