%0 Generic %1 richings2014nonequilibrium %A Richings, Alexander J. %A Schaye, Joop %A Oppenheimer, Benjamin D. %D 2014 %K atomic cooling ism molecular %T Non-equilibrium chemistry and cooling in the diffuse interstellar medium - II. Shielded gas %U http://arxiv.org/abs/1403.6155 %X 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 the gas itself, 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. We find that the transition from atomic to molecular hydrogen is triggered by H2 self-shielding. The transition column density is lower at higher density (or at higher pressure for gas clouds in pressure equilibrium) and at higher metallicity, due to an increase in the H2 fraction in the photodissociated region and hence a corresponding increase in the H2 self-shielding. We also compare the HI-H2 transition in our model to two prescriptions for molecular hydrogen formation that have been implemented in hydrodynamic simulations.

@misc{richings2014nonequilibrium, abstract = {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 the gas itself, 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. We find that the transition from atomic to molecular hydrogen is triggered by H2 self-shielding. The transition column density is lower at higher density (or at higher pressure for gas clouds in pressure equilibrium) and at higher metallicity, due to an increase in the H2 fraction in the photodissociated region and hence a corresponding increase in the H2 self-shielding. We also compare the HI-H2 transition in our model to two prescriptions for molecular hydrogen formation that have been implemented in hydrodynamic simulations.}, added-at = {2014-03-26T09:52:34.000+0100}, author = {Richings, Alexander J. and Schaye, Joop and Oppenheimer, Benjamin D.}, biburl = {https://www.bibsonomy.org/bibtex/2462a29692b9a7f0c5cd26ea7aa2869c7/miki}, description = {[1403.6155] Non-equilibrium chemistry and cooling in the diffuse interstellar medium - II. Shielded gas}, interhash = {5d3cf351170f1ec51716562ed38d9d09}, intrahash = {462a29692b9a7f0c5cd26ea7aa2869c7}, keywords = {atomic cooling ism molecular}, note = {cite arxiv:1403.6155Comment: 22 pages, 8 figures, submitted to MNRAS. Supplementary material can be found at http://noneqism.strw.leidenuniv.nl}, timestamp = {2014-03-26T09:52:34.000+0100}, title = {Non-equilibrium chemistry and cooling in the diffuse interstellar medium - II. Shielded gas}, url = {http://arxiv.org/abs/1403.6155}, year = 2014 }