Distinguishing between different mechanisms of FU-Orionis-type luminosity outbursts

Abstract

Aims. Accretion and luminosity bursts can be triggered by three distinct mechanisms: the magnetorotational instability (MRI) in the inner disk regions, clump infall in gravitationally fragmented disks, and close encounters with an intruder star. We study all three of these burst mechanisms to determine the disk kinematic characteristics that can help to distinguish between them. Methods. Numerical hydrodynamics simulations in the thin-disk limit were employed to model the bursts in disk environments that are expected for each burst mechanism. Results. We found that the circumstellar disks featuring accretion bursts can bear kinematic features that are distinct for different burst mechanisms, which can be useful when identifying the origin of a particular burst. The disks in the stellar encounter and clump-infall models are characterized by deviations from the Keplerian rotation of tens of per cent, while the disks in the MRI models are characterized by deviations of only a few per cent, which is mostly caused by the gravitational instability that fuels the MRI bursts. Velocity channel maps also show distinct kinks and wiggles, which are caused by gas disk flows that are particular to each considered burst mechanism. The deviations of velocity channels in the burst-hosting disks from a symmetric pattern typical of Keplerian disks are strongest for the clump-infall and collision models, and carry individual features that may be useful for the identification of the corresponding burst mechanism. The considered burst mechanisms produce a variety of light curves with the burst amplitudes varying in the Δm = 2.5−3.7 limits, except for the clump-infall model where Δm can reach 5.4, although the derived numbers may be affected by a small sample and boundary conditions. Conclusions. Burst-triggering mechanisms are associated with distinct kinematic features in the burst-hosting disks that may be used for their identification. Further studies including a wider model parameter space and the construction of synthetic disk images in thermal dust and molecular line emission are needed to constrain the mechanisms that lead to FU Orionis bursts.