THE ASTROPHYSICAL JOURNAL, 420:612-631, 1994 January 10

       PROPER MOTIONS AND RADIAL VELOCITIES IN THE GLOBULAR CLUSTER M22
                           AND THE CLUSTER DISTANCE

                               RUTH C. PETERSON
    UCO/Lick Observatories, University of California, Santa Cruz, CA 95064

                                      AND

                               KYLE M. CUDWORTH
  Yerkes Observatory, University of Chicago, Box 258, Williams Bay, WI 53191


                                   ABSTRACT

       The distance to the globular cluster M22 is reexamined from new
    radial-velocity data and revised proper motions.  Proper motions are
    rederived for 672 stars in the field of M22 using plate constant
    solutions from which stars of large proper motions have been removed,
    to reduce systematic errors which depend on coordinates.  Radial
    velocities good to 1 km/s have been obtained for 130 members of the
    globular cluster M22.  The mean radial velocity is -148.8 +/- 0.8 km/s.
    The radial velocity dispersion is 6.6 +/- 0.8 km/s in the mean, and
    shows little if any dependence on distance from the cluster center over
    a 3'-7' annulus.  Ignoring the small effect of rotation, a
    mass-to-light ratio M/L_V = 0.85 +/- 0.15 in solar units is derived.
    The lack of stars moving faster than the escape velocity suggests
    strongly that a value M/L_V ~ 1 applies to the cluster as a whole.
    This value is among the lowest observed for any globular cluster.
       Rotation is evident in both proper motions and radial velocities,
    and its position angle is in reasonable agreement with the major axis
    of the cluster's elongation.  Its amplitude diminishes with radius,
    from ~6 km/s for 1' <= r <= 3', to ~3 km/s for 3' <= r <= 7'.  Rotation
    must be carefully modeled to determine an accurate distance to the
    cluster.  From the simple approximation that the velocity dispersion is
    unaffected in the directions perpendicular to the streaming motion, a
    comparison of the dispersions in proper motion with those in radial
    velocity leads to a preliminary estimate for the distance of 2.6 +/-
    0.3 kpc, and a horizontal-branch magnitude M_V = 0.71 +/- 0.27, using
    the reddening value E(B-V) = 0.42 from M22 BHB stars.

    Subject headings: astrometry -- globular clusters: individual (M22) --
                      stars: distances -- techniques: radial velocities