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We report new Atacama Large Millimeter/submillimeter Array Band 3 (86-100GHz; $\sim$80 mas angular resolution) and Band 4 (146-160 GHz; $\sim$50 masangular resolution) observations of the dust continuum emission towards thearchetypal and ongoing accretion burst young stellar object FU Ori, whichsimultaneously covered its companion, FU Ori S. In addition, we presentnear-infrared (2-2.45 $\mu$m) observations of FU Ori taken with the GeneralRelativity Analysis via VLT InTerferometrY (GRAVITY; $\sim$1 mas angularresolution) instrument on the Very Large Telescope Interferometer (VLTI). Wefind that the emission in both FU Ori and FU Ori S at (sub)millimeter and nearinfrared bands is dominated by structures inward of $\sim$10 au radii. Wedetected closure phases close to zero from FU Ori with VLTI/GRAVITY, whichindicate the source is approximately centrally symmetric and therefore islikely viewed nearly face-on. Our simple model to fit the GRAVITY data showsthat the inner 0.4 au radii of the FU Ori disk has a triangular spectral shapeat 2-2.45 $\mu$m, which is consistent with the H$_{2}$O and CO absorptionfeatures in a $\dot{M}\sim$10$^{-4}$ $M_{\odot}\,yr^{-1}$, viscously heatedaccretion disk. At larger ($\sim$0.4-10 au) radii, our analysis shows thatviscous heating may also explain the observed (sub)millimeter and centimeterspectral energy distribution when we assume a constant, $\sim$10$^{-4}$$M_{\odot}\,yr^{-1}$ mass inflow rate in this region. This explains how theinner 0.4 au disk is replenished with mass at a modest rate, such that itneither depletes nor accumulates significant masses over its short dynamictimescale. Finally, we tentatively detect evidence of vertical dust settling inthe inner 10 au of the FU Ori disk, but confirmation requires more completespectral sampling in the centimeter bands.

Diagnosing 0.1-10 au scale morphology of the FU\,Ori disk using ALMA and VLTI/GRAVITY