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

Abstract

Aims. Accretion and luminosity bursts triggered by three distinct mechanisms: the magnetorotational instability in the inner disk regions, clump infall in gravitationally fragmented disks and close encounters with an intruder star, were studied to determine the disk kinematic characteristics that can help to distinguish between these burst mechanisms. 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 burst origin. The disks in the stellar encounter and clump-infall models are characterized by tens of percent deviations from the Keplerian rotation, whie the disks in the MRI models are characterized only a few percent deviation, 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 peculiar 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. Abridged.

Accepted for publication by Astronomy & Astrophysics

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