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 (A~ 432) 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 $\theta_{\rm E}=0.48\pm 0.01$ mas and the parallax $\pi_{\rm E}=0.12\pm 0.03$, leading to an estimate of the lens mass $M=0.50\pm0.14\,M_{\odot}$ and its distance to the observer $D_L=5.5\pm0.4$ 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 $(r_\perp,m_{\rm p})$, where $r_\perp$ is the projected planet-star separation and m_ p 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 $\Delta\chi^2$ 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.