1 Astrophysics and Planetary Science, The Niels Bohr Institute, Faculty of Science, Københavns Universitet2 MiNDSTEp Consortium3 Univ Toulouse, IRAP, CNRS, F-31400 Toulouse, France4 Chonnam Natl Univ, Inst Basic Sci Res, Dept Phys, Chonju 361763, South Korea5 Ohio State Univ, Dept Astron, Columbus, OH 43210 USA6 unknown7 Astrophysics and Planetary Science, The Niels Bohr Institute, Faculty of Science, Københavns Universitet
Context. Caustic crossing is the clearest signature of binary lenses in microlensing. In the present context, this signature is diluted by the large source star but a detailed analysis has allowed the companion signal to be extracted. Aims. MOA 2009-BLG-411 was detected on August 5, 2009 by the MOA-Collaboration. Alerted as a high-magnification event, it was sensitive to planets. Suspected anomalies in the light curve were not confirmed by a real-time model, but further analysis revealed small deviations from a single lens extended source fit. Methods. Thanks to observations by all the collaborations, this event was well monitored. We first decided to characterize the source star properties by using a more refined method than the classical one: we measure the interstellar absorption along the line of sight in five different passbands (VIJHK). Secondly, we model the lightcurve by using the standard technique: make (s,q,α) grids to look for local minima and refine the results by using a downhill method (Markov chain Monte Carlo). Finally, we use a Galactic model to estimate the physical properties of the lens components. Results. We find that the source star is a giant G star with radius 9 R⊙. The grid search gives two local minima, which correspond to the theoretical degeneracy s ≡ s-1. We find that the lens is composed of a brown dwarf secondary of mass MS = 0.05 M⊙ orbiting a primary M-star of mass MP = 0.18 M⊙. We also reveal a new mass-ratio degeneracy for the central caustics of close binaries. Conclusions. As far as we are aware, this is the first detection using the microlensing technique of a binary system in our Galaxy composed of an M-star and a brown dwarf.
Astronomy and Astrophysics, 2012, Vol 547, Issue November