Probing Red Giant Atmospheres with Gravitational Microlensing

Gravitational microlensing provides a new technique for studying the surfacesof distant stars. Microlensing events are detected in real time and can befollowed up with precision photometry and spectroscopy. This method isparticularly adequate for studying red giants in the Galactic bulge. Recentlywe developed an efficient method capable of computing the lensing effect forthousands of frequencies in a high-resolution stellar spectrum. Here wedemonstrate the effects of microlensing on synthesized optical spectra of redgiant model atmospheres. We show that different properties of the stellarsurface can be recovered from time-dependent photometry and spectroscopy of apoint-mass microlensing event with a small impact parameter. In this study weconcentrate on center-to-limb variation of spectral features. Measuring suchvariations can reveal the depth structure of the atmosphere of the source star.Comment: 23 pages with 11 Postscript figures, submitted to ApJ; Section 2 expanded, references added, text revise