Prof. Eric Herbst
The gas-phase chemistry of interstellar clouds is powered by ionization caused by primary cosmic rays, mainly protons, and secondary electrons. The cosmic ray ionization rate throughout a cloud, ζH, can be estimated based on the initial energy spectrum entering a cloud. However, there are a number of uncertainties, including poor knowledge of the flux of the lowest energy cosmic rays, which are the most critical for ionization. So, to most astrochemists, ζH is still regarded as a parameter with no dependence on depth into a cloud. For dense interstellar clouds in most regions of the galaxy, the best value of ζH lies in the range 10-17 – 10-16 s-1. This value, determined by comparison between molecular observations and the results of chemical simulations, has had a varied history. Recent studies of selected interstellar ions; namely H3 +, detected in the infrared, and OH+ and H2O+, detected in the far-infrared, indicate through chemical simulations that there are regions, including diffuse clouds in the galactic spiral arms, outflows from objects such as Orion KL, and much of the galactic center, where the cosmic ray ionization rate can be 1-3 orders of magnitude greater than the standard value. In my talk, I will discuss the role of cosmic rays in interstellar chemistry, and emphasize the apparent need for a large range of values for ζH in diverse sources.