# Compression of turbulent magnetized gas in Giant Molecular Clouds

## TL;DR

This study uses 3D MHD simulations to explore how compression in magnetized, turbulent gas affects energy dissipation and velocity dispersion in molecular clouds, revealing reduced dissipation and higher turbulence levels.

## Contribution

It introduces a novel simulation-based analysis of compressing, magnetized turbulence and develops an analytic model for the effective equation of state in such conditions.

## Key findings

- Reduced dissipation rate in compressing, magnetized turbulence.
- Higher equilibrium velocity dispersion compared to non-magnetized or non-compressing cases.
- Implications for turbulence driving and chemical processes in molecular clouds.

## Abstract

Interstellar gas clouds are often both highly magnetized and supersonically turbulent, with velocity dispersions set by a competition between driving and dissipation. This balance has been studied extensively in the context of gases with constant mean density. However, many astrophysical systems are contracting under the influence of external pressure or gravity, and the balance between driving and dissipation in a contracting, magnetized medium has yet to be studied. In this paper we present three-dimensional (3D) magnetohydrodynamic (MHD) simulations of compression in a turbulent, magnetized medium that resembles the physical conditions inside molecular clouds. We find that in some circumstances the combination of compression and magnetic fields leads to a rate of turbulent dissipation far less than that observed in non-magnetized gas, or in non-compressing magnetized gas. As a result, a compressing, magnetized gas reaches an equilibrium velocity dispersion much greater than would be expected for either the hydrodynamic or the non-compressing case. We use the simulation results to construct an analytic model that gives an effective equation of state for a coarse-grained parcel of the gas, in the form of an ideal equation of state with a polytropic index that depends on the dissipation and energy transfer rates between the magnetic and turbulent components. We argue that the reduced dissipation rate and larger equilibrium velocity dispersion produced by compressing, magnetized turbulence has important implications for the driving and maintenance of turbulence in molecular clouds, and for the rates of chemical and radiative processes that are sensitive to shocks and dissipation.