THE ASTRONOMICAL JOURNAL    VOLUME 111, NUMBER 3, PAGE 1128          MARCH 1996

      THE STELLAR INITIAL MASS FUNCTION IN THE GIANT H II REGION NGC 595

                               ELIOT M. MALUMUTH

  Astronomy Programs, Computer Sciences Corporation, Laboratory for Astronomy
   and Solar Physics, Goddard Space Flight Center, Greenbelt, Maryland 20771
                 Electronic mail: hrseliot@klatu.gsfc.nasa.gov

                     WILLIAM H. WALLER AND JOEL Wm. PARKER

            Hughes STX, Laboratory for Astronomy and Solar Physics,
            Goddard Space Flight Center, Greenbelt, Maryland 20771
    Electronic mail: waller@uit.gsfc.nasa.gov; parker@kuylym.gsfc.nasa.gov


                                   ABSTRACT

    As part of a larger effort to study the resolved and composite
    properties of the giant H II regions in Messier 33, we have analyzed
    multiband HST/WFPC-2 images of NGC 595 in terms of the ionizing
    cluster's resolved stellar population.  Photometric reductions of the
    PC images yield 100 stars in the UV image, 272 stars on the U image,
    345 stars on the B image, and 561 stars on the V image.  A total of 267
    stars are common to the U, B, and V images while 86 stars are detected
    on all 4 images.  Although some clustering is evident, the degree of
    central concentration is less than that seen in 30 Doradus.  The
    resulting U-B vs B-V diagram of the resolved stars is used to determine
    the reddening of each star.  The average reddening derived from this
    diagram is E(B-V) = 0.36 +/- 0.28 mag.  The dereddened color-M_V
    diagram is best fit by a model cluster having an age of 4.5 +/- 1.0
    Myr, and hence intitial masses no greater than 51 Msun.  A total of 13
    supergiant stars and 10 candidate WR stars [Drissen et al. (AJ, 105,
    1400, 1993), and references therein] are identified with M_V = -5 to -8
    mag.  The remainder are main-sequence O-type (98) and early B-type
    (>145) stars with M_V = -1 to -6 mag.  The ratio of WR to O stars is
    WR/O = 0.11 +/- 0.01, roughly the same as found in the core of 30
    Doradus.  The resulting luminosity function has a slope of alpha =
    -0.71.  The derived IMF has a slope of Gamma = -1.32 +/- 0.02 before
    subtracting a background component, and Gamma = -1.00 +/- 0.05 after
    subtracting a background based on photometry of the surrounding WF
    images.  Integration of the derived IMF down to a lower mass limit of
    4 Msun yields a total mass of 7350 Msun, while integration down to 0.1
    Msun yields a total mass of 18 000 Msun.  The total estimated ionizing
    luminosity is 5.0x10^50 photon/s, roughly half that which is inferred
    from the Halpha luminosity in this region.  This shortfall of ionizing
    photons can be reconciled by allowing for a spread in the stellar ages,
    and/or increasing the modeled EUV luminosity of the stars at the
    inferred cluster age.