M supergiants in the Milky Way and the Magellanic Clouds: colors, spectral types, and luminosities

The differences in metal abundances between the Milky Way, Large Magellanic Cloud (LMC), and Small Magellanic Cloud (SMC) affect most of the observable properties of the M supergiants in these galaxies; those in the SMC (which has the lowest metal abundance) have the earliest mean spectral type, while those of the Milky Way exhibit the latest mean spectral type. This is presently interpreted as a combination of two effects of differing metal abundance on the supergiant atmospheres: first, lower abundance stars of a given effective temperature have earlier MK spectral types due to reduced TiO abundance; second, the Hayashi track is shifted to hotter effective temperature at reduced metal abundance, thereby shifting the mean spectral type still earlier. The fact that the 10-micron excess decreases linearly with metal abundance suggests that mass loss rates are roughly the same for stars in all three galaxies, with the dust-to-gas ratio proportional to metal abundance.