Molecular hydrogen formation on interstellar dust grains

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.