RUPRECHT 147: THE OLDEST NEARBY OPEN CLUSTER AS A NEW BENCHMARK FOR STELLAR ASTROPHYSICS

Abstract

Ruprecht 147 is a hitherto unappreciated open cluster that holds great\npromise as a standard in fundamental stellar astrophysics. We have conducted a\nradial velocity survey of astrometric candidates with Lick, Palomar, and MMT\nobservatories and have identified over 100 members, including 5 blue\nstragglers, 11 red giants, and 5 double-lined spectroscopic binaries (SB2s). We\nestimate the cluster metallicity from spectroscopic analysis, using\nSpectroscopy Made Easy (SME), and find it to be [M/H] = +0.07 \\pm 0.03. We have\nobtained deep CFHT/MegaCam g'r'i' photometry and fit Padova isochrones to the\n(g' - i') and 2MASS (J - K) CMDs using the \\tau^2 maximum-likelihood procedure\nof Naylor (2009), and an alternative method using 2D cross-correlations\ndeveloped in this work. We find best fits for isochrones at age t = 2.5 \\pm\n0.25 Gyr, m - M = 7.35 \\pm 0.1, and A_V = 0.25 \\pm 0.05, with additional\nuncertainty from the unresolved binary population and possibility of\ndifferential extinction across this large cluster. The inferred age is heavily\ndependent by our choice of stellar evolution model: fitting Dartmouth and\nPARSEC models yield age parameters of 3 Gyr and 3.25 Gyr respectively. At\napproximately 300 pc and 3 Gyr, Ruprecht 147 is by far the oldest nearby star\ncluster.\n