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 \sim 3 magnitudes 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 magnitudes below the main sequence turnoff in the K _ { S } band .
After adopting a reddening of E ( B–V ) = 0.041 \pm 0.004 and a metal abundance of [ Fe/H ] = –0.009 \pm 0.009 based on a survey of published values , we fit the unevolved main sequence of M67 to field main sequence stars with 2MASS photometry and Hipparcos parallaxes .
This analysis yields distance moduli of ( m–M ) _ { K _ { S } } = 9.72 \pm 0.05 and ( m–M ) _ { 0 } = 9.70 \pm 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 make 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 main sequence turnoff ( TO ) and the point on the lower main sequence 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 \pm 0.347 ) – ( 0.259 \pm 0.037 ) *Log Age ( yrs ) .