Measuring the Rotation Speed of Giant Stars from Gravitational Microlensing

During some gravitational lensing events, the lens transits the face of thestar. This causes a shift in the apparent radial velocity of the star which isproportional to its rotation speed. It also changes the magnification relativeto what would be expected for a point source. By measuring both effects, onecan determine the rotation parameter $v\sin i$. The method is especially usefulfor K giant stars because these have turbulent velocities that are typicallylarge compared with their rotation speed. By making a series of radial velocitymeasurements, one can typically determine $v\sin i$ to the same accuracy as theindividual radial velocity measurements. There are approximately 10microlensing transit events per year which would be suitable to make thesemeasurements.Comment: 11 pages including 1 embedded figur