AAS 199th meeting, Washington, DC, January 2002
Session 75. Gravitational Lensing and Cosmic Distances
Oral, Tuesday, January 8, 2002, 2:00-3:30pm, International Ballroom Center

[75.04] First microlens mass measurement: PLANET photometry of EROS BLG-2000-5

J. H. An (Ohio State), PLANET Collaboration

We analyze PLANET photometric observations of the caustic-crossing binary-lens microlensing event, EROS BLG-2000-5, and find that modeling the observed light curve requires incorporation of the microlens parallax and the binary rotation. The projected Einstein radius (\tilde rE = 3.61±0.11 AU) is derived from the measurement of the microlens parallax, and we are also able to infer the angular Einstein radius (\thetaE = 1.38±0.12 mas) from the finite source effect on the light curve, combined with an estimate of the angular size of the source given by the source position in a color-magnitude diagram. The lens mass, M = 0.612±0.057 M\sun, is found by combining these two quantities. This is the first time that parallax effects are detected for a caustic-crossing event and also the first time that the lens mass degeneracy has been completely broken through photometric monitoring alone. The combination of the projected Einstein radius and the angular Einstein radius also allows us to conclude that the lens lies in the near side of the disk, within 2.6 kpc of the Sun. In addition, we derive the lens-source relative proper motion and show that it favors the source lying behind the bulge -- in the disk, but further study will be required to find the source location definitively.