CRAFT (Cosmic Ray Acceleration From Turbulence) in Molecular Clouds

TL;DR

This paper proposes a novel mechanism for generating low-energy cosmic rays in molecular clouds through turbulent reconnection, potentially explaining observed ionization rates without requiring external sources.

Contribution

It introduces first order Fermi-acceleration in turbulent reconnection as a local cosmic ray source within molecular clouds, filling a gap in understanding cosmic ray origins.

Findings

Energetic arguments support sufficient energy for ionization rates.

Reconnection-driven acceleration can produce homogeneous cosmic ray distribution.

Mechanism explains high ionization rates without external flux.

Abstract

Low-energy cosmic rays, in particular protons with energies below 1 GeV, are significant drivers of the thermochemistry of molecular clouds. However, these cosmic rays are also greatly impacted by energy losses and magnetic field transport effects in molecular gas. Explaining cosmic ray ionization rates of $10^{-16}$ s$^{-1}$ or greater in dense gas requires either a high external cosmic ray flux, or local sources of MeV-GeV cosmic ray protons. We present a new local source of low-energy cosmic rays in molecular clouds: first order Fermi-acceleration of protons in regions undergoing turbulent reconnection in molecular clouds. We show from energetic-based arguments there is sufficient energy within the magneto-hydrodynamic turbulent cascade to produce ionization rates compatible with inferred ionization rates in molecular clouds. As turbulent reconnection is a volume-filling process, the proposed mechanism can produce a near-homogeneous distribution of low-energy cosmic rays within molecular clouds.