Spitzer Opens New Path to Break Classic Degeneracy for Jupiter-Mass Microlensing Planet OGLE-2017-BLG-1140Lb
We analyze the combined Spitzer and ground-based data for OGLE-2017-BLG-1140 and show that the event was generated by a Jupiter-class (m_p\simeq 1.6 M_jup) planet orbiting a mid-late M dwarf (M\simeq 0.2 M_\odot) that lies D_LS\simeq 1.0 kpc in the foreground of the microlensed, Galactic-bar, source star. The planet-host projected separation is a_\perp \simeq 1.0 AU, i.e., well-beyond the snow line. By measuring the source proper motion U_s from ongoing, long-term OGLE imaging, and combining this with the lens-source relative proper motion U_rel derived from the microlensing solution, we show that the lens proper motion U_l=U_rel + U_s is consistent with the lens lying in the Galactic disk. We show that while the Spitzer and ground-based data are comparably well fitted by planetary (i.e., binary-lens, 2L1S) models and by binary-source (1L2S) models, the combination of Spitzer and ground-based data decisively favor the planetary model. This is a new channel to resolve the 2L1S/1L2S degeneracy, which can be difficult to break in some cases.