Infall Explains the Disk Kinematics of AB Aur without Gravitational Instability

Abstract Late-stage infall onto protoplanetary disks can produce large-scale spiral arms. In this Letter, 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 produced 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. We further show that a unified model invoking infall onto a central binary can explain the CO morphology and kinematics, scattered light spirals, and millimeter-continuum emission in AB Aur.