The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds

TL;DR

This paper investigates the high cosmic-ray ionization rates in diffuse interstellar clouds, proposing an enhanced low-energy cosmic-ray flux as an explanation, and explores its implications on various astrophysical phenomena.

Contribution

It introduces two cosmic-ray spectra that explain the high ionization rates and examines their effects on interstellar chemistry and energetics, challenging the role of supernova remnants.

Findings

Enhanced low-energy cosmic-ray flux can account for observed ionization rates.

Proposed spectra are consistent with some observables but suggest different cosmic-ray sources.

Implications include altered light element abundances and gamma-ray emissions.

Abstract

Diffuse interstellar clouds show large abundances of H_3^+ which can be maintained only by a high ionization rate of H_2. Cosmic rays are the dominant ionization mechanism in this environment, so the large ionization rate implies a high cosmic-ray flux, and a large amount of energy residing in cosmic rays. In this paper we find that the standard propagated cosmic-ray spectrum predicts an ionization rate much lower than that inferred from H_3^+. Low-energy (~10 MeV) cosmic rays are the most efficient at ionizing hydrogen, but cannot be directly detected; consequently, an otherwise unobservable enhancement of the low-energy cosmic-ray flux offers a plausible explanation for the H_3^+ results. Beyond ionization, cosmic rays also interact with the interstellar medium by spalling atomic nuclei and exciting atomic nuclear states. These processes produce the light elements Li, Be, and B, as well as gamma-ray lines. To test the consequences of an enhanced low-energy cosmic-ray flux, we adopt two physically-motivated cosmic-ray spectra which by construction reproduce the ionization rate inferred in diffuse clouds, and investigate the implications of these spectra on dense cloud ionization rates, light element abundances, gamma-ray fluxes, and energetics. One spectrum proposed here provides an explanation for the high ionization rate seen in diffuse clouds while still appearing to be broadly consistent with other observables, but the shape of this spectrum suggests that supernovae remnants may not be the predominant accelerators of low-energy cosmic rays.