Magnetic fields, stellar feedback, and the geometry of H II Regions

TL;DR

This paper reviews recent observations of magnetic fields in H II regions, highlighting how magnetic pressure influences the structure, dynamics, and evolution of these regions in star-forming environments.

Contribution

It synthesizes observational data to propose a model where magnetic fields shape the morphology and gas flows in H II regions during star formation.

Findings

Magnetic fields are compressed and resist expansion in H II regions.

Field lines tend to be perpendicular to the long axis of molecular clouds.

Magnetic fields influence gas flow and may enhance cosmic ray heating.

Abstract

Magnetic pressure has long been known to dominate over gas pressure in atomic and molecular regions of the interstellar medium. Here I review several recent observational studies of the relationships between the H^+, H^0 and H_2 regions in M42 (the Orion complex) and M17. A simple picture results. When stars form they push back surrounding material, mainly through the outward momentum of starlight acting on grains, and field lines are dragged with the gas due to flux freezing. The magnetic field is compressed and the magnetic pressure increases until it is able to resist further expansion and the system comes into approximate magnetostatic equilibrium. Magnetic field lines can be preferentially aligned perpendicular to the long axis of quiescent cloud before stars form. After star formation and pushback occurs ionized gas will be constrained to flow along field lines and escape from the system along directions perpendicular to the long axis. The magnetic field may play other roles in the physics of the H II region and associated PDR. Cosmic rays may be enhanced along with the field and provide additional heating of atomic and molecular material. Wave motions may be associated with the field and contribute a component of turbulence to observed line profiles.