The impact of attenuation on cosmic-ray chemistry

I. Abundances and chemical calibrators in molecular clouds

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg 412 96, Sweden
² Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, West Bengal 741246, India
³ Faculty of Physics, University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
⁴ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 12 August 2025
Accepted: 15 January 2026

Abstract

The chemistry of shielded molecular gas is primarily driven by energetic, charged particles dubbed cosmic rays (CRs), in particular those with energy levels under 1 GeV. CRs ionize molecular hydrogen and helium, the latter of which contributes greatly to the destruction of molecules. CR ionization initiates a wide range of gas-phase chemistry, including pathways important for the so-called carbon cycle, C⁺/C/CO. Therefore, the CR ionization rate, ζ, is fundamental in theoretical and observational astrochemistry. Although observational methods show a wide range of ionization rates –varying with the environment, and especially decreasing into dense clouds- astrochemical models often assume a constant rate. To address this limitation, we employed a post-processed gas-phase chemical model of a simulated dense molecular cloud that incorporates CR energy losses within the cloud. This approach allowed us to investigate changes in the abundance profiles of important chemical tracers and gas temperatures. Furthermore, we analyzed analytical calibrators for estimating ζ in dense molecular gas that are robust when tested against a full chemical network. Additionally, we provide improved estimations of the electron fraction in dense gas for better consistency with observational data and theoretical calibrations for UV-shielded regions.