Striations, integrals, hourglasses and collapse -- thermal instability driven magnetic simulations of molecular clouds

TL;DR

This study uses 3D MHD simulations to explore how thermal instability, magnetic fields, and gravity interact to form collapsing structures in molecular clouds, reproducing observed features like hourglass fields and dense clumps.

Contribution

It introduces detailed 3D AMR MHD simulations of molecular cloud formation, capturing realistic magnetic and density structures and dynamics.

Findings

Magnetic field-aligned striations form during collapse.

Hourglass magnetic field configurations develop in supercritical clouds.

Simulated density and velocity spectra match observations.

Abstract

The MHD version of the adaptive mesh refinement (AMR) code, MG, has been employed to study the interaction of thermal instability, magnetic fields and gravity through 3D simulations of the formation of collapsing cold clumps on the scale of a few parsecs, inside a larger molecular cloud. The diffuse atomic initial condition consists of a stationary, thermally unstable, spherical cloud in pressure equilibrium with lower density surroundings and threaded by a uniform magnetic field. This cloud was seeded with 10% density perturbations at the finest initial grid level around n=1.1 cm^{-3} and evolved with self-gravity included from the outset. Several cloud diameters were considered (100 pc, 200 pc and 400 pc) equating to several cloud masses (17,000 Msun, 136,000 Msun and 1.1x10^6 Msun). Low-density magnetic-field-aligned striations were observed as the clouds collapse along the field lines into disc-like structures. The induced flow along field lines leads to oscillations of the sheet about the gravitational minimum and an integral-shaped appearance. When magnetically supercritical, the clouds then collapse and generate hourglass magnetic field configurations with strongly intensified magnetic fields, reproducing observational behaviour. Resimulation of a region of the highest mass cloud at higher resolution forms gravitationally-bound collapsing clumps within the sheet that contain clump-frame supersonic (M~5) and super-Alfvenic (M_A~4) velocities. Observationally realistic density and velocity power spectra of the cloud and densest clump are obtained. Future work will use these realistic initial conditions to study individual star and cluster feedback.