The Effects of Magnetic Fields on the Dynamics of Radiation Pressure Dominated Massive Star Envelopes

TL;DR

This study uses 3D radiation magneto-hydrodynamic simulations to show that magnetic fields significantly influence energy transport, structure, and observable properties of radiation pressure dominated massive star envelopes, especially near the iron opacity peak.

Contribution

It demonstrates how magnetic fields amplify and support turbulent stellar envelopes, enhancing radiative energy transport and causing observable oscillations and velocity fluctuations.

Findings

Magnetic fields amplify and support turbulent density fluctuations.

Magnetic buoyancy increases vertical energy transport.

Stellar envelopes shrink and exhibit oscillations due to magnetic effects.

Abstract

We use three dimensional radiation magneto-hydrodynamic simulations to study the effects of magnetic fields on the energy transport and structure of radiation pressure dominated main sequence massive star envelopes at the region of the iron opacity peak. We focus on the regime where the local thermal timescale is shorter than the dynamical timescale, corresponding to inefficient convective energy transport. We begin with initially weak magnetic fields relative to the thermal pressure, from 100-1000G in differing geometries. The unstable density inversion amplifies the magnetic field, increasing the magnetic energy density to values close to equipartition with the turbulent kinetic energy density. By providing pressure support, the magnetic field's presence significantly increases the density fluctuations in the turbulent envelope, thereby enhancing the radiative energy transport by allowing photons to diffuse out through low density regions. Magnetic buoyancy brings small scale magnetic fields to the photosphere and increases the vertical energy transport with the energy advection velocity proportional to the Alfv\'en velocity, although in all cases we study photon diffusion still dominates the energy transport. The increased radiative and advective energy transport causes the stellar envelope to shrink by several scale heights. We also find larger turbulent velocity fluctuations compared to the purely hydrodynamic case, reaching $\approx$ 100 km/s at the stellar photosphere. The photosphere also shows vertical oscillations with similar averaged velocities and periods of a few hours. The increased turbulent velocity and oscillations will have strong impacts on the line broadening and periodic signals in massive stars.