The Effects of Cosmic Rays on the Chemistry of Dense Cores

TL;DR

This paper investigates how detailed modeling of cosmic ray interactions, including ionization and dissociation effects, significantly impacts the chemical composition predictions of dense pre-stellar cores, improving agreement with observations.

Contribution

It introduces enhanced cosmic ray treatments in the UCLCHEM code, accounting for ionization, dissociation, and excited species production, which were not previously included.

Findings

Cosmic ray treatments cause chemical abundance variations up to several orders of magnitude.

Ionization rate is the most influential factor affecting chemical compositions.

Enhanced treatments improve the match between model predictions and observational data.

Abstract

Cosmic rays are crucial for 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 the 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 needs 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 physical conditions. The ionization dependency is the most significant treatment, influencing chemical abundances through 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.