Small-scale magnetic fields of the spectroscopic binary T Tauri stars V1878 Ori and V4046 Sgr

Abstract

Aims. The goal of this study is to investigate the small-scale magnetic fields of the two spectroscopic binary T Tauri stars V1878 Ori and V4046 Sgr. This is done to complete the observational characterisation of the surface magnetic fields of these stars because only their large-scale magnetic fields have been studied with Zeeman Doppler imaging (ZDI) so far. Methods. To investigate the small-scale magnetic fields, the differential Zeeman intensification of near-infrared Ti I lines was investigated using high-resolution archival spectra obtained with the ESPaDOnS spectrograph at the CFHT. In order to study the binary components separately, the spectra were disentangled by considering observations taken at different orbital phases. The Zeeman-intensification analysis was performed based on detailed polarised radiative transfer calculations aided by the Markov chain Monte Carlo inference, treating magnetic field filling factors and other stellar parameters that could affect the spectra as free parameters. Results. The obtained average magnetic field strengths of the components of V1878 Ori are 1.33 ± 0.08 and 1.57 ± 0.09 kG, respectively. Previous ZDI studies of V1878 Ori recovered about 14 and 20% of this total magnetic field strength. For V4046 Sgr, the magnetic field strengths are 1.96 ± 0.18 and 1.83 ± 0.18 kG, respectively. In this case, about 12 and 9% of the total magnetic field strength was detected by ZDI. Conclusions. The small-scale magnetic field strengths obtained from Zeeman intensification are similar for the two components of each binary. This is in contrast to the large-scale magnetic fields obtained from ZDI investigations, performed using the same observations, which found that magnetic field strengths and topologies vary significantly in the components of the two binaries. While the large-scale field might look significantly different, the overall magnetic energy, primarily carried by the small-scale magnetic fields, appears to be quite similar. This indicates that the efficiency of the magnetic dynamo is comparable for the components of the two binaries.