We report the discovery of a medium-strength (∼0.5 kG) magnetic field on the young, massive star τ Sco (B0.2 V), which becomes the third-hottest magnetic star known. Circularly polarized Zeeman signatures are clearly detected in observations collected mostly with the ESPaDOnS spectropolarimeter, recently installed on the 3.6-m Canada-France-Hawaii Telescope; temporal variability is also clearly established in the polarimetry, and can be unambiguously attributed to rotational modulation with a period close to 41 d. Archival ultraviolet (UV) spectra confirm that this modulation repeats over time-scales of decades, and refine the rotation period to 41.033 ± 0.002 d.Despite the slow rotation rate of τ Sco, we none the less succeed in reconstructing the large-scale structure of its magnetic topology. We find that the magnetic structure is unusually complex for a hot star, with significant power in spherical-harmonic modes of degree up to 5. The surface topology is dominated by a potential field, although a moderate toroidal component is probably present. We fail to detect intrinsic temporal variability of the magnetic structure over the 1.5-yr period of our spectropolarimetric observations (in agreement with the stable temporal variations of the UV spectra), and infer that any differential surface rotation must be very small.The topology of the extended magnetic field that we derive from the photospheric magnetic maps is also more complex than a global dipole, and features in particular a significantly warped torus of closed magnetic loops encircling the star (tilted at about 90 • to the rotation axis), with additional, smaller, networks of closed-field lines. This topology appears to be consistent with the exceptional X-ray properties of τ Sco and also provides a natural explanation of the variability observed in wind-formed UV lines. Although we cannot completely rule out the possibility that the field is produced through dynamo processes of an exotic kind, we conclude that its magnetic field is most probably a fossil remnant from the star formation stage.Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National
Aims. We investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields. The aim of this study is to explore the weak part of the magnetic field distribution of Ap/Bp stars. Methods. Using the MuSiCoS and NARVAL spectropolarimeters at Télescope Bernard Lyot (Observatoire du Pic du Midi, France) and the cross-correlation technique Least Squares Deconvolution (LSD), we obtained 282 LSD Stokes V signatures of our 28 sample stars, in order to detect the magnetic field and to infer its longitudinal component with high precision (median σ = 40 G). Results. For the 28 studied stars, we obtained 27 detections of Stokes V Zeeman signatures from the MuSiCoS observations. Detection of the Stokes V signature of the 28th star (HD 32650) was obtained during science demonstration time of the new NARVAL spectropolarimeter at Pic du Midi. This result clearly shows that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. Furthermore, all detected magnetic fields correspond to longitudinal fields which are significantly greater than some tens of G. To better characterise the surface magnetic field intensities and geometries of the sample, we phased the longitudinal field measurements of each star using new and previously-published rotational periods, and modeled them to infer the dipolar field intensity (B d , measured at the magnetic pole) and the magnetic obliquity (β). The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G. We interpret this "magnetic threshold" as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars.
We report the detection, through spectropolarimetric observations, of a strong dipolar magnetic field of presumably fossil origin at the surface of the very young O star θ1 Ori C. The Stokes V signatures we detect are variable with time, the variations being consistent with rotational modulation. A detailed modelling of our observations indicates that this dipole field has an intensity of 1.1±0.1 kG and is inclined at 42°±6° with respect to the rotation axis (assumed to be inclined at 45° to the line of sight). We find, in particular, that the positive magnetic pole comes closest to the observer when the variable Hα emission component observed on this star reaches maximum strength. This discovery represents the first definite detection of a magnetic field in an O star, as well as the first detection of a fossil field in a very young star. We also investigate in this paper the magnetic confinement of the radiatively driven wind of θ1 Ori C in the context of the magnetically confined wind‐shock model of Babel & Montmerle. In the case of θ1 Ori C, this model predicts the formation of a large magnetosphere (extending as far as 2–3R∗), consisting of a very hot post‐shock region (with temperatures in excess of 10 MK and densities of about 1010–1011 cm‐3) generated by the strong collision of the wind streams from both stellar magnetic hemispheres, as well as a dense cooling disc forming in the magnetospheric equator. We find that this model includes most of the physics required to obtain a satisfactory level of agreement with the extensive data sets available for θ1 Ori C in the literature (and, in particular, with the recent X‐ray data and the phase‐resolved spectroscopic observations of ultraviolet and optical wind lines) provided that the mass‐loss rate of θ1 Ori C is at least 5 times smaller than that predicted by radiatively driven wind models. We finally show how new observations with the XMM or Chandra spacecraft could help us constrain this model much more tightly and thus obtain a clear picture of how magnetic fields can influence the winds of hot stars.
From observations collected with the ESPaDOnS spectropolarimeter, we report the discovery of magnetic fields at the surface of the mildly accreting classical T Tauri star (cTTS) V2129 Oph. Zeeman signatures are detected, both in photospheric lines and in the emission lines formed at the base of the accretion funnels linking the disc to the protostar, and monitored over the whole rotation cycle of V2129 Oph. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized line profiles. We reconstruct the large‐scale magnetic topology at the surface of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to be rather complex, with a dominant octupolar component and a weak dipole of strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to the rotation axis. The large‐scale field is anchored in a pair of 2‐kG unipolar radial field spots located at high latitudes and coinciding with cool dark polar spots at photospheric level. This large‐scale field geometry is unusually complex compared to those of non‐accreting cool active subgiants with moderate rotation rates. As an illustration, we provide a first attempt at modelling the magnetospheric topology and accretion funnels of V2129 Oph using field extrapolation. We find that the magnetosphere of V2129 Oph must extend to about 7R★ to ensure that the footpoints of accretion funnels coincide with the high‐latitude accretion spots on the stellar surface. It suggests that the stellar magnetic field succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of V2129 Oph. The magnetospheric geometry we derive qualitatively reproduces the modulation of Balmer lines and produces X‐ray coronal fluxes typical of those observed in cTTSs.
In this paper, we present new spectropolarimetric observations of the planet-hosting star τ Bootis, using ESPaDOnS and Narval spectropolarimeters at Canada-France-Hawaii Telescope and Telescope Bernard Lyot, respectively.We detected the magnetic field of the star at three epochs in 2008. It has a weak magnetic field of only a few gauss, oscillating between a predominant toroidal component in January and a dominant poloidal component in June and July. A magnetic polarity reversal was observed relative to the magnetic topology in 2007 June. This is the second such reversal observed in 2 years on this star, suggesting that τ Boo has a magnetic cycle of about 2 years. This is the first detection of a magnetic cycle for a star other than the Sun. The role of the close-in massive planet in the short activity cycle of the star is questioned.τ Boo has a strong differential rotation, a common trend for stars with shallow convective envelope. At latitude 40 • , the surface layer of the star rotates in 3.31 d, equal to the orbital period. Synchronization suggests that the tidal effects induced by the planet may be strong enough to force at least the thin convective envelope into corotation.τ Boo shows variability in the Ca II H & K and Hα throughout the night and on a nightto-night time-scale. We do not detect enhancement in the activity of the star that may be related to the conjunction of the planet. Further data are needed to conclude about the activity enhancement due to the planet.
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