We resolved FU Ori at 29-37 GHz using the JVLA with ~0.07 arcsec resolution, and performed the complementary JVLA 8-10
GHz observations, the SMA 224 GHz and 272 GHz observations, and compared with archival ALMA 346 GHz observations to
obtain the SEDs. Our 8-10 GHz observations do not find evidence for the presence of thermal radio jets, and constrain the
radio jet/wind flux to at least 90 times lower than the expected value from the previously reported bolometric
luminosity-radio luminosity correlation. The emission at >29 GHz may be dominated by the two spatially unresolved
sources, which are located immediately around FU Ori and its companion FU Ori S, respectively. Their deconvolved radii at
33 GHz are only a few au. The 8-346 GHz SEDs of FU Ori and FU Ori S cannot be fit by constant spectral indices (over
frequency). The more sophisticated models for SEDs suggest that the >29 GHz emission is contributed by a combination of
free-free emission from ionized gas, and thermal emission from optically thick and optically thin dust components. We
hypothesize that dust in the innermost parts of the disks (<~0.1 au) has been sublimated, and thus the disks are no more
well shielded against the ionizing photons. The estimated overall gas and dust mass based on SED modeling, can be as high
as a fraction of a solar mass, which is adequate for developing disk gravitational instability. Our present explanation
for the observational data is that the massive inflow of gas and dust due to disk gravitational instability or
interaction with a companion/intruder, was piled up at the few au scale due to the development of a deadzone with
negligible ionization. The piled up material subsequently triggered the thermal and the MRI instabilities when the
ionization fraction in the inner sub-au scale region exceeded a threshold value, leading to the high protostellar
accretion rate. We note that this mechanism may resolve the missing-disk-mass problem when comparing with the exoplanets
identified by Kepler satellite.
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