Discovery of a strong magnetic field on the O star HD 191612:: new clues to the future of θ1 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 theta(1) Ori C). The stability of the observed Zeeman signature over four nights of observation, together with the non-rotational shape of line profiles, argues 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-zero-age main-sequence magnetic O star theta(1) Ori C, but with an even stronger field ( about 15 kG at an age similar to that of theta(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.