THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 98:595-615, 1995 June

        ULTRAVIOLET AND OPTICAL OBSERVATIONS OF OB ASSOCIATIONS IN M31

         JESSE K. HILL, JOAN E. ISENSEE, RALPH C. BOHLIN, K. P. CHENG,
  P. M. N. HINTZEN, ROBERT W. O'CONNELL, MORTON S. ROBERTS, ANDREW M. SMITH,
                    ERIC P. SMITH, AND THEODORE P. STECHER


                                   ABSTRACT

        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, A&AS,
     96, 379) 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, ApJS, 82, 197).  Comparison
     with evolutionary models suggests a maximum initial mass ~85 Msun.
     Ultraviolet extinction within most M31 OB associations is found to
     follow an extinction curve similar to that of Hutchings et al. (1992,
     ApJ, 400, L35).  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.

     Subject headings: galaxies: individual (M31) -- galaxies: photometry
                       -- galaxies: star clusters -- ultraviolet: stars