Evolution and Pulsation of Red Supergiants at Different Metallicities

We have investigated the evolution and pulsation of 15, 20, and 30 M_☉ stars with metallicities Z = 0.0005-0.02, by use of up-to-date opacities, mass loss through the stellar wind, and overshooting from the convective core. We have calculated the blue-to-red (B/R) ratio of supergiants for our stellar models and found that both normal and overshooting models show a tendency of decreasing B/R ratio with increasing metallicity, which is entirely opposite to what has been observed in some stellar systems. Because incorporating overshooting can significantly reduce the B/R ratio, it seems to indicate that overshooting from the convective core is strong in extremely metal-poor stars and fades gradually away with increasing metal abundance. However, many physical ingredients can affect the B/R ratio of the supergiants; our solution is not complete. We have compared the theoretical period-luminosity (P-L) relations with some long-period variables (LPVs) observed in the LMC, M33, and the Milky Way (MW). The observed LPVs are found to be in good agreement with the P-L relation of the fundamental mode for the LMC and M33, but the LPVs in the Milky Way seem to have shorter periods and seem more likely to be the first overtone pulsators. We have found that the period-luminosity relations at different metallicities show a tendency of decreasing luminosity and increasing period with increasing metal abundance, and the correction to the luminosity at a given period can be approximated by δM_bol ~ 0.83δ log Z.