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Session 99 - Invited Talk.
Invited session, Thursday, January 18
1st Floor, La Villita Assembly Building
At least 6 gravitational microlensing experiments are now underway with several more planned. The primary motivation for initiating these experiments was to detect Massive Compact Halo Objects (MACHOs), a leading candidate for the dark matter in the Milky Way halo. The experiments have already placed important limits on MACHOs and may have detected them at a lower-than-expected level. However, the vast majority of the >100 events detected so far are not MACHOs, but ordinary stars. I discuss the short and long term prospects for using microlensing to learn about the stellar mass and luminosity functions and for finding planetary systems around the detected stars. Current microlensing data allow one to statistically measure the mass function of Galactic disk and bulge stars. Initial estimates are roughly consistent with traditional measurements in the solar neighborhood. Much more detailed comparisons will soon be possible. ``Parallax'' and ``proper motion'' measurements of lensing events will allow measurement of individual masses and so detect or rule out brown dwarfs. ``Pixel lensing'', a new technique for monitoring lensing events of unresolved stars, can be used to measure the mass function in other galaxies and to look for intra-cluster MACHOs in the Virgo cluster. Pixel lensing can also probe the luminosity function in external galaxies to well below the magnitude limit where individual stars are unresolved. An ambitious but feasible all-sky search for microlensing of quasars could measure the star-formation history of the universe. Closer to home, follow up observations of microlensing events are already being mounted in an attempt to find planetary systems. This is an excellent approach and is probably the only ground-based technique for finding Earth-mass planets around ordinary stars.
