Chemical Exchange Reactions and H₂ Formation in Dark Interstellar Clouds

Abstract

Stecher and Williams¹ have sought to explain the lack of 21 cm radiation in dark clouds on the basis of the formation of molecular H₂. We wish to point out several serious deficiencies in their proposed production mechanism. Their chemical exchange process gives an H₂ production rate¹ This rate is extremely sensitive to the assumed values of ⁿH and T. With their choice of values ⁿH = 10⁴ cm⁻³, T = 100.0° K, the timescale for complete recombination in the absence of dissociation is ∼ 10⁸ yr—already considerably longer than the time for gravitational collapse at this density. A lower density ⁿH < 10³ cm⁻³ or a slightly lower gas temperature T ≲ 80° K (which, indeed, would be appropriate if cooling by H₂ molecules is included) increases the recombination timescale to ≳ 10¹⁰ yr! These results are not at all surprising because Stecher and Williams adopt the highly dangerous procedure of using the extreme tail of an exponential distribution.