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

中文翻译：

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低金属度环境中红超巨星的温度

预计红超巨星 (RSG) 的温度将取决于金属丰度 (Z)，因此较低 Z 的 RSG 会变暖。在这项工作中，我们通过分析低 Z 星系 Wolf-Lundmark-Mellote 中的 RSG 来研究 Hayashi 极限的 Z 依赖性，并与小麦哲伦云和大麦哲伦云的较高 Z 环境中的 RSG 进行比较。我们通过拟合它们的光谱能量分布来确定每颗恒星的有效温度 (Teff)，如 VLT + SHOOTER 所观察到的，与 MARCS 模型大气。我们发现平均温度 $T_{\textrm {eff}_{\textrm {WLM}}}=4400\pm 202$ K, $T_{\textrm {eff}_{\textrm {SMC}}}=4130\ pm 103$ K，$T_{\textrm {eff}_{\textrm {LMC}}}=4140\pm 148$ K。通过种群综合分析，我们发现虽然日内瓦进化模型定性地再现了这一趋势，这些模型中的 RSG 系统性地太酷了。我们推测我们的结果可以通过标准太阳混合长度对 RSG 的不适用性来解释。