A brown dwarf orbiting an M-dwarf: MOA 2009-BLG-411L

Bachelet, E.; Fouqué, P.; Han, C.; Gould, A.; Albrow, M. D.; Beaulieu, J. P.; Bertin, E.; Bond, I. A.; Christie, G. W.; Heyrovský, D.; Horne, K.; Jørgensen, U. G.; Maoz, D.; Mathiasen, M.; Matsunaga, N.; Mccormick, J.; Menzies, J.; Nataf, D.; Natusch, T.; Oi, N.; Renon, N.; Tsapras, Y.; Udalski, A.; Yee, J. C.; Batista, V.; Bennett, D. P.; Brillant, S.; Caldwell, J. A. R.; Cassan, A.; Cole, A.; Cook, K. H.; Coutures, C.; Dieters, S.; Dominik, M.; Dominis Prester, D.; Donatowicz, J.; Greenhill, J.; Kains, N.; Kane, S. R.; Marquette, J. B.; Martin, R.; Pollard, K. R.; Sahu, K. C.; Street, R. A.; Wambsganss, J.; Williams, A.; Zub, M.; Bos, M.; Subo, Dong; Drummond, J.; Gaudi, B. S.; Graff, D.; Janczak, J.; Kaspi, S.; Kozłowski, S.; Lee, C. U.; Monard, L. A. G.; Muñoz, J. A.; Park, B. G.; Pogge, R. W.; Polishook, D.; Shporer, A.; Abe, F.; Botzler, C. S.; Fukui, A.; Furusawa, K.; Hearnshaw, J. B.; Itow, Y.; Korpela, A. V.; Ling, C. H.; Masuda, K.; Matsubara, Y.; Miyake, N.; Muraki, Y.; Ohnishi, K.; Rattenbury, N. J.; Saito, T. o.; Sullivan, D.; Sumi, T.; Suzuki, D.; Sweatman, W. L.; Tristram, P. J.; Wada, K.; Allan, A.; Bode, M. F.; Bramich, D. M.; Clay, N.; Fraser, S. N.; Hawkins, E.; Kerins, E.; Lister, T. A.; Mottram, C. J.; Saunders, E. S.; Snodgrass, C.; Steele, I. A.; Wheatley, P. J.; Bozza, Valerio; Browne, P.; Burgdorf, M. J.; Calchi Novati, S.; Dreizler, S.; Finet, F.; Glitrup, M.; Grundahl, F.; Harpsøe, K.; Hessman, F. V.; Hinse, T. C.; Hundertmark, M.; Liebig, C.; Maier, G.; Mancini, L.; Rahvar, S.; Ricci, D.; Scarpetta, Gaetano; Skottfelt, J.; Southworth, J.; Surdej, J.; Zimmer, F.

doi:10.1051/0004-6361/201219765

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.