The large-scale magnetic field and poleward mass accretion of the classical T Tauri star TW Hya
Article first published online: 17 AUG 2011
© 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS
Monthly Notices of the Royal Astronomical Society
Volume 417, Issue 1, pages 472–487, October 2011
How to Cite
Donati, J.-F., Gregory, S. G., Alencar, S. H. P., Bouvier, J., Hussain, G., Skelly, M., Dougados, C., Jardine, M. M., Ménard, F., Romanova, M. M., Unruh, Y. C. and the MaPP collaboration (2011), The large-scale magnetic field and poleward mass accretion of the classical T Tauri star TW Hya. Monthly Notices of the Royal Astronomical Society, 417: 472–487. doi: 10.1111/j.1365-2966.2011.19288.x
- Issue published online: 4 OCT 2011
- Article first published online: 17 AUG 2011
- Accepted 2011 June 20. Received 2011 June 20; in original form 2011 May 25
- techniques: polarimetric;
- stars: formation;
- stars: imaging;
- stars: individual: TW Hya;
- stars: magnetic field;
- stars: rotation
We report here results of spectropolarimetric observations of the ≃8 Myr classical T Tauri star (cTTS) TW Hya carried out with ESPaDOnS at the Canada–France–Hawaii Telescope in the framework of the ‘Magnetic Protostars and Planets’ programme, and obtained at two different epochs (2008 March and 2010 March). Obvious Zeeman signatures are detected at all times, both in photospheric lines and in accretion-powered emission lines. Significant intrinsic variability and moderate rotational modulation are observed in both photospheric and accretion proxies.
Using tomographic imaging, we reconstruct maps of the large-scale field of the photospheric brightness and the accretion-powered emission at the surface of TW Hya at both epochs. We find that the magnetic topology is mostly poloidal and axisymmetric with respect to the rotation axis of the star and that the octupolar component of the large-scale field (2.5–2.8 kG at the pole) largely dominates the dipolar component. This large-scale field topology is characteristic of partly convective stars, supporting the conclusion (from evolutionary models) that TW Hya already hosts a radiative core. We also show that TW Hya features a high-latitude photospheric cool spot overlapping with the main magnetic pole (and producing the observed radial velocity fluctuations); this is also where accretion concentrates most of the time, although accretion at lower latitudes is found to occur episodically.
We propose that the relatively rapid rotation of TW Hya (with respect to AA Tau-like cTTSs) directly reflects the weakness of the large-scale dipole, no longer capable of magnetically disrupting the accretion disc up to the corotation radius (at which the Keplerian period equals the stellar rotation period). We therefore conclude that TW Hya is in a phase of rapid spin-up as its large-scale dipole field progressively vanishes.