2003MNRAS.343..539P

The mean absolute magnitude of the local red clump (RC), M^RC_λ, is a very well-determined quantity owing to the availability of accurate Hipparcos parallaxes for several hundred RC stars, potentially allowing it to be used as an accurate extragalactic distance indicator. Theoretical models predict that the RC mean magnitude is dependent on both age and metallicity and, furthermore, that these dependences are non-linear. This suggests that a population correction, ΔM^RC_λ, based on the star formation rate (SFR) and age-metallicity relation (AMR) of the system in question, should be applied to the local RC magnitude before it can be compared to the RC in any other system in order to make a meaningful distance determination.

Using a sample of eight Galactic open clusters and the Galactic globular cluster 47 Tuc, we determine the cluster distances, and hence the RC absolute magnitude in V, I and K, by applying our empirical main-sequence (MS) fitting method, which utilizes a large sample of local field dwarfs with accurate Hipparcos parallaxes. The nine clusters have metallicities in the range -0.7 ≤[Fe/H]≤+0.02 and ages from 1 to 11 Gyr, enabling us to make a quantitative assessment of the age and metallicity dependences of ΔM^RC_λ predicted by the recent theoretical models of Girardi & Salaris and Salaris & Girardi. We find excellent agreement between the empirical data and the models in all three passbands, 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, ΔM^RC_λis 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. Using recent determinations of the SFR and AMR for four systems - the Large and Small Magellanic Clouds, Carina and the solar neighbourhood - we examine the quantity I^RC_obs-K^RC_obs, which is the difference between the mean magnitude of the RC in the I band and the K band. Comparing the theoretical predictions with the most recent observational data, we find complete agreement between the observations and the models, thus confirming even further the applicability of the population corrections predicted from theory.