2024MNRAS.535.3363D

This paper presents near-infrared spectropolarimetric and velocimetric observations of the young planet-hosting T Tauri star PDS 70, collected with SPIRou at the 3.6-m Canada-France-Hawaii Telescope from 2020 to 2024. Clear Zeeman signatures from magnetic fields at the surface of PDS 70 are detected in our data set of 40 circularly polarized spectra. Longitudinal fields inferred from Zeeman signatures, ranging from $-116$ to 176 G, are modulated on a time-scale of $3.008\pm 0.006$ d, confirming that this is the rotation period of PDS 70. Applying Zeeman-Doppler imaging to subsets of unpolarized and circularly polarized line profiles, we show that PDS 70 hosts low-contrast brightness spots and a large-scale magnetic field in its photosphere, featuring in particular a dipole component of strength 200-420 G that evolves on a time-scale of months. From the broadening of spectral lines, we also infer that PDS 70 hosts a small-scale field of $2.51\pm 0.12$ kG. Radial velocities derived from unpolarized line profiles are rotationally modulated as well, and exhibit additional longer term chromatic variability, most likely attributable to magnetic activity rather than to a close-in giant planet (with a 3$\sigma$ upper limit on its minimum mass of $\simeq$4 at a distance of $\simeq$0.2 au). We finally confirm that accretion occurs at the surface of PDS 70, generating modulated redshifted absorption in the 1083.3-nm He I triplet, and show that the large-scale magnetic field, often strong enough to disrupt the inner accretion disc up to the corotation radius, weakens as the star gets fainter and redder (as in 2022), suggesting that dust from the disc more easily penetrates the stellar magnetosphere in such phases.