The observational impact of dust trapping in self-gravitating discs

TL;DR

This paper develops a 3D semi-analytic model of self-gravitating discs with dust trapping, demonstrating that observable spiral structures can indicate gravitational instability even in lower-mass discs, using synthetic ALMA observations.

Contribution

It introduces a novel model combining dust trapping and self-gravity, enabling predictions of observable spiral structures in protoplanetary discs and applying it to real systems.

Findings

Discs with low mass ratios can produce observable spiral arms.

Signatures of gravitational instability are detectable with sufficient grain growth.

Multi-wavelength observations can reveal dust trapping and grain growth processes.

Abstract

We present a 3D semi-analytic model of self-gravitating discs, and include a prescription for dust trapping in the disc spiral arms. Using Monte-Carlo radiative transfer we produce synthetic ALMA observations of these discs. In doing so we demonstrate that our model is capable of producing observational predictions, and able to model real image data of potentially self-gravitating discs. For a disc to generate spiral structure that would be observable with ALMA requires that the disc's dust mass budget is dominated by millimetre and centimetre-sized grains. Discs in which grains have grown to the grain fragmentation threshold may satisfy this criterion, thus we predict that signatures of gravitational instability may be detectable in discs of lower mass than has previously been suggested. For example, we find that discs with disc-to-star mass ratios as low as $0.10$ are capable of driving observable spiral arms. Substructure becomes challenging to detect in discs where no grain growth has occurred or in which grain growth has proceeded well beyond the grain fragmentation threshold. We demonstrate how we can use our model to retrieve information about dust trapping and grain growth through multi-wavelength observations of discs, and using estimates of the opacity spectral index. Applying our disc model to the Elias 27, WaOph 6 and IM Lup systems we find gravitational instability to be a plausible explanation for the observed substructure in all 3 discs, if sufficient grain growth has indeed occurred.