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[124.01] MACHOS and Other Dark Subjects: What Has Been Learned Using Gravitational Lensing

C. Alcock (LLNL)

Baryonic material can exist in several dark forms: ``planets," brown dwarfs, very old degenerate dwarf stars, and neutron stars. Black holes are frequently added to this list, even though a black hole has no baryon number. These objects, most of which emit some light but at levels below present day detection thresholds, are collectively known as Machos. Several groups have exploited the gravitational microlens signature to search for Machos in the dark halo of the Milky Way. Over 350 microlensing events have been reported (most by the MACHO Project) of which about 30 are towards the Magellanic Clouds. The most straightforward interpretation of these events is that they represent a significant component of the dark halo of the Milky Way. Alternative explanations have been advanced, and it has proved difficult to distinguish between the various suggestions. A robust conclusion, however, is that objects of substellar mass do not comprise much of the dark matter in the halo of the Milky Way. Gravitational mirolensing has also been used to search for planets, and it is uniquely capable of probing the structure of stellar atmospheres. It is now possible to design and construct microlensing surveys that are much more powerful than any of the existing surveys. The outline and goals of such a survey will be discussed.