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A quite remarkable aspect of non-interacting O-stars with detected surfacemagnetic fields is that they all are very slow rotators. This paper uses thisunique property to first demonstrate that the projected rotational speeds ofmassive, hot stars, as derived using current standard spectroscopic techniques,can be severely overestimated when significant "macroturbulent" line-broadeningis present. This may, for example, have consequences for deriving thestatistical distribution of rotation rates in massive-star populations, and forthe use of these rates in stellar evolution models. It is next shown how suchmacroturbulence (seemingly a universal feature of hot, massive stars) ispresent in all but one of the magnetic O-stars, namely NGC 1624-2. Assumingthen a simple model in which NGC 1624-2's exceptionally strong, large-scalemagnetic field suppresses atmospheric motions down to layers where the magneticand gas pressures are comparable, first empirical constraints on the formationdepth of this enigmatic hot-star macroturbulence are derived. The resultssuggest an origin in the thin sub-surface convection zone of massive stars,consistent with a physical origin due to, e.g., stellar pulsations excited bythe convective motions.

proceedings of the international astronomical union

Constraining general massive-star physics by exploring the unique  properties of magnetic O-stars: Rotation, macroturbulence, and sub-surface  convection