Studies of luminous stars in nearby galaxies. VI. The brightest supergiants and the distance to M 33.
Eleven normal early type supergiants and 11 M type supergiants have been spectroscopically confirmed in M 33. The visual extinction for the supergiants, determined from spectroscopic and color data and from the neutral hydrogen column density, is about 0.5 mag greater than the foreground reddening from field stars. Internal reddening is a serious problem in external galaxies, and corrections must be applied for it when using stars as distance indicators. The luminosity calibration, visual magnitude approximately equal to -8, observed for the brightest M supergiants in other Local Group galaxies is adopted to derive a new and more reliable true distance modulus of 23.9 + or - 0.2 mag (24.7 mag visual) for M 33. The luminosities, color-magnitude diagrams, and evolution of the brightest stars are also discussed and compared with the most luminous Galactic and LMC supergiants.