Rings and Spirals in Protoplanetary Disks: The ALMA View of Planet Formation

Abstract

Exoplanet surveys have revealed remarkable diversity in planet compositions and orbital architectures, which are thought to be closely linked to where and when planets formed in protoplanetary disks. Leveraging the unprecedented spatial resolution and sensitivity of ALMA, we show that the dust and volatile distributions in disks are far more complex than previously thought, and we examine which chemical and dynamical processes are likely to have shaped these systems. We surveyed six nearby disks in H13CN, DCN, H13CO+, and DCO+. Because the D/H ratios of molecules are sensitive to their formation temperature, they are often used to infer the origins of volatile reservoirs. While the high D/H ratios measured in this survey are comparable to those in earlier stages of star formation, the disks' diverse molecular emission patterns suggest that a combination of warm and cold pathways contribute to chemical reprocessing in the disk. In the Disk Substructures at High Angular Resolution Project (DSHARP), we undertook the first high angular resolution ALMA disk survey. Observations of 20 disks at a resolution of 5 au indicate that annular gap and ring structures are common. Annular substructures can occur at virtually any radius where millimeter continuum emission is detected and range in widths from a few to tens of au. The absence of an obvious association with the disk thermal structure suggests that substructures do not occur preferentially near molecular snowlines. The annular substructures, however, bear strong resemblance to simulations of planet-disk interactions. If the observed gaps are indeed opened by planets, this would imply that wide-orbit giant planets can form quickly (within a million years) in disks. In a minority of cases, annular substructures co-exist with other types of substructures, including two-armed spirals and crescent-like emission. Millimeter continuum spiral arms have so far been detected preferentially in T Tauri disks without cavities, while spiral arms detected in scattered light typically occur in Herbig Ae disks with large central cavities and no millimeter spiral arm counterparts. This dichotomy suggests that multiple spiral arm formation processes, which may include perturbation by a companion or disk instabilities, are active in disks. In addition to surveys, we present detailed case studies of three notable systems: TW Hya, GM Aur, and RU Lup. Radiative transfer modeling of high-resolution 12CO observations toward TW Hya reveals strong radial variations in the column density profile, which may trace substructure in the underlying gas distribution or radially varying CO depletion in the upper layers of the disk. Multi-frequency continuum images of the TW Hya and GM Aur disks yield radial spectral index profiles with local maxima near continuum gaps and local minima near continuum rings. While low spectral indices have often been ascribed to grain growth and dust trapping, moderate-to-high localized optical depths appear to have a significant impact on disk spectral indices. Finally, whereas continuum observations detect a compact dust disk with axisymmetric gaps and rings around RU Lup, 12CO observations reveal non-Keplerian spiral arms stretching out to a radius of 1000 au and clumps located up to 1500 au from the star. The RU Lup system dramatically highlights the different views of disk structure provided by dust and gas observations, with gas observations suggesting that the planet formation environment is substantially more chaotic than implied by dust observations alone.