Chromatic and spectroscopic signatures of microlensing events as a tool for the gravitational imaging of stars.

The detection of microlensing events from stars in the Large Magellanic Cloud and in the Galactic bulge raises important constraints on the distribution of dark matter and on Galactic structure, although some events may be the result of a new type of intrinsic variability. When lenses are relatively close to the sources, it is predicted that chromatic and spectroscopic effects are likely to appear for a significant fraction of the microlensing events. These effects are due to the differential amplification of the limb and the centre of the stellar disc, and a systematic dependence with wavelength and time provides an unambiguous signature of a microlensing event (as opposed to a new type of intrinsic stellar variability). The author presents detailed predictions of the effects, using realistic model atmospheres. The observations of these effects provide a direct constraint on stellar atmospheres, allowing a three-dimensional reconstruction or imaging of its structure, a unique tool with which to test the current models of stellar atmospheres.