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. <BR /> 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. <BR /> 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. <BR /> Results: We find that the source star is a giant G star with radius 9 R<SUB>☉</SUB>. The grid search gives two local minima, which correspond to the theoretical degeneracy s ≡ s<SUP>-1</SUP>. We find that the lens is composed of a brown dwarf secondary of mass M<SUB>S</SUB> = 0.05 M<SUB>☉</SUB> orbiting a primary M-star of mass M<SUB>P</SUB> = 0.18 M<SUB>☉</SUB>. We also reveal a new mass-ratio degeneracy for the central caustics of close binaries. <BR /> 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. Appendix is available in electronic form at <A href="http://www.aanda.org">http://www.aanda.org</A>
Bachelet, E., Fouque, P., Han, C., Gould, A., Albrow, M.d., Beaulieu, J.-., et al. (2012). A brown dwarf orbiting an M-dwarf: MOA 2009-BLG-411L. ASTRONOMY & ASTROPHYSICS, 547, A55 [10.1051/0004-6361/201219765].
A brown dwarf orbiting an M-dwarf: MOA 2009-BLG-411L
Mancini L.;Ricci D.;
2012-01-01
Abstract
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. Appendix is available in electronic form at http://www.aanda.org
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