Protostellar accretion discs have cool, dense midplanes where externally
originating ionisation sources such as X-rays or cosmic rays are unable to
penetrate. This suggests that for a wide range of radii, MHD turbulence can
only be sustained in the surface layers where the ionisation fraction is
sufficiently high. A dead zone is expected to exist near the midplane, such
that active accretion only occurs near the upper and lower disc surfaces.
Recent work, however, suggests that under suitable conditions the dead zone may
be enlivened by turbulent transport of ions from the surface layers into the
dense interior.
In this paper we present a suite of simulations that examine where, and under
which conditions, a dead zone can be enlivened by turbulent mixing. We use
three-dimensional, multifluid shearing box MHD simulations, which include
vertical stratification, ionisation chemistry, ohmic resistivity, and
ionisation due to X-rays from the central protostar. We compare the results of
the MHD simulations with a simple reaction-diffusion model.
The simulations show that in the absence of gas-phase heavy metals, such as
magnesium, turbulent mixing has essentially no effect on the dead zone. The
addition of a relatively low abundance of magnesium, however, increases the
recombination time and allows turbulent mixing of ions to enliven the dead zone
completely beyond a distance of 5 AU from the central star, for our particular
disc model. During the late stages of protoplanetary disc evolution, when small
grains have been depleted and the disc surface density has decreased below its
high initial value, the structure of the dead zone may be significantly altered
by the action of turbulent transport.Comment: 20 pages, 11 figures, accepted for publication in A&A, high
resolution pdf available at
http://www.maths.qmul.ac.uk/~rpn/preprints/index.htm