Mass measurement of a single unseen star and planetary detection efficiency for OGLE 2007-BLG-050

Aims. We analyze OGLE-2007-BLG-050, a high magnification microlensing event (Ã432) whose peak occurred on 2 May, 2007, with pronounced finite-source and parallax effects. We compute planet detection efficiencies for this event in order to determine its sensitivity to the presence of planets around the lens star.Methods. Both finite-source and parallax effects permit a measurement of the angular Einstein radius E=0.48±0.01 mas and the parallax πE=0.12±0.03, leading to an estimate of the lens mass M=0.50±0.14 M and its distance to the observer DL=5.5±0.4 kpc. This is only the second determination of a reasonably precise (<30%) mass estimate for an isolated unseen object, using any method. This allows us to calculate the planetary detection efficiency in physical units (r,m), where r is the projected planet-star separation and mp is the planet mass.Results. When computing planet detection efficiency, we did not find any planetary signature, i.e. none of the planetary configurations provides a δX 2 improvement higher than 60, and our detection efficiency results reveal significant sensitivity to Neptune-mass planets, and to a lesser extent Earth-mass planets in some configurations. Indeed, Jupiter and Neptune-mass planets are excluded with a high confidence for a large projected separation range between the planet and the lens star, respectively [0.6-10] and [1.4-4] AU, and Earth-mass planets are excluded with a 10% confidence in the lensing zone, i.e. [1.8-3.1] AU. © 2009 ESO.