  Angular momentum is transported outwards through an accretion disc by
magnetohydrodynamical (MHD) turbulence thus allowing material to accrete on to
the central object. The magneto-rotational instability (MRI) requires a minimum
ionisation fraction to drive turbulence in a disc. The inner parts of the disc
around a young stellar object are sufficiently hot to be thermally ionised.
Further out, cosmic rays ionise the surface layers and a dead zone forms at the
mid-plane where the disc is too cool for the MRI to operate. The surface
density in the turbulent active layer is often assumed to be constant with
radius because the cosmic rays penetrate a constant layer. However, if a
critical magnetic Reynolds number, Re_{M,crit}, is used to determine the extent
of the dead zone, the surface density in the layer generally increases with
radius. For small critical magnetic Reynolds number of order 1, the constant
layer approximation may be a reasonable fit. However, MHD simulations suggest
the critical magnetic Reynolds number may be much larger, of order 10^4.
Analytical fits for the surface density in the magnetic active layer show that
\Sigma_m \propto Re_{M,crit}^{-2} R^{9/2} T^{\,2}$, at temperature T and radius
R, are a good fit for higher critical magnetic Reynolds number. For the
metallicity variation between our galaxy, the LMC and the SMC, there should be
no significant difference in the extent of the dead zone. Observations suggest
an increase in the lifetime of the disc with decreasing metallicity that cannot
be explained by the dead zone structure (ignoring possible differences in dust
abundances).
