An empirical test of the theoretical population corrections to the Red Clump absolute magnitude

Abstract

The mean absolute magnitude of the local red clump (RC) is a very well determined quantity due to the availability of accurate HIPPARCOS parallaxes for several hundred RC stars, potentially allowing it to be used as an accurate extra-galactic distance indicator. Theoretical models predict that the RC mean magnitude has non-linear dependencies on both age and metallicity. This suggests that a population correction, based on the star formation rate (SFR) and age-metallicity relation (AMR) of a particular system, should be applied to the local RC magnitude before it can be compared to the RC in that system in order to make a meaningful distance determination. Using a sample of 8 Galactic open clusters and the GC 47 Tuc, we determine the cluster distances, and hence the RC absolute magnitude in V, I and K, by applying our empirical main sequence fitting method, which utilizes a large sample of local field dwarfs with accurate HIPPARCOS parallaxes. The age and metallicity range of these 9 clusters enable us to make a quantitative assessment of the age and metallicity dependencies of the population corrections predicted by the theoretical models of Girardi & Salaris (2001). We find excellent agreement between the empirical data and the models in all 3 pass-bands, with no statistically significant trends or offsets, thus fully confirming the applicability of the models to single-age, single-metallicity stellar populations. Since, from the models, the population correction is a complicated function of both metallicity and age, if this method is used to derive distances to composite populations, it is essential to have an accurate assessment of the SFR and AMR of the system in question, if errors of several tenths of a magnitude are to be avoided.