Importance of thermal diffusion in the gravo-magnetic limit cycle

TL;DR

This study demonstrates that thermal diffusion, especially turbulent diffusion, significantly influences the duration and frequency of accretion bursts in protoplanetary discs with dead zones, altering the conditions for these outbursts.

Contribution

It highlights the importance of including turbulent and radiative thermal diffusion in models of disc outbursts, showing their impact on burst characteristics and parameter ranges.

Findings

Turbulent diffusion reduces burst duration to less than 10^3 years.

Including heat transport extends the infall rate range for bursts.

Prandtl numbers around 10 suggest turbulence dominates heat transport.

Abstract

We consider the role of thermal diffusion due to turbulence and radiation on accretion bursts that occur in protoplanetary discs which contain dead zones. Using 1D viscous disc models we show that diffusive radial transport of heat is important during the gravo-magnetic limit cycle, and can strongly modify the duration and frequency of accretion outbursts. When the Prandtl number is large - such that turbulent diffusion of heat is unimportant - radial radiative diffusion reduces the burst duration compared to models with no diffusive transport of heat. When the Prandtl number is small ($\lesssim 25$) we find that turbulent diffusion dominates the radial transport of heat, reducing the burst duration to $\lesssim 10^3$ years as well as increasing the burst frequency. Furthermore, inclusion of radial transport of heat extends the range of infall rates under which the disc undergoes accretion bursts from $10^{-8}$ to $10^{-6}$ M$_\odot$ yr$^{-1}$ with no diffusion, to $10^{-8}$ to $\gtrsim10^{-4}$ M$_\odot$ yr$^{-1}$ with radiative and strong turbulent diffusion. The relative roles of radiative and turbulent thermal diffusion are likely to vary during an accretion burst, but simple estimates suggest that the expected Prandtl numbers are of the order of 10 in protoplanetary discs, and hence that turbulent diffusion is likely to be an important process during accretion outbursts due to the gravo-magnetic limit cycle.