Cosmic ray penetration in diffuse clouds

TL;DR

This paper models cosmic ray transport in diffuse clouds, revealing how their spectrum differs from the interstellar medium below about 100 MeV, impacting ionization and gamma-ray production.

Contribution

It provides a detailed analysis of cosmic ray spectra inside diffuse clouds by solving the transport equation including diffusion, advection, and ionization losses.

Findings

Cosmic ray spectrum inside clouds differs below 100 MeV.

Spectrum becomes harder or softer depending on the interstellar spectrum slope.

The spectral change is independent of model details.

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 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 interstellar medium if the latter is a power law $\propto p^{-s}$ with $s$ larger (smaller) than $\sim 0.42$.