Magnetometry of the classical T Tauri star GQ Lup: non-stationary dynamos and spin evolution of young Suns

International audienceWe report here results of spectropolarimetric observations of the classical T Tauri star (cTTS) GQ Lup carried out with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT) in the framework of the 'Magnetic Protostars and Planets' (MaPP) programme, and obtained at two different epochs (2009 July and 2011 June). From these observations, we first infer that GQ Lup has a photospheric temperature of 4300 ± 50 K and a rotation period of 8.4 ± 0.3 d; it implies that it is a 1.05 ± 0.07 M⊙ star viewed at an inclination of ≃30°, with an age of 2-5 Myr and a radius of 1.7 ± 0.2 R⊙, and has just started to develop a radiative core. Large Zeeman signatures are clearly detected at all times, both in photospheric lines and in accretion-powered emission lines, probing longitudinal fields of up to 6 kG and hence making GQ Lup the cTTS with the strongest large-scale fields known as of today. Rotational modulation of Zeeman signatures, also detected both in photospheric and accretion proxies, is clearly different between our two runs; we take this as further evidence that the large-scale fields of cTTSs are evolving with time and thus that they are produced by non-stationary dynamo processes. Using tomographic imaging, we reconstruct maps of the large-scale field, of the photospheric brightness and of the accretion-powered emission at the surface of GQ Lup at both epochs. We find that the magnetic topology is mostly poloidal and axisymmetric with respect to the rotation axis of the star; moreover, the octupolar component of the large-scale field (of polar strength 2.4 and 1.6 kG in 2009 and 2011, respectively) dominates the dipolar component (of polar strength ≃1 kG) by a factor of ≃2, consistent with the fact that GQ Lup is no longer fully convective. GQ Lup also features dominantly poleward magnetospheric accretion at both epochs. The large-scale dipole component of GQ Lup is however not strong enough to disrupt the surrounding accretion disc further than about half-way to the corotation radius (at which the Keplerian period of the disc material equals the stellar rotation period), suggesting that GQ Lup should rapidly spin up like other similar partly convective cTTSs. We finally report a 0.4 km s-1 radial velocity change for GQ Lup between 2009 and 2011, suggesting that a brown dwarf other than GQ Lup B may be orbiting GQ Lup at a distance of only a few au's