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

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 Flash with extensions to metaldependent 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.

Physical effects on compact high-velocity clouds in the circumgalactic medium

“…Such a result has not been observed in the clouds simulated by Heitsch & Putman (2009). In Sander & Hensler (2019) we have shown that self-gravity makes a distinct difference in the evolution of even homogeneous clouds with low densities and far below their Jeans masses.…”

“…However, for later stages (> 40 Myr) differences become striking. Furthermore, in Sander & Hensler (2019) we have shown that self-gravity leads to a core-halo density profile even for resting (i.e. without dynamical interaction with the ambient medium) and initially homogeneous clouds with mass below their Bonnor-Ebert mass.…”

“…So, neglecting magnetic fields in our models does not contradict the observations. It further has been shown theoretically by Sander & Hensler (2019) that the effective heat flux, i.e. integrated over the cloud surface, is reduced to only 68 per cent of its undisturbed value by a strong magnetic dipole field.…”

“…If the total mass becomes a significant fraction of the (temperaturedependent) Jeans mass M J for a particular (r), self-gravity cannot be neglected as it relevantly affects the dynamics of all mass elements. However, it has been shown in Sander & Hensler (2019) that self-gravity has a measurable effect on the evolution of even clouds with masses well below their Jeans mass.…”

The role of the halo magnetic field on accretion through High-Velocity Clouds

“…The mass of our cloud is an order of magnitude below its Jeans mass. Recently, however, Sander & Hensler (2019 showed that selfgravity can have a significant effect on clouds below the Jeans mass if they have a cuspy density profile. In that case, the self-gravity inhibits stripping such that the cloud loses gas more slowly but also decreases the efficiency of condensation lowering the overall amount of cold gas.…”