Chemical effects of H2 formation excitation

• Published in: Astrophysics and space science Vol. 286; no. 3-4; pp. 487 - 499
• Main Authors: Garrod, Rt, Rawlings, Jmc, Williams, Da
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 2003
• Subjects: Astrophysics; Chemistry; Hydrogen
• ISSN: 0004-640X; 1572-946X
• DOI: 10.1023/A:1026376615183

The effects of the production on dust grain surfaces of molecular hydrogen in excited states have been investigated. On the assumption that all of the H^sub 2^ formed on the surface of grains has a sufficient level of excitation too vercome the energy barriers in the formation reactions for the important OH and CH^sup +^ radicals, we consider the likely abundances of excited H^sub 2^ (H^sub 2^^sup *^), OH and CH^sup +^ in various situations. Two different models are employed; the first links the H^sub 2^^sup *^ abundance directly to that of H^sub 2^ using a steady-state approximation, whilst the second considers the time-dependence of H^sub 2^^sup *^. The second model is applied to gas that has been subjected to a strong isothermal shock (specifically, the shock-induced collapse of a diffuse cloud), which results in an extreme (high density, high atomic hydrogen abundance) environment. In general, it is found that the presence of the excited H^sub 2^ has only marginal effects on the chemistry of interstellar clouds. However, in the isothermal shock model, the abundances of CH^sup +^ are significantly enhanced, but only on short timescales, whilst the effects on the OH abundances are smaller, but last longer. We conclude that other than in such exceptional environments there are no obvious chemical signatures of the formation of H^sub 2^^sup *^.[PUBLICATION ABSTRACT]