Non-equilibrium chemistry and cooling in the diffuse interstellar medium

A. J. Richings, J. Schaye and B. D. Oppenheimer

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Paper 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 ionisation) equilibrium. We present a reaction network that can be used to solve for this thermo-chemical evolution. Our model follows the evolution of all ionisation states of the 11 elements that dominate the cooling rate, along with important molecules including 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 ionisation and heating and photochemical reactions. We focus on conditions typical for the diffuse ISM, with densities of 10-2 cm-3 < nH < 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 photoionisation code Cloudy. We identify the major coolants in interstellar gas to be CII, SiII and FeII, along with OI and H2 at densities nH > 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.


Dominant coolants in chemical equilibrium
Molecular abundances
Non-equilibrium cooling functions

Paper II: Shielded gas


We extend the non-equilibrium model for the chemical and thermal evolution of diffuse interstellar gas presented in Richings et al. (2014) to account for shielding from the UV radiation field. We attenuate the photochemical rates by dust and by gas, including absorption by HI, H2 , HeI, HeII and CO where appropriate. We then use this model to investigate the dominant cooling and heating processes in interstellar gas as it becomes shielded from the UV radiation. We consider a one-dimensional plane-parallel slab of gas irradiated by the interstellar radiation field, either at constant density and temperature or in thermal and pressure equilibrium. The dominant thermal processes tend to form three distinct regions in the clouds. At low column densities cooling is dominated by ionised metals such as SiII, FeII, FeIII and CII, which are balanced by photoheating, primarily from HI. Once the hydrogen-ionising radiation becomes attenuated by neutral hydrogen, photoelectric dust heating dominates, while CII becomes dominant for cooling. Finally, dust shielding triggers the formation of CO and suppresses photoelectric heating. The dominant coolants in this fully shielded region are H2 and CO. The column density of the HI-H2 transition predicted by our model is lower at higher density (or at higher pressure for gas clouds in pressure equilibrium) and at higher metallicity, in agreement with previous PDR models. We also compare the HI-H2 transition in our model to two prescriptions for molecular hydrogen formation that have been implemented in hydrodynamic simulations.


Comparison with Cloudy


Black, J. H. 1987, ASSL, 134, 731
Haardt, F., & Madau, P. 2001, in Neumann D. M., Tran J. T. V., eds, XXIst Moriond Astrophys. Meeting, Clusters of Galaxies and the High Redshift Universe Observed in X-rays Editions Frontieres, Paris, 64

Last modified: Sun Jun 8 11:30:44 2014