A MiMeS analysis of the magnetic field and circumstellar environment of the weak-wind O9 sub-giant star HD 57682
J.H. Grunhut, G.A. Wade, W.L.F. Marcolino, V. Petit, MiMeS Collaboration
aa r X i v : . [ a s t r o - ph . S R ] S e p Active OB starsProceedings IAU Symposium No. 272, 2010A.C. Editor, B.D. Editor & C.E. Editor, eds. c (cid:13) A MiMeS analysis of the magnetic field andcircumstellar environment of the weak-windO9 sub-giant star HD 57682
J.H. Grunhut , G.A. Wade , W.L.F. Marcolino , V. Petit , and theMiMeS Collaboration Kingston, Canada; Marseille, France; West Chester, USA
Abstract.
I will review our recent analysis of the magnetic properties of the O9IV star HD 57682,using spectropolarimetric observations obtained with ESPaDOnS at the Canada-France-Hawaiitelescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. Idiscuss our most recent determination of the rotational period from longitudinal magnetic fieldmeasurements and H α variability - the latter obtained from over a decade’s worth of profes-sional and amateur spectroscopic observations. Lastly, I will report on our investigation of themagnetic field geometry and the effects of the field on the circumstellar environment. Keywords. instrumentation: polarimeters, techniques: spectroscopic, stars: magnetic fields,stars: rotation, stars: individual (HD 57682)
1. Introduction
The presence of strong, globally-organized magnetic fields in hot, massive stars is rare.To date, only a handful of massive O-type stars are known to host magnetic fields. In2009, Grunhut et al. reported the discovery of a strong magnetic field in the weak-windO9IV star HD 57682 from the presence of Zeeman signatures in mean Least-SquaresDeconvolved (LSD) Stokes V profiles. Their analysis of the IU E and optical spectradetermined the following atmospheric and wind properties: T eff = 34 . g ) =4 . ± . R = 7 . +2 . − . R ⊙ , M = 17 +19 − M ⊙ , and log( ˙ M ) = − . ± . M ⊙ yr − .
2. Temporal Variability
Both the longitudinal magnetic field and H α equivalent width of HD 57682 are stronglyvariable. In addtion to our 17 ESPaDOnS observations, we’ve also utilized H α observa-tions from amateur spectroscopy from the BeSS database, as well archival ESO UVESand FEROS observations dating back over a decade. A period search of these data re-sulted in a period of ∼
31 d, consistent with the rotational period estimated by Grunhutet al. (2009). However, the magnetic data could not be reasonable phased with this pe-riod. Ultimately, adopting a period of 63.58 d (twice the period obtained from the H α data) resulted in a coherent phasing of all the data at our disposal, as shown in Fig. 1.The longitudinal magnetic field appears to vary sinusoidally, consistent with a magneticfield dominated by a strong dipolar component. The H α equivalent width shows a double-wave pattern with peak emission occurring at the magnetic crossover phases (i.e. whenthe longitudinal field is null).The photometric light curve from Hipparcos shows no apparent variability. This likelyindicates that the column density of the magnetically confined plasma is relatively lowat eclipse phases. 11920 J.H. Grunhut et al. Phase (P=63.58 d) -2000200400 B z ( G ) Phase (P=63.58 d) H α E Q W Phase (P=63.58 d) H I P M a g Figure 1.
Phased longitudinal magnetic field measurements (left), H α equivalent width vari-ation (middle), and Hipparcos photometry (right), for HD 57682. Different colours indicatedifferent epochs of observations. Figure 2. Left: χ landscape as a function of dipole field strength ( B d ) and obliquity angle ( β ).Shown are the intervals corresponding to 1, 2, 3, 4, and 5 σ using i = 22 ◦ . Right:
Phased H α residual variations relative to an LTE model. Note the variability is likely due to a magneticallyconfined wind.
3. Magnetic Geometry and Circumstellar Environment
Using the longitudinal component of the magnetic field, measured from the mean LSDStokes V and I profiles, we are able to fit a dipole model, characterized by the magneticfield strength at the poles ( B d ) and the angle of obliquity of the magnetic axis relativeto the rotation axis ( β ). Using the v sin i and radius as determined by Grunhut et al.(2009) and assuming rigid rotation, we can infer an inclination angle i = 22 +17 ◦− . Usingthis value we obtain the χ landscape as a function of B d and β shown in Fig. 2. Takinginto account the range of possible inclination angles, we find that B d = 1 − β = 94 ◦ ± ◦ .In Fig. 2 we also show the residual variations of H α phased with the adopted rota-tional period. We conclude based on the characteristics of this dynamic spectrum that themagnetic field is exerting strong confinement (confinement parameter η ∗ ∼ − ; ud-Doula & Owocki 2002) on the weak wind of HD 57682, resulting in the H α variability.However, the slow rotation is likely unable to centrifugally support a stable magneto-sphere. Therefore, the plasma that is present likely has a relatively short residence timein the magnetosphere, and must therefore be continually replenished (see Townsend etal. these proceedings). References
Grunhut et al., 2009,
MNRAS , 400, L94ud-Doula, A., Owocki S.P., 2002,