Proper motions & radial velocities in M22

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 ~6km/s for 1'<=r<= 3', to ~3km/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+/-0.3kpc, and a horizontal-branch magnitude M_V=0.71+/-0.27, using the reddening value E(B-V)=0.42 from M22 BHB stars.