Morphological and Kinematical Diagnostic of FU Orionis-type Outburst Mechanisms

TL;DR

This work investigates how to diagnose FU Orionis-type outburst mechanisms by linking hydrodynamic burst models (MRI-triggered, clump infall, and intruder flyby) to synthetic ALMA-like observations of the C$^{18}$O $J=3-2$ line. Using FEOSAD-based 2D hydrodynamics and RADMC-3D radiative transfer, it shows that line morphology alone poorly traces gas density due to temperature structure, but residual velocity and velocity-channel diagnostics reveal clear, model-specific signatures. GI-induced small residuals accompany MRI-driven spirals, while clump infall produces spiral-shaped expansion and the intruder yields highly asymmetric, large-amplitude velocity features, all of which can be detectable at ~30 au with ALMA. These kinematic diagnostics offer practical tools for identifying the physical mechanisms behind FU Orionis-type outbursts and for distinguishing among competing burst scenarios in young stellar objects.

Abstract

We investigated the possibility of determining the mechanism of the FU Orionis-type outburst based on molecular line observations of protoplanetary disks with synthetic observations of distinct numerical burst models. The morphology of the synthetic $\mathrm{C^{18}O}$ 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 $\mathrm{C^{18}O}$ 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 (GI), 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.

Figures (16)

• Figure 1: Simulated spatial distributions of gas surface density (top row), gas temperature (middle row), and gas scale heights (bottom row) at the time of burst onset for the MRI-triggered burst (left), clump infall (center), and close stellar flyby (right). The arrows point to the infalling clump causing the burst and the intruder.
• Figure 2: Mass accretion rates and total luminosities in the three burst models considered. The panels from top to bottom correspond to the MRI-triggered burst, clump accretion, and close flyby, respectively. The black circles indicate the exact times during the bursts considered in this work.
• Figure 3: Surface density (color) and velocity field (vectors) in the MRI model (left), clump-infall model (middle), and close stellar flyby model (right). The Keplerian rotation is subtracted from the model velocity. Note the different spatial scales to better resolve the disk spatial morphology and velocity fields. The disk rotates counterclockwise. The scale bar is in g cm$^{-2}$.
• Figure 4: Integrated intensity maps of the synthetic C^18O $J=3\hbox{--}2$ emission for the three models without (top row) and with (bottom row) beam convolution. From left to right, maps for the MRI, clump-infall, and intruder models are presented. The dotted curves indicate the locations of the high-density spiral arms in the first two models. The crosses mark positions of the central protostars and the intruder. In the bottom row, contour levels are $3$, 6, 12, and $24\times\sigma$, where $\sigma = 2.0$, 2.2, and $2.6~\mjypbm$ for the MRI, clump-infall, and the intruder models, respectively. The filled circle in the bottom left panel denotes the beam size of $0\farcs1$.
• Figure 5: Mean-velocity maps of the synthetic C^18O emission (first row); expected Keplerian velocity fields after projection onto the plane of the sky (second row); residual-velocity maps of the raw simulation data, which are computed using the true gas velocity from the simulations (third row); and residual-velocity maps of the synthetic C^18O emission (forth row). From left to right, maps for the MRI, clump-infall and intruder models are presented. We note that color scales of the residual maps are different for all models. The boundaries between the disks and envelopes, which are defined by a density threshold of $0.1~\mathrm{g~cm^{-2}}$, are denoted by contours. The envelope regions are masked in the residual maps for better visualizations. The dotted curves indicate the locations of the high-density spiral arms. Black crosses indicate the protostellar positions, and green crosses denote positions of an inner clump or the intruder. Contours in the zoom-in view of the residual maps for the MRI model denote zero residual velocities.