Galactocentric Distance With the OGLE and Hipparcos Red Clump Stars

We compare red clump stars with parallaxes known to better than 10% in the Hipparcos catalog with the OGLE red clump stars in Baade's Window, corrected for interstellar reddening. There are $\sim 600$ and $\sim 10,000$ 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_{I,m} = -0.18 $, and the dispersion $ \sigma _{RC} = 0.24 $. This allows a single step determination of the distance to the galactic center and gives $R_0 = 7.97 \pm 0.08 $ kpc (formal statistical error). We discuss a variety of systematic effects which make the true error significantly larger. The local stars are relatively blue and have a small color dispersion: $ < (V-I) > = 1.01 $, $ \sigma _{(V-I)} = 0.08 $, while for the bulge stars $ < (V-I)_0 > = 1.22 $, $\sigma_{(V-I)_0} = 0.14 $. Presumably, the bulge population has a broader range and a higher average metallicity than the local disk population.