2015A&A...578A.124N

The ethynyl radical (C₂H) is one of the first radicals to be detected in the interstellar medium. Its higher rotational transitions have recently become available with the Herschel Space Observatory. We aim to constrain the physical parameters of the C₂H emitting gas toward the Orion Bar. We analyze the C₂H line intensities measured toward the Orion Bar CO⁺ Peak and Herschel/HIFI maps of C₂H, CH, and HCO⁺ and a NANTEN map of [CI]. We interpret the observed C₂H emission using the combination of Herschel/HIFI and NANTEN data with radiative transfer and PDR models. Five rotational transitions of C₂H (from N=6-5 up to N=10-9) have been detected in the HIFI frequency range toward the CO⁺ peak of the Orion Bar. Based on the five detected C₂H transitions, a single component rotational diagram analysis gives a rotation temperature of ∼64K and a beam-averaged C₂H column density of 4x10¹³cm^–2. The rotational diagram is also consistent with a two-component fit, resulting in rotation temperatures of 43±0.2K and 123±21K and in beam-averaged column densities of ∼8.3x10¹³cm^–2 and ∼2.3x10¹³cm^–2 for the three lower-N and for the three higher-N transitions, respectively. The measured five rotational transitions cannot be explained by any single parameter model. According to a non-LTE model, most of the C₂H column density produces the lower-N C₂H transitions and traces a warm (T_kin∼100-150K) and dense (n(H₂)∼10⁵-10⁶cm^–3) gas. A small fraction of the C₂H column density is required to reproduce the intensity of the highest-N transitions (N=9-8 and N=10-9) originating in a high-density (n(H₂)∼5x10⁶cm^–3) hot (T_kin∼400K) gas. The total beam-averaged C₂H column density in the model is 10¹⁴cm^–2. A comparison of the spatial distribution of C₂H to those of CH, HCO⁺, and [CI] shows the best correlation with CH. Both the non-LTE radiative transfer model and a simple PDR model representing the Orion Bar with a plane-parallel slab of gas and dust suggest that C₂H cannot be described by a single pressure component, unlike the reactive ion CH⁺, which was previously analyzed toward the Orion Bar CO⁺ peak. The physical parameters traced by the higher rotational transitions (N=6-5, ..., 10-9) of C₂H may be consistent with the edges of dense clumps exposed to UV radiation near the ionization front of the Orion Bar.