In an effort to improve the utility of the helium-burning red clump luminosity as a distance indicator, we explore the sensitivity of the K-band red clump absolute magnitude M-K (RC) to metallicity and age. We rely upon JK photometry for 14 open clusters and two globular clusters from the Second Incremental Data Release of the Two Micron All Sky Survey (2MASS) Point Source Catalog. The distances, metallicities, and ages of the open clusters are all on an internally consistent system, while the K ( RC) values are measured from the 2MASS data. For clusters younger than similar to2 Gyr, M-K(RC) is insensitive to metallicity but shows a dependence on age. In contrast, for clusters older than similar to2 Gyr, M-K(RC) is influenced primarily by the metallicity of the population and shows little or no dependence on the age. Theoretical red clump models based on the formalism of Girardi et al. reinforce this finding. Over comparable metallicity and age ranges, our average M-K(RC) value is in accord with that based on solar neighborhood red clump stars with Hipparcos parallaxes. Lastly, we compute the distance to the open cluster NGC 2158 using our red clump calibration. Adopting an age of 1.6 +/- 0.2 Gyr and Fe/H = 0.24 +/- 0.06, our calibration yields a distance of (m-M)(V) = 14.38 +/- 0.09. References: 25
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