Be star discs: powered by a non-zero central torque

TL;DR

This paper proposes a physically motivated model for Be star discs using a non-zero torque boundary condition, suggesting magnetic activity on the star supplies both mass and angular momentum, better explaining disc formation and evolution.

Contribution

It introduces a non-zero torque boundary condition for Be star disc models, incorporating magnetic activity as a source of mass and angular momentum, improving upon previous zero-torque models.

Findings

Non-zero torque boundary condition better models disc evolution.

Magnetic flaring events can supply sufficient mass and angular momentum.

Small-scale magnetic fields may drive observed disc dynamics.

Abstract

Be stars are rapidly rotating B stars with Balmer emission lines that indicate the presence of a Keplerian, rotationally supported, circumstellar gas disc. Current disc models, referred to as "decretion discs", make use of the zero torque inner boundary condition typically applied to accretion discs, with the 'decretion' modelled by adding mass to the disc at a radius of about two per cent larger than the inner disc boundary. We point out that, in this model, the rates at which mass and energy need to be added to the disc are implausibly large. What is required is that the disc has not only a source of mass but also a continuing source of angular momentum. We argue that the disc evolution may be more physically modelled by application of the non-zero torque inner boundary condition of Nixon & Pringle (2020), which determines the torque applied at the boundary as a fraction of the advected angular momentum flux there and approaches the accretion and decretion disc cases in the appropriate limits. We provide supporting arguments for the suggestion that the origin of the disc material is small-scale magnetic flaring events on the stellar surface, which, when combined with rapid rotation, can provide sufficient mass to form, and sufficient angular momentum to maintain, a Keplerian Be star disc. We discuss the origin of such small-scale magnetic fields in radiative stars with differential rotation. We conclude that small-scale magnetic fields on the stellar surface, may be able to provide the necessary mass flux and the necessary time-dependent torque on the disc inner regions to drive the observed disc evolution.