Absolute magnitude of the red clump and its applications in distance determination.

Red clump giants are intermediate-age core helium-burning stars.Theoretical models predict that their luminosities weakly depends on their age and chemical composition.Small variance in intrinsic luminosities and weakly dependence on chemical composition and age make them an extremely useful distance indicators. Accurate parallaxes for several hundred red clump giants in the solar neighborhood provided by Hipparcos satellite have been used to estimate the mean absolute I-band and K-band magnitudes. Several I-band and K-band mean absolute magnitudes have been reported based on local red clump giants and the dependence on metallicity and color have also been investigated.Accurate distance to the Galactic center,local group,and some star clusters in our Galaxy have been measured using red clump giants as reliable standard candles.By using the red clump giants population as a tracer of inner Galaxy,some researchers have analysed the structure of the bar. As all stellar distance indicators,the main worry for red clump giants lies in the possible population effects.Several studies have indicated that the luminosity dependence of the red clump giants on age and metallicity can cause significant difference in mean absolute magnitude between different stellar populations. More red clump giants with accurate parallaxes,high-quality photometry and reliable metallicities are needed to calibrate the mean absolute magnitude and the population effects.In the near future the GAIA astrometric data(position and distance),photometric data(luminosity),and spectroscopic data(metallicity,distance and extinction),will resolve this ambiguity.