By Williams D.A., Viti S.

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3, are also the ions created by cosmic ray ionisation. Thus, the effect of strong X-ray ionisation on the chemistry has some similarities to that driven by a high flux of cosmic rays. However, because high-energy X-rays penetrate the cloud more readily than stellar UV, the H/H2 transition generally occurs much deeper into a cloud 30 3 Astrochemical Processes than in the case of a PDR with the same energy flux in the UV. In high-density and high-flux cases, H2 may remain a minor component until depths into a cloud corresponding to AV ∼ 103 mag.

The first three drivers create new and efficient chemical pathways, while the third injects energy into the gas to allow barriers to reaction to be overcome. 2, are readily penetrated by starlight; this is the main driver of chemistry in such regions. 1 top, but cosmic rays are almost unimpeded as they pass through such regions, and they drive much of the chemistry. 3. 1 bottom, injects heat and drives a characteristic chemistry in the interface between the jet and the ambient gas. Of course, in many situations some or all of the drivers may be acting together, while in others the chemistry may be dominated by one driver, such as a very intense UV field in a region of massive star formation or an unusually high flux of cosmic rays generated in energetic regions of an active galaxy.

Astronomical Spectroscopy: An Introduction to the Atomic and Molecular Physics of Astronomical Spectra. London: Imperial College Press. 1 What Drives Cosmic Chemistry? It is a relatively straightforward matter to use freely available computer codes and lists of chemical reactions to compute abundances of molecular species for many types of interstellar or circumstellar region. For example, the UDfA, Ohio, and KIDA websites (see Chapter 9) provide lists of relevant chemical reactions and reaction rate data.

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