Dead Zones around Young Stellar Objects: FU Orionis Outbursts and Transition Discs

TL;DR

This study uses simulations to explore how dead zones in accretion discs around young stars influence outbursts like FU Orionis events and the formation of transition discs with inner holes.

Contribution

It provides a detailed model of dead zone formation and evolution, linking magnetic Reynolds number thresholds to disc stability and star growth phenomena.

Findings

Dead zones form when magnetic Reynolds number exceeds ~10^4.

Dead zones can trigger gravo-magneto instabilities leading to accretion outbursts.

Late-stage dead zones may explain observed transition discs with inner holes.

Abstract

We perform global time-dependent simulations of an accretion disc around a young stellar object with a dead zone (a region where the magneto-rotational instability cannot drive turbulence because the material is not sufficiently ionised). For infall accretion rates on to the disc of around 10^-7 Msun/yr, dead zones occur if the critical magnetic Reynolds number is larger than about 10^4. We model the collapse of a molecular gas cloud. At early times when the infall accretion rate is high, the disc is thermally ionised and fully turbulent. However, as the infall accretion rate drops, a dead zone may form if the critical magnetic Reynolds number is sufficiently large, otherwise the disc remains fully turbulent. With a dead zone the disc can become unstable to the gravo-magneto instability. The mass of the star grows in large accretion outbursts that may explain FU Orionis events. At late times there is not sufficient mass in the disc for outbursts to occur but the dead zone becomes even more prominent as the disc cools. Large inner dead zones in the later stages of disc evolution may help to explain observations of transition discs with an inner hole.