Diagnosing 0.1-10 au scale morphology of the FU\,Ori disk using ALMA and
  VLTI/GRAVITY
  We report new Atacama Large Millimeter/submillimeter Array Band 3 (86-100
GHz; $\sim$80 mas angular resolution) and Band 4 (146-160 GHz; $\sim$50 mas
angular resolution) observations of the dust continuum emission towards the
archetypal and ongoing accretion burst young stellar object FU Ori, which
simultaneously covered its companion, FU Ori S. In addition, we present
near-infrared (2-2.45 $\mu$m) observations of FU Ori taken with the General
Relativity Analysis via VLT InTerferometrY (GRAVITY; $\sim$1 mas angular
resolution) instrument on the Very Large Telescope Interferometer (VLTI). We
find that the emission in both FU Ori and FU Ori S at (sub)millimeter and near
infrared bands is dominated by structures inward of $\sim$10 au radii. We
detected closure phases close to zero from FU Ori with VLTI/GRAVITY, which
indicate the source is approximately centrally symmetric and therefore is
likely viewed nearly face-on. Our simple model to fit the GRAVITY data shows
that the inner 0.4 au radii of the FU Ori disk has a triangular spectral shape
at 2-2.45 $\mu$m, which is consistent with the H$_{2}$O and CO absorption
features in a $\dot{M}\sim$10$^{-4}$ $M_{\odot}\,yr^{-1}$, viscously heated
accretion disk. At larger ($\sim$0.4-10 au) radii, our analysis shows that
viscous heating may also explain the observed (sub)millimeter and centimeter
spectral energy distribution when we assume a constant, $\sim$10$^{-4}$
$M_{\odot}\,yr^{-1}$ mass inflow rate in this region. This explains how the
inner 0.4 au disk is replenished with mass at a modest rate, such that it
neither depletes nor accumulates significant masses over its short dynamic
timescale. Finally, we tentatively detect evidence of vertical dust settling in
the inner 10 au of the FU Ori disk, but confirmation requires more complete
spectral sampling in the centimeter bands.
