Rigorous Theory for Secondary Cosmic-Ray Ionization

Abstract

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, , as a function of the local primary ionization rate, ζ p ( N ). The ratio 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 versus N is practically insensitive to their particular shape and thus is a general characteristic of the secondary CR ionization in dense gas.