Carbon lines towards Orion A

The physical processes that shape the structure of the interstellar medium can be studied in detail using bright photodissociation regions (PDRs) as laboratories. The energetics and physical properties of PDRs can be determined through carbon radio recombination lines (CRRLs) and the 158 micron-[CII] line. The nimble SOFIA observatory with its upGREAT instrument produced a velocity resolved one square degree map of the Orion nebula in [CII]. Motivated by the SOFIA observations and the development of new models describing the intensity of CRRLs we revisited the relation between [CII] and CRRLs towards three PDRs in the Orion nebula. We used constant density and constant pressure PDR models to predict the line properties. For a radiation field G0>100 PDR models predict an offset between the [CII] and CRRLs, implying that they trace the opposite ends of the C+ layer. We find that constant pressure PDR models are able to explain the observed line intensities while being consistent with independent measurements of the PDR properties (strength of the radiation field and density), but that requires a thermal pressure in the C+ layer that is a factor of four larger than previous estimates. These results are qualitatively similar to the predictions of photoevaporating PDR models. We also apply a similar analysis to the gas in Orion's Veil and determine the gas temperature, density and the photoelectric heating efficiency on two parsec scales.