Non-equilibrium chemistry and cooling in the diffuse interstellar medium - I. Optically thin regime

An accurate treatment of the multiphase interstellar medium (ISM) in hydrodynamic galaxy simulations requires that we follow not only the thermal evolution of the gas, but also the evolution of its chemical state, including its molecular chemistry, without assuming chemical (including ionization) equilibrium. We present a reaction network that can be used to solve for this thermo-chemical evolution. Our model follows the evolution of all ionization states of the 11 elements that dominate the cooling rate, along with important molecules such as H₂ and CO, and the intermediate molecular species that are involved in their formation (20 molecules in total). We include chemical reactions on dust grains, thermal processes involving dust, cosmic ray ionization and heating and photochemical reactions. We focus on conditions typical for the diffuse ISM, with densities of 10^-2 cm^-3≲ n_H≲10⁴ cm^-3 and temperatures of 10² K ≲ T ≲ 10⁴ K, and we consider a range of radiation fields, including no UV radiation. In this paper, we consider only gas that is optically thin, while paper II considers gas that becomes shielded from the radiation field. We verify the accuracy of our model by comparing chemical abundances and cooling functions in chemical equilibrium with the photoionization code CLOUDY. We identify the major coolants in diffuse interstellar gas to be C II, Si II and Fe II, along with O I and H₂ at densities n_H ≳ 10² cm^-3. Finally, we investigate the impact of non-equilibrium chemistry on the cooling functions of isochorically or isobarically cooling gas. We find that, at T < 10⁴ K, recombination lags increase the electron abundance above its equilibrium value at a given temperature, which can enhance the cooling rate by up to two orders of magnitude. The cooling gas also shows lower H₂ abundances than in equilibrium, by up to an order of magnitude.