A new look at the old star cluster NGC6791

Abstract

We present comprehensive cluster membership and g′r′ photometry of the prototypical old, metal-rich Galactic star cluster NGC6791. The proper-motion catalog contains 58,901 objects down to g′ 24, limited to a circular area of radius 30′. The highest precision of the proper motions is 0.08 mas yr-1. Our proper motions confirm cluster membership of all main and also some rare constituents of NGC6791. The total number of probable cluster members down to g′ = 22 (MV +8) is 4800, corresponding to M tot 5000 M. New findings include an extended horizontal branch in this cluster. The angular radius of NGC6791 is at least 15′ (the effective radius is Rh ≃ 44 while the tidal radius is rt ≃ 23′). The luminosity function of the cluster peaks at 4.5 and then steadily declines toward fainter magnitudes. Our data provide evidence that differential reddening may not be ignored in NGC6791. © 2011. The American Astronomical Society. All rights reserved.