Rigorous theory for secondary cosmic-ray ionization
Alexei V. Ivlev, Kedron Silsbee, Marco Padovani, Daniele Galli
The energy spectrum of electrons produced in molecular gas by interstellar cosmic rays (CRs) is rigorously calculated as a function of gas column density N traversed by the CRs. This allows us to accurately compute the local value of the secondary ionization rate of molecular hydrogen, ζsec(N), as a function of the local primary ionization rate, ζp(N). The ratio ζsec/ζp increases monotonically with N, and can considerably exceed the value of ≈0.67 commonly adopted in the literature. For sufficiently soft interstellar spectra, the dependence ζsec/ζp versus N is practically insensitive to their particular shape and thus is a general characteristic of the secondary CR ionization in dense gas.