Can a cosmic ray carrot explain the ionization level in diffuse molecular clouds?

Abstract

Low energy cosmic rays are the major ionization agents of molecular clouds. However, it has been shown that, if the cosmic ray spectrum measured by Voyager 1 is representative of the whole Galaxy, the predicted ionization rate in diffuse clouds fails to reproduce data by 1-2 orders of magnitude, implying that an additional source of ionization must exist. One of the solutions proposed to explain this discrepancy is based on the existence of an unknown low energy (in the range 1 keV-1 MeV, not probed by Voyager) cosmic ray component, called carrot when first hypothesized by Reeves and collaborators in the seventies. Here we investigate the energetic required by such scenario. We show that the power needed to maintain such low energy component is comparable of even larger than that needed to explain the entire observed cosmic ray spectrum. Moreover, if the interstellar turbulent magnetic field has to sustain a carrot, through second-order Fermi acceleration, the required turbulence level would be definitely too large compared to the one expected at the scale resonant with such low energy particles. Our study basically rules out all the plausible sources of a cosmic ray carrot, thus making such hidden component unlikely to be an appealing and viable source of ionization in molecular clouds.

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Cited by in corpus (11)

• Low energy cosmic rays
• Gamma rays as probes of cosmic-ray propagation and interactions in galaxies
• Stochastic Fluctuations of Low-Energy Cosmic Rays and the Interpretation of Voyager Data
• Stochasticity of Cosmic Rays from Supernova Remnants and the Ionization Rates in Molecular Clouds
• The cosmic ray ionisation and $\gamma$-ray budgets of star-forming galaxies
• Constraining the cosmic ray spectrum in the vicinity of the supernova remnant W28: from sub-GeV to multi-TeV energies
• Cosmic ray driven galactic winds
• Cosmic rays cannot explain the high ionisation rates in the Galactic centre
• Galactic cosmic ray propagation through M dwarf planetary systems
• Constraining the low-energy cosmic ray flux in the central molecular zone from MeV nuclear deexcitation line observations
• High Energy Emission from the Galactic Center