THE ASTRONOMICAL JOURNAL    VOLUME 110, NUMBER 4, PAGE 1649        OCTOBER 1995

             HUBBLE SPACE TELESCOPE PLANETARY CAMERA OBSERVATIONS
              OF THE STELLAR POPULATIONS NEAR THE NUCLEUS OF M33

                              KENNETH J. MIGHELL

      Department of Astronomy, Columbia University, 538 W. 120th Street,
                           New York, New York 10027
               Electronic mail: mighell@figaro.phys.columbia.edu

                                R. MICHAEL RICH

      Department of Astronomy, Columbia University, 538 W. 120th Street,
                           New York, New York 10027
                 Electronic mail: rmr@cuphyd.phys.columbia.edu

                                   ABSTRACT

    We have studied the stellar populations near the nucleus of the nearby
    spiral galaxy M33 using archival observations of the Hubble Space
    Telescope Planetary Camera in the F555W and F785LP passbands.  The I vs
    (V-I) color-magnitude diagram reveals a young stellar population with a
    blue main sequence and red supergiants, and stars in a blue loop that
    are Cepheid candidates.  An old red giant branch is clearly visible,
    with a break at I = 20.7 mag which is, within the observational errors,
    identical to the giant branch termination point of the Mould & Kristian
    (1986, ApJ, 305, 591) halo field.  However, red giants are seen up to I
    = 19.5 mag, and many bright red giants are undetected in the V band.
    We find evidence for a wide abundance spread of at least 1.5 dex from
    metal-poor (~M15) to metal-rich (>= 47 Tucanae) in the Population II
    stars, in sharp contrast to the M33 halo field of Mould & Kristian
    (1986) which is metal-poor (~M92) with a narrow abundance range.  We
    show that the various stellar populations have different spatial
    distributions.  The Population II stars are more centrally concentrated
    than the Population I stars.  The red giants brighter than I = 20.7
    (the termination point for a Galactic globular cluster giant branch)
    are found to be more centrally concentrated than the (presumably) old
    red giants.  The most-centrally concentrated bright Population II
    giants are probably associated with the many bright red giants that are
    undetected in the V band.  These infrared-bright giants were probably
    created in an intermediate-age burst of star formation that took place
    well after the formation of the oldest halo stars but long before the
    recent burst of star formation seen in nearby OB associations.  This
    stellar population probably corresponds to the bulge identified in the
    infrared H band by Minniti et al. (1993, ApJ, 410, L79).