SETI public: On the Ionisation Fraction in Protoplanetary Disks I and II

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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
\\
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)

Paper: astro-ph/0509553
Date: Mon, 19 Sep 2005 16:37:52 GMT (72kb)

Title: On the Ionisation Fraction in Protoplanetary Disks II: The Effect of
Turbulent Mixing on Gas--phase Chemistry

Authors: Martin Ilgner and Richard P. Nelson

Categories: astro-ph

Comments: 11 pages, 7 figures; accepted for publication in A & A
\\
We calculate the ionisation fraction in protostellar disk models using two
different gas-phase chemical networks, and examine the effect of turbulent
mixing by modelling the diffusion of chemical species vertically through the
disk. The aim is to determine in which regions of the disk gas can couple to
a
magnetic field and sustain MHD turbulence. We find that the effect of
diffusion
depends crucially on the elemental abundance of heavy metals (magnesium)
included in the chemical model. In the absence of heavy metals, diffusion
has
essentially no effect on the ionisation structure of the disks, as the
recombination time scale is much shorter than the turbulent diffusion time
scale. When metals are included with an elemental abundance above a
threshold
value, the diffusion can dramatically reduce the size of the magnetically
decoupled region, or even remove it altogther. For a complex chemistry the
elemental abundance of magnesium required to remove the dead zone is 10(-10)
-
10(-8). We also find that diffusion can modify the reaction pathways, giving
rise to dominant species when diffusion is switched on that are minor
species
when diffusion is absent. This suggests that there may be chemical
signatures
of diffusive mixing that could be used to indirectly detect turbulent
activity
in protoplanetary disks. We find examples of models in which the dead zone
in
the outer disk region is rendered deeper when diffusion is switched on.
Overall
these results suggest that global MHD turbulence in protoplanetary disks may
be
self-sustaining under favourable circumstances, as turbulent mixing can help
maintain the ionisation fraction above that necessary to ensure good
coupling
between the gas and magnetic field.

\\ ( http://arXiv.org/abs/astro-ph/0509553 , 72kb)

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