http://arxiv.org/abs/1405.3903
We study the spatial distribution and abundance of C2H in the prototypical high UV-illumination PDR, the Orion Bar. We analyse 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 rotation diagram analysis gives a rotation temperature of 67 K and a C2H column density of 5×10^13 cm^-2. A non-LTE radiative transfer model with a C2H column density of 10^14 cm^-2, an H2 volume density of 10^6 cm^-3, a kinetic temperature of 400 K, and an electron density of 10 cm^-3 is required to fit the two highest rotational transitions of C2H. This model gives a reasonable fit to the lower-N transitions as well, however, the N=6-5,…,8-7 transitions are more consistent with lower kinetic temperatures and H2 volume densities (Tkin~150 K and n(H2)=5×10^5 cm^-3). A comparison of the spatial distribution of C2H to those of CH, HCO+, and [CI] shows the best correlation with CH. For Orion Bar conditions, C2H traces a relatively narrow region at intermediate depths, equivalent to visual extinctions between 0.5 and 2 in a model with a pressure of 10^8 cm^-3 K. This region is part of the region where the CH abundances peak, which makes CH a relatively good tracer of C2H. This is supported by the spatial correlation seen in the observations. The spatial distribution of the observed species compared to PDR models suggests a high-pressure model (~10^8 cm^-3 K) for the Orion Bar, similar to CH+, SH+, and OH+. The Orion Ridge is more consistent with a lower pressure model.
Z. Nagy, V. Ossenkopf, F. Tak, et. al.
Fri, 16 May 14
33/55
Comments: Submitted to A&A, abstract abridged
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