WIYN Open Cluster Study. III. The Observed Variation of the Red Clump Luminosity and Color with Metallicity and Age

The recent controversy regarding the variation of the I-band red clump (RC) luminosity (M^RC_I) with metallicity and age has prompted us to examine this question, using precise photometry of Galactic open clusters. Because these data extend well down to the unevolved main sequence, it is possible to use main-sequence fitting to estimate the cluster distance moduli, thereby providing a straightforward measure of the absolute magnitude of the red clump. Using this technique and comparing the observed quantities with theoretical ones, we draw the following conclusions. Cluster M^RC_I values are much less sensitive to metal abundance than their M^RC_V counterparts, making the former more amenable for use as a distance indicator. However, the range in the observed values of M^RC_I is much larger than that expected from the measurement errors alone. This fact suggests that the value of M^RC_I is influenced to a significant degree by properties of the stellar population, such as metal abundance and age. In particular, the observed variation of M^RC_I and M^RC_V with metallicity and age, as inferred from the open clusters in this study, agrees qualitatively with the predictions of the theoretical models. These results indicate that application of the solar-neighborhood M^RC_I to red clumps of other stellar populations with a different age/abundance mix, such as the Magellanic Clouds, may produce spurious results.