The Effects of Cosmic Rays on the Chemistry of Dense Cores

Abstract

Cosmic rays are crucial to the chemistry of molecular clouds and their evolution. They provide essential ionizations, dissociations, heating, and energy to the cold, dense cores. As cosmic rays pierce through clouds they are attenuated and lose energy, which leads to a dependency on the column density of a system. The detailed effects these particles have on the central regions still need to be fully understood. Here, we revisit how cosmic rays are treated in the UCLCHEM chemical modeling code by including both ionization rate and H2 dissociation rate dependencies alongside the production of cosmic ray induced excited species and we study in detail the effects of these treatments on the chemistry of pre-stellar cores. We find that these treatments can have significant effects on chemical abundances, up to several orders of magnitude, depending on the physical conditions. The ionization dependency is the most significant treatment influencing chemical abundances through the increased presence of ionized species, grain desorptions, and enhanced chemical reactions. Comparisons to chemical abundances derived from observations show the new treatments reproduce these observations better than the standard handling. It is clear that more advanced treatments of cosmic rays are essential to chemical models and that including this type of dependency provides more accurate chemical representations.