Magnetic inhibition of convection in O star envelopes

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.