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We investigate the physics and chemistry of the gas and dust in densephoton-dominated regions (PDRs), along with their dependence on theilluminating UV field. Using Herschel-HIFI observations, we study the gasenergetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023is the prototype of a PDR illuminated by a B2V star and is one of the keytargets of Herschel. Our approach consists in determining the energetics of theregion by combining the information carried by the mid-IR spectrum (extinctionby classical grains, emission from very small dust particles) with that of themain gas coolant lines. In this letter, we discuss more specifically theintensity and line profile of the 158 micron (1901 GHz) [CII] line measured byHIFI and provide information on the emitting gas. We show that both the [CII]emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs)arise from the regions located in the transition zone between atomic andmolecular gas. Using the Meudon PDR code and a simple transfer model, we findgood agreement between the calculated and observed [CII] intensities. HIFIobservations of NGC 7023 provide the opportunity to constrain the energetics atthe surface of PDRs. Future work will include analysis of the main coolant line[OI] and use of a new PDR model that includes PAH-related species.

Gas morphology and energetics at the surface of PDRs: new insights with Herschel observations of NGC 7023