Probing Red Giant Atmospheres with Gravitational Microlensing

Gravitational microlensing provides a new technique for studying the surfaces of distant stars. Microlensing events are detected in real time and can be followed up with precision photometry and spectroscopy. This method is particularly adequate for studying red giants in the Galactic Bulge. We show that different properties of the stellar surface can be recovered from timedependent photometry and spectroscopy of a point-mass microlensing event with a small impact parameter. An emphasis is placed on center-to-limb variation of spectral features, which reveals the depth structure of the atmosphere of the source star. A model of the MACHO Alert 95-30 event source is used to illustrate the technique and the implications for constructing model atmospheres of red giants. Subject headings: gravitational lensing — red giants — stars: atmospheres Submitted to The Astrophysical Journal, February 1999