Microlensing photométrique et détermination des masses stellaires

Abstract

Microlensing was suggested for stellar mass determination of nearby stars by several authors (e.g. Paczyński). There are two aspects in gravitational microlensing: photometry and astrometry. Here only the photometric aspect, i.e. magnification of a background source by a stellar lens, will be considered. The first study in this domain was done by Paczyński. An attempt to investigate some observational constraints (Alard et al.) takes its origin in this study. It will be shown here that blending of the source by the lens not only induces a degeneracy which has been pointed out by Woźniak and Paczyński and can be removed by measuring the flux of the source outside the microlensing event, but also strongly reduces the photometric cross section as well as the duration of microlensing events. The expected number of events decreases strongly with blending. Blending effects can be reduced by selecting faint lens candidates. Unfortunately however, it is difficult to monitor a sufficient number of lens candidates per night, and restricting oneself to nearby and high proper motion objects would lead to a very small number of lens candidates. In the case of short duration events, the light curve must be sampled at short time intervals. When sampling daily, the condition is very strong and the expected rate of events very small. Observing from space would increase significantly the rate, but this gain is still insufficient. We conclude that the event rate will be very small and that the project does not seem to be feasible, at least under current observing conditions and even in the near future.