Towards an understanding of the Of?p star HD 191612: optical spectroscopy

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 II λ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with P orb = 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.

An ‘analytic dynamical magnetosphere’ formalism for X-ray and optical emission from slowly rotating magnetic massive stars

“…As an explicit example of the potential diagnostic power of our model, Figure 7 shows quite remarkably good fits to the observed Hα light-curve and dynamic spectra (Howarth et al 2007) of the slowly rotating magnetic O-star HD191612 (Wade et al 2011;Sundqvist et al 2012). We use the stellar parameters derived for HD191612 by Howarth et al (2007) (effective escape speed ve = 600 km/s and stellar radius R = 14.5R ), and a maximum loop-closure radius set additionally by the observed dipole magnetic field strength (Howarth et al 2007;Wade et al 2011), Rc ≈ rA ≈ η 1/4 R ≈ 3.5R for wind confinement-parameter η ≡ B 2 R 2 /(ṀB=0v∞) (ud-Doula & Owocki 2002).…”

“…The strong compression, and relatively slow infall speed, means the density is much higher than in the wind upflow, and the several 10kK equilibrium temperature means it can efficiently radiate in hydrogen emission line series, leading then to prominent optical emission in lines like Hα (Howarth et al 2007;Sundqvist et al 2012) that are much stronger than from the wind. Moreover it is cool enough that there is a significant X-ray boundfree opacity from abundant, partially ionized heavy elements like CNO and Fe, which can thus play a role in attenuating the X-rays emitted from the hot post-shock gas .…”

“…We use the stellar parameters derived for HD191612 by Howarth et al (2007) (effective escape speed ve = 600 km/s and stellar radius R = 14.5R ), and a maximum loop-closure radius set additionally by the observed dipole magnetic field strength (Howarth et al 2007;Wade et al 2011), Rc ≈ rA ≈ η 1/4 R ≈ 3.5R for wind confinement-parameter η ≡ B 2 R 2 /(ṀB=0v∞) (ud-Doula & Owocki 2002). Since RC depends only weakly on mass loss and wind terminal speed (to the 1/4 power) and all other parameters here are known from observations, we may safely keep the closure radius fixed during the fitting.…”

New findings on the prototypical Of?p stars

“…While the previous observations were taken at orbital phases φ orb = 0.83-0.96 and at phases φ cyc = 0.09-0.44 in the 538d cycle, the new exposure corresponds to a phase φ cyc = 0.13 in the line-profile/photometric cycle of 538d and a phase φ orb = 0.55 in the orbital cycle (Fig. 6, ephemeris taken from Howarth et al 2007). The new data thus enable us to check whether the flux variations are linked to the circumstellar, possibly magnetic, structures surrounding the star or to colliding winds in a binary.…”

“…In the past, stars displaying this feature were classified by Walborn (1972) in a separate category, dubbed Of?p. In recent years, this category has attracted a lot of attention, as one can see from the numerous publications (Walborn et al 2003(Walborn et al , 2004Donati et al 2006;Howarth et al 2007;Hubrig et al 2008;Nazé et al 2001Nazé et al , 2004Nazé et al , 2007Nazé et al , 2008a. They dealt with the peculiar properties of these stars (for a summary see Nazé et al 2008b) and led to a modification of the definition of these stars.…”