Abstract
Abstract FU Orionis objects are low-mass pre-main sequence stars characterized by dramatic outbursts several magnitudes in brightness. These outbursts are linked to episodic accretion events in which stars gain a significant portion of their mass. The physical processes behind these accretion events are not yet well understood. The archetypal FU Ori system, FU Orionis, is composed of two young stars with detected gas and dust emission. The continuum emitting regions have not been resolved until now. Here, we present 1.3 mm observations of the FU Ori binary system using the Atacama Large Millimeter/submillimeter Array. The disks are resolved at 40 mas resolution. Radiative transfer modeling shows that the emission from FU Ori north (primary) is consistent with a dust disk with a characteristic radius of ∼11 au. The ratio between the major and minor axes shows that the inclination of the disk is ∼37°. FU Ori south is consistent with a dust disk of similar inclination and size. Assuming the binary orbit shares the same inclination angle as the disks, the deprojected distance between the north and south components is 0.″6, i.e., ∼250 au. Maps of 12CO emission show a complex kinematic environment with signature disk rotation at the location of the northern component, and also (to a lesser extent) for FU Ori south. The revised disk geometry allows us to update FU Ori accretion models, yielding a stellar mass and mass accretion rate of FU Ori north of 0.6 M ⊙ and 3.8 × 10−5 M ⊙ yr−1, respectively.
Highlights
Protostars build a significant fraction of their final mass through an accretion disk fed by a surrounding envelope
Radiative transfer modeling shows that the emission from FU Ori north is consistent with a dust disk with a characteristic radius of ∼11 au
While most young stellar objects have luminosities that are significantly lower than expected from steady protostellar disk accretion, some exhibit episodes of high activity, increasing their optical brightness by several orders of magnitude on timescales as short as one year
Summary
Protostars build a significant fraction of their final mass through an accretion disk fed by a surrounding envelope. While most young stellar objects have luminosities that are significantly lower than expected from steady protostellar disk accretion (the so-called luminosity problem; Kenyon et al 1990; Evans et al 2009), some exhibit episodes of high activity, increasing their optical brightness by several orders of magnitude on timescales as short as one year (see Audard et al 2014 for a review). Accretion at this stage can reach up to 10−4 Me yr−1. This would imply that a significant fraction of the mass of a young protostar is accreted in an episodic way, with prolonged periods of low accretion (Zhu et al 2009; Vorobyov & Basu 2010; Bae et al 2014)
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