INITIAL MASS FUNCTIONS FROM ULTRAVIOLET STELLAR PHOTOMETRY - A COMPARISON OF LUCKE AND HODGE OB ASSOCIATIONS NEAR 30-DORADUS WITH THE NEARBY FIELD

UV stellar photometry is presented for 1563 stars within a 40' circular field in the LMC, excluding the 10' x 10' field centered on R136 investigated earlier by Hill et al. (1993). Magnitudes are computed from images obtained by the Ultraviolet Imaging Telescope in bands centered at 1615 angstrom and 2558 angstrom. Stellar masses and extinctions are estimated for the stars in associations using the evolutionary models of Schaerer et al. (1993), assuming the age is 4 Myr and that the local LMC extinction follows the Fitzpatrick (1985) 30 Dor extinction curve. The estimated slope of the initial mass function (IMF) for massive stars (> 15 M.) within the Lucke and Hodge (LH) associations is GAMMA = - 1.08 +/- 0.2. Initial masses and extinctions for stars not within LH associations are estimated assuming that the stellar age is either 4 Myr or half the stellar lifetime, whichever is larger. The estimated slope of the IMF for massive stars not within LH associations is GAMMA = - 1.74 +/- 0.3 (assuming continuous star formation), compared with GAMMA = - 1.35, and GAMMA = - 1.7 +/- 0.5, obtained for the Galaxy by Salpeter (1955) and Scalo (1986), respectively, and GAMMA = - 1.6 obtained for massive stars in the Galaxy by Garmany, Conti, & Chiosi (1982). The shallower slope of the association IMF suggests that not only is the star formation rate higher in associations, but that the local conditions favor the formation of higher mass stars there. We make no corrections for binaries or incompleteness.