THE ASTROPHYSICAL JOURNAL 425:122-126, 1994 April 10

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

      JESSE K. HILL, JOAN E. ISENSEE, ROBERT H. CORNETT, RALPH C. BOHLIN,
           ROBERT W. O'CONNELL, MORTON S. ROBERTS, ANDREW M. SMITH,
                            AND THEODORE P. STECHER

                                   ABSTRACT

       UV stellar photometry is presented for 1563 stars within a 40'
    circular field in the LMC, excluding the 10'x10' 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 A and 2558 A.  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 Msun) 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.

    Subject headings: Magellanic Clouds -- open clusters and associations:
                      individual (30 Doradus) -- stars: luminosity
                      function, mass function -- ultraviolet: stars