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Ruprecht 147 is a hitherto unappreciated open cluster that holds greatpromise as a standard in fundamental stellar astrophysics. We have conducted aradial velocity survey of astrometric candidates with Lick, Palomar, and MMTobservatories and have identified over 100 members, including 5 bluestragglers, 11 red giants, and 5 double-lined spectroscopic binaries (SB2s). Weestimate the cluster metallicity from spectroscopic analysis, usingSpectroscopy Made Easy (SME), and find it to be [M/H] = +0.07 \pm 0.03. We haveobtained deep CFHT/MegaCam g'r'i' photometry and fit Padova isochrones to the(g' - i') and 2MASS (J - K) CMDs using the \tau^2 maximum-likelihood procedureof Naylor (2009), and an alternative method using 2D cross-correlationsdeveloped in this work. We find best fits for isochrones at age t = 2.5 \pm0.25 Gyr, m - M = 7.35 \pm 0.1, and A_V = 0.25 \pm 0.05, with additionaluncertainty from the unresolved binary population and possibility ofdifferential extinction across this large cluster. The inferred age is heavilydependent by our choice of stellar evolution model: fitting Dartmouth andPARSEC models yield age parameters of 3 Gyr and 3.25 Gyr respectively. Atapproximately 300 pc and 3 Gyr, Ruprecht 147 is by far the oldest nearby starcluster.

Ruprecht 147: The oldest nearby open cluster as a new benchmark for stellar astrophysics