Gravitational Fragmentation of Expanding Shells. II. Three-dimensional Simulations

TL;DR

This paper uses three-dimensional simulations to study how expanding shells driven by HII regions undergo gravitational fragmentation, revealing earlier and larger unstable modes than predicted by simple models.

Contribution

It provides high-resolution 3D simulation results showing the growth of gravitational instabilities in expanding shells, extending previous linear analyses with new analytic formulae.

Findings

Perturbations grow earlier than linear predictions.

Most unstable mode wavenumber is larger than in thin-shell analysis.

Deformation of contact discontinuity accompanies instability development.

Abstract

We investigate the gravitational fragmentation of expanding shells driven by HII regions using the three-dimensional Lagrangian simulation codes based on the Riemann solver, called Godunov smoothed particle hydrodynamics. The ambient gas is assumed to be uniform. In order to attain high resolution to resolve the geometrically thin dense shell, we calculate not the whole but a part of the shell. We find that perturbations begin to grow earlier than the prediction of the linear analysis under the thin-shell approximation. The wavenumber of the most unstable mode is larger than that in the thin-shell linear analysis. The development of the gravitational instability is accompanied by the significant deformation of the contact discontinuity. These results are consistent with a linear analysis presented by Iwasaki et al. (2011) that have taken into account the density profile across the thickness and approximate shock and contact discontinuity boundary conditions. We derive useful analytic formulae for the fragment scale and the epoch when the gravitational instability begins to grow.