Spiral formation caused by late infall onto protoplanetary disks

¹ Institut für Theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
² Dipartimento di Fisica, Università degli Studi di Milano, Via Giovanni Celoria 16, 20133 Milano, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 December 2025
Accepted: 3 March 2026

Abstract

The classical picture that planet formation occurs in protoplanetary disks that are isolated from their environment is undergoing a major shift toward a more connected picture. An increasing number of evolved disks is found to interact actively with their environment, often showing various types of spiral structures. We investigated whether these spirals can be a direct result of ongoing late infall using the grid-based 3D hydrodynamics code FARGO3D. We performed a detailed analysis of the spiral properties and appearance in scattered light and CO line emission using radiative transfer modeling with the code RADMC3D. In scattered light, we find well-defined spirals with a few arms (m = 2) and more flocculent structures: the gradual accretion of gas remnants after a major accretion event is most successful in the former, whereas active accretion via streamers favors the latter. The m = 2 spirals we find have a very low pattern speed, making them easily discernible from spirals caused by a perturber. We also find spiral patterns in the ¹²CO residual motions, but their morphology does not match the one found in scattered light. The disk perturbations are strongest in the upper layers (z > 4H), which is reflected in the reduced amplitude of the residual motions in the more optically thin ¹³CO emission. Moreover, we find that the formation of m = 2 spirals is not promoted in disks with lower mass, even though they are more susceptible to deeper kinematic perturbations. While the late-infall streamers directly affect planet formation through the delivery of fresh material, we show that the midplane remains unperturbed unless the infalling mass is on the same order of magnitude as the disk mass. Planet formation can therefore only be affected by late infall through secondary mechanisms that lead to dust trapping or the generation of turbulence starting from surface-level perturbations.