Mapping Microlensing and Stellar Variability in M31

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CrottsAST-0070882The principal investigator and colleagues propose to study micolensing events in M31, using the background stars in that galaxy to identify microlenses in both M31 itself and in our own galaxy. The rate of lensing events is expected to be higher in M31 than in our galaxy or the LMC because the inclination of M31 allows us to see through a greater column density of stars. In addition to lensing objects, the surveys will also find intrinsically variable stars (such as Mira variables). A key question today in astrophysics is what composes the dark matter known to exist in galaxies. A crucial clue comes from observations which indicate that a large fraction of the mass of our Galaxy may be due to "Massive, Compact Halo Objects" (MACHOs), an interpretation which is highly controversial and difficult to understand. This proposal is to fund a three-year observational program, involving our nearest neighboring, large galaxy, M31 in Andromeda, which will settle the question of whether a large part of galaxy dark matter is composed of stellar massed objects in the halo. These results can be obtained well in advance of competing, proposed approaches, and settle the current quandary within the context of a galaxy separate from our own, indicating whether the MACHO halo is a universal phenomenon. The proposed study can address unsettled questions from Galactic surveys about the structure and content of the central Bulge, as well as produce a comprehensive assay of variable stars in M31, whichcan be used for several, new astronomical studies. Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC).****