THE ASTROPHYSICAL JOURNAL, 448:179-194, 1995 July 20

       THE INTERMEDIATE STELLAR MASS POPULATION IN R136 DETERMINED FROM
               HUBBLE SPACE TELESCOPE PLANETARY CAMERA 2 IMAGES

                               DEIDRE A. HUNTER
       Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001

                                EDWARD J. SHAYA
     Department of Physics, University of Maryland, College Park, MD 20742

                                JON A. HOLTZMAN
       Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001

                                ROBERT M. LIGHT
     IPAC, California Institute of Technology, 100-22, Pasadena, CA 91125

                                      AND

                      EARL J. O'NEIL, JR. AND ROGER LYNDS
   Kitt Peak National Observatory, National Optical Astronomy Observatories,
                       P.O. Box 26732, Tucson, AZ 85726


                                   ABSTRACT

        We have analyzed Hubble Space Telescope (HST) images of the
     compact, luminous star cluster R136 in the LMC that were taken with
     the refurbished HST and new Wide Field/Planetary Camera.  These images
     allow us to examine the stellar population in a region of unusually
     intense star formation at a scale of 0.01 pc.  We have detected stars
     to 23.5 in F555W and have quantified the stellar population to an
     M_555.0 of 0.9 or a mass of 2.8 Msun.  Comparisons of HR diagrams with
     isochrones that were constructed for the HST flight filter system from
     theoretical stellar evolutionary tracks reveal massive stars, a main
     sequence to at least 2.8 Msun, and stars with M_555.0 >= 0.5 still on
     pre-main sequence tracks.  The average stellar population is fit with
     a 3-4 Myr isochrone.  Contrary to expectations from star formation
     models, however, the formation period for the massive stars and lower
     mass stars appear to largely overlap.  We have measured the IMF for
     stars 2.8-15 Msun in three annuli from 0.5-4.7 pc from the center of
     the cluster.  The slopes of the IMF in all three annuli are the same
     within the uncertainties, thus, showing no evidence for mass
     segregation beyond 0.5 pc.  Furthermore, the combined IMF slope, -1.22
     +/- 0.06, is close to a normal Salpeter IMF.  The lower mass limit
     must be lower than the limits of our measurements: <= 2.8 Msun beyond
     0.5 pc and <= 7 Msun within 0.1 pc.  This is contrary to some
     predictions that the lower mass limit could be as high as 10 Msun in
     regions of intense massive star formation.  Integrated properties of
     R136 are consistent with its being comparable to a rather small
     globular cluster when such clusters were the same age as R136.  From
     the surface brightness profile, an upper limit for core radius of 0.02
     pc is set.  Within a radius of 0.4 pc we estimate that there have been
     roughly 20 crossing times and relaxation should be well along.  Within
     0.5 pc crowding prevents us from detecting the intermediate mass
     population, but there is a hint of an excess of stars brighter than
     M_555.0 = -5 and of a deficit in the highest mass stars between 0.6 pc
     and 1.2 pc.  This would be consistent with dynamical segregation.

     Subject headings: Hertzsprung-Russell diagram -- Magellanic Clouds --
                       open clusters and associations: individual (R136) --
                       stars: evolution -- stars: luminosity function, mass
                       function