Magnetic inhibition of convection in O-star envelopes

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 similar to 55 and similar to 30 M-circle dot 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 that a polar dipole strength of 16.5 +/- 5.9 kG is required to suppress convection, in good agreement with the value similar to 20 kG measured by spectropolarimetry.