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

Abstract

We present relation of the mean I-band brightness of red clump stars on metallicity. Red clump stars were proposed to be a very attractive standard candle for distance determination. The calibration is based on 284 nearby red giant stars whose high quality spectra allowed to determine accurate individual metal abundances. High quality parallaxes (\sigma_\pi / \pi < 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 a weak dependence of the mean I-band brightness on metallicity (about 0.13 mag/dex). What is more important, the range of metallicity of the Hipparcos sample partially overlaps with metallicity of field giants in the LMC, thus making it possible to determine the distance to the LMC by almost direct comparison of brightness of the local Hipparcos red clump giants with that of LMC stars. Photometry of field red clump giants in nine low extinction fields of the LMC halo collected during the OGLE-II microlensing survey compared with the Hipparcos red clump stars data yields the distance modulus to the LMC: (m-M)_LMC=18.24+/-0.08 mag.Comment: 11 pages. Latex+psfig. Accepted for publictation in ApJ Letters. Major revision: 30% larger sample of Hipparcos red giants with spectroscopic metallicities and extended photometry of the LMC field red clump stars (9 lines-of-sight). Due to journal space limitation, sections on comparison of the LMC red clump distance modulus with previous determinations and relations between the color indices and metallicity removed from this versio