Magnetic fields and the location of the PDR

I review recent studies of the emission-line regions in Orion and M 17. Both have similar geometries, a bubble of hot shocked gas surrounding the central star cluster, with H+, H0, and H2 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 H0 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 H0 region. The magnetic pressure in the H0 region balances the momentum in starlight and together they set the location of the H0 region. The picture is that, when the star cluster formed, it created a bubble of ionized gas which expanded and compressed surrounding H0 and H2 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