The surprising magnetic topology of τ Sco: fossil remnant or dynamo output?

By J. Donati, I. Howarth, M. Jardine, P. Petit, C. Català, J. Landstreet, J. Bouret, E. Alecian, J. R. Barnes, T. Forveille and 2 more. 2006

365 citations recorded by the source corpus.

We report the discovery of a medium-strength (~0.5 kG) magnetic field on the young, massive star ? Sco (B0.2V), 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 ? 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 ? 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 Research Council of Canada, the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. E-mail: donati@ast.obs-mip.fr (J-FD); idh@star.ucl.ac.uk (IDH); mmj@st-andrews.ac.uk (MMJ); petit@ast.obs-mip.fr (PP); claude.catala@obspm.fr (CC); jlandstr@uwo.ca (JDL); jean-claude.bouret@oamp.fr (J-CB); evelyne.alecian@obspm.fr (EA); jrb3@st-andrews.ac.uk (JRB); forveill@cfht.hawaii.edu (TF); fpaletou@ast.obs-mip.fr (FP); manset@cfht.hawaii.edu (NM)

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/536fab9af8ea793df650b744f840cacbc861e1a6

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

Citations in the corpus

Large-scale magnetic topologies of late M dwarfs★: Magnetic topologies of late M dwarfs (2010, methodology): “The magnetic field is decomposed into its poloidal and toroidal components and described as a set of spherical harmonics-like coefficients, as implemented by Donati et al. (2006b).”
Magnetic cycles of the planet-hosting star τ Bootis – II. A second magnetic polarity reversal (2009, methodology): “We use the most recent version of the code (Donati et al. 2006), which describes the field by its poloidal and toroidal components, both expressed in terms of spherical harmonic expansions.”
Magnetospheric accretion on the T Tauri star BP Tauri (2008, methodology): “By measuring the polarisation spectrum of a star and by monitoring its modulation throughout the whole rotation cycle, the large-scale topology of the parent surface magnetic field can be retreived (e.g., Donati 2001; Donati et al. 2006b).”
Present-day cosmic abundances - A comprehensive study of nearby early B-type stars and implications for stellar and Galactic evolution and interstellar dust models (2012, background): “Only three sample stars have a confirmed magnetic field,βCep (Henrichs et al. 2000),ζ Cas (Neiner et al. 2003) andτSco (Donati et al. 2006).”
X-ray spectroscopy of stars (2009, background): “…(1992), Chlebowski and Garmany (1991), Berghöfer et al (1996), (9)Raassen et al (2008), Bouret et al (2008), (10) Mewe et al (2003), Cohen et al (2003), Donati et al (2006b), (11) Stelzer et al (2005), Petit et al (2008), (12) Cassinelli et al (1994), Hubrig et al (2006), (13) Henrichs et al…”
Large-scale magnetic topologies of early M dwarfs (2008, methodology): “A follow-up study confirmed this point and further demonstrated that the magnetic configuration of V374 Peg is apparently stable on timescales of ≃ 1 yr (Morin et al. 2008a).”
Mass loss from hot massive stars (2008, background): “We note thatτ Sco (a prototypical weak-winded star) reachesη∗ ≈ 80 for the measured field strength of 500 G (Donati et al 2006), i.e., extreme effects are to be expected.”
Large-scale magnetic topologies of late M dwarfs⋆ (2008, methodology): “This result is further confirmed by the first detailed spectropolarimetric observations of the very active fully-convective star V374 Peg by Donati et al. (2006) and Morin et al. (2008) (hereafter M08) who measure very weak differential rotation (about 1/10th of the solar surface shear).”
Magnetic cycles of the planet-hosting star τ Bootis (2008, methodology): “Circularly polarised spectra of stars with stable magnetic topologies (e.g., chemically peculiar stars and hotter equivalents, like τ Sco, Donati et al. 2006) were observed during both runs to check that the instrument behaves properly and yields nominal field strengths and polarities.”
Magnetic Fields of Nondegenerate Stars (2009, methodology): “…of early-type stars off the main-sequence may be studied observationally by following stars of a given mass at different evolutionary stages (e.g., Donati et al. 2006b; Bouret et al. 2008); the very recent detection of unusually strong large-scale magnetic fields in active giants(Aurière et al.…”
Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph (2007, methodology): “More details about this description can be found in Donati et al. (2006). range of rotation periods (as we did in Sec.”
Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold (2008, methodology): “…here the modelling strategy of Donati & Brown (1997), including also the spherical harmonic expansion of the surface magnetic field implemented by Donati et al. (2006) in order to easily distinguish between the poloidal and toroidal components of the reconstructed magnetic field distribution.”