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Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7615s&link_type=abstract
HST Proposal ID #7615
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Hst Proposal Id #7615 Stellar Populations
Scientific paper
Gravitational microlensing offers a unique and direct method of determining the masses of stars and other compact objects down to planetary masses. We propose to apply this method to the globular cluster M22, ideally placed against a dense background of bulge stars, to measure its mass function, especially below the hydrogen ignition limit. Compared to the ongoing microlensing studies, our program has the enormous advantage that the location of both the lens and the source stars along the line of sight are known a priori. This removes the uncertainties that affect the interpretation of both the bulge and the LMC microlensing events, and permits a direct determination of the lens mass. Our program consists of 41 separate WFPC2 observations of 3 fields near the core of M22, approximately at 3 day intervals to cover a range of lens masses between 0.02 and 1 M_odot , and repeated over three years. Three additional orbits will be required the first year in order to obtain dithered master images to be used for photometric comparisons. Depending on the slope of the mass function, between 5 and 20 total events will be detected. Our observations can clearly detect brown dwarfs as short time-scale events { t < 8 days}; non-detection of such events will provide a firm upper limit of on the mass fraction between 0.02 and 0.08 M_odot. The observations will also provide a clear indication of the slope of the mass function over the range 0.02 to 1M_odot. Parallel observations with STIS will provide very useful data to some of the scientific spin-off projects, although they are not crucial to the main goal of the program. As a byproduct of these observations, we will also obtain a thorough census of variable stars with periods between 3 and 1000 days in the core and outer region of M22.
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