ULTRAVIOLET IMAGING TELESCOPE PHOTOMETRY OF MASSIVE STARS - THE OB ASSOCIATION NGC-206 IN M31

The Ultraviolet Imaging Telescope (UIT) obtained UV images of the giant M31 OB association NGC 206. Magnitudes in bands at 1520 and 2490 angstrom were obtained for 30 massive stars, which demonstrate the effectiveness of UIT for photometry of moderately crowded hot stars to V approximately 21. The UV colors and magnitudes observed for stars in NGC 206 place them in the region of the color-magnitude diagram occupied by evolutionary models for 30-60 M. stars, after correcting for extinction. The brighter stars are systematically redder than the fainter stars, indicating that they are supergiants of age approximately 4 Myr, while the fainter, bluer stars are nearer age zero. The relative numbers of probable supergiants measured by us and the number of probable main-sequence O stars measured from optical images are in agreement with the relative lifetimes. Calculated UIT colors are presented for a library of standard star spectra constructed from IUE and ground-based observations.