Gemma Gonzalez-Tora and colleagues in their recent paper, have analysed a total of 28 red supergiants (RSGs) observed with VLTXSHOOTER from the neighbouring galaxies, Small Magellanic Cloud (SMC), Large Magellanic Cloud (LMC) and Wolf-Lundmark-Mellote (WLM), by fitting the flux of the SED regions free from molecular features. They found that, the average effective temperature of red supergiants depends on metallicity for WLM and MCs.
Red supergiants (RSGs) are stars with a supergiant luminosity class (Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. The effective temperature (Teff) of RSGs is thought to be a manifestation of the Hayashi limit. The Hayashi limit fixes the minimum effective temperature of the star and its maximum radius, where the star is known to have its most extended convective region and still maintaining hydrostatic equilibrium. Several theoretical predictions showed that, temperatures of red supergiants (RSGs) depends on metallicity (Z) in such a way that lower-metallicity RSGs are warmer.
Now, Gemma Gonzalez-Tora and colleagues investigated the metallicity-dependence of the Hayashi limit by analysing RSGs in the low-Z galaxy Wolf-Lundmark-Mellote (WLM), and compared it with the RSGs in the higher-Z environments of the Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC).
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They determine the effective temperature (Teff) of each star (as shown in table 1 & 2 above) by fitting their spectral energy distributions, as observed by VLT+SHOOTER, with MARCS model atmospheres and found average effective temperatures of 4400 K, 4130 K, and 4140 K for Wolf-Lundmark-Mellote, Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC), respectively. This trend of increasing average effective temperature with decreasing metallicity is in qualitative agreement with theoretical predictions.
“From results obtained, it is clear that, the average effective temperature of RSGs depends on metallicity for WLM and the MCs.”
Finally, it has also been found from population synthesis analysis that, there is a systematic offset between expected and observed temperatures of RSGs at all metallicities. Specifically, RSGs in evolutionary models are too cool by ∼ 200 K. This could be due to a wrong estimation of the mixing length parameter for 1D models of massive stars.
Reference: Gemma González-Torà, Ben Davies, Rolf-Peter Kudritzki, Bertrand Plez, “The temperatures of red supergiants in low metallicity environments”, Arxiv, pp. 1-23, 2021. https://arxiv.org/abs/2106.01807
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