Viscous heating in the disk of the outbursting star FU Orionis

Abstract

Context. FU Orionis is the archetypal FUor star, a subclass of young stellar object (YSO) that undergo rapid brightening events, often gaining 4-6 magnitudes on timescales of days. This brightening is often associated with a massive increase in accretion; one of the most ubiquitous processes in astrophysics from planets and stars to super-massive black holes. We present multi-band interferometric observations of the FU Ori circumstellar environment, including the first J-band interferometric observations of a Y SO. Aims. We investigate the morphology and temperature gradient of the innermost regions of the accretion disk around FU Orionis. We aim to characterise the heating mechanisms of the disk and comment on potential outburst triggering processes. Methods. Recent upgrades to the MIRC-X instrument at the CHARA array allowed the first dual-band J and H observations of YSOs. Using baselines up to 331 m, we present high angular resolution data of a YSO covering the near-infrared bands J, H, and K. The unprecedented spectral range of the data allows us to apply temperature gradient models to the innermost regions of FU Ori. Results. We spatially resolve the innermost astronomical unit of the disk and determine the exponent of the temperature gradient of the inner disk to T ∝ r −0.74±0.02. This agrees with theoretical work that predicts T ∝ r −0.75 for actively accreting, steady state disks, a value only obtainable through viscous heating within the disk. We find a disk which extends down to the stellar surface at 0.015 ± 0.007 au where the temperature is found to be 5800 ± 700 K indicating boundary layer accretion. We find a disk inclined at 32 ± 4 • with a minor-axis position angle of 34 ± 11 •. Conclusions. We demonstrate that J-band interferometric observations of YSOs are feasible with the MIRC-X instrument at CHARA. The temperature gradient power-law derived for the inner disk is consistent with theoretical predictions for steady-state, optically thick, viciously heated accretion disks.

Inferring possible magnetic field strength of accreting inflows in EXor-type objects from scaled laboratory experiments

“…The similarity detailed above was made between the experiments and the EXor object detailed in Giannini et al (2017). We find that a similarly good scalability can be found between our experimental measurements and the FU Orionis star, whose characterization was recently improved by Labdon et al (2020). In that case, taking the temperature value to be around 2 kK at the FU Orionis truncation radius, and assuming an averaged density of ∼ 10 13 cm −3 and a velocity of ∼ 120 km/s for the accretion stream, we find that this stream scales that produced by the high-power laser produced plasma (detailed in Table 1) for a ∼ 200 G magnetic field strength value in the astrophysical case.…”

Inferring possible magnetic field strength of accreting inflows in EXor-type objects from scaled laboratory experiments

“…The similarity detailed above was made between the experiments and the EXor object detailed in Giannini et al (2017). We find that a similarly good scalability can be found between our experimental measurements and the FU Orionis star, whose characterization was recently improved by Labdon et al (2021). In that case, taking the temperature value to be around 2 kK at the FU Orionis truncation radius, and assuming an averaged density of ∼10 13 cm −3 and a velocity of ∼120 km s −1 for the accretion stream, we find that this stream scales that produced by the high-power laser produced plasma (detailed in Table 1) for a ∼200 G magnetic field strength value in the astrophysical case.…”

The young protostellar disk in IRAS16293-2422 B is hot and shows signatures of gravitational instability

“…The presence of a positive temperature gradient at small radii seems to be also the case in the evolved and gravitationally unstable (Dong et al 2016) FU Ori disk (Lizano et al 2016;Labdon et al 2021). The high-resolution (0.1-10 au) observations reported by show the inner parts of the disk in FU Ori with temperatures above 700 K at radii 3 au.…”