Disruption of circumstellar discs by large-scale stellar magnetic fields

TL;DR

This study uses MHD simulations to show that even modest stellar magnetic fields can rapidly disrupt circumstellar discs around Be stars, explaining the lack of observed large-scale magnetic fields in these stars.

Contribution

The paper demonstrates through simulations that stellar magnetic fields of 10-100 G can significantly disrupt or destroy Be star discs, providing a new explanation for observational findings.

Findings

A 10 G magnetic field can significantly disrupt the disc.

A 100 G magnetic field can completely destroy the disc in days.

Disc disruption efficacy scales with plasma beta, independent of other stellar parameters.

Abstract

Spectropolarimetric surveys reveal that 8-10\% of OBA stars harbor large-scale magnetic fields, but thus far no such fields have been detected in any classical Be stars. Motivated by this, we present here MHD simulations for how a pre-existing Keplerian disc -- like that inferred to form from decretion of material from rapidly rotating Be stars -- can be disrupted by a rotation-aligned stellar dipole field. For characteristic stellar and disc parameters of a near-critically rotating B2e star, we find that a polar surface field strength of just 10 G can significantly disrupt the disc, while a field of 100 G, near the observational upper limit inferred for most Be stars, completely destroys the disc over just a few days. Our parameter study shows that the efficacy of this magnetic disruption of a disc scales with the characteristic plasma beta {(defined as the ratio between thermal and magnetic pressure)} in the disc, but is surprisingly insensitive to other variations, e.g. in stellar rotation speed, or the mass loss rate of the star's radiatively driven wind. The disc disruption seen here for even a modest field strength suggests that the presumed formation of such Be discs by decretion of material from the star would likely be strongly inhibited by such fields; this provides an attractive explanation for why no large-scale fields are detected from such Be stars.