ULTRAVIOLET IMAGING TELESCOPE AND OPTICAL EMISSION-LINE OBSERVATIONS OF H-II REGIONS IN M81

Images of the type Sab spiral galaxy M81 were obtained in far-UV and n ear-UV bands by the Ultraviolet Imaging Telescope (UIT) during the Ast ro-1 Spacelab mission of 1990 December. Magnitudes in the two UV bands are determined for 52 H II regions from the catalog of Petit, Sivan, & Karachentsev (1988). Fluxes of the Halpha and Hbeta emission lines a re determined from CCD images. Extinctions for the brightest H II regi ons are determined from observed Balmer decrements. Fainter H II regio ns are assigned the average of published radio-Halpha extinctions for several bright H II regions. The radiative transfer models of Witt, Th ronson, & Capuano (1992) are shown to predict a relationship between B almer Decrement and Halpha extinction consistent with observed line an d radio fluxes for the brightest 7 H II regions and are used to estima te the UV extinction. Ratios of Lyman continuum flux (computed from th e extinction corrected Halpha flux) to the extinction corrected far-UV flux are compared with ratios predicted by model spectra computed for IMF slope equal to -1.0 and stellar masses ranging from 5 to 120 M(.) . Ages and masses are estimated by comparing the Halpha and far-UV flu xes and their ratio with the models. The total of the estimated stella r masses for the 52 H II regions is 1.4 x 10(5) M(.). The star-formati on rate inferred for M81 from the observed UV and Halpha fluxes is low for a spiral galaxy at approximately 0.13 M(.) yr-1, but consistent w ith the low star-formation rates obtained by Kennicutt (1983) and Cald well et al. (1991) for early-type spirals.