Galactocentric Distance with the Optical Gravitational Lensing Experiment and HIPPARCOS Red Clump Stars

Abstract

We compare red clump stars with parallaxes known to better than 10% in the Hipparcos catalog and corrected for interstellar extinction with the OGLE red clump stars in Baade's Window, also corrected for interstellar extinction. There are ~600 and ~10,000 such stars in the two data sets, respectively. We find empirically that the average I-band magnitude of red clump stars does not depend on their intrinsic color in the range 0.8<(V-I)₀<1.4. The red clump luminosity function is well represented by a Gaussian with the peak at M_I0,m=-0.28 and the dispersion σ_RC~0.2 mag. This allows a single-step determination of the distance to the Galactic center and gives R₀=8.4+/-0.4 kpc. The number of red clump stars is so large that the formal statistical error is only ~1%. The local stars are relatively blue and have a small color dispersion: <(V-I)>=1.01 and σ_(V-I)=0.08, while for the bulge stars <(V-I)₀>=1.22 and σ_(V-I)0=0.14. Presumably, the bulge population has a broader range and a higher average metallicity than the local disk population.