C2H observations toward the Orion Bar

Nagy, Zsófia and Ossenkopf, V. and Van, der Tak F. F. S. and Faure, A. and Makai, Z. and Bergin, E. A. (2015) C2H observations toward the Orion Bar. ASTRONOMY & ASTROPHYSICS, 578. ISSN 0004-6361

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Abstract

Context. 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. Aims. We aim to constrain the physical parameters of the C2H emitting gas toward the Orion Bar. Methods. 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. Results. 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 similar to 64 K and a beam-averaged C2H column density of 4 x 10(13) cm(-2). The rotational diagram is also consistent with a two-component fit, resulting in rotation temperatures of 43 +/- 0.2 K and 123 +/- 21 K and in beam-averaged column densities of similar to 8.3 x 10(13) cm(-2) and similar to 2.3 x 10(13) cm(-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 similar to 100-150 K) and dense (n(H-2) similar to 10(5)-10(6) cm(-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(H-2) similar to 5 x 10(6) cm(-3)) hot (T-kin similar to 400 K) gas. The total beam-averaged C2H column density in the model is 10(14) cm(-2). A comparison of the spatial distribution of C2H to those of CH, HCO+, and [Ci] shows the best correlation with CH. Conclusions. 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.

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