Magnetospheric accretion on the fully-convective classical T Tauri star DN Tau

We report here results of spectropolarimetric observations of the classical T Tauri star DN Tau carried out (at 2 epochs) with ESPaDOnS at the Canada–France–Hawaii Telescope within the ‘Magnetic Protostars and Planets’ programme. We infer that DN Tau, with a photospheric temperature of 3950 ± 50 K, a luminosity of 0.8 ± 0.2 L_⊙ and a rotation period of 6.32 d, is a ≃2 Myr-old fully convective 0.65 ± 0.05 M_⊙ star with a radius of 1.9 ± 0.2 R_⊙, viewed at an inclination of 35 ± 10°. Clear circularly polarized Zeeman signatures are detected in both photospheric and accretion-powered emission lines, probing longitudinal fields of up to 1.8 kG (in the He i D₃ accretion proxy). Rotational modulation of Zeeman signatures, detected  both in photospheric and accretion lines, is different between our two runs, providing further evidence that fields of classical T Tauri stars are generated by non-stationary dynamos. 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 DN Tau at both epochs. We find that the magnetic topology is mostly poloidal, and largely axisymmetric, with an octupolar component (of polar strength 0.6–0.8 kG) 1.5–2.0 times larger than the dipolar component (of polar strength ≃0.3–0.5 kG). DN Tau features dominantly poleward accretion at both epochs. The large-scale dipole component of DN Tau is however too weak to disrupt the surrounding accretion disc further than 65–90 per cent of the corotation radius (at which the disc Keplerian period matches the stellar rotation period), suggesting that DN Tau is already spinning up despite being fully convective.