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 near-UV bands by the Ultraviolet Imaging Telescope (UIT) during the Astro-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 are determined from CCD images. Extinctions for the brightest H II regions are determined from observed Balmer decrements. Fainter H II regions are assigned the average of published radio-Halpha extinctions for several bright H II regions. The radiative transfer models of Witt, Thronson, & Capuano (1992) are shown to predict a relationship between Balmer Decrement and Halpha extinction consistent with observed line and radio fluxes for the brightest 7 H II regions and are used to estimate the UV extinction. Ratios of Lyman continuum flux (computed from the 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 fluxes and their ratio with the models. The total of the estimated stellar masses for the 52 H II regions is 1.4 x 10(5) M(.). The star-formation 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 with the low star-formation rates obtained by Kennicutt (1983) and Caldwell et al. (1991) for early-type spirals.