Color-Magnitude Diagram Distribution of the Bulge Red Clump Stars -- Evidence for the Galactic Bar

Stanek K.Z., Mateo M., Udalski A., Szymanski M.,
Kaluzny J., and Kubiak M.

1994, Astrophys. J. Letters, 429, L73 (ADS).


The color-magnitude diagrams of approximately 5 x 105 stars obtained for 13 fields toward the Galactic bulge with the Optical Gravitational Lensing Experiment (OGLE) project reveal a well-defined population of bulge red clump stars. We find that the distributions of the extinction-adjusted apparent magnitudes of the red clump stars in fields lying at l = plus or minus 5 deg in Galactic longitude differ by 0.37 plus or minus 0.025 mag. Assuming that the intrinsic luminosity distribution of the red clump stars is the same on both sides of the Galactic center, this implies that the distances to the red clump stars in the two fields differ ba a factor 1.185 plus or minus 0.015. A plausible explanation of the observed difference in the luminosity distribution is that the Galactic bulge is a triaxial structure, or bar, which is inclined to the line of sight by no more than 45 deg, with the part of the bar at the positive Galactic longitude being closer to us. This agrees rather well with other studies indicating the presence of the bar in the center of the Galaxy. Color-magnitude diagram data are accessible over the computer network with anonymous ftp.