Wiyn open cluster study. LV. Astrometry and membership in NGC 6819

Abstract

We present proper motions and astrometric membership analysis for 15,750 stars around the intermediate-age open cluster NGC 6819. The accuracy of relative proper motions for well-measured stars ranges from ∼0.2 mas yr-1 within 10' of the cluster center to 1.1 mas yr-1 outside this radius. In the proper motion vector-point diagram, the separation between the cluster members and field stars is convincing down to V ∼ 18 and within 10' from the cluster center. The formal sum of membership probabilities indicates a total of ∼2500 cluster members down to V ∼ 22. We confirm the cluster membership of several variable stars, including some eclipsing binaries. The estimated absolute proper motion of NGC 6819 is μxa bs = -2.6±0.5 andμya bs = -4.2 ±0.5 mas yr-1. A cross-identification between the proper motion catalog and a list of X-ray sources in the field of NGC 6819 resulted in a number of new likely optical counterparts, including a candidate CV. For the first time we show that there is significant differential reddening toward NGC 6819. © 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.