2017A&A...599A..22N

Context. Photon dominated regions (PDRs) are interfaces between the mainly ionized and mainly molecular material around young massive stars. Analysis of the physical and chemical structure of such regions traces the impact of far-ultraviolet radiation of young massive stars on their environment.
Aims. We present results on the physical and chemical structure of the prototypical high UV-illumination edge-on Orion Bar PDR from an unbiased spectral line survey with a wide spectral coverage which includes lines of many important gas coolants such as [CII], [CI], and CO and other key molecules such as H₂CO, H₂O, HCN, HCO⁺, and SO.
Methods. A spectral scan from 480-1250GHz and 1410-1910GHz at 1.1MHz resolution was obtained by the HIFI instrument on board the Herschel Space Observatory. We obtained physical parameters for the observed molecules. For molecules with multiple transitions we used rotational diagrams to obtain excitation temperatures and column densities. For species with a single detected transition we used an optically thin LTE approximation.In the case of species with available collisional rates, we also performed a non-LTE analysis to obtain kinetic temperatures, H₂ volume densities, and column densities.
Results. About 120 lines corresponding to 29 molecules (including isotopologues) have been detected in the Herschel/HIFI line survey, including 11 transitions of CO, 7 transitions of ¹³CO, 6 transitions of C¹⁸O, 10 transitions of H₂CO, and 6 transitions of H₂O. The rotational temperatures are in the range between ∼22 and ∼146K and the column densities are in the range between 1.8x10¹²cm^–2 and 4.5x10¹⁷cm^–2. For species with at least three detected transitions and available collisional excitation rates we derived a best fit kinetic temperature and H₂ volume density. Most species trace kinetic temperatures in the range between 100 and 150K and H₂ volume densities in the range between 10⁵ and 10⁶cm^–3. The species with temperatures and/or densities outside this range include the H₂CO transitions tracing a very high temperature (315K) and density (1.4x10⁶cm^–3) component and SO corresponding to the lowest temperature (56K) measured as a part of this line survey.
Conclusions. The observed lines/species reveal a range of physical conditions (gas density/temperature) involving structures at high density/high pressure, making the traditional clump/interclump picture of the Orion Bar obsolete.