Localized Interstellar Molecular Hydrogen

Abstract

Stecher and Williams¹ have claimed that the critical grain temperature for extensive H₂ formation and deposition of solid hydrogen mantles in dense clouds is T≃2.3° K—lower than our estimate² of ≈3° K based on thermodynamic arguments. We wish to point out that their conclusion is far from definitive and depends critically on several arbitrary assumptions. First, they presume that the surfaces of interstellar grains duplicate closely the properties of pure graphite surfaces in regard to physical absorption of hydrogen molecules. It is to be expected that interstellar graphite surfaces, if they are free, are exceptionally rich in dislocations and high energy impurity sites which would drastically alter their surface chemistry and adsorption properties. Such imperfections would make a surface much more efficient for trapping adsorbed atoms and molecules. Indeed, the surface chemistry involved in the formation of the first two or three monomolecular layers must in reality be expected to deviate considerably from the “ideal” situation described by Stecher and Williams¹. Another possibility they did not consider is that the surfaces on which the hydrogen condenses in dense clouds are of an icy nature, appropriate to the composite graphite/ice grains.