Proper motions and membership probabilities of stars in the region of globular cluster NGC 6809^⋆,⋆⋆

¹ Aryabhatta Research Institute of Observational Sciences, Manora Peak, 263129 Nainital, India
e-mail: devesh@aries.res.in; rkant@aries.res.in
² Dipartimento di Astronomia, Universita di Padova, Vicolo dell’ Osservatorio 2, 35122 Padova, Italy
e-mail: andrea.bellini@unipd.it

Received: 29 March 2012
Accepted: 9 May 2012

Abstract

Context. NGC 6809 is a luminous metal-poor halo globular cluster that is relatively easy to study due to its proximity and low concentration. Because of its high Galactic latitude (b =  −23°), interstellar reddening and contamination is not very high.

Aims. We aim to determine the relative proper motion and membership probability of the stars in the wide area of globular cluster NGC 6809. To target cluster members reliably during spectroscopic surveys and both spatial and radial distributions in the cluster outskirts without including field stars, a good proper motion and membership probability catalogue of NGC 6809 is required.

Methods. The archival data of two epochs with a time-base line of 7.1 years have been collected with Wide Field Imager (WFI) mounted on the 2.2 m MPG/ESO telescope. The CCD images of both epochs have been reduced using the astrometric techniques as described in Anderson et al. (2006). The calibrated UBVI magnitudes have been derived using Stetson’s secondary standard stars.

Results. We derived the relative proper motion and membership probabilities for  ~12 600 stars in the field of globular cluster NGC 6809. The measurement error in proper motions for the stars of V ~ 17 mag is 2.0 mas yr^-1, gradually increasing up to  ~3 mas yr^-1 at V = 20 mag. We also provide the membership probability for the published different types of sources in NGC 6809. An electronic catalogue with proper motion and membership probability for the stars will be available to the astronomical community.