Magnetic cycles of the planet-hosting star τ Bootis

By J. Donati, C. Moutou, R. Fares, D. Bohlender, C. Català, M. Deleuil, E. Shkolnik, A. Cameron, M. Jardine, Gordon A. H. Walker. 2008

152 citations recorded by the source corpus.

We have obtained new spectropolarimetric observations of the planet-hosting star τ Bootis, using the ESPaDOnS and NARVAL spectropolarimeters at the Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL). With this data set, we are able to confirm the presence of a magnetic field at the surface of τ Boo and map its large-scale structure over the whole star. The large-scale magnetic field is found to be fairly complex, with a strength of up to 10 G; it features a dominant poloidal field and a small toroidal component, the poloidal component being significantly more complex than a dipole. The overall polarity of the magnetic field has reversed with respect to our previous observation (obtained a year before), strongly suggesting that τ Boo is undergoing magnetic cycles similar to those of the Sun. This is the first time that a global magnetic polarity switch is observed in a star other than the Sun; given the infrequent occurrence of such events in the Sun, we speculate that the magnetic cycle period of τ Boo is much shorter than that of the Sun. Our new data also allow us to confirm the presence of differential rotation, both from the shape of the line profiles and the latitudinal shearing that the magnetic structure is undergoing. The differential rotation surface shear that τ Boo experiences is found to be 6 to 10 times larger than that of the Sun, in good agreement with recent claims that differential rotation is strongest in stars with shallow convective zones. We propose that the short-magnetic cycle period is due to the strong level of differential rotation. With a rotation period of 3.0 and 3.9 d at the equator and pole, respectively, τ Boo appears as the first planet-hosting star whose rotation (at intermediate latitudes) is synchronized with the orbital motion of its giant planet (period 3.3 d). Assuming that this synchronization is not coincidental, it suggests that the tidal effects induced by the giant planet can be strong enough to force the thin convective envelope (though not the whole star) into corotation. We also detect time-dependent activity fluctuations on τ Boo, but cannot unambiguously determine whether they are intrinsic to the star or induced by the planet; more observations of similar type are needed to determine the role of the close-in giant planet orbiting τ Boo on both the activity enhancements and the magnetic cycle of the host star.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/e7c6588b505cea8d46e80bf7a6f49608826d2fde

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In Astronomy and Astrophysics

1. J. Donati
2. J. Landstreet
3. Gordon I. Ogilvie
4. A. Barker
5. J. Morin
6. E. Shkolnik
7. D. Bohlender
8. R. Fares

Citations in the corpus

• Magnetic Fields of Nondegenerate Stars (2009, methodology): “Very recently, first evidence for global polarity switches was reported in a staro her than the Sun, namely the Jupiterhosting F8 starτ Boo (Donati et al. 2008d).”
• Transiting exoplanets from the CoRoT space mission . VI. CoRoT-Exo-3b: the first secure inhabitant of the brown-dwarf desert (2008, unknown): “The observed rotational veloc- ity of the star is indeed compatible with this hypothesis and the phenomenon has recently been reported for two other massive planets orbiting an F-star in circular orbit: τ Boo (Donati et al. 2008) and CoRoT-Exo-4b (Aigrain et al. 2008).”
• On the tidal evolution of Hot Jupiters on inclined orbits (2009, background): “For an F-dwarf (like τ Boo), it has been proposed that only the outer convective envelope (of mass fraction ∼ ?) participates in angular momentum exchange with the orbit (Marcy et al. 1997; Dobbs-Dixon et al. 2004; Donati et al. 2008).”
• Magnetic cycles of the planet-hosting star τ Bootis – II. A second magnetic polarity reversal (2009, methodology): “…a good candidate for detecting any planet-induced activity signatures (of presumably very low amplitude). τ Boo has a weak magnetic field of few Gauss (Catala et al. 2007; Donati et al. 2008), that switched its polarity in a year, what makes the study of the magnetic field of this star interesting.”
• Stellar magnetism: empirical trends with age and rotation (2014, background): “Donati et al. (2008a) suggested that the step in the saturation level between early-Ms and mid-Ms is caused by different efficiencies at producing large-scale versus smallscale fields, where rapidly-rotating mid-M dwarfs generate fields on larger spatial scales than early-M dwarfs (see also Reiners…”