3D MHD Simulation of a Pulsationally-Driven MRI Decretion Disc

TL;DR

This study uses 3D MHD simulations to show how pulsations on rapidly rotating, weakly magnetized stars can generate Keplerian discs with winds, supporting the viscous decretion disc model for Be star formation.

Contribution

It demonstrates that pulsationally driven magnetic processes can produce Be star discs with properties consistent with the viscous decretion disc paradigm, including a coupled wind.

Findings

Magnetic fields enable efficient angular momentum transport in the disc.

Simulations produce a Keplerian disc with a surface wind driven by magnetic torques.

Results align with 1D viscous disc models but reveal complex multi-phase structures.

Abstract

We explore the pulsationally driven orbital mass ejection mechanism for Be star disc formation using isothermal, 3D magnetohydrodynamic (MHD) and hydrodynamic simulations. Non-radial pulsations are added to a star rotating at 95\% of critical as an inner boundary condition that feeds gas into the domain. In MHD, the initial magnetic field within the star is weak. The hydrodynamics simulation has limited angular momentum transport, resulting in repeating cycles of mass accumulation into a rotationally-supported disc at small radii followed by fall-back onto the star. The MHD simulation, conversely, has efficient (Maxwell $\alpha_{\rm M}$ $\sim$ 0.04) angular momentum transport provided by both of turbulent and coherent magnetic fields; a slowly decreting midplane driven by the magnetorotational instability and a supersonic wind on the surface of the disc driven by global magnetic torques. The angle and time-averaged properties near the midplane agree reasonably well with a 1D viscous decretion disc model with a modified $\tilde\alpha=0.5$, in which the gas transitions from a subsonic thin disc to a supersonic spherical wind at the critical point. 1D models, however, cannot capture the multi-phase decretion/angular structure seen in our simulations. Our results demonstrate that, at least under certain conditions, non-radial pulsations on the surface of a rapidly rotating, weakly magnetized star can drive a Keplerian disc with the basic properties of the viscous decretion disc paradigm, albeit coupled to a laminar wind away from the midplane. Future modeling of Be star discs should consider the possible existence of such a surface wind.