WIYN Open Cluster Study. X. The K-Band Magnitude of the Red Clump as a Distance Indicator

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 ~2 Gyr, M_K(RC) is insensitive to metallicity but shows a dependence on age. In contrast, for clusters older than ~2 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.