OGLE-2017-BLG-0406: ${\it Spitzer}$ Microlens Parallax Reveals
  Saturn-mass Planet orbiting M-dwarf Host in the Inner Galactic Disk
  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 ${\it
Spitzer}$ 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 ${\it Spitzer}$-"only" data each provide very strong one-dimensional (1-D)
constraints on the 2-D microlens parallax vector $\bf{\pi_{\rm E}}$. When
combined, these yield a precise measurement of $\bf{\pi_{\rm E}}$, and so of
the masses of the host $M_{\rm host}=0.56\pm0.07\,M_\odot$ and planet $M_{\rm
planet} = 0.41 \pm 0.05\,M_{\rm Jup}$. The system lies at a distance $D_{\rm
L}=5.2 \pm 0.5 \ {\rm kpc}$ 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 $a_{\perp} =
3.5 \pm 0.3 \ {\rm au}$, 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 ${\it Gaia}$
proper-motion measurement of the source suffers from a catastrophic
$10\,\sigma$ error.
