  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.

Astrophysics