The Optical Gravitational Lensing Experiment. Red Clump Stars as a Distance Indicator

We present calibration of the mean I-band brightness of red clump stars on metallic-ity. Red clump stars were proposed to be a very attractive standard candle for distance determination. The calibration is based on 218 nearby red giant stars whose high quality spectra allowed to determine accurate individual metal abundances. High quality par-allaxes (σπ/π < 10%) and photometry of these very bright stars come from Hipparcos measurements. Metallicity of the sample covers a large range: −0.6<[Fe/H] <+0.2 dex. We find only a weak dependence of the mean I-band magnitude on metallicity (0.15 mag/dex) in good agreement with previous empirical and theoretical work. What is more important, the range of metallicity of the Hipparcos sample overlaps with the metallicity of field stars in the LMC, thus making it possible to determine the distance to the LMC by direct comparison of brightness of the local Hipparcos red clump giants with that of LMC stars. Photometry of field red clump giants in four low extinction regions of the LMC halo collected during the OGLE-II microlensing survey compared with the Hipparcos red clump stars data yields the final (free from population effects) " red-clump method " distance modulus to the LMC: (m − M) = 18.23 ± 0.05 mag. So far, this is the most robust and accurate distance determination to the LMC. We also present empirical relations between the color indices U − B, B − V and V − I and metallicity of nearby red clump stars.