MOA-2010-BLG-073L: An M-Dwarf with a Substellar Companion at the Planet/Brown Dwarf Boundary

Abstract

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. Analyzing the pre-event source lightcurve, we demonstrate that it is an irregular variable over time scales >200d. Its dereddened color, $(V-I{)}_{S,0}$, is 1.221$\pm$0.051mag and from our lens model we derive a source radius of 14.7$\pm$1.3 $R_{\odot }$, 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$\pm$0.0006. The Einstein crossing time of the event, $T_{\mathrm{E}}=44.3$\pm$0.1d,wassufficientlylongthatthelightcurveexhibitedparallaxeffects.Inaddition,thesourcetrajectoryrelativetothelargecausticstructureallowedtheorbitalmotionofthelenssystemtobedetected.CombiningtheparallaxwiththeEinsteinradius,wewereabletoderivethedistancetothelens,$D_L$=2.8$\pm$0.4kpc,andthemassesofthelensingobjects.TheprimaryofthelensisanM-dwarfwith$M_{L,p}$=0.16$\pm0.03M_{\odot}$whilethecompanionhas$M_{L,s}$=11.0$\pm2.0M_{\rm{J}}$ putting it in the boundary zone between planets and brown dwarfs.

24 pages, 8 figures, best viewed in colour, accepted by ApJ