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Massive, hot OB-stars show clear evidence of strong macroscopic broadening(in addition to rotation) in their photospheric spectral lines. This paperexamines the occurrence of such "macro-turbulence" in slowly rotating O-starswith strong, organised surface magnetic fields. Focusing on the CIV 5811A line,we find evidence for significant macro-turbulent broadening in all stars exceptNGC1624-2, which also has (by far) the strongest magnetic field. Instead, thevery sharp CIV lines in NGC1624-2 are dominated by magnetic Zeeman broadening,from which we estimate a dipolar field of approximately 20 kG. By contrast,magnetic broadening is negligible in the other stars (due to their weaker fieldstrengths, on order 1 kG), and their CIV profiles are typically very broad andsimilar to corresponding lines observed in non-magnetic O-stars. Quantifyingthis by an isotropic, Gaussian macro-turbulence, we derive vmac = 2.2 (+-0.9/2.2) km/s for NGC1624-2, and vmac = 20-65 km/s for the rest of the magneticsample. We use these observational results to test the hypothesis that thefield can stabilise the atmosphere and suppress the generation ofmacro-turbulence down to stellar layers where the magnetic pressure PB and thegas pressure Pg are comparable. Using a simple grey atmosphere to estimate thetemperature T0 at which PB = Pg, we find that T0 > Teff for all investigatedmagnetic stars, but that T0 reaches the ~160000 K layers associated with theiron opacity-bump in hot stars only for NGC1624-2. This is consistent with theview that the responsible physical mechanism for photospheric O-starmacro-turbulence may be stellar gravity-mode oscillations excited bysub-surface convection zones, and suggests that a sufficiently strong magneticfield can suppress such iron-bump generated convection and associatedpulsational excitation.

On magnetic inhibition of photospheric macro-turbulence generated in the iron-bump opacity zone of O-stars