A Magnetically Supported Photodissociation Region in M17

The southwestern (SW) part of the Galactic H II region M17 contains anobscured ionization front that is most easily seen at infrared and radiowavelengths. It is nearly edge-on, thus offering an excellent opportunity tostudy the way in which the gas changes from fully ionized to molecular asradiation from the ionizing stars penetrates into the gas. M17 is also one ofthe very few H II regions for which the magnetic field strength can be measuredin the photodissociation region ( PDR) that forms the interface between theionized and molecular gas. Here we model an observed line of sight through thegas cloud, including the H+, H0 (PDR), and molecular layers, in a fullyself-consistent single calculation. An interesting aspect of the M17 SW bar isthat the PDR is very extended. We show that the strong magnetic field that isobserved to be present inevitably leads to a very deep PDR, because thestructure of the neutral and molecular gas is dominated by magnetic pressure,rather than by gas pressure, as previously had been supposed.We also show thata wide variety of observed facts can be explained if a hydrostatic geometryprevails, in which the gas pressure from an inner X-ray hot bubble and theoutward momentum of the stellar radiation field compress the gas and itsassociated magnetic field in the PDR, as has already been shown to occur in theOrion Nebula. The magnetic field compression may also amplify the localcosmic-ray density. The pressure in the observed magnetic field balances theoutward forces, suggesting that the observed geometry is a natural consequenceof the formation of a star cluster within a molecular cloud.Comment: Published as 2007, ApJ,658,111