Tycho-Gaia Astrometric Solution Parallaxes and Proper Motions for Five
  Galactic Globular Clusters
  We present a pilot study of Galactic globular cluster (GC) proper motion (PM)
determinations using Gaia data. We search for GC stars in the Tycho-Gaia
Astrometric Solution (TGAS) catalogue from Gaia Data Release 1 (DR1), and
identify five members of NGC104 (47 Tucanae), one member of NGC5272 (M3), five
members of NGC6121 (M4), seven members of NGC6397, and two members of NGC6656
(M22). By taking a weighted average of member stars, fully accounting for the
correlations between parameters, we estimate the parallax (and, hence,
distance) and PM of the GCs. This provides a homogeneous PM study of multiple
GCs based on an astrometric catalog with small and well-controlled systematic
errors, and yields random PM errors similar to existing measurements. Detailed
comparison to the available Hubble Space Telescope (HST) measurements generally
shows excellent agreement, validating the astrometric quality of both TGAS and
HST. By contrast, comparison to ground-based measurements shows that some of
those must have systematic errors exceeding the random errors. Our parallax
estimates have uncertainties an order of magnitude larger than previous
studies, but nevertheless imply distances consistent with previous estimates.
By combining our PM measurements with literature positions, distances, and
radial velocities, we measure Galactocentric space motions for the clusters and
find that these also agree well with previous analyses. Our analysis provides a
framework for determining more accurate distances and PMs of Galactic GCs using
future Gaia data releases. This will provide crucial constraints on the near
end of the cosmic distance ladder and provide accurate GC orbital histories.
