Highenergy Cosmicray Diffusion in Molecular Clouds: A Numerical Approach
Abstract
The propagation of highenergy cosmic rays (CRs) through giant molecular clouds constitutes a fundamental process in astronomy and astrophysics. The diffusion of CRs through these magnetically turbulent environments is often studied through the use of energydependent 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 highenergy astrophysics community to model CR 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 ~50 TeV particle energies for a purely turbulent field. These results have important consequences for the analysis of CR processes based on TeV emission spectra associated with dense molecular clouds.
 Publication:

The Astrophysical Journal
 Pub Date:
 December 2010
 DOI:
 10.1088/0004637X/725/1/515
 arXiv:
 arXiv:1010.0059
 Bibcode:
 2010ApJ...725..515F
 Keywords:

 cosmic rays;
 diffusion;
 ISM: clouds;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 Accepted for publication in The Astrophysical Journal