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

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.

Galactic cosmic-ray maps at different energies added for illustration

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• Anomalous Ionization in the Central Molecular Zone by sub-GeV Dark Matter
• Cosmic rays cannot explain the high ionisation rates in the Galactic centre
• Imprints of PeV cosmic-ray sources on the diffuse gamma-ray emission
• The first detection of cosmic-ray excited H${}_2$ in interstellar space
• Sub-keV dark matter can strongly ionize molecular clouds
• Energetic particles and high-energy processes in cosmological filaments and their astronomical implications
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