3.5 Microlensing

Gravitational imaging of stellar photospheres is a potentially useful tool for stellar astrophysics. Detecting spots in ways different from, e. g., rotationally induced modulations of the stellar brightness or spectral line profiles could bring considerable insight into their properties. For instance, microlensing can help to disentangle the temperature and spot area. Moreover, it provides an opportunity for probing starspots on the surfaces of slow rotating stars, which are unsuitable candidates for the Doppler imaging technique.

A microlensing event has been recently used for high-precision, multicolour measuring a limb darkening profile on the surface of a K3 giant and revealed a significant disagreement between the observed and predicted from atmospheric models disk brightness distribution (Fields et al., 2003 ). The Optical Gravitational Lensing Experiment (OGLE) survey of bulge giants provided photometric evidence for stellar spots via microlensing events (Udalski et al., 1995 ). Modelling of the spots on the stars selected from the OGLE database was undertaken by Guinan et al. (1997 ). Further theoretical investigation of possible detections of stellar spots by gravitational microlensing was carried out by Heyrovský and Sasselov (2000 ) and Hendry et al. (2002 ). They concluded that starspots generally produce a clear signature only for transit events. Moreover, this signature is strongly suppressed by limb darkening for spots close to the limb, although spots may still be clearly detected for favourable lens trajectories. It was found also that in order for microlensing events to be an effective starspot imaging tool, intensive temporal sampling throughout the transit is needed.