THE ASTRONOMICAL JOURNAL    VOLUME 113, NUMBER 2, PAGE 669     FEBRUARY 1997

       GLOBULAR CLUSTER PHOTOMETRY WITH THE HUBBLE SPACE TELESCOPE. VI.
        WF/PC-I OBSERVATIONS OF THE STELLAR POPULATIONS IN THE CORE OF
                                M13 (NGC 6205)

                    RANDI L. COHEN AND PURAGRA GUHATHAKURTA

                UCO/Lick Observatory, University of California,
                         Santa Cruz, California 95064
             Electronic mail: randi@ucolick.org, raja@ucolick.org

                                  BRIAN YANNY

        Fermi National Accelerator Laboratory, Batavia, Illinois 60510
                     Electronic mail: yanny@sdss.fnal.gov

                              DONALD P. SCHNEIDER

  Department of Astronomy & Astrophysics, The Pennsylvania State University,
                      University Park, Pennsylvania 16802
                      Electronic mail: dps@astro.psu.edu

                                JOHN N. BAHCALL

           Institute for Advanced Study, Princeton, New Jersey 08540
                       Electronic mail: jnb@sns.ias.edu

    We study the dense core of the globular cluster Messier 13 (NGC 6205)
    using pre-refurbishment Planetary Camera-I images obtained with the
    Hubble Space Telescope. Short exposures (60 s) through the F555W and
    F785LP filters (similar to Johnson V and I, respectively) have been
    used to obtain V and I photometry of 2877 stars brighter than V~20 in a
    1.25 arcmin^2 region of the cluster including its core and extending
    out to r~66 arcsec (2.3 pc) from its center. The sample is complete to
    V ~= 18.3 (the main sequence turnoff) and the 1 sigma photometric error
    is about 0.1 mag. We find 15 blue straggler star candidates and 10
    other possible blue stragglers in this region of M13. Their specific
    frequency is in the range F_(BSS)=0.04-0.07, comparable to what is
    observed near the centers of other dense clusters. A comparison between
    M13's observed V band stellar luminosity function and a theoretical
    model (Bergbusch & Vandenberg, 1992, ApJS, 81, 163) for the luminosity
    function of an old, metal-poor cluster shows that the model predicts
    too few of the brightest red giants (V~12.5-15) by a factor of two
    relative to subgiants/turnoff stars (>6 sigma effect). The radial
    distributions of red giants, blue stragglers, and subgiants are
    consistent with one another, and are well fit by a King profile of core
    radius r_(core)=38 arcsec +/- 6 arcsec (90% confidence limits) or 1.3
    pc. Stars in the blue horizontal branch of M13, however, appear to be
    centrally depleted relative to other stellar types. We combine data
    from three dense ``King model clusters,'' M13, M3, and 47 Tuc, and two
    post core collapse clusters, M30 and M15, and compare the distributions
    of various stellar types as a function of (r/r_(half light)) and
    (r/r_(core)). The horizontal branch stars in the combined sample appear
    to be centrally depleted relative to the giants (97% significance)-this
    depletion is only a 1-2 sigma effect in each of the clusters taken
    individually. The blue stragglers in the combined sample are centrally
    concentrated relative to the giants. (Copyright) 1997 American
    Astronomical Society.