The observational impact of dust trapping in self-gravitating discs

Abstract

We present a 3D semi-analytical 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 (Atacama Large Millimeter/submillimeter Array) 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 multiwavelength 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 three discs, if sufficient grain growth has indeed occurred.

Particle Dynamics in 3D Self-gravitating Disks I: Spirals

“…particle size. If particles are dominated by their aerodynamic properties (gas-to-particle drag forces only), one might expect only the more well-coupled species to match the gas and decoupled particles to have more axisymmetric structure (Gibbons et al 2012;Cadman et al 2020). Two-dimensional simulations at longer cooling timescales have shown decreasing pitch angle (Shi et al 2016), but we do not observe a considerable decrease in θ with increasing β.…”

“…Previous work which did not include the gravity from gas to dust (e.g. Gibbons et al 2012;Cadman et al 2020) suggested that large particles form axisymmetric rings in self-gravitating disks, while in our simulations we will show that large particles still form spirals. Thus, there are several differences between our work and Gibbons et al (2012).…”

“…The spiral structure of self-gravitating disks are often studied for their gas dynamics (Cossins et al 2009;Michael et al 2012), but the effect of self-gravity with respect to solid material is an often neglected part of meshbased simulations, due in part to the computational expense and because of the minor effect in low-mass disks. Without considering the self-gravity effects from the gas onto the dust and between the dust particles themselves, larger dust species will remain decoupled from the gas and form axisymmetric structures, and this may be the expected outcome in many cases (Cadman et al 2020). However, as we will show, when the effects of the gas self-gravity is included on the dust, even large species will form non-axisymmetric structure similar to the gas.…”

Constraining protoplanetary disc mass using the GI wiggle

“…However, the origin of the spiral morphology remains unclear. Possibilities that have been investigated include companions, both stellar and planetary, (Dong et al 2015;Fung & Dong 2015;Dong & Fung 2017;Forgan et al 2018b;Price et al 2018b;Huang et al 2018;Dong et al 2018;Veronesi et al 2019;Shen et al 2020) and gravitational instability (GI) , 2005Cossins et al 2009;Dong et al 2015;Hall et al 2016;Meru et al 2017;Forgan et al 2018b;Hall et al 2018;Huang et al 2018;Hall et al 2019Hall et al , 2020Cadman et al 2020b;Chen et al 2021). While it has been demonstrated that GI can be responsible for spiral morphology of some observed discs, it requires the disc-to-star mass ratio, 𝑞, be 0.1 for the spirals to be observable (Cossins et al 2010;Dipierro et al 2014;Dong et al 2015;Hall et al 2016;Kratter & Lodato 2016;Hall et al 2019).…”

Spiral structures in gravito-turbulent gaseous disks

“…These spiral arms are believed to induce variability in the mass accretion rate (Lodato & Rice 2005;Vorobyov & Basu 2005, 2015, and they could play important roles in planet formation via dust grain accumulation (Rice et al 2004;Dipierro et al 2015), thermal processing and collisional fragmentation (Boss & Durisen 2005;Boley & Durisen 2008;Ida et al 2008;Booth & Clarke 2016). The morphology of observed spirals may also be used to discriminate self-gravitating disks and to constrain their mass and temperature distributions (Dong et al 2015(Dong et al , 2016(Dong et al , 2018Hall et al 2016;Meru et al 2017;Forgan et al 2018;Cadman et al 2020).…”