Simple Organic Chemistry in the Horsehead Nebula


Paper:	Simple Organic Chemistry in the Horsehead Nebula
Volume:	420, Bioastronomy 2007: Molecules, Microbes and Extraterrestrial Life
Page:	43
Authors:	Goicoechea, J. R.; Pety, J.; Gerin, M.; Hily-Blant, P.; Teyssier, D.; Roueff, E.
Abstract:	We present our latest results on carbon chemistry in the Horsehead nebula, one of the most famous objects in the sky and a unique laboratory to understand the chemistry of interstellar clouds} illuminated by UV radiation} from nearby stars. Photodissociation regions (PDRs) are interesting intermediate media between diffuse and dense dark clouds, thus enabling astrochemists to probe a large variety of physical and chemical processes. In particular, our high resolution astronomical observations show that the Horsehead edge is a realistic template to determine the molecular inventory in PDRs and to investigate the photostability of simple organic molecules}. In this contribution we show that simple carbon chains and rings (CCH, c-C₃H₂ and C₄H) are tightly spatially correlated with each other and with the emission of polycyclic aromatic hydrocarbons (PAHs). We show how molecules such as HCO⁺ start to be enriched in deuterium (DCO⁺/HCO⁺ > 0.02) as the gas cools down in the densest and UV protected prestellar condensations. We also determine the gas phase sulfur abundance in the UV irradiated gas from CS and HCS⁺ observations and chemical modeling. We finally present the first results of our search of gas phase species with a probable dust grain surface origin (e.g., H₂S). We stress the need of well conceived astronomical observations together with models that treat consistently both the photochemistry of simple organic species and the radiative transport of their emission lines.