Paper: astro-ph/0509550
Date: Mon, 19 Sep 2005 15:54:39 GMT (230kb)

Title: On the Ionisation Fraction in Protoplanetary Disks I: Comparing
Different Reaction Networks

Authors: Martin Ilgner and Richard P. Nelson

Categories: astro-ph

Comments: 21 pages, 23 figures, accepted for publication in A & A
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We calculate the ionisation fraction in protostellar disk models using a
number of different chemical reaction networks, including gas-phase and
gas-grain reaction schemes. The disk models we consider are conventional
alpha-disks, which include viscous heating and radiative cooling. The primary
source of ionisation is assumed to be X-ray irradiation from the central star.
We consider a number of gas-phase chemical networks. In general we find that
the simple models predict higher fractional ionisation levels and more
extensive active zones than the more complex models. When heavy metal atoms are
included the simple models predict that the disk is magnetically active
throughout. The complex models predict that extensive regions of the disk
remain magnetically uncoupled even with a fractional abundance of magnesium of
10(-8). The addition of submicron sized grains with a concentration of 10(-12)
causes the size of the dead zone to increase dramatically for all kinetic
models considered. We find that the simple and complex gas-grain reaction
schemes agree on the size and structure of the resulting dead zone. We examine
the effects of depleting the concentration of small grains as a crude means of
modeling the growth of grains during planet formation. We find that a depletion
factor of 10(-4) causes the gas-grain chemistry to converge to the gas-phase
chemistry when heavy metals are absent. 10(-8) is required when magnesium is
included. This suggests that efficient grain growth and settling will be
required in protoplanetary disks, before a substantial fraction of the disk
mass in the planet forming zone between 1 - 10 AU becomes magnetically active
and turbulent.

\\ ( http://arXiv.org/abs/astro-ph/0509550 , 230kb)