AST 98-02984 Arlin P. Crotts The PI is continuing a survey of gravitational microlensing of stars in M31. This complements the surveys for MACHOs (massive compact halo objects) in the direction of the Large Magellanic Cloud and the Galactic Bulge. The PI has already constructed the major hardware for this effort, has written the software to find variable stars in highly crowded fields, and has large blocks of telescope time at the 1.8 meter Vatican Advanced Technology Telescope and the MDM 1.3 meter telescope. Using M31 to search for MACHOs is particularly interesting because it is the only external spiral galaxy that can be successfully probed for dark matter in this way. It is different from the Galaxy in that sightlines through the halo have associated with them a possibly high optical depth (or probability per star) of halo microlensing. This promises to produce a stronger halo lensing signal that the Galactic case. Furthermore, this signal is variable across the face of the galaxy, and this can be used to further constrain halo dark matter distribution models. It is likely that this localization of the halo mass will also lead to better estimates of the source-lens-observer geometry, and hence the component masses of the objects acting as lenses ***
