THE ASTROPHYSICAL JOURNAL, 347:251-265, 1989 December 1

          THE NONTHERMAL STELLAR DYNAMICS OF THE GLOBULAR CLUSTER M15

                               RUTH C. PETERSON
          Whipple Observatory, Smithsonian Astrophysical Observatory

                                PATRICK SEITZER
                       Space Telescope Science Institute

                                      AND

                               KYLE M. CUDWORTH
                   Yerkes Observatory, University of Chicago


                                   ABSTRACT

       The velocity dispersion as a function of radius in the globular
    cluster M15 is derived from measurements of 120 individual stars
    between 0.1' and 4.6' of the cluster center, and from the integrated
    light of the central cusp.  The stellar measurements, with an
    individual accuracy of 1 km/s, indicate a mean cluster velocity of
    -107.1 +/- 0.9 km/s, and a mean velocity dispersion of 9.0 +/- 0.6
    km/s.  The velocity dispersion inside 1' varies with radius.  The
    dispersion of the 27 stars within 20" is 14.2 +/- 1.9 km/s, while that
    of the 30 stars between 0.5' and 1.2' is 8.4 +/- 1.4 km/s.  The cusp
    itself appears to have a dispersion of at least 25 km/s.  Except for
    its greater velocity broadening, the spectrum of the integrated light
    of the cusp is indistinguishable from that formed by superposition of
    the individual M15 giant spectra, demonstrating that the excess light
    at the center is due primarily to the normal M15 population.
       The increase of the dispersion toward the center is not consistent
    with truncated isothermal (King-Michie) models, for which the velocity
    dispersion is nearly constant within a few core radii and then falls
    steeply beyond.  The observed behavior is difficult to reconcile as
    well with current post-core collapse models, which are also isothermal
    and whose central energy sources are modest.  It is indicative of a
    nonthermal energy distribution with substantial heating in the central
    regions; this is consistent with a central black hole of about 1000
    Msun.

    Subject headings: clusters: globular -- radial velocities -- stars:
                      stellar dynamics