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Astron. Astrophys. 338, 995-1005 (1998)

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Gas and grain chemistry in a protoplanetary disk

K. Willacy * 1, H.H. Klahr 2, T.J. Millar 1 and Th. Henning 2

1 Department of Physics, UMIST, P.O. Box 88, Manchester, England, UK
2 Astrophysical Institute and University Observatory, Schillergässchen 3, D-07745 Jena, Germany

Received 21 April 1998 / Accepted 29 July 1998

Abstract

The chemistry in a protoplanetary accretion disk is modelled between a radius of 100 and 0.1 AU of the central object. We find that interaction of the gas with the dust grains is very important, both by removing a large fraction of the material from the gas in the outer regions and through the chemical reactions which can occur on the dust grain surfaces. In addition, collision with grains neutralises gaseous ions effectively and keeps the ionization fraction low. This results in a chemistry which is dominated by neutral-neutral reactions, even if ionization is provided by cosmic rays or by the decay of radioactive isotopes. We model the effects of two desorption processes with very different efficiencies and find that while these produce similar results over much of the disk for many species, some molecules are extremely sensitive to the nature of the desorption and may one day be used as an observational test for the desorption process.

Key words: stars: formation – stars: pre – main sequence – accretion, accretion disks – molecular processes – dust, extinction

* Present address: Jet Propulsion Laboratory, California Institute of Technology, MS 169-327, Pasadena CA 91109

Send offprint requests to: K. Willacy

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998
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