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.
Highlights
Magnetic fields have a significant impact on the life of low-mass stars
We essentially focus on simple observables that can be derived from these profiles, and in particular on equivalent widths, radial velocity (RV) and longitudinal magnetic fields, and look at how they vary with time – both in terms of rotational modulation and intrinsic variability
Our paper presents dual-epoch spectropolarimetric observations of the cTTS DN Tau aimed at unveiling the large-scale magnetic topology present at the surface of the protostar, at looking for potential long-term temporal variations of this magnetic topology, and at investigating how it impacts accretion from the inner regions of the accretion disc to the stellar surface
Summary
Magnetic fields have a significant impact on the life of low-mass stars. For instance, they are known to give rise to various activity phenomena throughout the atmospheres of cool stars (depending on their mass and rotation rates), including dark spots and bright plages at stellar surfaces, high-temperature outer atmospheric layers (e.g. chromospheres and coronae), energetic flares resulting from recon-. At a later formation stage, low-mass protostars that still possess a massive accretion disc (the so-called classical T Tauri stars or cTTSs) host magnetic fields strong enough to disrupt the central regions of their discs and to control accretion through discrete funnels linking the inner discs to the stellar surfaces. In this process, magnetic fields can brake the rotation of cTTSs via a star/disc coupling mechanism whose physical nature is still a matter of speculation (e.g. Bouvier et al 2007; Mohanty & Shu 2008; Romanova et al 2011; Matt et al 2012; Zanni & Ferreira 2013). After a brief description of our dual-epoch MaPP observations (Section 2) and a short summary of what is known (and relevant to this paper) on DN Tau (Section 3), we describe in more details the rotational modulation and intrinsic variability we observe (Section 4), carry-out the modelling of the large-scale magnetic field, brightness and accretion maps of DN Tau (Section 5) and summarize the results and their implications for our understanding of magnetospheric accretion processes of cTTSs and their impact on the angular momentum evolution of low-mass stars (Section 6)
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