Mapping the Merging Zone of Late Infall in the AB Aur Planet-forming System

Late infall events challenge the traditional view that planet formation occurs without external influence. Here we present deep Atacama Large Millimeter/submillimeter Array 12CO J = 2–1 and SO JN = 56–45 observations toward AB Aurigae (AB Aur), a Class II disk system with strong signs of gravitational instability and ongoing planet formation. By applying Keplerian and anti-Keplerian masks, we separate disk-like and non-disk-like motions of 12CO, considering the two outputs as the “disk” and “exo-disk” (out of disk) emission components, respectively. The disk component of 12CO extends to ∼1600 au in radius and exhibits a stunningly rich architecture of global spiral structure. The exo-disk emission consists predominantly of three spiral structures—S1, S2, and S3—whose projections are cospatial with the disk. We successfully reproduce their trajectories with a ballistic accretion flow model, finding that S1 and S2 (both redshifted) are infalling toward the disk from in front, and S3 (blueshifted) is infalling from behind. Where the terminal ends of S1 and S2 become indistinguishable from the disk, we observe a brightness peak in SO emission 2.5× the azimuthal average of a background SO ring. This merging zone lies within a relatively confined region 15°–100∘ east of north, and between ∼150 and 300 au from the star, at scales relevant to where planet candidates have been previously identified. The AB Aur system provides a unified picture of late infall inducing replenishment of the disk, triggering gravitational instability, and modifying the conditions of forming planets.