Modeling the Galactic Bar Using Red Clump Giants

The color-magnitude diagrams of $\sim 7 \times 10^5$ stars obtained for 12fields across the Galactic bulge with the OGLE project reveal a well-definedpopulation of bulge red clump giants. We find that the distributions of theapparent magnitudes of the red clump stars are systematically fainter whenmoving towards lower galactic $l$ fields. The most plausible explanation ofthis distinct trend is that the Galactic bulge is a bar, whose nearest end liesat positive galactic longitude. We model this Galactic bar by fitting for allfields the observed luminosity functions in the red clump region of thecolor-magnitude diagram. We find that almost regardless of the analyticalfunction used to describe the 3-D stars distribution of the Galactic bar, theresulting models have the major axis inclined to the line of sight by$20-30\deg$, with axis ratios corresponding to $x_0:y_0:z_0=3.5:1.5:1$. Thisputs a strong constraint on the possible range of the Galactic bar models.Gravitational microlensing can provide us with additional constrains on thestructure of the Galactic bar.