OGLE-2017-BLG-0406: $Spitzer$ Microlens Parallax Reveals Saturn-mass Planet orbiting M-dwarf Host in the Inner Galactic Disk

Abstract

We report the discovery and analysis of the planetary microlensing event OGLE-2017-BLG-0406, which was observed both from the ground and by the $Spitzer$ satellite in a solar orbit. At high magnification, the anomaly in the light curve was densely observed by ground-based-survey and follow-up groups, and it was found to be explained by a planetary lens with a planet/host mass ratio of $q=7.0\times 10^{-4}$ from the light-curve modeling. The ground-only and $Spitzer$-"only" data each provide very strong one-dimensional (1-D) constraints on the 2-D microlens parallax vector ${\pi }_{\mathrm{E}}$. When combined, these yield a precise measurement of ${\pi }_{\mathrm{E}}$, and so of the masses of the host $M_{\mathrm{host}}=0.56\pm 0.07M_{\odot }$ and planet $M_{\mathrm{planet}}=0.41\pm 0.05M_{\mathrm{Jup}}$. The system lies at a distance $D_{\mathrm{L}}=5.2\pm 0.5\mathrm{kpc}$ from the Sun toward the Galactic bulge, and the host is more likely to be a disk population star according to the kinematics of the lens. The projected separation of the planet from the host is $a_{\perp }=3.5\pm 0.3\mathrm{au}$, i.e., just over twice the snow line. The Galactic-disk kinematics are established in part from a precise measurement of the source proper motion based on OGLE-IV data. By contrast, the $Gaia$ proper-motion measurement of the source suffers from a catastrophic $10\sigma$ error.

40 pages, 12 figures, 10 tables, accepted for publication in The Astronomical Journal

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