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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents



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ID: 225835.0, MPI für Astronomie / Publikationen_mpia
Reduction of chemical networks. II. Analysis
of the fractional ionisation in protoplanetary
discs
Authors:Semenov, D.; Wiebe, D.; Henning, Th.
Date of Publication (YYYY-MM-DD):2004
Title of Journal:Astronomy and Astrophysics
Volume:417
Start Page:93
End Page:106
Review Status:not specified
Audience:Not Specified
Abstract / Description:We analyse the evolution of the fractional
ionisation in a steady-state protoplanetary disc
over 106 yr. We consider a disc model
with a vertical temperature gradient and with
gas-grain chemistry including surface reactions.
The ionisation due to stellar X-rays, stellar
and interstellar UV radiation, cosmic rays and
radionuclide decay is taken into account. Using
our reduction schemes as a tool for the
analysis, we isolate small sets of chemical
reactions that reproduce the evolution of the
ionisation degree at representative disc
locations with an accuracy of 50%-100%. On the
basis of fractional ionisation, the disc can be
divided into three distinct layers. In the dark
dense midplane the ionisation degree is
sustained by cosmic rays and radionuclides only
and is very low, ‚‚lap;10-12. This
region corresponds to the so-called ``dead
zone'' in terms of the angular momentum
transport driven by MHD turbulence. The
ionisation degree can be accurately reproduced
by chemical networks with about 10 species and a
similar number of reactions. In the intermediate
layer the chemistry of the fractional ionisation
is driven mainly by the attenuated stellar
X-rays and is far more complicated. For the first
time, we argue that surfacehydrogenation of long
carbon chains can be of crucial importance for
the evolution of the ionisation degree in
protoplanetary discs. In the intermediate layer
reduced networks contain more than a 100 species
and hundreds of reactions. Finally, in the
unshielded low-density surface layer of the disc
the chemical life cycle of the ionisation degree
comprises a restricted set of
photoionisation-recombination processes. It is
sufficient to keep about 20 species and reactions
in reduced networks. Furthermore, column
densities of key molecules are calculated and
compared to the results of other recent studies
and observational data. The relevance of our
results to the MHD modelling of protoplanetary
discs is discussed.
Free Keywords:astrochemistry; stars: formation; molecular
; processes; ISM: molecules; ISM: abundances
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
External Affiliations:K‚‚ouml;nigstuhl 17, 69117 Heidelberg, Germany
<EMAIL>semenov@mpia-hd.mpg.de</EMAIL>),
AB(Institute of Astronomy of the RAS,
Pyatnitskaya St. 48, 119017 Moscow, Russia
<EMAIL>dwiebe@inasan.ru</EMAIL>), AC(Max Planck
Institute for Astronomy, K‚‚ouml;nigstuhl 17,
69117 Heidelberg, Germany
<EMAIL>henning@mpia.de</EMAIL>)
Identifiers:URL:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bi... [ID No:1]
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