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It is well known that the polarization signal in microlensing events of hotstars is larger than that of main-sequence stars. Most hot stars rapidly rotatearound their stellar axes. The stellar rotation makes ellipticity andgravity-darkening effects which break the spherical symmetry of the sourceshape and the circular symmetry of the source surface brightness respectively.Hence, it causes a net polarization signal for the source star. Thispolarization signal should be considered in polarimetry microlensing of fastrotating stars. For moderate rotating stars, lensing can magnify or evencharacterize small polarization signals due to the stellar rotation throughpolarimetry observations. The gravity-darkening effect due to a rotating sourcestar makes asymmetric perturbations in polarimetry and photometry microlensingcurves whose maximum happens when the lens trajectory crosses the projectedposition of the rotation pole on the sky plane. The stellar ellipticity makes atime shift (i) in the position of the second peak of the polarimetry curves intransit microlensing events and (ii) in the peak position of the polarimetrycurves with respect to the photometry peak position in bypass microlensingevents. By measuring this time shift via polarimetry observations ofmicrolensing events, we can evaluate the ellipticity of the projected sourcesurface on the sky plane. Given the characterizations of the FOcal Reducer andlow dispersion Spectrograph (FORS2) polarimeter at Very Large Telescope (VLT)telescope, the probability of observing this time shift is so small. The moreaccurate polarimeters of the next generation may likely measure these timeshifts and evaluate the ellipticity of microlensing source stars.

Stellar rotation effects in polarimetric microlensing