Rings and gaps in protoplanetary disks: planets or snowlines?

TL;DR

High-resolution ALMA observations reveal dust rings in protoplanetary disks, and this study explores whether planets or snowlines cause these structures, emphasizing the complexity of interpreting gas and dust features.

Contribution

The paper demonstrates that dust rings can be explained by snowlines without planets and highlights the importance of detailed modeling in interpreting CO emission data.

Findings

Gas temperature increases inside dust gaps, implying deeper gas gaps if planets are present.

Snowline models can reproduce dust rings without requiring planetary companions.

Interpreting CO emission in disks is complex and requires careful analysis.

Abstract

High resolution ALMA observations of protoplanetary disks have revealed that many, if not all primordial disks consist of ring-like dust structures. The origin of these dust rings remains unclear, but a common explanation is the presence of planetary companions that have cleared gaps along their orbit and trapped the dust at the gap edge. A signature of this scenario is a decrease of gas density inside these gaps. In recent work, Isella et al. 2016 derived drops in gas density consistent with Saturn-mass planets inside the gaps in the HD163296 disk through spatially resolved CO isotopologue observations. However, as CO abundance and temperature depends on a large range of factors, the interpretation of CO emission is non-trivial. We use the physical-chemical code DALI to show that the gas temperature increases inside dust density gaps, implying that any gaps in the gas, if present, would have to be much deeper, consistent with planet masses higher than a Jupiter mass. Furthermore, we show that a model with increased grain growth at certain radii, as expected at a snowline, can reproduce the dust rings in HD163296 equally well without the need for companions. This scenario can explain both younger and older disks with observed gaps, as gaps have been seen in systems as young <1 Myr. While the origin of the rings in HD163296 remains unclear, these modeling results demonstrate that care has to be taken when interpreting CO emission in protoplanetary disk observations.