Stochasticity of Cosmic Rays from Supernova Remnants and the Ionization Rates in Molecular Clouds

TL;DR

This paper models the variability of cosmic-ray spectra from supernova remnants to explain the unexpectedly high ionization rates observed in molecular clouds, highlighting the importance of stochastic effects.

Contribution

It introduces a statistical model of cosmic-ray spectra from supernova remnants to account for observed ionization rate discrepancies.

Findings

Stochastic effects can explain high observed ionization rates.

Modelled ionization rate distribution aligns with observational data.

Variability in cosmic-ray spectra is significant for molecular cloud chemistry.

Abstract

Cosmic rays are the only agent able to penetrate into the interior of dense molecular clouds. Depositing (part of) their energy through ionisation, cosmic rays play an essential role in determining the physical and chemical evolution of star-forming regions. To a first approximation their effect can be quantified by the cosmic-ray induced ionization rate. Interestingly, theoretical estimates of the ionization rate assuming the cosmic-ray spectra observed in the local interstellar medium result in an ionization rate that is one to two orders of magnitude below the values inferred from observations. However, due to the discrete nature of sources, the local spectra of MeV cosmic rays are in general not representative for the spectra elsewhere in the Galaxy. Such stochasticity effects have the potential of reconciling modelled ionization rates with measured ones. Here, we model the distribution of low-energy cosmic-ray spectra expected from a statistical population of supernova remnants in the Milky Way. The corresponding distribution for the ionization rate is derived and confronted with data. We find that the stochastic uncertainty helps with explaining the surprisingly high ionization rates observed in many molecular clouds.