Infall Explains the Disk Kinematics of AB Aur Without Gravitational Instability

TL;DR

The paper addresses whether late-stage infall can account for the non-Keplerian kinematics and spiral features observed in AB Aur without invoking GI. It combines 3D SPH simulations of streamer-disk interactions with Monte Carlo radiative transfer to produce synthetic $^{13}$CO (2-1) channel maps and velocity residuals. The results show that infalling material induces radial and vertical velocity perturbations that generate wiggles in channel maps and radially converging/diverging flows along spiral arms, mirroring GI-like signatures. Comparisons with AB Aur suggest that the observed kinematic patterns can be explained by infall, though GI may still operate in outer disk regions, underscoring the need for multi-wavelength diagnostics to disentangle the contributing processes.

Abstract

Late-stage infall onto protoplanetary disks can produce large scale spiral arms. In this paper we used 3D smoothed particle hydrodynamics and radiative transfer simulations to study the kinematic perturbations induced in disks by infalling material. We found that deviations from Keplerian rotation are predominantly in the radial and vertical velocity components, spatially correlated with spiral arms in the gas surface density. The infall produces observable wiggles in the channel maps, analogous to those produce by the gravitational instability (GI), along with large-scale arcs and filaments. GI induced spiral arms produce radial velocity perturbations that point towards the center of the spiral arm owing to their higher self-gravity. We found a similar signature from infall-induced spiral arms, despite not including self-gravity in our simulation. Our study suggests that recent evidence of GI in the kinematics of the disk around AB Aur may instead be due to the observed infall, without the need for invoking GI.

Figures (4)

• Figure 1: Vertically integrated surface density (top row), midplane radial velocities (middle row) and midplane vertical velocities (bottom row) for three snapshots of our single stream simulation (Run 1). Spiral arms generated by the initial shock front of the streamer hitting the disk are seen in the first column and are labelled S1 and S2. Spiral arms intermediate to S1 and S2 are generated due to radially converging flows and are labelled S3. Spiral arms S1, S2, and S3, are highlighted with green dot-dashed, blue dashed, and solid purple lines, respectively.
• Figure 2: The Infall Wiggle: $^{13}$CO channel maps from Run 1 made with different assumptions on the velocity field, produced at $t \sim 7250$ years (snapshot between the middle and right columns of Figure \ref{['fig:velocity']}). The top row directly uses the radial ($v_r$), azimuthal ($v_\phi$), and vertical velocities ($v_z$) from the simulation. The second row assumes $v_\phi$ is at the Keplerian value and $v_r = v_z = 0$. The third row uses $v_\phi$ from the simulation, but $v_r = v_z = 0$. The fourth row uses $v_r$ from the simulation, with $v_z = 0$ and $v_\phi$ set to Keplerian. The final row assumes $v_z$ from the simulation, $v_r=0$, and $v_\phi$ is Keplerian.
• Figure 3: Polarized scattered light intensity (top row), moment 0 residuals (middle row) and moment 1 residuals (bottom row) of three simulation snapshots at times $t = [5600, 7250, 8950]$ years. Residuals were obtained as in speedie2024. Velocity residuals along the disk major axis show converging flows towards the location of spiral arms mapped in scattered light.
• Figure 4: The observations of AB Aur with VLT/SPHERE and ALMA (top row) with the mock versions of our model (bottom panel). In both AB Aur and our model, spiral arms seen in the scattered light correlate with patterns in the velocity residuals and spiral structures seen in the moment 0 residuals. The inset panels in the middle and right columns show the location of spiral structure close to the minor axis of the disk, where radial perturbations dominate over azimuthal perturbations. We see radially converging flows in our model that match those seen in AB Aur.