Spiral formation caused by late infall onto protoplanetary disks

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 amount of evolved disks are found to be actively interacting with their environment, often showing various types of spiral structures. In this work, we aim to investigate if these spirals can be a direct result of ongoing late infall using the grid-based 3D hydrodynamics code FARGO3D. We perform a detailed analysis of the spiral properties and appearance in scattered light and CO line emission using the radiative transfer code RADMC3D. In scattered light, we find both well-defined spirals with few arms (m=2) and more flocculent structures: The gradual accretion of gas remnants after a major accretion event has the most success 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 $^{12}$CO 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 by the reduced amplitude of the residual motions in the more optically thin $^{13}$CO emission. Moreover, we find that the formation of m=2 spirals is not promoted in disks with lower mass, despite being more susceptible to deeper kinematic perturbations. While the late-infall streamers impact planet formation directly through the delivery of fresh material, we show that the midplane remains unperturbed unless the infalling mass is of the same order of magnitude as the disk mass. Planet formation can therefore only be impacted by late infall through secondary mechanisms that lead to dust trapping or the generation of turbulence starting from surface-level perturbations.

Figures (26)

• Figure 1: Polarized scattered light intensity in four snapshots of simulation 1. The solid gray line denotes the 1m″ contour, the gray dashed line is the 0.1m″ contour, and the gray dotted line the 0.01m″ contour. The panel numbering corresponds to the phases of the cloudlet encounter. The white arrow direction denotes the original orbital velocity of the cloudlet.
• Figure 2: Azimuthal Fourier decomposition of the polarized scattered light intensity, normed by the total intensity. $m=\tilde{\phi}$ is the Fourier transform of the azimuthal coordinate, and the two panels show the transformation for two radii. The white dashed lines separate the phases of the encounter, labeled by the white numbers.
• Figure 3: Polarized scattered light intensity as a function of azimuth and time for two different radii. The white dashed lines and numbers denote the different encounter phases, as in Fig. \ref{['fig:cldl_spectrum']}. The colored dots show the azimuthal peaks, colored in gray if not used for further consideration, otherwise by membership to one of the spiral arms. The solid white lines show the linear fit used to determine the pattern speed.
• Figure 4: Moment 1 residuals from Keplerian motion of the $^{12}$CO line emission. The top row shows three snapshots, taken at the same time as in Fig. \ref{['fig:cldl_spirals_rphi']}, where the camera is oriented face-on. The bottom row instead shows images where the camera has an inclination of 30°. The black lines show polarized scattered light contours, as in Fig. \ref{['fig:cldl_spirals_rphi']}. The panel numbers correspond to the evolutionary phase.
• Figure 5: Same as Fig. \ref{['fig:cldl_13co']}, but for $^{13}$CO. We note that the value range of the color bar is reduced.