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

Previous observations have shown that the less than or similar to 10 au, greater than or similar to 400 K hot inner 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. Jansky Very Large Array in 2020 June-July, September, and November. We also performed complementary optical photometric monitoring observations. We found that the dust thermal emission from the hot inner disk mid-plane of FU Ori has been approximately stationary and the maximum dust grain size is greater than or similar to 1.6 mm in this region. If the hot inner disk of FU Ori, which is inward of the 150-170 K water snowline, is turbulent (e.g., corresponding to a Sunyaev & Shakura viscous alpha ( t ) greater than or similar to 0.1), or if the actual maximum grain size is still larger than the lower limit we presently constrain, then as suggested by the recent analytical calculations and the laboratory measurements, water-ice-free dust grains may be stickier than water-ice-coated dust grains in protoplanetary disks. Additionally, we find that the free-free emission and the Johnson B- and V-band magnitudes of these binary stars were brightening in 2016-2020. The optical and radio variability might be related to the dynamically evolving protostellar- or disk-accretion activities. Our results highlight that the hot inner disks of outbursting objects are important laboratories for testing models of dust grain growth. Given the active nature of such systems, to robustly diagnose the maximum dust grain sizes, it is important to carry out coordinated multiwavelength radio observations.

Automated summary

Through radio observations and optical monitoring, it was found that the dust thermal emission from the hot inner disk of FU Ori is stable, with maximum dust grain size exceeding 1.6 mm. The hot inner disk may be turbulent, and water-ice-free dust grains could be stickier than water-ice-coated ones. Additionally, the free-free emission and optical brightness of the binary stars increased from 2016 to 2020, possibly linked to evolving protostellar or disk accretion activities.