* Astronomy

Title: MOA-2010-BLG-073L: An M-Dwarf with a Substellar Companion at the Planet/Brown Dwarf Boundary
Authors: R. A. Street, J.-Y. Choi, Y. Tsapras, C. Han, K. Furusawa, M. Hundertmark, A. Gould, T. Sumi, I. A. Bond, D. Wouters, R. Zellem, A. Udalski, C. Snodgrass, K. Horne, M. Dominik, P. Browne, N. Kains, D. M. Bramich, D. Bajek, I. A. Steele, F. Abe, D. P. Bennett, C. S. Botzler, P. Chote, M. Freeman, A. Fukui, P. Harris, Y. Itow, C. H. Ling, K. Masuda, Y. Matsubara, N. Miyake, Y. Muraki, T. Nagayama, S. Nishimaya, K. Ohnishi, N. Rattenbury, To. Saito, D. J. Sullivan, D. Suzuki, W. L. Sweatman, P. J. Tristram, K. Wada, P. C. M. Yock, M. K. Szymanski, M. Kubiak, G. Pietrzynski, I. Soszynski, R. Poleski, K. Ulaczyk, L. Wyrzykowski, J. Yee, I.-G. Shin, C.-U. Lee, J. Skowron, L. Andrade De Almeida, D. L. DePoy, B. S. Gaudi, L.-W. Hung, F. Jablonski, S. Kaspi, N. Klein, K.-H. Hwang, J.-R. Koo, D. Maoz, et al. (64 additional authors not shown)

We present an analysis of the anomalous microlensing event, MOA-2010-BLG-073, announced by the Microlensing Observations in Astrophysics survey on 2010-03-18. 
This event was remarkable because the source was previously known to be photometrically variable. Analysing the pre-event source lightcurve, we demonstrate that it is an irregular variable over time scales >200d. Its dereddened colour, (V-I)_{S,0}, is 1.221±0.051mag and from our lens model we derive a source radius of 14.7±1.3 solar radii, suggesting that it is a red giant star. 
We initially explored a number of purely microlensing models for the event but found a residual gradient in the data taken prior to and after the event. This is likely to be due to the variability of the source rather than part of the lensing event, so we incorporated a slope parameter in our model in order to derive the true parameters of the lensing system. 
We find that the lensing system has a mass ratio of q=0.0654±0.0006. The Einstein crossing time of the event, T_{E}=44.3±0.1d, was sufficiently long that the lightcurve exhibited parallax effects. In addition, the source trajectory relative to the large caustic structure allowed the orbital motion of the lens system to be detected. Combining the parallax with the Einstein radius, we were able to derive the distance to the lens, D_L=2.8±0.4kpc, and the masses of the lensing objects. The primary of the lens is an M-dwarf with M_{L,p}=0.16±0.03 solar masses while the companion has M_{L,s}=11.0±2.0 Jupiter masses putting it in the boundary zone between planets and brown dwarfs.

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