Regensburg 2007 – wissenschaftliches Programm
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DY: Fachverband Dynamik und Statistische Physik
DY 18: Brownian motion and transport II
DY 18.6: Vortrag
Dienstag, 27. März 2007, 18:00–18:15, H3
Hydrogen recombination on interstellar dust: A first-passage problem with disorder — •Ingo Lohmar and Joachim Krug — Institute for Theoretical Physics, University of Cologne, Germany
Hydrogen recombination on the surface of interstellar dust grains is both a crucial ingredient in the complex astrochemistry of gas clouds and a prime example for the effect of a confined geometry on diffusion-mediated reactions. We have studied this problem theoretically: based on the first-passage problem of two diffusing (and desorbing) random walkers on the surface, we formulate a consistent revised definition of the sweeping rate of a single atom, which allows us to introduce the spatial aspects of the problem to standard master or rate equation treatments. For simple (spherical and homogeneous) grains, the effect on the recombination efficiency can be calculated exactly and is appreciable compared to previous approximations [1].
The astrophysical puzzle in this context is that H2 recombination still occurs efficiently at temperatures higher than predicted by the theory. The commonly accepted reason is the complex surface structure of the dust grains, which leads to a spatially inhomogeneous binding strength for reactants. While numerical simulations routinely confirm this explanation, analytical results for such first-passage problems in truly disordered environments are rare and limited to certain classes of systems. Our aim is to improve the theory in this respect, both for fundamental reasons as well as in view of the applicability to astrophysics.
[1] I. Lohmar, J. Krug, Month. Not. R. Astron. Soc. 370, 1025 (2006)