Galactocentric Distance with the OGLE and [ITAL]Hipparcos[/ITAL] Red Clump Stars

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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 MI0,m = -0.28 and the dispersion σRC ≈ 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.

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