Viscous decretion discs around rapidly rotating stars

TL;DR

This paper models steady viscous decretion discs around rapidly rotating stars, showing how angular momentum transfer from various mechanisms can sustain large, stable discs extending beyond ten times the stellar radius.

Contribution

It introduces a new model for star-disc systems driven by angular momentum transfer via low frequency modes, expanding understanding of decretion disc formation around fast rotators.

Findings

Discs can extend beyond 10 stellar radii.

Steady solutions depend on angular momentum supply.

Multiple disc configurations are possible.

Abstract

We discuss steady viscous Keplerian decretion discs around rapidly rotating stars. We assume that low frequency modes, which may be excited by the opacity bump mechanism, convective motion in the core, or tidal force if the star is in a binary system, can transport an enough amount of angular momentum to the region close to the stellar surface. Under this assumption, we construct a star-disc system, in which there forms a viscous decretion disc around a rapidly rotating star because of the angular momentum supply. We find a series of solutions of steady viscous decretion discs around a rapidly rotating star that extend to $R_{\rm disc}\gtsim~10R_*$ with $R_*$ being the equatorial radius of the star, depending on the amount of angular momentum supply.