Ultraviolet Imaging Telescope Photometry of Massive Stars: The OB Association NGC 206 in M31
O'Connell, Robert W.; Bohlin, Ralph C.; Smith, Andrew M.; Roberts, Morton S.; Stecher, Theodore P.; Neff, Susan G.; Hill, Jesse K.; Pfarr, Barbara B.; Isensee, Joan E.
United States
Abstract
The Ultraviolet Imaging Telescope (UIT) obtained UV images of the giant M31 OB association NGC 206. Magnitudes in bands at 1520 and 2490 A were obtained for 30 massive stars, which demonstrate the effectiveness of UIT for photometry of moderately crowded hot stars to V ~ 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_sun_ stars, after correcting for extinction. The brighter stars are systematically redder than the fainter stars, indicating that they are supergiants of age ~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.