The Effective Temperatures and Physical Properties of Magellanic Cloud Red Supergiants: The Effects of Metallicity

Abstract

We present moderate-resolution spectrophotometry of 36 red supergiants(RSGs) in the LMC and 37 RSGs in the SMC. Using the MARCS atmospheremodels to fit this spectrophotometry, we determine the stars' physicalproperties and compare the results to evolutionary models. The(V-R)_{0} broadband colors agree with those from fitting theoptical spectrophotometry, but (V-K)_{0} results showmetallicity-dependent systematic differences in the physical properties.We conclude that this is likely due to the limitations of static 1Dmodels, and accept that there is still some uncertainty in the effectivetemperature scale. We find that the temperature scales for Milky Way,LMC, and SMC K-type supergiants agree with each other, while for M-typesupergiants the LMC and SMC scales are cooler than the Galactic scale by50 and 150 K, respectively. Since spectral classification of RSGs isbased on TiO line strengths, it follows that stars with lower abundancesof these elements must be cooler in order to have the line strengthsassociated with a given spectral type. However, this effect is notsufficient to explain the shift in average spectral type between thethree galaxies. Instead, metallicity's effect on the coolest extent ofstellar evolution is primarily responsible. Our results bring RSGs intomuch better agreement with stellar evolutionary theory, although the SMCRSGs show a considerably larger spread in effective temperatures at agiven luminosity than the LMC stars. This is expected due to the largereffects of rotational mixing in lower metallicity stars. We also findthat the reddening distribution of RSGs in the Clouds is skewedsignificantly toward higher values, consistent with our recent findingthat Galactic RSGs show extra extinction due to circumstellar dust.