Unstable magnetospheric accretion on the T Tauri star TW Hya

Abstract: In this paper we present new spectropolarimetric and velocimetric observations of the prototypical classical T Tauri star TW Hya obtained with SPIRou at the Canada-France-Hawaii Telescope, expanding our previous monitoring over two new seasons (2024 and 2025). We confirm that the large-scale magnetic field of TW Hya varied with time, and find that it showed fluctuations on a timescale of about a year in addition to the longer term variations outlined in the previous study. Using Zeeman-Doppler imaging, we obtain that the large-scale field of TW Hya mostly consisted of a poloidal dipole of mean polar strength 0.83 kG, inclined at an average 17degr to the rotation axis. We also find that the radial velocities of TW Hya, once fully filtered from telluric contamination, were dominated by rotational modulation induced by activity, with residuals of 32 m/s rms. No signal from a putative close-in planet is found, with an upper limit on the planet mass ranging from 0.33 to 0.98 Mjup for distances of 0.053 to 0.41 au from the central star. Emission lines indicate that the mass accretion rate was equal to $10<sup>{-8.33\pm0.20}$ Msun/yr on average, with peak-to-peak fluctuations by a factor of ~5 from season to season. This confirms that accretion onto TW Hya is unstable, with the magnetospheric gap carved by the large-scale field at the center of the disk extending on average no further than 33-40% of the corotation radius where the disk Keplerian angular velocity equals the rotation rate at the stellar surface.

• The paper reveals that TW Hya's accretion is unstable, with rapid magnetic field variations observed over multiple seasons.
• It uses Zeeman-Doppler imaging and Gaussian Process Regression to derive detailed magnetic topologies and accretion diagnostics.
• Findings constrain inner disk planet candidates and validate MHD models predicting unstable, non-axisymmetric accretion flows.

Unstable Magnetospheric Accretion and Magnetic Field Evolution in TW Hya

Introduction and Context

The study presents an extensive spectropolarimetric and velocimetric monitoring campaign of the classical T Tauri star TW Hya, leveraging SPIRou at CFHT over multiple observing seasons (2019–2025). TW Hya is the nearest and best-characterized cTTS, with a well-studied transition disk, and is a key laboratory for understanding magnetospheric accretion, magnetic field evolution, and potential planet formation during late-stage stellar evolution. The campaign doubled the available high S/N near-IR spectra, enabling high-cadence tracing of the stellar magnetic topology and accretion processes with unprecedented temporal fidelity.

Observational Methodology

The dataset comprises 164 SPIRou spectra, reduced with three independent pipelines (Libre-ESpRIT, APERO, and LBL), with typical S/N = 140–410 in the H-band. Full Stokes I and V timeseries allowed detailed Zeeman-Doppler imaging (ZDI) and RV analysis. Atmospheric parameters and small-scale surface magnetic field intensities were modeled with ZeeTurbo. Time series of line veiling, temperature proxies, and emission/absorption line diagnostics (He I 1083 nm, Pa$\beta$, Br$\gamma$) tracked accretion behavior and inner wind variability. RVs were corrected for telluric contamination and BERV correlations via Gaussian Process Regression (GPR).

Magnetic Field Topology, Rotation, and Variability

The longitudinal field $B_\ell$ exhibited significant seasonal evolution and rotational modulation ($P_\text{rot}=3.587\pm0.009$ d), with strong quasi-periodic components on $\sim$1 yr timescales. ZDI reconstructions yielded a large-scale topology persistently dominated by a poloidal dipole (0.67–1.07 kG), inclined by $7^\circ$–$29^\circ$ with respect to the rotation axis, and octupolar components at lower amplitude (∼40% of dipole strength). The field structure was highly axisymmetric ($\gtrsim$80% poloidal energy), but showed abrupt topology changes both within and between seasons, including polarity switches and rapid morphological shifts, especially during 2024–2025.

The small-scale surface-averaged fields ($\langle B_\text{small} \rangle=2.6$–$2.9$ kG) were consistent with values from independent Zeeman broadening analyses ($\gamma$0 kG). This coherence supports scenarios in which large- and small-scale fields co-exist and evolve independently due to distributed dynamo action, differential rotation, and a nearly pole-on system orientation [see also e.g., Yadav et al., 2015, (Yadav et al., 2015)].

Accretion Regime and Temporal Accretion Variability

Line veiling and emission/absorption line diagnostics traced a highly variable but persistently present accretion flow. Median near-IR veiling was low ($\gamma$1–$\gamma$2), but both the mass accretion rate and line profile morphologies (redshifted absorption in He I 1083 nm and Pa$\gamma$3) varied substantially between and within seasons. Using updated scaling laws [Fiorellino et al. 2025, (Fiorellino et al., 25 Sep 2025)], the derived mass accretion rates were $\gamma$4 (peak-to-peak factor ∼5 by season), with the minimum value observed in 2024 ($\gamma$5).

There was no persistent rotational modulation of the accretion rate, consistent with the nearly pole-on geometry and highly unstable, variable magnetosphere–disk boundary. The He I and Pa$\gamma$6 redshifted absorption signals, however, did show rotational modulation during specific intervals, supporting models with accretion flows funneled towards localized star–disk interaction regions, but frequently disrupted by changes in the large-scale field and inner disk clumpiness.

Magnetospheric Truncation and Accretion Stability

Combining the time-averaged dipolar field strength, stellar parameters, and accretion rate, the analysis finds that the magnetospheric truncation radius ($\gamma$7 $\gamma$8) is consistently small compared to the co-rotation radius ($\gamma$9 measured, 0.33±0.06 in recent 3D MHD simulations [Romanova et al. 2025b, (Romanova et al., 23 Jan 2025)]). This ratio is a robust determinant of the accretion regime, with values ≲0.5 predicting unstable, non-axisymmetric tongues of disk material penetrating the magnetosphere (e.g., [Blinova et al., 2016, (Menabde et al., 2015)]). The observational manifestation is in rapid, aperiodic fluctuations in redshifted absorption profiles and the absence of stable periodicity in the TESS and ground-based light curves.

Planet Detection Limits and Disk–Planet Interaction

High-cadence, telluric-corrected RV timeseries, filtered for activity signals using GPR, yielded no compelling evidence for inner planets, placing stringent mass upper limits: $B_\ell$0–$B_\ell$1 at 0.053–0.41 au, under the circular, co-planar scenarios. This directly excludes candidate close-in planets previously suggested at an ∼8 d period and reduces the likelihood of Jovian-mass bodies in the inner clearing capable of strongly shaping the inner disk. At longer orbital periods, the sensitivity degrades rapidly due to temporal sampling, but the mass limits at 1–2 au are of order a few $B_\ell$2—insufficient to entirely rule out gap-forming planets inferred from disk structure [Mentiplay et al. 2019, (Mentiplay et al., 2018); Andrews et al. 2016, (Andrews et al., 2016)].

Implications and Future Directions

The high-cadence, multi-seasonal SPIRou monitoring definitively demonstrates that accretion onto TW Hya is persistently unstable, directly linking rapid magnetospheric topology evolution to observable accretion variability. The observed ratio $B_\ell$3 and detailed line-profile behavior provide observational confirmation of MHD predictions for unstable accretion, including impulsive tongue-like flows and rapid time variability [Romanova et al. 2025b, (Romanova et al., 23 Jan 2025); Blinova et al., 2016, (Menabde et al., 2015)].

The lack of confirmed close-in giant planets further constrains migration and clearing scenarios, while the combined magnetic and RV data confirm that any candidate sub-Jovians are unlikely to reach sufficient mass/accretion luminosity to modify the inner disk on dynamical timescales. The data underline the need for continued simultaneous near-IR and optical (SPIRou/ESPaDOnS) monitoring to decouple photospheric, chromospheric, and accretion-related variability, and to exploit new instrumental capabilities (e.g., VISION/Wenaokeao) for definitive magnetic–accretion–planet connection diagnostics.

Continued high-cadence ZDI and emission-line spectroscopy of TW Hya and comparison samples will refine constraints on time-dependent MHD models of unstable accretion and disk dissipation. The empirical mapping of large-scale topology lifetimes and their impact on accretion stability provides crucial benchmarks for next-generation models of PMS stellar evolution and planet-disk interaction.

Conclusion

This comprehensive campaign establishes TW Hya as a clear case of unstable magnetospheric accretion, with rapid and pronounced magnetic and accretion variability governed by the interplay between a strong, evolving large-scale field and a highly variable inner disk. The results support current MHD predictions and set stringent upper limits on the mass of inner disk planets. The interplay of dynamo action, disk truncation, and unstable accretion revealed in TW Hya stands as a paradigm for late PMS stellar evolution, and ongoing monitoring will be essential to further understand the timescale and feedbacks of accretion, magnetic field evolution, and planet formation in cTTSs.

References:

• Donati et al., "Unstable magnetospheric accretion on the T Tauri star TW Hya" (2607.11674)
• Romanova et al., 3D MHD simulations of TW Hya (Romanova et al., 23 Jan 2025)
• Fiorellino et al., PENELLOPE: Empirical accretion diagnostics (Fiorellino et al., 25 Sep 2025)
• Yadav et al., Large- and small-scale magnetic field co-existence (Yadav et al., 2015)
• Mentiplay et al., Disk gap modeling in TW Hya (Mentiplay et al., 2018)
• Andrews et al., Gaps in the TW Hya disk (Andrews et al., 2016)
• Blinova et al., Unstable accretion tongues and observable variability (Menabde et al., 2015)