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.
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References
Stecher, T. P., and Williams, D. A., Nature, 219, 1349 (1968).
Field, G. B., Somerville, W. B., and Dressler, K., Ann. Rev. Astron. Astrophys., 4, 207 (1966).
Stecher, T. P., and Williams, D. A., Astrophys. J., 149, L29 (1967).
Garzoli, S. L., and Varsavsky, C. M., Astrophys. J., 145, 79 (1966).
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SOLOMON, P., WICKRAMASINGHE, N. Chemical Exchange Reactions and H2 Formation in Dark Interstellar Clouds. Nature 220, 1214–1215 (1968). https://doi.org/10.1038/2201214a0
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DOI: https://doi.org/10.1038/2201214a0
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