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Session 32 - General ISM.
Oral session, Wednesday, January 07
Recent experiments by Pirronello et al.(1997a,b), performed on realistic interstellar dust grain analogues in the temperature range between 5 and 18 K, have shown that: (a) in the range between 10 and 15 K molecular hydrogen formation rates are up to one order of magnitude lower than previously assumed; (b) at the lowest temperatures, upto about 9 - 10 K, mobility of adsorbed H atoms is much lower than previously believed. Tunneling by itself is not high enough to assure the required mobility and thermal activation is needed. A careful analysis of the kinetics of molecular hydrogen formation shows that two distinct physical regimes do exist. In one, characterized by low H mobility and coverage, recombination rate is low and is proportional to the square of H adatom concentration on the surface. In the other, characterized by high mobility and relatively high coverage, the probability to form molecular hydrogen is high and virtually independent on adatom coverage. Implications of this analysis for the understanding of processes dominating in the ISM are discussed.
Pirronello, V., Biham, O., Liu, C., Shen, L. and Vidali, G. 1997a, Ap.J. 483, L131 Pirronello, V., Liu, C., Shen, L., Vidali, G., 1997b, Ap.J. 475, L69.
Work supported by NASA Grant NAG5-4998.
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