Cosmic-ray slowing down in molecular clouds: Effects of heavy nuclei

Abstract

Context. A cosmic ray (CR) spectrum propagated through ISM contains very few low-energy (<100 MeV) particles. Recently, a local CR spectrum, with strong low energy components, has been proposed to be responsible for the over production of H + molecule in some molecular clouds. Aims. We aim to explore the effects of the chemical composition of low-energy cosmic rays (CRs) when they slow down in dense molecular clouds without magnetic fields. We considered both ionization and solid material processing rates. Methods. We used galatic CR chemical composition from proton to iron. We propagated two types of CR spectra through a cloud made of H2: those CR spectra with different contents of low energy CRs and those assumed to be initially identical for all CR species. The stopping and range of ions in matter (SRIM) package provided the necessary stopping powers. The ionization rates were computed with cross sections from recent semi-empirical laws, while effective cross sections were parametrized for solid processing rates using a power law of the stopping power (power 1 to 2). Results. The relative contribution to the cloud ionization of proton and heavy CRs was found identical everywhere in the irradiated cloud, no matter which CR spectrum we used. As compared to classical calculations, using protons and high-energy behaviour of ionization processes (Z 2 scaling), we reduced absolute values of ionization rates by few a tens of percents but only in the case of spectrum with a high content of low-energy CRs. We found, using the same CR spectrum, the solid material processing rates to be reduced between the outer and inner part of thick cloud by a factor 10 (as in case of the ionization rates) or by a factor 100, depending on the type of process.