Insights into the physics when modeling cold gas clouds in a hot plasma

TL;DR

This paper investigates the physical processes affecting cold gas clouds in hot plasma, emphasizing the importance of self-gravity and magnetic fields, through numerical simulations and analytical studies, to improve modeling accuracy.

Contribution

It provides a detailed analysis of self-gravity effects and heat conduction suppression, challenging common assumptions and highlighting the need for careful physical process consideration in models.

Findings

Self-gravity significantly influences cloud evolution, even in low-mass clouds.

Jeans and Bonnor-Ebert criteria are only sufficient, not necessary, for self-gravity effects.

Magnetic dipole fields reduce heat conduction by about 32%, with limited impact at higher strengths.

Abstract

This paper aims at studying the reliability of a few frequently raised but not proven arguments for the modeling of cold gas clouds embedded in or moving through a hot plasma and at sensitizing modelers to a more careful consideration of unavoidable acting physical processes and their relevance. At first, by numerical simulations we demonstrate the growing effect of self-gravity on interstellar clouds and, by this, moreover argue against their initial setup as homogeneous. We apply the adaptive-mesh refinement code {\sc Flash} with extensions to metal-dependent radiative cooling and external heating of the gas, self-gravity, mass diffusion, and semi-analytic dissociation of molecules and ionization of atoms. We show that the criterion of Jeans mass or Bonnor-Ebert mass, respectively, provides only a sufficient but not a necessary condition for self-gravity to be effective, because even low-mass clouds are affected on reasonable dynamical timescales. The second part of this paper is dedicated to analytically study the reduction of heat conduction by a magnetic dipole field.We demonstrate that in this configuration, the effective heat flow, i.e. integrated over the cloud surface, is suppressed by only $32$ per cent by magnetic fields in energy equipartition and still insignificantly for even higher field strengths.