Towards an understanding of the Of?p star HD 191612: optical spectroscopy
Walborn, Nolan R.; Donati, J. -F.; Herrero, A.; Morel, T.; Nazé, Yaël; Reig, P.; Howarth, Ian D.; Antokhin, I.; Rauw, G.; Negueruela, I.; Bohlender, D.; Puls, Joachim; Kolka, I.; Lennon, Danny J.; Annuk, K.; Bond, H.; Georgiev, L.; Gies, D.; Harmer, D.; McDavid, D.
United Kingdom, United States, Spain, Germany, Belgium, Estonia, Russia, Canada, France, Mexico, Greece
Abstract
We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe-O8fp). The Balmer and He I lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He II absorptions and metal lines (including many selective emission lines but excluding He IIλ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with Porb= 1542 d, e= 0.45. We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a ~O8 giant with a ~B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.