The Formation of Population I Stars, Part II. The Formation of Molecular Hydrogen in Interstellar Matter

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Abstract
This paper is essentially an investigation of the possibility of molecule-formation (chiefly H 2 ) by surface-reactions on interstellar dust-grains. The process has often been mentioned but here we seek to derive as precise an expression as possible for the rate of formation in terms of the properties of the grains. The effect upon the rate of a collision between interstellar clouds is also considered. The case of interstellar CH is discussed; it seems that some of the difficulties encountered by Bates and Spitzer may be over-come if we recognize the possibility of the enhanced production of CH in the conditions produced by a collision between clouds. If this is accepted, it indicates that the type of process here considered does actually take place with a fairly high efficiency. The case of H 2 is then considered. An efficiency comparable to that inferred in the case of CH would give a proportion of H 2 in ordinary interstellar matter in satisfactory agreement with Kahn's estimate of the amount required to cool the matter after heating by cloud-collisions. Such an efficiency would also explain why free atomic hydrogen would not be found with a density more than the order 100 atom cm −3 . If H 2 is produced at the inferred rate, then if an ordinary interstellar cloud is compressed by a factor about 100 the material will mostly take the form of H 2 . This is of much significance for the process of star formation. Moreover, this significance is likely to remain, even if the process of H 2 -formation has about the lowest efficiency that could be admitted.

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