Cosmic ray ionization of the interstellar medium

Abstract

We consider a cosmic ray spectrum that is a power law in momentum down to a cutoff and derive a lower cutoff corresponding to $E_{kin}\sim (30-60)$ MeV from the observed ionization rates in nearby diffuse clouds. While the real spectra of cosmic rays may not be so simple, we argue that one expects a substantial change in the spectra at such energies and that, at first approximation, a power law spectra with a lower cutoff is appropriate. Such a description of the cosmic rays in the interstellar medium is not only theoretically more attractive than the spectra used in the literature, but is also supported by recent observations.

(to appear in MNRAS) 8 pages, Plain TeX, 2 figures available upon request, Max Planck Institut f. Radioastronomie preprint # 562

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