نبذة مختصرة : International audience ; Context. Classical T Tauri stars are thought to accrete material from their surrounding protoplanetary disks through funnel flows along their magnetic field lines. The classical T Tauri stars with high accretion rates (∼10 −7 M ⊙ yr −1 ) are ideal targets for testing this magnetospheric accretion scenario in a sustained regime. Aims. We constrained the accretion-ejection phenomena around the strongly accreting northern component of the S CrA young binary system (S CrA N) by deriving its magnetic field topology and its magnetospheric properties, and by detecting ejection signatures, if any. Methods. We led a two-week observing campaign on S CrA N with the ESPaDOnS optical spectropolarimeter at the Canada-France-Hawaii Telescope. We recorded 12 Stokes I and V spectra over 14 nights. We computed the corresponding least-squares deconvolution (LSD) profiles of the photospheric lines and performed Zeeman-Doppler imaging (ZDI). We analyzed the kinematics of noticeable emission lines, namely He I λ 5876 and the first four lines of the Balmer series, which are known to trace the accretion process. Results. We found that S CrA N is a low-mass (0.8 M ⊙ ) young (∼1 Myr) and fully convective object exhibiting strong and variable veiling (with a mean value of 7 ± 2), which suggests that the star is in a strong accretion regime. These findings could indicate a stellar evolutionary stage between Class I and Class II for S CrA N. We reconstructed an axisymmetric large-scale magnetic field (∼70% of the total energy) that is primarily located in the dipolar component, but has significant higher poloidal orders. From the narrow emission component radial velocity curve of He I λ 5876, we derived a stellar rotation period of P * = 7.3 ± 0.2 days. We found a magnetic truncation radius of ∼2 R * which is significantly closer to the star than the corotation radius of ∼6 R * , suggesting that S CrA N is in an unstable accretion regime. That the truncation radius is quite smaller than the size of the Br γ line ...
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