Removing Degeneracy of Microlensing Light Curves through Narrowband Photometry of Giants

The standard light curve of a microlensing event provides only twoconstraints on the six unknown parameters of the lens. We show that narrow-bandphotometry during a microlensing event of a giant star can in additiondetermine the angular radius of the Einstein ring and the proper motion of thelens. This possibility results from the fact that the extended atmospheres ofgiants emit the cores of resonant lines primarily from a narrow ring (limbbrightening). The radius of the emission ring can be determined to a precisionof 20% by detailed spectroscopic observations of the source after the lensingevent has ended. A considerable fraction of the clump giant stars in the bulgehave a ring radius >10^{12} cm, within the range of Einstein radii forsub-solar mass lenses. The extended thin ring also provides a sensitive probeof possible planetary companions as it sweeps across the lens plane. The ringsignature can be detected photometrically, using a narrow-band filter centeredon the CaII K line at 3933 A.Comment: 13 pages, uuencoded file, 4 figures include