# The Astrochemical Impact of Cosmic Rays in Protoclusters I: Molecular Cloud Chemistry

## TL;DR

This study models how cosmic ray attenuation influences molecular cloud chemistry in protoclusters, revealing significant effects on ion abundances and molecular emissions, with implications for interpreting astrochemical observations.

## Contribution

It introduces coupled cosmic ray attenuation and chemical evolution models for protocluster environments, highlighting the impact on ionization rates and molecular abundances.

## Key findings

- Ion abundances are highly sensitive to cosmic ray treatment.
- Classic cosmic ray models underpredict ion column densities by an order of magnitude.
- H3+ based ionization estimates often underpredict actual rates.

## Abstract

We present astrochemical photo-dissociation region models in which cosmic ray attenuation has been fully coupled to the chemical evolution of the gas. We model the astrochemical impact of cosmic rays, including those accelerated by protostellar accretion shocks, on molecular clouds hosting protoclusters. Our models with embedded protostars reproduce observed ionization rates. We study the imprint of cosmic ray attenuation on ions for models with different surface cosmic ray spectra and different star formation efficiencies. We find that abundances, particularly ions, are sensitive to the treatment of cosmic rays. We show the column densities of ions are under predicted by the `classic' treatment of cosmic rays by an order of magnitude. We also test two common chemistry approximations used to infer ionization rates. We conclude that the approximation based on the H$_3^+$ abundance under predicts the ionization rate except in regions where the cosmic rays dominate the chemistry. Our models suggest the chemistry in dense gas will be significantly impacted by the increased ionization rates, leading to a reduction in molecules such as NH$_3$ and causing H$_2$-rich gas to become [C II] bright.