Two applications of Galactic gravitational microlensing : probing the atmosphere of bulge stars and searching for extrasolar planets

A Galactic microlensing event occurs when a foreground object (called the lens) crosses the line-of-sight of a background star (the source). Depending on the microlensing event properties, the method has proven to be a valuable tool (1) to probe the stellar atmosphere of source stars and (2) to search for extrasolar planets around microlenses. In the first application, the source-star targets are usually red giants in the Galactic bulge (6-8 kpc), for which we could derive precisely their limb-darkening coefficients, to be compared to model predictions. When combined to spectroscopic observations, one can even measure the variation of the equivalent-width of individual spectral lines across the star disk, hence probing different optical depths, as we achieved for the event OGLE-2002-BLG-069. The comparison of our measurements to stellar atmosphere models has triggered new ideas on how to compute limb-darkening coefficients, or alternative laws based on a principal component analysis of stellar atmosphere models. A second important application is the search for extrasolar planets around the lens. Galactic gravitational microlensing has proven to be a valuable tool to detect Jovian- to super-Earth-mass planets with orbital radii of a few AUs. It is, for now, the only method that opens a window onto this mass/orbit regime. Microlensing has already led to four published detections of extrasolar planets, one of them being OGLE-2005-BLG-390Lb, a planet of only 5.5 M_earth orbiting its M-dwarf host star at 2.6 AU. Very recent observations (March-October 2007) provided more planetary candidates, still under study, that will double the number of detections. For non-planetary microlensing events observed from 1995 to 2006 we compute detection efficiency diagrams, which can then be used to derive an estimate of the Galactic abundance of cool planets in the mass regime from Jupiters to sub-Neptunes. In both applications, since the signals involved are of very short duration (few hours), an intense and continuous monitoring is required. This is achieved by ground-based networks of telescopes (such as PLANET/RoboNET) following up targets, which are identified as microlensing events by single dedicated telescopes (OGLE and MOA).

I will start with a brief overview of the method, the teams involved and the current observational setup. Then I will present the main results we have obtained when it was possible to study the source star’s amosphere, and their implications.

Secondly, I will review what could be achieve with microlensing in the search for extrasolar planets, present our recent discoveries and discuss the wider implications for extrasolar planetary research.