Suppression of photospheric velocity fluctuations in strongly magnetic O stars in radiation-magnetohydrodynamic simulations

Context . O stars generally show clear signs of strong line broadening (in addition to rotational broadening) in their photospheric absorption lines (typically referred to as macroturbulence), believed to originate in a turbulent sub-surface zone associated with enhanced opacities due to the recombination of iron-group elements (at T ~ 150–200 kK). O stars with detected global magnetic fields also display such macroturbulence; the sole exception to this is NGC 1624-2, which also has the strongest (by far) detected field of the known magnetic O stars. It has been suggested that this lack of additional line broadening is due to NGC 1624-2’s exceptionally strong magnetic field, which might be able to suppress the turbulent velocity field generated in the iron opacity peak zone. Aims . We tested this hypothesis based on two-dimensional (2D), time-dependent, radiation magnetohydrodynamical (RMHD) box- in-a-star simulations for O stars that encapsulate the deeper sub-surface atmosphere (down to T ~ 400 kK), the stellar photosphere, and the onset of the supersonic line-driven wind in one unified approach. To study the potential suppression of atmospheric velocity fluctuations, we extended our previous non-magnetic O star radiation-hydrodynamic (RHD) simulations to include magnetic fields of varying strengths and orientations. Methods . We used MPI-AMRVAC , which is a Fortran 90 based publically available parallel finite volume code that is highly modular. We used the recently added RMHD module to perform all our simulations here. Results . For moderately strong magnetic cases (~1 kG), the simulated atmospheres are highly structured and characterised by large root-mean-square velocities, and our results are qualitatively similar to those found in previous non-magnetic studies. By contrast, we find that a strong horizontal magnetic field in excess of 10 kG can indeed suppress the large velocity fluctuations, and can thus stabilise (and thereby also inflate) the atmosphere of a typical early O star in the Galaxy. On the other hand, an equally strong radial field is only able to suppress horizontal motions, and as a consequence these models exhibit significant radial fluctuations. Conclusions . Our simulations provide an overall physical rationale as to why NGC 1624-2 with its strong ~20 kG dipolar field lacks the large macroturbulent line broadening that all other known slowly rotating early O stars exhibit. However, our simulations also highlight the importance of field geometry for controlling the atmospheric dynamics in massive and luminous stars that are strongly magnetic, tentatively suggesting latitudinal dependence of macroturbulence and basic photospheric parameters.