Radiative transfer revisited for emission lines in photon dominated regions

Abstract

Context. Transfer in lines controls the gas cooling of photon dominated regions (PDR) and provides many of the observational constraints that are available for their modelling. Aims. The interpretation of infrared and radio observations by the new generation of instruments, such as Herschel, requires sophisticated line radiative-transfer methods. The effect of dust emission on the excitation of molecular species in molecular regions is investigated in detail to explicitly show the origin of various approximations used in the literature. Application to H2O is emphasised. Methods. The standard 1D radiative transfer equation is written as a function of the space variable (as opposed to the usual optical depth). This permits to simultaneously consider all pumping contributions to a multi-level species in a non-uniform slab of dust and gas. This treatment is included in the Meudon PDR Code (available at http://aristote.obspm.fr/MIS/). Results. Infrared emission from hot grains at the edge of the PDR may penetrate deep inside the cloud, providing an efficient radiation source to excite some species at a location where cold grains no longer emit. This leads to non-negligible differences with classical escape probability methods for some lines, e.g. water. Cooling efficiency does not follow directly from line emissivities. The infrared pumping contribution leads to a higher excitation that enhances collisional de-excitation and reduces cooling efficiency.