Proper motions and radial velocities in the globular cluster M22 and the cluster distance

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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(-1) have been obtained for 130 members of the globular cluster M22. The mean radial velocity is -148.8+/-0.8 km s(-1) The radial velocity dispersion is 6.6+/-0.8 km s(-1) 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) similar to 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 similar to 6 km s(-1) for 1'less than or equal to r less than or equal to 3', to similar to 3 km s(-1) for 3'less than or equal to r less than or equal to 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.

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