High Energy Cosmic-ray Diffusion in Molecular Clouds: A Numerical Approach

Abstract

The propagation of high-energy cosmic rays through giant molecular clouds constitutes a fundamental process in astronomy and astrophysics. The diffusion of cosmic-rays through these magnetically turbulent environments is often studied through the use of energy-dependent diffusion coefficients, although these are not always well motivated theoretically. Now, however, it is feasible to perform detailed numerical simulations of the diffusion process computationally. While the general problem depends upon both the field structure and particle energy, the analysis may be greatly simplified by dimensionless analysis. That is, for a specified purely turbulent field, the analysis depends almost exclusively on a single parameter -- the ratio of the maximum wavelength of the turbulent field cells to the particle gyration radius. For turbulent magnetic fluctuations superimposed over an underlying uniform magnetic field, particle diffusion depends on a second dimensionless parameter that characterizes the ratio of the turbulent to uniform magnetic field energy densities. We consider both of these possibilities and parametrize our results to provide simple quantitative expressions that suitably characterize the diffusion process within molecular cloud environments. Doing so, we find that the simple scaling laws often invoked by the high-energy astrophysics community to model cosmic-ray diffusion through such regions appear to be fairly robust for the case of a uniform magnetic field with a strong turbulent component, but are only valid up to $\sim 50$ TeV particle energies for a purely turbulent field. These results have important consequences for the analysis of cosmic-ray processes based on TeV emission spectra associated with dense molecular clouds.

Accepted for publication in The Astrophysical Journal

References in corpus (7)

• Fermi LAT Observations of Supernova Remnants Interacting with Molecular Clouds
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• Numerical propagation of high energy cosmic rays in the Galaxy I: technical issues
• Enhanced Cosmic Ray Flux and Ionization for Star Formation in Molecular Clouds Interacting with Supernova Remnants
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Cited by in corpus (16)

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• Global diffusion of cosmic rays
• Turbulence-Level Dependence of Cosmic-Ray Parallel Diffusion
• Regimes of cosmic-ray diffusion in Galactic turbulence
• Particle Transport in intense small scale magnetic turbulence with a mean field
• A young SNR illuminating nearby Molecular Clouds with cosmic rays
• Leaked GeV CRs from a broken shell: Explaining 9 years Fermi-LAT data of SNR W28
• Effective Shielding of $\lesssim$ 10 GeV Cosmic Rays from Dense Molecular Clumps
• Anisotropic cosmic-ray diffusion in isotropic Kolmogorov turbulence
• A Numerical Assessment of Cosmic-ray Energy Diffusion through Turbulent Media
• Diffusive Cosmic-ray Acceleration in Sagittarius A*
• Assessing the Feasibility of Cosmic-Ray Acceleration by Magnetic Turbulence at the Galactic Center
• Effects of Turbulence on Cosmic Ray Propagation in Protostars and Young Star/Disk Systems
• Acceleration of cosmic rays in presence of magnetohydrodynamic fluctuations at small scales
• Cosmic Ray Propagation in Turbulent Spiral Magnetic Fields associated with Young Stellar Objects