The fragmentation of expanding shells II: Thickness matters

TL;DR

This paper analytically and numerically investigates how the finite thickness of expanding shells influences gravitational instability, revealing that pressure confinement significantly affects the size of unstable fragments.

Contribution

It introduces a new dispersion relation accounting for shell thickness and pressure confinement, improving predictions over the standard thin-shell analysis.

Findings

Thicker shells with high pressure support smaller unstable fragments.

The new dispersion relation aligns well with 3D hydrodynamic simulations.

Shell thickness and confining pressure critically influence gravitational instability.

Abstract

We study analytically the development of gravitational instability in an expanding shell having finite thickness. We consider three models for the radial density profile of the shell: (i) an analytic uniform-density model, (ii) a semi-analytic model obtained by numerical solution of the hydrostatic equilibrium equation, and (iii) a 3D hydrodynamic simulation. We show that all three profiles are in close agreement, and this allows us to use the first model to describe fragments in the radial direction of the shell. We then use non-linear equations describing the time-evolution of a uniform oblate spheroid to derive the growth rates of shell fragments having different sizes. This yields a dispersion relation which depends on the shell thickness, and hence on the pressure confining the shell. We compare this dispersion relation with the dispersion relation obtained using the standard thin-shell analysis, and show that, if the confining pressure is low, only large fragments are unstable. On the other hand, if the confining pressure is high, fragments smaller than predicted by the thin-shell analysis become unstable. Finally, we compare the new dispersion relation with the results of 3D hydrodynamic simulations, and show that the two are in good agreement.