GRAVITATIONAL MICROLENSING OF STARS OF NO NZERO ANGULAR SIZE

The possibility of employing gravitational-microlensing observations f or high angular resolution studies of stars is explored. The decrease in the amplification factor of a gravitational lens with increasing an gular distance from its optical axis and the finite angular diameter o f a stellar disk result in a divergence of the microlensing light curv e from the curve corresponding to a point source. The effects of finit e stellar angular sizes and limb darkening on the observed microlensin g curve are treated. It is shown that the angular radius of a star, me asured in fractions of the Einstein-cone radius, can be determined usi ng a nonlinear least-squares fitting procedure. The currently availabl e methods of solving this problem, which allow point estimates and con fidence intervals to be obtained for the parameters, are discussed. Th e wavelength dependence of limb darkening results in a chromatic effec t during microlensing observations of stars, generally manifested itse lf as reddening. Color variations in the standard broad-band UBV syste m are estimated for various microlensing conditions. The effect is rat her small, with color variations being no larger than 0(m).015. Howeve r, it can be enhanced by a special choice of the detector passbands th at ensure a maximum limb-darkening difference. An important feature of the effect is its dependence only on the ratio of the angle between t he lens and the true direction toward the star to the stellar angular radius. Given the angular diameter of the star, the angular radius of the Einstein cone of a gravitational lens can be estimated by comparin g the observed and calculated color indices and flux amplification fac tors.