THE DISTRIBUTION OF GALACTIC DISK STARS IN BAADE WINDOW

The color-magnitude diagram of approximately 3 X 10(5) stars obtained for Baade's Window towards the Galactic bulge with the OGLE project reveals a surprisingly narrow main sequence due to galactic disk stars at a distance of d approximately 2 kpc, i.e., at the location of the Sagittarius spiral arm. A more careful analysis indicates there is an excess in the number of disk stars by a factor approximately 2 between us and d approximately 2.5 kpc and a rapid drop by a factor approximately 10 beyond that distance. It is unlikely that the observed structure is an artifact of the interstellar extinction, but careful determination of the extinction is needed before the structure is firmly established. The narrow main sequence extends down to stars as faint as M(V) almost-equal-to 7, i.e., it is composed of old stars. This is not expected in a conventional disk model, or in a conventional model of a spiral structure. However, a strong concentration of old stars towards spiral arms has been noticed in some other galaxies, like M51 [Rix & Rieke, ApJ, 418, 123 (1993)], with the near-infrared surface photometry. We have also found that the relative distribution of red clump and red giant stars in the Galactic bulge implies that there is a relatively young stellar population present there. Color-magnitude diagram data are accessible over the computer network with anonymous ftp.