Star-disk interactions in the strongly accreting T Tauri star S CrA N

Abstract

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 λ5876 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 λ5876, 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 Brγ line emitting region, as measured with the GRAVITY interferometer (∼8 R_*), supports the presence of outflows, which is nicely corroborated by the line profiles presented in this work.
Conclusions: The findings from spectropolarimetry are complementary to those provided by optical long-baseline interferometry, allowing us to construct a coherent view of the innermost regions of a young, strongly accreting star. The strong and complex magnetic field reconstructed for S CrA N is inconsistent with the observed magnetic signatures of the emission lines associated with the postshock region, however. We recommend a multitechnique synchronized campaign of several days to place more constrains on a system that varies on a timescale of about one day.

PI (Perraut) Program 18AF12. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.