Inner walls or vortices? Crescent-shaped asymmetries in ALMA observations of protoplanetary discs

TL;DR

This study interprets crescent-shaped asymmetries in protoplanetary discs as emission from their inner cavity walls, providing insights into their vertical structure and challenging the common vortex hypothesis.

Contribution

It introduces a geometric model of inner cavity wall emission to explain asymmetries, improving fit over axisymmetric models and estimating disc scale heights.

Findings

Inner cavity walls can explain crescent asymmetries in protoplanetary discs.

Asymmetries are predominantly on the far-side, supporting a geometric origin.

Estimated dust scale heights are 0.4 au at 37 au for CIDA 9 and 0.2 au at 12 au for RY Tau.

Abstract

Crescent-shaped asymmetries are common in millimetre observations of protoplanetary discs and are usually attributed to vortices or dust overdensities. However, they often appear on a single side of the major axis and roughly symmetric about the minor axis, suggesting a geometric origin. In this work, we interpret such asymmetries as emission from the exposed inner cavity walls of inclined discs and use them to characterise their vertical extent. Here we focus on the discs around CIDA 9 and RY Tau, first modelling their observations in visibility space with a simple geometric prescription for the walls, and then exploring more detailed radiative transfer models. Accounting for the wall emission yields significantly better residuals than purely axisymmetric models, and we estimate the dust scale height of these systems to be 0.4 au at 37 au for CIDA 9 and 0.2 au at 12 au for RY Tau. Finally, we identify crescent-shaped asymmetries in twelve discs, nine of which have constraints on their orientation - in all cases, the asymmetry appears on the far-side of the disc, lending support to the hypothesis that they are due to their inner rims. Modelling this effect in larger samples of discs will help to build a statistical view of their vertical structure.