RADIAL-VELOCITIES OF STARS IN THE GLOBULAR-CLUSTER M14 AND THE CLUSTER DISTANCE

The internal stellar velocity distribution of the globular cluster M4 is evaluated from nearly 200 new radial velocity measurements good to 1 km s(-1) and a rederivation of existing proper motions. The mean radial velocity of the cluster is 70.9 +/- 0.6 km s(-1). The velocity dispersion is 3.5 +/- 0.3 km s(-1) 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) approximate to 4 is adopted, as has been suggested recently by several authors.