2025ApJ...995..129S

We investigated the possibility of determining the mechanism of FU Orionis-type outbursts based on molecular line observations of protoplanetary disks with synthetic observations of distinct numerical burst models. The morphology of the synthetic C¹⁸O emission is sensitive to gas temperature and does not coincide with the actual gas disk structures, particularly in the magnetorotational instability (MRI) and clump-infall models, which exhibit peculiar temperature distributions. This highlights the need for careful interpretation of morphologies of line emission from disks under accretion outbursts. The synthetic C¹⁸O emission of each model exhibits distinct kinematic features that can be used to distinguish outburst scenarios. In the MRI model, kinematic features of the gravitational instability, which fuels MRI-driven accretion bursts, are small in both amplitude and spatial extent, resulting in no prominent local features in the residual velocity map at a typical distance for FU Orionis-type objects. In contrast, the clump-infall model shows a clear sign of gas expansion along a spiral, which is caused by exchange of angular momentum between an infalling clump and surrounding gas. The intruder model exhibits a highly asymmetric velocity structure with respect to the systemic velocity of the primary protostar in velocity channel maps. These distinct kinematic features may serve as promising diagnostics for distinguishing the physical mechanisms responsible for FU Orionis-type outbursts.