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 $(m_p\simeq 1.6,M_{\rm jup})$planetorbitingamid-lateMdwarf$(M\simeq 0.2,M_\odot)$thatlies$D_{LS}\simeq 1.0,$\mathrm{k}\mathrm{p}\mathrm{c}$$ in the foreground of the microlensed, Galactic-bar, source star. The planet-host projected separation is $a_{\perp }\simeq 1.0\mathrm{a}\mathrm{u}$, i.e., well-beyond the snow line. By measuring the source proper motion ${\mathbf{\mu }}_s$ from ongoing, long-term OGLE imaging, and combining this with the lens-source relative proper motion ${\mathbf{\mu }}_{\mathrm{r}\mathrm{e}\mathrm{l}}$ derived from the microlensing solution, we show that the lens proper motion ${$\mathbf{\mu }$}l={$\mathbf{\mu }$}$\mathrm{r}\mathrm{e}\mathrm{l}$ + {$\mathbf{\mu }$}_s$ is consistent with the lens lying in the Galactic disk, although a bulge lens is not ruled out. 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.