Magnetic inhibition of convection in O star envelopes

TL;DR

This paper proposes a criterion for magnetic fields to suppress convection in massive O star envelopes, showing that strong magnetic fields can influence stellar evolution and explaining observed phenomena like the absence of macroturbulence.

Contribution

It introduces a new criterion for magnetic suppression of convection in massive stars and applies it to NGC 1624-2, linking magnetic field strength to stellar evolution effects.

Findings

Magnetic suppression of convection is possible up to ~55 Msun in ZAMS stars.

A magnetic field of ~16.5 kG can suppress convection in NGC 1624-2, matching observations.

Magnetic suppression can significantly alter the evolution of evolved stars.

Abstract

It has been suggested that the absence of macroturbulence in the atmosphere of NGC 1624 - 2 is due its strong magnetic field (the strongest known for a massive O star) suppressing convection in its outer layers, removing the mechanism thought responsible for the observed macroturbulence in stars with lower field strengths. Here, we develop and apply a criterion for a uniform magnetic field to suppress convection in stellar envelopes in which radiation pressure is a significant contributor to hydrostatic balance. We find upper mass limits of ~55 Msun and ~30 Msun for magnetic suppression to be possible in Zero Age Main Sequence and Terminal Age Main Sequence stars, respectively. For evolved stars, magnetic suppression of convection can significantly alter the stars' evolution. For NGC 1624 - 2, we find a polar dipole strength of 16.5 +/- 5.9 kG is required to suppress convection, in good agreement with the value ~20 kG measured by spectropolarimetry.