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

¹ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: davide.massari@oabo.inaf.it
² University of Groningen, Kapteyn Astronomical Institute, 9747 AD Groningen, The Netherlands
³ Herzberg Astronomy and Astrophysics, National Research Council Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada
⁶ Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifca 1, 00133 Roma, Italy
⁷ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy

Received: 18 July 2016
Accepted: 19 September 2016

Abstract

Aims. For the first time the astrometric capabilities of the Gemini Multi-Conjugate Adaptive Optics System (GeMS) facility at the Gemini South Adaptive Optics Imager (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 the Hubble Space Telescope (HST) for a sample of its stars are already available, allowing 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-Advanced Camera for Survey (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 (μ_αcosδ,μ_δ) = (4.09,−3.41)mas yr^-1, which matches previous HST/ACS measurements with a very good accuracy of 0.03 mas yr^-1 and with a comparable precision (rms of 0.43 mas yr^-1).

Conclusions. This study successfully demonstrates that high-quality proper motions can be measured for relatively large fields of view (85′′× 85′′) with MCAO-assisted, ground-based cameras and provides a first, successful test of the performances of GeMS on multi-epoch data.