H2 reformation in post-shock regions

H-2 formation is an important process in post-shock regions, since H-2 is an active participant in the cooling and shielding of the environment. The onset of H-2 formation therefore has a strong effect on the temperature and chemical evolution in the post-shock regions. We recently developed a model for H-2 formation on a graphite surface in warm conditions. The graphite surface acts as a model system for grains containing large areas of polycyclic aromatic hydrocarbon structures. Here, this model is used to obtain a new description of the H-2 formation rate as a function of gas temperature that can be implemented in molecular shock models. The H-2 formation rate is substantially higher at high gas temperatures as compared to the original implementation of this rate in shock models, because of the introduction of H atoms which are chemically bonded to the grain (chemisorption). Since H-2 plays such a key role in the cooling, the increased rate is found to have a substantial effect on the predicted line fluxes of an important coolant in dissociative shocks [O i] at 63.2 and 145.5 mu m. With the new model, a better agreement between the model and observations is obtained. Since one of the goals of Herschel/PACS will be to observe these lines with higher spatial resolution and sensitivity than the former observations by Infrared Space Observatory-LWS, this more accurate model is very timely to help with the interpretation of these future results.