Non-linear diffusion of cosmic rays escaping from supernova remnants: Cold partially neutral atomic and molecular phases

Abstract

We aim to elucidate cosmic ray (CR) propagation in the weakly ionized environments of supernova remnants (SNRs) basing our analysis on the cosmic ray cloud (CRC) model developed by \citet{2013ApJ...768...73M} and \citet{2016MNRAS.461.3552N}. We solved two transport equations simultaneously: one for the CR pressure and one for the Alfv\'en wave energy density where CRs are initially confined in the SNR shock. Cosmic rays trigger a streaming instability and produce slab-type resonant Alfv\'en modes. The self-generated turbulence is damped by ion-neutral collisions and by noncorrelated interaction with Alfv\'en modes generated at large scales. We show that CRs leaking in cold dense phases such as those found in cold neutral medium (CNM) and diffuse molecular medium (DiM) can still be confined over distances of a few tens of parsecs from the CRC center for a few thousand years. At 10 TeV, CR diffusion can be suppressed by two or three orders of magnitude. This effect results from a reduced ion-neutral collision damping in the decoupled regime. We calculate the grammage of CRs in these environments. We find that in both single and multi-phase setups at 10 GeV, CNM and DiM media can produce grammage in the range 10-20 $\rm{g/cm^2}$ in the CNM and DiM phases. At 10 TeV, because of nonlinear propagation the grammage increases to values in the range 0.5-20 $\rm{g/cm^2}$ in these two phases. We also present preliminary calculations in inhomogeneous interstellar medium combining two or three different phases where we obtain the same trends.