EARLY estimates of the maximum temperature, T M, of interstellar dust grains (IDGs) consistent with efficient H2 formation on their surface, yielded values near 10 K (refs 1-3). The expression for T M is Chemical equation represents where E H 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, n H 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 T M to be unacceptably low (see, for example, refs 5 and 4), and have led to proposals of new or revised mechanisms for interstellar H2 formation 5-9. 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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