Studies of luminous stars in nearby galaxies. VIII. The Small Magellanic Cloud.

The spectra, colors, and positions on the H-R diagrams of the Small Magellanic (SMC) supergiants of all spectral types are examined and compared with corresponding data for supergiant populations in the LMC and Milky Way for clues to the role of chemical composition (i.e., low metallicity) on their evolution. A comparison of the observed luminosity function shows that while the solar neighborhood and the LMC are essentially the same, the SMC is significantly different, especially at the upper end. These differences are not due to chemical-composition variations. The relative numbers of supergiants of different luminosities vary most closely with the mass of the galaxy. The blue to red supergiant ratio in the SMC confirms the trends observed in the Galaxy and LMC, and is probably affected by chemical abundance differences, especially in the lower luminosity intervals. In addition, it is found that a group of supergiants in the SMC, with spectral types B8-A5, have anomalous colors and hydrogen lines too strong for their luminosities.