Discovery of a strong magnetic field on the O star HD 191612: New clues to the future of θ1 Orionis C

Abstract

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 θ1 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 θ1 Ori C, but with an even stronger field (about 15 kG at an age similar to that of θ1 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. © 2005 The Authors. Journal compilation © 2005 RAS.