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Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.212k&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
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1
Scientific paper
The deuterium enrichment of many molecules in molecular clouds currently is a topic of intense debate. Theories of molecule deuteration in molecular clouds mostly deal with gas-phase related processes. However, often it is hard to explain the high abundances of multi-deuterated species even when relatively extreme physical conditions are applied. We propose a completely different mechanism of deuteration enrichment - the diffusion of hydrogen in the frozen mantles of interstellar dust grains. This mechanism may be explained as follows. Firstly, many hydrogen-rich molecules are accumulated in a porous grain mantle. Cosmic-ray induced photons dissociate these molecules, producing smaller fragments, including H and H_2. Unlike other species, hydrogen is capable of tunneling right through the amorphous mantle lattice and escaping to surface and gas phases. We argue that this outward diffusion is more important than the inward diffusion into the mantle from the surface. The heavier deuterium has a significantly lower diffusion rate and it mostly remains in the mantle. This leads to overall depletion of hydrogen in the mantle and a relative enrichment of deuterium. Secondly, cosmic rays and cosmic-ray induced photons release light, mobile molecules on the surfaces of the pores in the mantle. They provide an active chemistry that ensures the interchange of deuterium and other atoms between the molecules. The active subsurface chemistry and diffusion of hydrogen leads to a unique mantle-level molecular composition that perhaps might be regarded as the third chemical phase of the interstellar medium. We present a simple model to provide a rough evaluation of this mechanism of deuteration enrichment.
Kalvans J.
Shmeld I.
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