Cosmic ray penetration in diffuse clouds

TL;DR

This paper models cosmic ray spectra in diffuse clouds, revealing significant differences from the interstellar medium at energies below 100 MeV, which impacts ionization rates and gamma-ray emissions.

Contribution

It provides a detailed solution to the cosmic ray transport equation in diffuse clouds, highlighting spectral modifications at low energies and their effects on ionization.

Findings

Cosmic ray spectrum inside diffuse clouds differs below 100 MeV from the ISM.

The spectrum becomes harder or softer depending on the ISM spectral index.

Ionization rates in diffuse clouds are significantly affected by low-energy cosmic rays.

Abstract

Cosmic rays are a fundamental source of ionization for molecular and diffuse clouds, influencing their chemical, thermal, and dynamical evolution. The amount of cosmic rays inside a cloud also determines the $\gamma$-ray flux produced by hadronic collisions between cosmic rays and cloud material. We study the spectrum of cosmic rays inside and outside of a diffuse cloud, by solving the stationary transport equation for cosmic rays including diffusion, advection and energy losses due to ionization of neutral hydrogen atoms. We found that the cosmic ray spectrum inside a diffuse cloud differs from the one in the interstellar medium (ISM) for energies smaller than $E_{br}\approx 100$ MeV, irrespective of the model details. Below $E_{br}$, the spectrum is harder (softer) than that in the ISM if the latter is a power law $\propto p^{-s}$ with $s$ larger (smaller) than $\sim0.42$. As a consequence also the ionization rate due to CRs is strongly affected. Assuming an average Galactic spectrum similar to the one inferred from AMS-2 and Voyager 1 data, we discuss the resulting ionization rate in a typical diffuse cloud.