Discovery of a strong magnetic field on the O star HD 191612: new clues to the future of θ 1 Orionis C? ⋆

From observations made with the ESPaDOnS spectropolarimeter, recently installed on the 3.6-m Canada–France–Hawaii Telescope, we report the discovery of a strong magnetic field in the Of?p spectrum variable HD 191612 – only the second known magnetic O star (following � 1 Ori C). The stability of the observed Zeeman signature over four nights of observation, together with the non-rotational shape of line profiles, argue that the rotation period of HD 191612 is significantly longer than the 9-d value previously proposed. We suggest that the recently identified 538-d spectral-variability period is the rotation period, in which case the observed line-of-sight magnetic field of 220 ± 38 G implies a large-scale field (assumed dipolar) with a polar strength of about 1.5 kG. If confirmed, this scenario suggests that HD 191612 is, essentially, an evolved version of the near-ZAMS magnetic O star � 1 Ori C, but with an even stronger field (about 15 kG at an age similar to that of � 1 Ori C). We suggest that the rotation rate of HD 191612, which is exceptionally slow by accepted O-star standards, could be due to angular-momentum dissipation through a magnetically confined wind.

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