ULTRAVIOLET PHOTOMETRY OF OB ASSOCIATIONS IN M31

Near-UV (2490 angstrom) and far-UV (1520 angstrom) magnitudes are obtained for 76 massive stars in 24 OB associations in the central and southern portions of M31 from images obtained by the Ultraviolet Imaging Telescope during the Astro 1 spacelab mission. UV photometry is important because the most massive stars are brightest in the UV, the sensitivity to interstellar extinction is largest there, and because stars detected in the far-UV are unlikely to be foreground Galactic stars. A comparison is made with the previous UIT photometry of 30 stars in the giant association NGC 206 (A78). The brightest stars in the far-UV band in NGC 206 are approximately 1 mag brighter than the brightest stars in any of the other associations. The most likely explanation for the brighter stars in NGC 206 is the lower extinction, together with more recent formation of at least some massive stars. Extinctions are estimated from the relation between E(B-V) and the distance from the center of M31 derived by Hodge and Lee from optical CCD stellar photometry, although the extinction of NGC 206 is assumed to follow the reddening model of Hutchings, Bianchi, and Massey. From evolutionary models we estimate lower limits to the maximum stellar mass at approximately 60-100 M. in NGC 206, A29, A61, A63, A130, and A132. For other associations, the limits are in the range approximately 20-55 M.. The association with the most measured stars other than NGC 206 is A21, with 15. The brightest star in A21 is 2 mag fainter than the brightest star in NGC 206 with mass approximately 40 M., if it is a supergiant at the age of maximum far-UV luminosity. The faintness of the stars in A21 is only partially accounted for by increased extinction, according to the relation of Hodge and Lee. Ages are estimated at 3-11 Myr for the most recent star formation in each association, if the star with the brightest m152 magnitude is a blue supergiant. A mass range is computed for the brightest star in the far-UV band by assuming that it is either on the zero-age main sequence (maximum mass) or a supergiant with maximum far-UV luminosity (minimum mass).