2008MNRAS.386..688J

The nature of the magnetic coupling between T Tauri stars and their discs determines not only the mass accretion process but possibly the spin evolution of the central star. We have taken a recently published surface magnetogram of one moderately accreting T Tauri star (V2129 Oph) and used it to extrapolate the geometry of its large-scale field. We determine the structure of the open (wind-bearing) field lines, the closed (X-ray bright) field lines and the relatively small subset of field lines that pass through the equatorial plane inside the Keplerian corotation radius and which are therefore available to accrete.

We consider a series of models in which the stellar magnetic field is opened up by the outward pressure of the hot coronal gas at a range of radii or source surfaces. As the source surface is increased, accretion takes place along progressively simpler field structures and impacts on progressively fewer sites at the stellar surface. This is consistent with the observed variation in the Caii IRT and Hei lines which suggests that accretion in the visible hemisphere is confined to a single high-latitude spot. By determining the density and velocity of the accretion flows, we find that in order to have most of the total mass accretion rate impacting on a single high-latitude region we need disc material to accrete from approximately 7R_solar, close to the Keplerian corotation radius at 6.8R_solar.

We also calculate the coronal density and X-ray emission measure. We find that both the magnitude and rotational modulation of the emission measure increase as the source surface is increased. For the field structure of V2129 Oph which is dominantly octupolar, the emission forms a bright, high-latitude ring that is always in view as the star rotates. Since the accretion funnels are not dense enough to cause significant scattering of coronal X-ray photons, they provide only a low rotational modulation of around 10 per cent at most.