9518(  1) - 12/06/01 12:32  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      9518
                                                       Version: 1
                                                       Check-in Date: 06-Dec-2001
                                                                      11:43:34

1.Proposal Title:
Astrometric Gravitational Microlensing: an HST perspective
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2. Proposal For  3. Cycle
AR               11
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4. Investigators
                                                                                     Contact?
    PI: Bohdan Paczynski                  Princeton University Observatory
   CoI: Dr. Shude Mao                     Jodrell Bank Observatory                      N

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5. Abstract

Gravitational microlensing is developing into an important astrophysical tool with
diverse applications. In the coming years, about 1000 microlensing events will be
discovered annually, many in real-time. However, many applications suffer from the
degeneracy between the lens mass, transverse velocity and the distances. The
astrometric signature of microlensing partially breaks the degeneracy. The High-
Resolution Camera of the Advanced Camera for Surveys will be an ideal instrument for
measuring this effect. We propose to conduct a theoretical study to prepare for such
observations. These observations will lead to the first unique determination of the
lens mass, and have significant impacts on the nature of the lenses toward the LMC,
the model of the Galactic bar and the census of stellar mass black holes in the
Galaxy. We will study what are the best microlensing events for HST astrometric
studies, and the optimal strategy (frequency and duration) for such HST monitoring.
The lensed stars lie preferentially on the far side of the Galactic bar due to the
enhanced lensing probability by the stars on the near side. This bias leads to
systematic differences in the proper motions and radial velocities of the lensed
stars relative to the random stars in the Galactic bar. We will quantify these
differences and their dependence on the bar model.
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                                                9518(  1) - 12/06/01 12:32  - [  2]

            Summary Form for Proposal  9518

  Item                         Used in this proposal
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  Proposal Category            AR
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