H2 reformation in post-shock regions

TL;DR

This paper presents a new model for H2 formation on graphite surfaces at warm temperatures, significantly improving shock models by increasing the formation rate and better matching observational data of cooling lines.

Contribution

A novel H2 formation rate model based on chemisorption on graphite surfaces, enhancing shock model accuracy and observational agreement.

Findings

H2 formation rate is higher at elevated temperatures due to chemisorption.

The new model improves agreement between shock predictions and observed [O I] line fluxes.

Enhanced H2 formation impacts cooling and chemical evolution in post-shock regions.

Abstract

H2 formation is an important process in post-shock regions, since H2 is an active participant in the cooling and shielding of the environment. The onset of H2 formation therefore has a strong effect on the temperature and chemical evolution in the post shock regions. We recently developed a model for H2 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 H2 formation rate as a function of gas temperature that can be implemented in molecular shock models. The H2 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 H2 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 micron. With the new model a better agreement between 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 ISO-LWS, this more accurate model is very timely to help with the interpretation of these future results.