Viscous Heating In The Disk Of The Outbursting Star Fu Orionis

Context. FU Orionis is the archetypal FUor star, a subclass of young stellar objects (YSOs) that undergo rapid brightening events, often gaining between four and six magnitudes on timescales of days. This brightening is often associated with a massive increase in accretion, which is one of the most ubiquitous processes in astrophysics for bodies ranging 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 YSO.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 have allowed for 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 allowed us to apply temperature gradient models to the innermost regions of FU Ori.Results. We spatially resolved the innermost astronomical unit of the disk and determine the exponent of the temperature gradient of the inner disk to T proportional to r(-0.74 +/- 0.02). This agrees with theoretical works that predict T proportional to r(-0.75) for actively accreting, steady-state disks, which is a value only obtainable through viscous heating within the disk. We found a disk that extends down to the stellar surface at 0.015 +/- 0.007 au, where the temperature is found to be 5800 +/- 700 K. We found a disk inclined at 32 +/- 4 degrees with a minor-axis position angle of 34 +/- 11 degrees.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.