C₂H observations toward the Orion Bar^⋆

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: zsofia.nagy.astro@gmail.com
² Department of Physics and Astronomy, University of Toledo, 2801 West Bancroft Street, Toledo, OH 43606, USA
³ Kapteyn Astronomical Institute, University of Groningen, PO box 800, 9700 AV Groningen, The Netherlands
⁴ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
⁵ Université Joseph Fourier/CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
⁶ University of Michigan, Ann Arbor, MI 48197, USA

Received: 15 May 2014
Accepted: 20 April 2015

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 ~64 K and a beam-averaged C₂H column density of 4 × 10¹³ 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 ~8.3 × 10¹³ cm^-2 and ~2.3 × 10¹³ 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 ~ 100−150 K) and dense (n(H₂) ~ 10⁵−10⁶ 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(H₂) ~5 × 10⁶ cm^-3) hot (T_kin ~ 400 K) gas. The total beam-averaged C₂H column density in the model is 10¹⁴ 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.