The fragmentation of expanding shells III: Oligarchic accretion and the mass spectrum of fragments

TL;DR

This paper uses SPH simulations to study the gravitational fragmentation of expanding shells, revealing that in the non-linear phase, clump growth is dominated by oligarchic accretion, leading to a skewed mass spectrum.

Contribution

It introduces the concept of oligarchic accretion in shell fragmentation and compares analytical predictions with simulation results across different regimes.

Findings

Clump mass function matches analytical predictions in linear regime.

Sink mass function is skewed towards high-mass objects in non-linear regime.

Oligarchic accretion dominates the growth of bound objects.

Abstract

We use SPH simulations to investigate the gravitational fragmentation of expanding shells through the linear and non--linear regimes. The results are analysed using spherical harmonic decomposition to capture the initiation of structure during the linear regime; the potential-based method of Smith et al. (2009) to follow the development of clumps in the mildly non-linear regime; and sink particles to capture the properties of the final bound objects during the highly non-linear regime. In the early, mildly non--linear phase of fragmentation, we find that the clump mass function still agrees quite well with the mass function predicted by the analytic model. However, the sink mass function is quite different, in the sense of being skewed towards high-mass objects. This is because, once the growth of a condensation becomes non-linear, it tends to be growing non-competitively from its own essentially separate reservoir; we call this Oligarchic Accretion.