C₂H observations toward the Orion Bar

Abstract

Context. The ethynyl radical (C 2 H) 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 C 2 H emitting gas toward the Orion Bar. Methods. We analyze the C 2 H line intensities measured toward the Orion Bar CO + Peak and Herschel/HIFI maps of C 2 H, CH, and HCO + and a NANTEN map of [Ci]. We interpret the observed C 2 H emission using the combination of Herschel/HIFI and NANTEN data with radiative transfer and PDR models. Results. Five rotational transitions of C 2 H (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 C 2 H transitions, a single component rotational diagram analysis gives a rotation temperature of ∼64 K and a beam-averaged C 2 H column density of 4 × 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 ∼8.3 × 10 13 cm −2 and ∼2.3 × 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 C 2 H column density produces the lower−N C 2 H transitions and traces a warm (T kin ∼ 100−150 K) and dense (n(H 2 ) ∼ 10 5 −10 6 cm −3 ) gas. A small fraction of the C 2 H 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 ) ∼ 5×10 6 cm −3 ) hot (T kin ∼ 400 K) gas. The total beam-averaged C 2 H column density in the model is 10 14 cm −2 . A comparison of the spatial distribution of C 2 H 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 planeparallel slab of gas and dust suggest that C 2 H 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 C 2 H may be consistent with the edges of dense clumps exposed to UV radiation near the ionization front of the Orion Bar.

Hydrocarbon Emission Rings in Protoplanetary Disks Induced by Dust Evolution

“…In addition, our models, for both sources, and Kama et al (2016b) have a stronger UV field on the disk surface due to dust settling; in our case we find G 0 1 in layers where C 2 H is forming. This, and the known association of C 2 H (and other hydrocarbons) with photodissociation regions (Pilleri et al 2013;Nagy et al 2015), argues that the UV field is also an important factor in hydrocarbon production.…”

“…In the dense interstellar medium the reactive C 2 H radical has been characterized as a tracer of transition zones within PDRs where the gas is rich in atoms and simple molecules with short reactive lifetimes (see Nagy et al 2015, for a recent study). Thus in Figure 5 there is a dramatic difference in the overall C 2 H distribution.…”

Synthetic observations of star formation and the interstellar medium

“…It is viewed approximately edge on and is relatively close at ∼ 410 pc, making it the "prototypical" PDR. The list of papers computing theoretical emission measures with a focus on the gas properties is huge, including Tielens & Hollenbach (1985a), Bregman et al (1989), Jansen et al (1995), Young Owl et al (2000), Walmsley et al (2000), Pellegrini et al (2009), Habart et al (2010, Ascasibar et al (2011), Goicoechea et al (2011Goicoechea et al ( , 2016Goicoechea et al ( , 2017, Melnick et al (2012), Nagy et al (2013Nagy et al ( , 2015Nagy et al ( , 2017, van der Tak et al (2013), and Cuadrado et al (2015). All of these studies are 1D and typically aim to reproduce the emission features of some subset of the large variety of atomic and molecular species detected towards the Orion Bar.…”

Herschel/HIFI spectral line survey of the Orion Bar

“…Two H 18 2 O transitions detected as part of this line survey allowed the ortho-to-para ratio of water to be measured toward the Orion Bar (Choi et al 2014). We also analysed six high-N transitions (N = 6-5, ..., 10-9) of C 2 H as a part of this line survey and found them to trace warm and dense gas including a component with densities of n(H 2 ) ∼ 5 × 10 6 cm −3 and temperatures of T kin ∼ 400 K (Nagy et al 2015b). The already studied molecules detected in this line survey suggest a complex structure with disparate gas components traced by different species.…”