Astrometry with MCAO: HST-GeMS proper motions in the globular cluster NGC 6681

TL;DR

This paper demonstrates that the MCAO system GeMS on Gemini-South can accurately measure stellar proper motions over large fields, matching HST results and validating ground-based astrometry for future studies.

Contribution

First successful test of GeMS MCAO system for measuring proper motions in a globular cluster, achieving accuracy comparable to HST.

Findings

GeMS achieves 0.03 mas/yr accuracy in proper motions.

Ground-based MCAO can measure proper motions over large fields.

Results match previous HST measurements with high precision.

Abstract

Aims: for the first time the astrometric capabilities of the Multi-Conjugate Adaptive Optics (MCAO) facility GeMS with the GSAOI camera on Gemini-South are tested to quantify the accuracy in determining stellar proper motions in the Galactic globular cluster NGC 6681. Methods: proper motions from HST/ACS for a sample of its stars are already available, and this allows us to construct a distortion-free reference at the epoch of GeMS observations that is used to measure and correct the temporally changing distortions for each GeMS exposure. In this way, we are able to compare the corrected GeMS images with a first-epoch of HST/ACS images to recover the relative proper motion of the Sagittarius dwarf spheroidal galaxy with respect to NGC 6681. Results: we find this to be (\mu_{\alpha}cos\delta, \mu_{\delta}) = (4.09,-3.41) mas/yr, which matches previous HST/ACS measurements with a very good accuracy of 0.03 mas/yr and with a comparable precision (r.m.s of 0.43 mas/yr). Conclusions: this study successfully demonstrates that high-quality proper motions can be measured for quite large fields of view (85 arcsec X 85 arcsec) with MCAO-assisted, ground-based cameras and provides a first, successful test of the performances of GeMS on multi-epoch data.