AAS 200th meeting, Albuquerque, NM, June 2002
Session 74. Stellar Atmospheres and Circumstellar Material
Display, Thursday, June 6, 2002, 9:20am-4:00pm, SW Exhibit Hall

## [74.20] Gravitational Microlensing of Circumstellar Envelopes

J. E. Bjorkman (University of Toledo), R. Ignace (University of Iowa), J. F. L. Simmons (University of Glasgow, & Centre Universitaire, Luxembourg)

We discuss the flux and polarization signals obtained from the microlensing of stars with extended circumstellar envelopes by a single point-mass lens. We find that stars with extended envelopes will show a high level of variable polarisation (up to 10%), even if they are spherically symmetric. Since the stellar envelopes most likely to be lensed are produced by red giant winds, we include the effects of a central cavity (representing the dust condensation radius) within the scattering envelope. In this paper we show that, even if the event is not a photospheric transit, the combination of photometric and polarimetric light curves determines the angular Einstein radius of the lens, \thetaE, the lens impact parameter, \theta0, and the lens proper motion, \muL (both direction and magnitude). Basically, the level of polarization at flux maximum determines the lens impact parameter, the flux amplification gives the Einstein radius, and the variation of the polarisation position angle determines both the magnitude and direction of the lens proper motion. Since the lens proper motion can place kinematic constraints on the distance to the lens, we conclude polarization measurements of the microlensing event can help constrain the allowable range of lens masses.

Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.