SIMBAD references

Massive, hot OB-stars show clear evidence of strong macroscopic broadening (in addition to rotation) in their photospheric spectral lines. This paper examines the occurrence of such `macroturbulence' in slowly rotating O-stars with strong, organized surface magnetic fields. Focusing on the Civ 5811 Å line, we find evidence for significant macroturbulent broadening in all stars except NGC 1624-2, which also has (by far) the strongest magnetic field. Instead, the very sharp Civ lines in NGC 1624-2 are dominated by magnetic Zeeman broadening, from which we estimate a dipolar field ∼ 20 kG. By contrast, magnetic broadening is negligible in the other stars (due to their weaker field strengths, on the order of 1 kG), and their Civ profiles are typically very broad and similar to corresponding lines observed in non-magnetic O-stars. Quantifying this by an isotropic, Gaussian macroturbulence, we derive vmac = 2.2 ±^0.9_2.2kms-1 for NGC 1624 and vmac ~ 20-65kms-1 for the rest of the magnetic sample.

We use these observational results to test the hypothesis that the field can stabilize the atmosphere and suppress the generation of macroturbulence down to stellar layers where the magnetic pressure PB and the gas pressure Pg are comparable. Using a simple grey atmosphere to estimate the temperature T0 at which PB = Pg, we find that T0 > Teff for all investigated magnetic stars, but that T0 reaches the ∼ 160 000 K layers associated with the iron opacity bump in hot stars only for NGC 1624-2. This is consistent with the view that the responsible physical mechanism for photospheric O-star macroturbulence may be stellar gravity-mode oscillations excited by sub-surface convection zones, and it suggests that a sufficiently strong magnetic field can suppress such iron-bump generated convection and associated pulsational excitation.