Magnetic fields and the location of the PDR

TL;DR

This paper reviews how magnetic fields influence the structure and location of photodissociation regions (PDRs) in star-forming regions like Orion and M17, highlighting the role of magnetic pressure in balancing radiation forces.

Contribution

It introduces a model where magnetic pressure in H^0 regions balances radiation pressure, simplifying the understanding of PDR parameters and their physical regulation.

Findings

Magnetic fields in H^0 regions are 10-100 times stronger than in the diffuse ISM.

Magnetic pressure supports the H^0 region against radiation pressure.

The model explains the placement of PDRs based on magnetic and radiation pressure balance.

Abstract

I review recent studies of the emission-line regions in Orion and M17. Both have similar geometries, a bubble of hot shocked gas surrounding the central star cluster, with H^+, H^0, and H_2 regions, often referred to as H II regions, PDRs, and molecular clouds, forming successive shells on the surface of a molecular cloud. The magnetic fields in the H^0 regions have been measured with 21 cm Zeeman polarization and are found to be 1 -- 2 dex stronger than the field in the diffuse ISM. The regions appear to be in rough hydrostatic equilibrium. The H^+ region is pushed away from the star cluster by starlight radiation pressure. Since most starlight is in ionizing radiation, most of its outward push will act on the H^+ region and then on to the H^0 region. The magnetic pressure in the H^0 region balances the momentum in starlight and together they set the location of the H^0 region. The picture is that, when the star cluster formed, it created a bubble of ionized gas which expanded and compressing surrounding H^0 and H_2 regions. The magnetic field was amplified until its pressure was able to support the momentum in starlight. This offers a great simplification in understanding the underlying physics that establishes parameters for PDR models.