Modelling the Galactic Bar Using Red Clump Giants

The color-magnitude diagrams of ∼ 7×105 stars obtained for 12 fields across the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump giants. We find that the distributions of the apparent magnitudes of the red clump stars are systematically fainter when moving towards lower galactic l fields. The most plausible explanation of this distinct trend is that the Galactic bulge is a bar, whose nearest end lies at positive galactic longitude. We model this Galactic bar by fitting for all fields the observed luminosity functions in the red clump region of the color-magnitude diagram. We find that almost regardless of the analytical function used to describe the 3-D stars distribution of the Galactic bar, the resulting models have the major axis inclined to the line of sight by 20− 30 deg, with axis ratios corresponding to x0 : y0 : z0 = 3.5 : 1.5 : 1. This puts a strong constraint on the possible range of the Galactic bar models. Gravitational microlensing can provide us with additional constrains on the structure of the Galactic bar. Subject headings: stars: HR diagram – stars: statistics – Galaxy: general – Galaxy: structure On leave from N. Copernicus Astronomical Center, Bartycka 18, Warszawa 00–716, Poland