The absolute proper motion and Galactic orbit of M3

Abstract

The combination of Schmidt plates and an automatic measuring machine provides a powerful method for investigating the proper motions of both the Galactic halo globular clusters and the nearby Galactic dwarf spheroidal satellites. We describe the first direct measurement of the absolute proper motion of a Galactic globular cluster with respect to a large number of background galaxies. Five pairs of Tautenburg Schmidt plates centred on the cluster M3, with epoch differences from 20 to 23 yr, were scanned using the automatic plate measuring facility in Cambridge. The proper motion determination was based on a stepwise regression method with generalized third-order polynomials and used between 1200 and 2200 galaxies for each pair of plates to define an inertial reference frame. The mean absolute proper motion of the cluster is found to be |${\mu}_{x} = -0.31 \pm 0.02\enspace \text {and}\enspace {\mu}_{y} = -0.23 \pm 0.04\enspace \text {arcsec}\, ({100\, \text {yr}})^{-1}$|⁠. Using this proper motion, together with the known radial velocity and distance to M3, we can infer the orbital parameters of the globular cluster in a Galactocentric inertial system.