A concordant scenario to explain FU Orionis from deep centimeter and millimeter interferometric observations

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
e-mail: baobabyoo@gmail.com
² Department of Astrophysics, University of Vienna, 1180 Vienna, Austria
³ Research Institute of Physics, Southern Federal University, 344090 Rostov-on-Don, Russia
⁴ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁵ Department of Physics, State University of New York at Fredonia, 280 Central Ave, Fredonia, NY 14063, USA
⁶ Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, 106 Taipei, Taiwan
⁷ Instituto de Radioastronomía y Astrofísica, UNAM, A.P. 3-72, Xangari, 58089 Morelia, Mexico
⁸ Astrobiology Center, NINS, 181-8588 Tokyo, Japan
⁹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, PO Box 67, 1525 Budapest, Hungary
¹⁰ Max-Planck-Institut för Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Department of Astronomy and RESCEU, University of Tokyo, 113-8654 Tokyo, Japan
¹² National Astronomical Observatory of Japan, NINS, 181-8588 Tokyo, Japan
¹³ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹⁴ Division of Particle and Astrophysical Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602 Aichi, Japan
¹⁵ Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK

Received: 15 December 2016
Accepted: 24 January 2017

Abstract

Aims. The aim of this work is to constrain properties of the disk around the archetype FU Orionis object, FU Ori, with as good as ~25 au resolution.

Methods. We resolved FU Ori at 29–37 GHz using the Karl G. Jansky Very Large Array (JVLA) in the A-array configuration, which provided the highest possible angular resolution to date at this frequency band (~007). We also performed complementary JVLA 8–10 GHz observations, Submillimeter Array (SMA) 224 GHz and 272 GHz observations, and compared these with archival Atacama Large Millimeter Array (ALMA) 346 GHz observations to obtain the spectral energy distributions (SEDs).

Results. 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 frequencies higher than 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, which is two orders of magnitude smaller in linear scale than the gaseous disk revealed by the previous Subaru-HiCIAO 1.6 μm coronagraphic polarization imaging observations. We are struck by the fact that these two spatially compact sources contribute to over 50% of the observed fluxes at 224 GHz, 272 GHz, and 346 GHz. The 8–346 GHz SEDs of FU Ori and FU Ori S cannot be fit by constant spectral indices (over frequency), although we cannot rule out that it is due to the time variability of their (sub)millimeter fluxes.

Conclusions. The more sophisticated models for SEDs considering the details of the observed spectral indices in the millimeter bands 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 longer 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 instability and the magnetorotational instability when the ionization fraction in the inner sub-au scale region exceeded a threshold value, leading to the high protostellar accretion rate.