MEASURING THE ROTATION SPEED OF GIANT STARS FROM GRAVITATIONAL MICROLENSING

During some gravitational lensing events, the lens transits the disk o f the star. This event causes a shift in the apparent radial velocity of the star proportional to the star's rotation speed. The magnificati on of such an event is different from that expected for a point source . By measuring both effects, one can determine the rotation parameter upsilon sin i. The method is especially useful for K-giant stars becau se these stars have turbulent velocities that are typically large comp ared with their rotation speed. By making a series of radial velocity measurements, one can typically determine upsilon sin i to the same ac curacy as the individual radial velocity measurements. There are appro ximately 10 microlensing transit events per year that would be suitabl e for these measurements.