Issue |
A&A
Volume 445, Number 1, January I 2006
|
|
---|---|---|
Page(s) | 205 - 222 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20053678 | |
Published online | 13 December 2005 |
On the ionisation fraction in protoplanetary disks
I. Comparing different reaction networks
Astronomy Unit, Queen Mary, University of London, Mile End Road, London E1 4NS, UK e-mail: m.ilgner@qmul.ac.uk
Received:
21
June
2005
Accepted:
13
September
2005
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
α-disks, which include viscous heating and radiative cooling.
The primary source of ionisation is assumed to be X-ray irradiation
from the central star. For most calculations we adopt a specific disk
model (with accretion rate
yr-1
and
), and examine the predictions made by the
chemical networks concerning the ionisation fraction, magnetic Reynolds
number, and spatial extent of magnetically active regions. This is to
aid comparison between the different chemical models.
We consider a number of gas-phase chemical networks. The simplest is
the five species model proposed by Oppenheimer & Dalgarno (1974). We
construct more complex models by extracting species and reactions from
the UMIST data base. 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 (“dead”) even when the fractional abundance of
magnesium
. This is because of the large number of
molecular ions that are formed, which continue to dominate the
recombination with free electrons in the presence of magnesium.
The addition of submicron sized grains with a concentration
of
causes the size of the “dead zone” to increase
dramatically for all kinetic models considered, as the grains are highly
efficient at sweeping up the free electrons. We find that the simple and
complex gas-grain reaction schemes agree on the size and structure of the
resulting “dead zone”, as the grains play a dominant role in determining
the ionisation fraction. 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. When magnesium is included a depletion factor of
10-8 is required to reproduce the gas-phase ionisation fraction. 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. Only after this has occurred can gas-phase chemical models be used
to predict reliably the ionisation degree in protoplanetary disks.
Key words: accretion, accretion disks / magnetohydrodynamics (MHD) / solar system: formation / stars: pre-main sequence
© ESO, 2005
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