Microlensing of Blended Stellar Images

The current modelling of single microlensing light curves neglects the possibility that only a fraction of the light is due to the lensed star, the remaining being due to a close, unresolved blend, which may be related or unrelated to the lens. Unfortunately, the effects of blending are significant as all microlensing experiments choose very crowded fields as their targets. In this paper we point out a strong degeneracy of the fitting procedure which makes it practically impossible to detect the presence of a blend by purely photometric means, except in a small part of the parameter space. Some blends may be detected by astrometric means, but the majority have to be corrected for statistically. The luminosity function reaching well below the ground based detection limit (with the HST) would be very helpful. The statistics of binary stars in the target population is also important and this could be determined with the repeating microlensing events. If no correction is made then the event time scales, the lens masses, and the optical depth are all systematically underestimated. Subject headings: galaxy: structure – ISM: extinction – photometry