Jason L. Curtis, Angie Wolfgang, Jason T. Wright, John M. Brewer, John A. Johnson
Ruprecht 147 is a hitherto unappreciated open cluster that holds great promise as a standard in fundamental stellar astrophysics. We have conducted a radial velocity survey of astrometric candidates with Lick, Palomar, and MMT observatories and have identified over 100 members, including 5 candidate blue stragglers, 11 red giants, and 5 SB2 binaries. We estimate the cluster metallicity from spectroscopic analysis, using Spectroscopy Made Easy (SME), and find it to be [M/H] = +0.08 \pm 0.03. We have obtained deep CFHT/MegaCam g'r'i'z' photometry and fit Padova isochrones to the (g' - i') and 2MASS (J - K) CMDs, using the \tau^2 maximum-likelihood procedure of Naylor (2009). We find best fits for isochrones at age t = 2.5 \pm 0.25 Gyr, m \pm M = 7.35 \pm 0.1, and A_V = 0.25 \pm 0.05, with significant uncertainty from the unresolved binary population and possibility of differential extinction across this large cluster. Our preferred model does not simultaneously fit the main sequence turnoff and the red giant branch in the optical CMD. We investigate alternative solutions and find that an older, closer and less extinguished Padova model with age t = 3.5 Gyr, m - M = 7.0, and A_V = 0.10 appears to better match the overall optical CMD (particularly the red giant branch). We do not favor this model because it poorly fits the upper main sequence in the optical, the age is inconsistent with our spectroscpic results, and our preferred model better fits the NIR CMD. Still, we cannot yet conclusively rule out the older solution. At 250 - 300 pc and an age of 2.5 - 3.5 Gyr, Ruprecht 147 is by far the oldest nearby star cluster.
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http://arxiv.org/abs/1206.6533
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