Non-equilibrium chemistry and cooling in the diffuse interstellar medium

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

Comparison with Cloudy

We consider a one-dimensional plane-parallel slab of gas with constant density and constant temperature that is illuminated from one side by the Black (1987) interstellar radiation field. In the figures below we compare the predictions from our model in chemical equilibrium (solid curves) with the 'big H2' model of Cloudy (dashed curves), in which the level populations of 1893 rovibrational states of the H2 model are followed and the photodissociation rates from these states via the Solomon process are calculated self-consistently. In the top panel of each figure, we also show the molecular abundances predicted by the 'small H2' model of Cloudy (dot-dot-dashed curves), in which the rovibrational levels of H2 in the ground electronic state are approximated by a two-level system: a ground state and a vibrationally excited state. In each figure we plot, as a function of total hydrogen column density, the molecular and ion abundances in chemical equilibrium (top two panels) and the cooling and heating rates (bottom two panels). These were calculated for three different densities (nH, tot = 1 cm-3, 100 cm-3 and 104 cm-3) and two different temperatures (T = 100 K and 300 K) at solar metallicity. The figure at nH, tot = 100 cm-3 and T = 300 K corresponds to figure 2 of paper II.

nH, tot (cm-3)
T = 100 K
T = 300 K

Last modified: Mon Mar 24 17:18:19 2014