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.
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