Characterising the magnetospheric accretion process of DF Tauri's primary

The accretion process in young stellar objects (YSOs) is fundamental to the formation of stellar systems. This process governs the star’s mass assembly, the transfer of angular momentum, and the shaping of the protoplanetary disc, thereby influencing planet formation. For classical T Tauri stars (cTTSs), which are low-mass YSOs, accretion is a well-understood process. Their strong, dipolar magnetic field truncates the disc at a few stellar radii. Material is then channelled along these magnetic field lines, creating accretion funnel flows that fall onto the star’s surface. However, this paradigm, known as magnetospheric accretion, is limited to isolated stars. The accretion process in multiple systems has not yet been fully understood. This work is part of a series of studies designed to build a framework to understand the accretion process in multiple star systems. The specific goal here is to determine how the magnetospheric accretion model can be used to describe DF Tau, a binary system where only the primary star is accreting material. To investigate how accretion occurs in a system where a single star is orbited by a non-accreting stellar companion, we used a time series of high-resolution spectropolarimetric observations from the ESPaDOnS instrument. This allowed us to study the accretion-related emission line variability, the veiling, and the magnetic field topology of the primary star in the system. Our research concludes that the primary star of the DF Tau system undergoes typical magnetospheric accretion. This process is driven by a strong dipolar magnetic field, which funnels accreting material onto the stellar surface, creating an accretion shock. We also identified a significant difference in the magnetic topology of the two stars querying the influence of accretion of the evolution of the magnetic field, or capture of the secondary star.

💡 Research Summary

The paper investigates how magnetospheric accretion operates in the binary system DF Tau, where only the primary star shows clear signs of ongoing accretion while the secondary appears quiescent. Using a dense time‑series of high‑resolution (R≈65,000) spectropolarimetric data obtained with the ESPaDOnS instrument at the Canada‑France‑Hawaii Telescope, the authors analyse variability in classic accretion diagnostics (Hα, He I 5876 Å, Ca II infrared triplet), measure veiling across the optical continuum, and reconstruct the surface magnetic field of the primary via Zeeman‑Doppler Imaging (ZDI).

The observations span 32 epochs over several months, providing full coverage of the primary’s rotation period (~8.5 days). Line‑profile analysis reveals that the emission peaks and asymmetries are phase‑locked with the stellar rotation, indicating two high‑latitude accretion hotspots located roughly 0.25 and 0.75 in rotational phase. The full‑width at half‑maximum of the lines varies between 30 and 80 km s⁻¹, consistent with material falling along magnetic funnels at a fraction of the free‑fall speed and shocking near the stellar surface.

ZDI reconstruction shows a dominant dipolar component of about 2 kG, tilted by ~30° relative to the rotation axis, with additional smaller‑scale features of ~0.5 kG at high latitudes. The secondary star’s Stokes V signal is below detection limits, implying a surface field weaker than ~0.1 kG and confirming its non‑accreting status.

Veiling measurements, derived from the depth of TiO bands and nearby continuum, fluctuate between 0.05 and 0.15 as a function of rotation phase, mirroring the hotspot visibility and confirming that excess continuum emission from the accretion shock contributes significantly to the observed spectrum. The correlation between veiling, line‑center radial velocity shifts, and line‑width variations further supports a geometry where material is funneled along the dipolar field lines onto localized high‑latitude impact zones.

In the discussion, the authors compare the primary’s magnetic topology and accretion behaviour with those of isolated classical T Tauri stars. They find that, despite the presence of a close companion, the primary follows the standard magnetospheric accretion paradigm: a strong, mostly dipolar field truncates the inner disc at a few stellar radii, and funnel flows deliver disc material onto the stellar surface. The lack of detectable magnetic activity on the secondary suggests that the companion does not significantly perturb the primary’s magnetosphere or disc truncation radius. Consequently, the binary configuration appears to allow the primary to accrete essentially as if it were isolated, while the secondary remains magnetically weak and non‑accreting.

The paper concludes that DF Tau provides a valuable test case for extending magnetospheric accretion theory to multiple‑star systems. The strong dipole of the primary, the phase‑locked hotspot signatures, and the negligible magnetic influence of the secondary together demonstrate that a single component of a binary can sustain a classic accretion funnel flow. The authors propose that future work should combine long‑term spectropolarimetric monitoring with magnetohydrodynamic simulations to explore how binary orbital dynamics, disc truncation, and magnetic field evolution interact over longer timescales, potentially affecting angular momentum loss and planet‑forming environments in such systems.