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

TL;DR

This study uses spectropolarimetric observations to map the magnetic field and accretion processes of the fully-convective T Tauri star DN Tau, revealing a mostly poloidal, axisymmetric magnetic topology and poleward accretion, with implications for stellar spin-up.

Contribution

It provides the first detailed magnetic topology maps of DN Tau at two epochs, demonstrating non-stationary dynamo activity and the magnetic field's role in accretion and stellar spin evolution.

Findings

Magnetic topology is mostly poloidal and axisymmetric.

The large-scale magnetic field is dominated by an octupolar component.

DN Tau's magnetic field is too weak to prevent spin-up despite being fully convective.

Abstract

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 3,950+-50 K, a luminosity of 0.8+-0.2 Lsun and a rotation period of 6.32 d, is a ~2Myr-old fully-convective 0.65+-0.05 Msun star with a radius of 1.9+-0.2 Dsun, viewed at an inclination of 35+-10degr. 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 He1 D3 accretion proxy). Rotational modulation of Zeeman signatures, detected both in photospheric and accretion lines, is different between our 2 runs, providing further evidence that fields of cTTSs 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) 1.5-2.0x 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% 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.