Gravity Versus Magnetic Fields in Forming Molecular Clouds

TL;DR

This study uses magnetohydrodynamics simulations to explore the roles of magnetic and gravitational forces in the formation and early evolution of molecular clouds, revealing that envelopes are magnetically supported while dense cores are gravity-dominated.

Contribution

It provides a self-consistent simulation-based analysis of magnetic field structures and dynamics in forming molecular clouds, highlighting the differing roles of magnetic support and gravity.

Findings

Cloud envelopes are magnetically supported with aligned magnetic fields.

Dense cores are dominated by gravity with varying magnetic field orientations.

Gravity exceeds other energies in dense regions, driving collapse.

Abstract

Magnetic fields are dynamically important in the diffuse interstellar medium. Understanding how gravitationally bound, star-forming clouds form requires modeling of the fields in a self-consistent, supernova-driven, turbulent, magnetized, stratified disk. We employ the FLASH magnetohydrodynamics code to follow the formation and early evolution of clouds with final masses of 3-8 $\times 10^3 M_{\odot}$ within such a simulation. We use the code's adaptive mesh refinement capabilities to concentrate numerical resolution in zoom-in regions covering single clouds, allowing us to investigate the detailed dynamics and field structure of individual self-gravitating clouds in a consistent background medium. Our goal is to test the hypothesis that dense clouds are dynamically evolving objects far from magnetohydrostatic equilibrium. We find that the cloud envelopes are magnetically supported with field lines parallel to density gradients and flow velocity, as indicated by the histogram of relative orientations and other statistical measures. In contrast, the dense cores of the clouds are gravitationally dominated, with gravitational energy exceeding internal, kinetic, or magnetic energy and accelerations due to gravity exceeding those due to magnetic or thermal pressure gradients. In these regions field directions vary strongly, with a slight preference towards being perpendicular to density gradients, as shown by three-dimensional histograms of relative orientation.