Surface Imaging by Microlensing - Exploring Depth in Stellar Atmospheres

Gravitational microlensing is a new technique for studying the surfaces of distant stars. A point mass lens, usually a low-mass star from the disk, will typically resolve the surface of a red giant in the Galactic bulge, as well as amplify its brightness by a factor of 10 or more. Such events are now detected in real time and can be followed up with precision photometry and spectroscopy. The motivation for stellar imaging via microlensing lies in its unique ability to provide center-to-limb variation of spectral diagnostic lines. Such variation maps into a variation with depth in the stellar atmosphere - a unique and very valuable constraint for modeling the structure of red giants' atmospheres. We illustrate the technique on a recent successful observational campaign and discuss the implications for current stellar models.