Proper motions and structural parameters of the Galactic globular
  cluster M71
  By exploiting two ACS/HST datasets separated by a temporal baseline of ~7
years, we have determined the relative stellar proper motions (providing
membership) and the absolute proper motion of the Galactic globular cluster
M71. The absolute proper motion has been used to reconstruct the cluster orbit
within a Galactic, three-component, axisymmetric potential. M71 turns out to be
in a low latitude disk-like orbit inside the Galactic disk, further supporting
the scenario in which it lost a significant fraction of its initial mass. Since
large differential reddening is known to affect this system, we took advantage
of near-infrared, ground-based observations to re-determine the cluster center
and density profile from direct star counts. The new structural parameters turn
out to be significantly different from the ones quoted in the literature. In
particular, M71 has a core and a half-mass radii almost 50% larger than
previously thought. Finally we estimate that the initial mass of M71 was likely
one order of magnitude larger than its current value, thus helping to solve the
discrepancy with the observed number of X-ray sources.
