SPIRou monitoring of the protostar V347 Aur: Binarity, magnetic fields, pulsed dynamo and accretion

We present in this paper an analysis of near-infrared observations of the 0.3 M⊙ protostar V347 Aur collected with the SPIRou high-resolution spectropolarimeter and velocimeter at the 3.6-m Canada-France-Hawaii Telescope from October 2019 to April 2023. From a set of 79 unpolarized and circularly polarized spectra of V347 Aur to which we applied Least-Squares Deconvolution (LSD), we derived radial velocities and longitudinal fields, along with their temporal variations over our monitoring campaign of 1258 d. Our data show that V347 Aur is an eccentric binary system with an orbital period 154.6 ± 0.7 d, experiencing strong to extreme accretion events near periastron. The companion is a 29.0 ± 1.6${\rm M}_{2\!_{4}}$ brown dwarf, a rare member of the brown dwarf desert of close companions around M dwarfs. We detect weak longitudinal fields (<100 G) at the surface of V347 Aur, significantly weaker than those of more evolved prototypical T Tauri stars. These fields show small-amplitude rotational modulation, indicating a mainly axisymmetric parent large-scale magnetic topology, and larger fluctuations at half the orbital period, suggesting that what we dub a ‘pulsed dynamo’ triggered by orbital motion and pulsed accretion operates in V347 Aur. Applying Zeeman-Doppler imaging to our circularly polarised LSD profiles, we find that the large-scale field of V347 Aur is mainly toroidal for most of our observations, with the toroidal component switching sign near periastron and apoastron. The weak large-scale dipole (≃ 30 G) is not able to disrupt the disc beyond 1.3 R⋆ even at the lowest accretion rates, implying longitudinally distributed (rather than localized) accretion at the surface of the protostar.