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

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: hugo.nowacki@univ-grenoble-alpes.fr
² Departamento de física, Universidade Federal de Minas Gerais, Belo Horizonte, MG 31270-901, Brazil
³ University of Tartu, Faculty of Science and Technology, Tartu Observatory, 51003 Tartu, Estonia
⁴ Department of Physics and Astronomy, Uppsala University, Box 516 75120 Uppsala, Sweden
⁵ Science Division, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

Received: 9 June 2023
Accepted: 3 August 2023

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.