THE ASTROPHYSICAL JOURNAL, 443:124-135, 1995 April 10

             RADIAL VELOCITIES OF STARS IN THE GLOBULAR CLUSTER M4
                           AND THE CLUSTER DISTANCE

                               RUTH C. PETERSON
     Astronomy and Astrophysics Board of Study, University of California,
                 Santa Cruz, CA 95064; peterson@lick.ucsc.edu

                                      AND

                     RICHARD F. REES AND KYLE M. CUDWORTH
         Yerkes Observatory, The University of Chicago, P.O. Box 258,
                            Williams Bay, WI 53191


                                   ABSTRACT

       The internal stellar velocity distribution of the globular cluster
    M4 is evaluated from nearly 200 new radial velocity measurements good
    to 1 km/s and a rederivation of existing proper motions.  The mean
    radial velocity of the cluster is 70.9 +/- 0.6 km/s.  The velocity
    dispersion is 3.5 +/- 0.3 km/s at the core, dropping marginally towards
    the outskirts.  Such a low internal dispersion is somewhat at odds with
    the cluster's orbit, for which the perigalacticon is sufficiently close
    to the galactic center that the probability of cluster disruption is
    high; a tidal radius two-thirds the currently accepted value would
    eliminate the discrepancy.  The cluster mass-to-light ratio is also
    small, M/L_V = 1.0 +/- 0.4 in solar units.  M4 thus joins M22 as a
    cluster of moderate mass and concentration with a mass-to-light ratio
    among the lowest known.
       The astrometric distance to the cluster is also smaller than
    expected, 1.72 +/- 0.14 kpc.  This is only consistent with conventional
    estimates of the luminosity of horizontal branch stars provided an
    extinction law R = A_V/E(B-V) ~ 4 is adopted, as has been suggested
    recently by several authors.

    Subject headings: globular clusters: individual (M4) -- stars:
                      kinematics -- techniques: radial velocities