Galactic Aspects of Gravitational Microlensing: High Magnification Events and Stellar Limb-darkening of a Source Star in the Galactic bulge

Abstract

Gravitational microlensing provides a powerful tool to search for extrasolar planets of stars at distances of order of several kpc. The suspicion of a planetary signal in the two high magnification events OGLE 2006-BLG-245 and MOA 2006-BLG-099 led us to perform a detailed modelling and analysis of those two events. Based on the comparison of single-lens and binary-lens models, we demonstrate that the observed light curve deviations are not caused by a planetary companion. Our modelling and analysis of four other high magnification events illustrate the possibility to study detection efficiencies of microlensing data sets to planetary companions. We also present a detailed study of the single-lens OGLE 2004-BLG-482 microlensing event, used to measure the brightness profile of the background lensed star located in the Galactic bulge. We performed data reduction and analysis of well sampled observations of this event obtained by the PLANET, OGLE and MicroFUN collaborations in the I, R and clear filters. We also used a high resolution spectrum obtained with VLT/UVES close to the peak of the light curve to determine the fundamental parameters of the source star, that we find to be a cool red M3 giant with Teff = 3667±150K, logg = 2.1±1.0. We then performed a detailed microlensing modelling of the light curve to measure linear limb-darkening coefficients and to provide new diagnostics of such measurements through microlensing. We compare our results to model-atmosphere predictions based on limb-darkening coefficients for the corresponding stellar parameters. Our limb-darkening measurements agree very well with predictions of the model atmosphere, for both linear limb-darkening laws and alternative limb-darkening profiles based on a principal component analysis of ATLAS stellar atmosphere models.