Orateur
Prof.
Eric Herbst
Description
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.
