Molecular hydrogen formation on interstellar dust grains

Abstract

EARLY estimates of the maximum temperature, TM, of interstellar dust grains (IDGs) consistent with efficient H2 formation on their surface, yielded values near 10 K (refs 1–3). The expression for TM is where EH is the binding energy of hydrogen to the dust grain 400 K, S is the sticking coefficient of H on the grain 0.5, v is the average velocity of H atoms in the dust clouds 105 cm s−1, nH is the hydrogen density in the cloud 102 cm−3, A is the surface area of a dust grain 10−9cm2, and τ0 6×10−13s. Observations have shown this TM to be unacceptably low (see, for example, refs 5 and 4), and have led to proposals of new or revised mechanisms for interstellar H2 formation5–9. Here we point out that for a certain class of gas–solid systems the characteristic desorption lifetime, τ0 , is many orders of magnitude longer than that used in refs 1–3. For grains made of these materials the original “physisorption model”1–3 will work for grain temperatures up to 30 K.

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COHEN, S. Molecular hydrogen formation on interstellar dust grains. Nature 261, 215–216 (1976). https://doi.org/10.1038/261215a0

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