DEEP 2MASS PHOTOMETRY OF M67 AND CALIBRATION OF THE MAIN-SEQUENCE J-KS COLOR DIFFERENCE AS AN AGE INDICATOR

We present an analysis of Two Micron All Sky Survey (2MASS) calibration photometry of the old open cluster M67 (NGC 2682). The proper-motion-cleaned color-magnitude diagram (CMD) resulting from these data extends similar to 3 mag deeper than one based on data from the point source catalog. The CMD extends from above the helium-burning red clump to a faint limit that is more than 7 mag below the main-sequence (MS) turnoff (TO) in the K-S band. After adopting a reddening of E(B - V)= 0.041 +/- 0.004 and a metal abundance of [Fe/H] = -0.009 +/- 0.009 based on a survey of published values, we fit the unevolved MS of M67 to field MS stars with 2MASS photometry and Hipparcos parallaxes. This analysis yields distance moduli of (m - M)K-S = 9.72 +/- 0.05 and (m - M)(0) = 9.70 +/- 0.05, which are consistent with published values. We compare the theoretical isochrones of Girardi et al. and Dotter et al. to the CMD of M67 and comment on the relative merits of each set of models. These comparisons suggest an age between 3.5 and 4.0 Gyr for M67. The depth of the M67 data makes them ideal for the calibration of a new age indicator that has recently been devised by Calamida et al.-the difference in (J - K-S) color between the MS TO and the point on the lower MS where it turns down (TD) and becomes nearly vertical [Delta(J - K-S)(TD)(TO)]. Coupled with deep 2MASS photometry for three other open clusters, NGC 2516, M44, and NGC 6791, we calibrate Delta(J - K-S) in terms of age and find Delta(J - K-S)(TD)(TO) = (3.017 +/- 0.347) - (0.259 +/- 0.037) x log age (yr).