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 and corrected for the interstellar extinction, with the OGLE red clump stars in Baade’s Window also corrected for the 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 = −0.26, 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 a formal statistical error is only ∼ 1%. The local stars are relatively blue and have a small color dispersion: 〈(V − I)〉 = 1.01, σ(V −I) = 0.08, while for the bulge stars 〈(V − I)0〉 = 1.22, σ(V −I)0 = 0.14. Presumably, the bulge population has a broader range and a higher average metallicity than the local disk population. Subject headings: galaxy: center – galaxy: fundamental parameters – galaxy: solar neighborhood – galaxy: stellar content – stars: horizontal-branch