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Previous observations have shown that the $\lesssim$10 au, $\gtrsim$400 K hotinner disk of the archetypal accretion outburst young stellar object, FU Ori,is dominated by viscous heating. To constrain dust properties in this region,we have performed radio observations toward this disk using the Karl G. JanskyVery Large Array (JVLA) in 2020 June-July, September, and November. We alsoperformed complementary optical photometric monitoring observations. We foundthat the dust thermal emission from the hot inner disk mid-plane of FU Ori hasbeen approximately stationary and the maximum dust grain size is $\gtrsim$1.6mm in this region. If the hot inner disk of FU Ori which is inward of the150-170 K water snowline is turbulent (e.g., corresponding to a Sunyaev &Shakura viscous $\alpha_{t}\gtrsim$0.1), or if the actual maximum grain size isstill larger than the lower limit we presently constrain, then as suggested bythe recent analytical calculations and the laboratory measurements, water-icefree dust grains may be stickier than water-ice coated dust grains inprotoplanetary disks. Additionally, we find that the free-free emission and theJohnson B and V bands magnitudes of these binary stars are brightening in2016-2020. The optical and radio variability might be related to thedynamically evolving protostellar or disk accretion activities. Our resultshighlight that hot inner disks of outbursting objects are importantlaboratories for testing models of dust grain growth. Given the active natureof such systems, to robustly diagnose the maximum dust grain sizes, it isimportant to carry out coordinated multi-wavelength radio observations.

Millimeter-sized Dust Grains Appear Surviving the Water-sublimating Temperature in the Inner 10 au of the FU Ori Disk