# Proper motions and structural parameters of the Galactic globular cluster M71

## TL;DR

This study uses HST data to measure M71's proper motions, determine its orbit within the Milky Way, and revise its structural parameters, revealing a disk-like orbit and a larger core radius than previously known.

## Contribution

It provides new proper motion measurements, a refined orbit within the Galactic potential, and updated structural parameters of M71 based on near-infrared observations.

## Key findings

- M71 has a disk-like orbit inside the Galactic disk.
- The cluster's core and half-mass radii are nearly 50% larger than previous estimates.
- The initial mass of M71 was likely ten times its current mass.

## Abstract

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.