MICROLENSING BY THE GALACTIC BAR

We compute the optical depth and duration distribution of microlensing events toward Baade's window in a model composed of a Galactic disk a nd a bar. The bar model is a self-consistent dynamical model built out of individual orbits that has been populated to be consistent with th e COBE maps of the Galaxy and kinematic observations of the Galactic b ulge. We find that most of the lenses are in the bulge with a line-of- sight distance 6.25 kpc (adopting R(0) = 8 kpc). The microlensing opti cal depth of a 2 x 10(10) M. bar plus a truncated disk is (2.2 +/- 0.4 5) x 10(-6), consistent with the large optical depth (3.2 +/- 1.2) x 1 0(-6) found by Udalski et al. (1994). This model optical depth is enha nced over the predictions of axisymmetric models by Kiraga and Paczyns ki (1994) by slightly more than a factor of 2, since the bar is elonga ted along the line of sight. The large Einstein radius and small trans verse velocity dispersion also predict a longer event duration in the self consistent bar model than in the Kiraga-Paczynski model. The even t rate and duration distribution also depend on the lower mass cutoff of the lens mass function. With a 0.1 M. cutoff, five to seven events (depending on the contribution of disk lenses) with a logarithmic mean duration of 20 days are expected for the Optical Gravitational Lensin g Experiment (OGLE) according to our model, while Udalski et al. (1994 ) observed nine events with durations from 8 to 62 days. On the other hand, if most of the lenses are brown dwarfs, our model predicts too m any short-duration events. A Kolmogorov-Smirnov test finds only 7% pro bability for the model with 0.01 M. cutoff to be consistent with curre nt data.