Implementation of chemistry in the Athena++ code

TL;DR

This paper introduces a chemistry module in the Athena++ astrophysical simulation code, enabling detailed modeling of chemical processes, heating, cooling, and radiation transfer in the interstellar medium with validation through various tests.

Contribution

The authors developed and integrated a flexible chemistry framework into Athena++, supporting multiple networks and coupled radiation and cosmic-ray processes, with validation for astrophysical applications.

Findings

Validated accuracy and convergence of the chemistry module

Successfully simulated photo-dissociation regions and shocks

Enabled realistic 3D turbulent ISM simulations

Abstract

Chemistry plays a key role in many aspects of astrophysical fluids. Atoms and molecules are agents for heating and cooling, determine the ionization fraction, serve as observational tracers, and build the molecular foundation of life. We present the implementation of a chemistry module in the publicly available magneto-hydrodynamic code Athena++. We implement several chemical networks and heating and cooling processes suitable for simulating the interstellar medium (ISM). A general chemical network framework in the KIDA format is also included, allowing the user to easily implement their own chemistry. Radiation transfer and cosmic-ray ionization are coupled with chemistry and solved with the simple six-ray approximation. The chemical and thermal processes are evolved as a system of coupled ODEs with an implicit solver from the CVODE library. We perform and present a series of tests to ensure the numerical accuracy and convergence of the code. Many tests combine chemistry with gas dynamics, including comparisons with analytic solutions, 1D problems of the photo-dissociation regions and shocks, and realistic 3D simulations of the turbulent ISM. We release the code with the new public version of Athena++, aiming to provide a robust and flexible code for the astrochemical simulation community.