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Multi-line Study of the Photo-dissociation Region M17-SW with SOFIA

Published onJun 01, 2020
Multi-line Study of the Photo-dissociation Region M17-SW with SOFIA

Photo-dissociation regions (PDRs) are the places where the radiative feedback by massive stars on molecular clouds happens. The molecular gas is photo-dissociated and then ionized by UV radiation. The UV radiation below 13.6 eV heats the gas via the photo-electric effect on dust grains. Cooling happens through the dust continuum and a handful of far infrared (FIR) fine-structure lines. FIFI-LS, the FIR spectrometer onboard the US-German airborne observatory, SOFIA, can map these main cooling processes efficiently. We observed the well-studied edge-on PDR, M17-SW, with high spatial resolution in all major FIR cooling lines of the ionized and neutral medium. By comparing the observed line intensities to model predictions we mapped the physical conditions of the ionized and neutral layer of the PDR by The [OIII] line ratio readily provides the electron density maps in the ionized layer just above the PDR. The analysis of the [OI], [CII], CO and continuum emission with the PDR Tool box allowed us to map the gas density and UV radiation field strength over the region. We also estimated the optical depth effects to the [OIII]63μm line and the contribution of the PDR to the ubiquitous [CII] emission for each map position. While the applied model is comparatively simple, a consistent picture of the spatial variation of the physical parameters over the mapped region could be derived. Based on these findings the processes and energetics in the PDR can be studied further possibly by applying more detailed models together with more data from other wavelengths.

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