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

\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_0,m} = -0.26

, and the dispersion

\sigma _{RC} \approx 0.2

mag. This allows a single step determination of the distance to the galactic center and gives

R_0 = 8.4 \pm 0.4

kpc. The number of red clump stars is so large that a formal statistical error is only

\sim 1%

. 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.

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