A Magnetically-Supported PDR in M17

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Accepted for publication in the Astrophysical Journal

Scientific paper

The SW part of the bright Galactic H II region M17 contains an obscured ionization front that is most easily seen at infrared and radio wavelengths. This "SW bar" has received considerable attention because the ionization front is seen nearly edge-on, thus offering an excellent opportunity to study the way in which the gas changes from fully ionized to molecular as radiation from the ionizing stars penetrates into it. M17 also is one of the very few H II regions for which the magnetic field strength can be measured in the photodissociation region (the "PDR") that forms the interface between the ionized and molecular gas. Here we carefully model an observed line of sight through the gas cloud, including the H+, H0 (PDR) and molecular layers, in a fully self-consistent single calculation. An interesting aspect of the M17 SW bar is that the PDR is very extended. We show that the relatively strong magnetic field which is observed to be present inevitably leads to a very deep PDR, because the structure of the neutral and molecular gas is dominated by magnetic pressure, rather than by gas pressure as previously had been supposed. We also show that a wide variety of observed facts can be explained if a hydrostatic geometry prevails, in which the gas pressure from an inner x-ray hot bubble and the outward momentum of the stellar radiation field compresses the gas and its associated magnetic field in the PDR, as has already been shown to occur in the Orion Nebula. The magnetic field compression may also amplify the local cosmic ray density by a factor of 300. The pressure in the observed magnetic field just balances the outward forces, suggesting that the observed geometry is a natural consequence of the formation of a star cluster within a molecular cloud.

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