Dead Zones around Young Stellar Objects: Dependence on Physical Parameters

TL;DR

This paper investigates how the size of dead zones in accretion discs around young stellar objects depends on physical parameters like magnetic Reynolds number and temperature, challenging common assumptions and providing analytical models.

Contribution

It introduces analytical fits for the surface density of active layers considering higher critical magnetic Reynolds numbers, refining previous models of dead zone extent.

Findings

Surface density in active layers increases with radius for high Re_{M,crit}

Analytical fits accurately model surface density dependence on Re_{M,crit}, radius, and temperature

Metallicity variations do not significantly affect dead zone size, but disc lifetime trends do

Abstract

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).