Hubble Space Telescope Astrometry of m4 and the Galactic Constant V₀/R₀

Abstract

From multiepoch WFPC2/HST observations we present astrometric measurements of stars in the Galactic globular cluster M4 (NGC 6121) and in the foreground and background. The presence of an extragalactic point source allows us to determine the absolute proper motion of the cluster and, through use of the field stars in this region only 18° from the Galactic center, to measure the difference between the Oort constants, A-B. We find (μ_αcosδ, μ_δ)_J2000.0=(-13.21+/-0.35, -19.28+/-0.35) mas yr^-1, and A-B=V₀/R₀=27.6+/-1.7 km s^-1 kpc^-1.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.