Removing Degeneracy of Microlensing Light Curves through Narrowband Photometry of Giants

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Abstract
The standard light curve of a microlensing event provides only two constraints on the five unknown parameters of the lens. We show that narrowband photometry during a microlensing event of a giant star can in addition determine the angular radius of the Einstein ring and the proper motion of the lens. This possibility results from the fact that the extended atmospheres of giants emit the cores of resonance lines primarily from a narrow ring (limb brightening). A considerable fraction of the clump giants in the bulge have a ring radius 1012 cm, within the range of Einstein radii for subsolar mass lenses. The extended thin ring also provides a sensitive probe of possible planetary companions as it sweeps across the lens plane. The ring signature can be detected photometrically, using a narrowband filter centered on resonance lines like Ca II λ3933.
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