Dependence of fragmentation in self-gravitating accretion discs on small scale structure

TL;DR

This paper introduces a new framework for understanding fragmentation in self-gravitating accretion discs, emphasizing the role of spiral structures and different collapse pathways, supported by 2D and 3D simulations.

Contribution

It proposes a novel fragmentation criterion based on spiral structure properties and distinguishes two collapse routes, supported by analytical predictions and numerical simulations.

Findings

Fragmentation occurs at $eta < 3$ in 2D simulations due to force smoothing.

3D simulations show fragmentation at higher $eta$ via quasistatic contraction.

Analytic prediction of fragmentation at $eta extless 12$ based on contraction against spiral disruption.

Abstract

We propose a framework for understanding the fragmentation criterion for self-gravitating discs which, in contrast to studies that emphasise the `gravoturbulent' nature of such discs, instead focuses on the properties of their quasi-regular spiral structures. Within this framework there are two evolutionary paths to fragmentation: i) collapse on the free-fall time, which requires that the ratio of cooling time to dynamical time ($\beta$) $< 3$ and ii) quasistatic collapse on the cooling time at a rate that is sufficiently fast that fragments are compact enough to withstand disruption when they encounter spiral features in the disc. We perform 2D grid simulations which demonstrate numerically converged fragmentation at $\beta < 3$ (in good agreement with Paardekooper et al. (2011) and others) and argue that this is a consequence of the fact that such simulations smooth the gravitational force on the scale $H$, the scale height of the disc. Such simulations thus only allow fragmentation via route i) above since they suppress the quasistatic contraction of fragments on scales $< H$; the inability of fragments to contract to significantly smaller scales then renders them susceptible to disruption at the next spiral arm encounter. On the other hand, 3D simulations indeed show fragmentation at higher $\beta$ via route ii). We derive an analytic prediction of fragmentation by route ii) when $\beta \lesssim 12$, based on the requirement that fragments must contract sufficiently to withstand disruption by spiral arms. We also discuss the necessary numerical requirements on both grid based and SPH codes if they are to model fragmentation via route ii).