Simple Organic Chemistry in the Horsehead Nebula

摘要

We present our latest results on carbon chemistry in the Horse-head 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 arid to investigate the photostability of simple organic molecules. In this contribution we show that simple carbon chains and rings (CCH, c-C_3H_2 and C_4H) 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 (DC0+ /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~+ observa-tions 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., H_2S). 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.