GRAVITATIONAL MICROLENSING OF THE GALACTIC BULGE STARS

The optical depth to microlensing on the ordinary disk stars is approximately 4 x 10(-7) for the Galactic bulge stars visible in "Baade's window." This means that at any given time one out of approximately 2.5 x 10(6) bulge stars must be brightened by at least 0.3 mag. The duration of events is proportional to the square root of mass of the lensing star, and it is in the range of 1 week to 1 month for the lensing stars of 0.1-1.0 M .. The average number of events with the amplitude exceeding 0.3 mag is expected to be approximately 4 per year per 10(6) bulge stars. If there are brown dwarfs in the Galactic disk with masses in the range 0.01-0.1 M., with the total mass density equal to the total mass density of the ordinary stars, then the combined optical depth to microlensing of the bulge stars is approximately 8 x 10(-7), the average number of events with the amplitude exceeding 0.3 mag is expected to be approximately 17 per year per 10(6) bulge stars, and the time scales of events are in the range of 3-20 days.