GRAVITATIONAL MICROLENSING BY THE HALO OF THE ANDROMEDA GALAXY

It has been shown by Paczynski that gravitational microlensing is a us eful method to detect brown dwarfs in the dark halo of our galaxy. At present several experiments are carried out to monitor several million stars in the Large Magellanic Cloud with the aim to find such microle nsing events. Here I discuss the possibility to use as targets stars i n the Andromeda galaxy. Such an experiment would be sensitive to both the brown dwarfs in our halo and in the one of M31. The optical depth tau to gravitational microlensing due to brown dwarfs in the halo of M 31 turns out to be approximately 10(-6), which is comparable to the va lue Of tau for our own halo. I also compute the microlensing rate and the average lensing duration and consider moreover the dependence of t au on the shape of the halo.