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 “ macro-turbulence ” in slowly rotating O-stars with strong , organised surface magnetic fields .
Focusing on the C iv 5811 Å line , we find evidence for significant macro-turbulent broadening in all stars except NGC 1624-2 , which also has ( by far ) the strongest magnetic field .
Instead , the very sharp C iv lines in NGC 1624-2 are dominated by magnetic Zeeman broadening , from which we estimate a dipolar field \sim 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 C iv profiles are typically very broad and similar to corresponding lines observed in non-magnetic O-stars .
Quantifying this by an isotropic , Gaussian macro-turbulence , we derive v _ { mac } = 2.2 \pm ^ { 0.9 } _ { 2.2 } km / s for NGC-1624 , and v _ { mac } \approx 20 - 65 km / s for the rest of the magnetic sample .

We use these observational results to test the hypothesis that the field can stabilise the atmosphere and suppress the generation of macro-turbulence down to stellar layers where the magnetic pressure P _ { B } and the gas pressure P _ { g } are comparable .
Using a simple grey atmosphere to estimate the temperature T _ { 0 } at which P _ { B } = P _ { g } , we find that T _ { 0 } > T _ { eff } for all investigated magnetic stars , but that T _ { 0 } reaches the \sim 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 macro-turbulence may be stellar gravity-mode oscillations excited by sub-surface convection zones , and suggests that a sufficiently strong magnetic field can suppress such iron-bump generated convection and associated pulsational excitation .