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Abstract: The Herschel Space Observatory opens the sky for observations in the farinfrared at high spectral and spatial resolution. A particular class ofmolecules will be directly observable; light diatomic hydrides and their ionsCH, OH, SH, NH, CH+, OH+, SH+, NH+. These simple constituents are importantboth for the chemical evolution of the region and as tracers of high-energyradiation. If outflows of a forming star erode cavities in the envelope,protostellar far UV FUV; 6 < E gamma < 13.6 eV radiation may escape throughsuch low-density regions. Depending on the shape of the cavity, the FUVradiation then irradiates the quiescent envelope in the walls along theoutflow. The chemical composition in these outflow walls is altered byphotoreactions and heating via FUV photons in a manner similar to photodominated regions PDRs.In this work, we study the effect of cavity shapes, outflow density, and of adisk with the two-dimensional chemical model of a high-mass young stellarobject introduced in the second paper in this series.We find that the shape of the cavity is particularly important in theinnermost part of the envelope, where the dust temperatures are high enough >100 K for water ice to evaporate. If the cavity shape allows FUV radiation topenetrate this hot-core region, the abundance of FUV destroyed species e.g.water is decreased. In particular, diatomic hydrides and their ions CH$+, OH+and NH+ are enhanced by many orders of magnitude in the outflow walls due tothe combination of high gas temperatures and rapid photodissociation of moresaturated species. The enhancement of these diatomic hydrides is sufficient fora detection using the HIFI and PACS instruments onboard Herschel. The effect ofX-ray ionization on the chemistry is found to be small, due to the much largerluminosity in FUV bands compared to X-rays.



Author: Simon Bruderer ETH Zurich, Arnold O. Benz ETH Zurich, P. Stäuber ETH Zurich, Steven D. Doty Denison University

Source: https://arxiv.org/







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