Ultraviolet and Optical Observations of OB Associations in M31

Images of the central and southern parts of the local group Sb spiral galaxy M31 were obtained by the Ultraviolet Imaging Telescope during the Astro-1 Spacelab mission. Stellar photometry is obtained for 611 stars in 59 van den Bergh associations in the near-UV A1 band (central wavelength 2490 A), and for 130 of these stars in the far-UV B1 band (central wavelength 1520 A). In addition, B-band magnitudes for 456 of the stars in ~30 associations, together with U- and R-band photometry for ~290 stars in ~15 associations are determined from CCD images. B-magnitudes from the catalog of Magnier et al. (1992) are given for 137 stars and R- magnitudes for 233 stars. Stellar positions are obtained from the ground- based CCD images and from UIT images. Total fluxes in the UV bands are measured for 63 associations. Finding charts are presented in the NUV and FUV bands, as well as one optical band. Tables give the images utilized in this investigation, the associations observed, with positions, total UV fluxes, extinctions estimated from color-magnitude diagrams, and stellar positions with photometry in up to five bands. UV color magnitude diagrams are presented with extinctions estimated by comparing the observed association color magnitude diagrams with colors and magnitudes of early-type stars computed from the IUE spectral atlas of Fanelli et al. (1992). Comparison with evolutionary models suggests a maximum initial mass ~85 M_sun_. Ultraviolet extinction within most M31 OB associations is found to follow an extinction curve similar to that of Hutchings et al. (1992). The variation in the fraction of near-UV stars also measured in the far-UV among the associations is probably caused by variations in age, extinction, and the degree of foreground contamination.