Time Lag between Accretion and Wind Events in the T Tauri Star RY Tau

The results of spectroscopic and photometric monitoring of the classical T Tauri star RY Tau are presented. The observation series span 220 nights from 2013 to 2024. During the observation period, the star’s brightness varied within the range of V=9-11 mag. The rotation axis of the “star + accretion disk” system is tilted at a large angle, so the line of sight intersects the wind region and accreting flows in the star’s magnetosphere. Variability in the short-wavelength wing of the Halpha emission line and the profile of the D NaI resonance doublet are analyzed. It is shown that the wind and accretion flows vary on a time scale of approximately 20 days. When the predominant flow direction changes, a time lag is observed: initially, accretion increases, and after two days, absorption in the line-of-sight wind decreases. It is concluded that the spectral line profiles are formed in the magnetospheric accretion flows and the conical wind originating from the boundary of the star’s magnetosphere. The time lag is determined by the tilt of the magnetic dipole and the opening angle of the conical wind. It is assumed that RY Tau operates in an unstable propeller mode, and fluctuations in the accretion and wind flows are caused by density waves in the accretion disk.

💡 Research Summary

This paper presents the results of an extensive 11‑year spectroscopic and photometric monitoring campaign of the classical T Tauri star RY Tau, covering 220 nights between 2013 and 2024. The authors obtained high‑resolution (R≈27 000) spectra of the Hα emission line and the Na I D resonance doublet using the 2.6 m ZTSh telescope at the Crimean Astrophysical Observatory, supplemented by observations from several other facilities (1.21 m Kourovka, 2.5 m NOT, 2.2 m CAHA). Simultaneous BVRI photometry was acquired on the same nights, with typical uncertainties of ±0.02 mag.

RY Tau is a bright (V≈9–11 mag), rapidly rotating (v sin i≈52 km s⁻¹, rotation period ≈2.84 d) CTTS whose rotation axis is inclined by ≈60–65° to the line of sight. This geometry ensures that the observer’s line of sight passes through both the magnetospheric accretion streams and the inner disk wind (a conical wind launched at the magnetosphere–disk boundary). The star’s fundamental parameters are Teff≈5945 K, L≈9.6 L⊙, M≈2.0 M⊙, R≈2.9 R⊙, and the corotation radius is ≈10 R⊙.

The analysis focuses on two quantitative diagnostics extracted from each spectrum: (1) Fb, the flux measured in the short‑wavelength (blue) wing of Hα, which traces the column density of the line‑of‑sight wind; and (2) D1r, the equivalent width of the red‑shifted absorption component of the Na I D1 line, which traces the density of the magnetospheric accretion flow. The blue‑wing flux is derived from the line equivalent width scaled by the contemporaneous V magnitude, while the D1r measurement isolates the red wing (excluding the narrow interstellar component) over a 1.5 Å interval (≈76 km s⁻¹).

Time series of Fb and D1r reveal variability on a characteristic timescale of ~20 days. To determine causality, the authors constructed uniformly sampled daily series (filling gaps with empty entries) and performed a cross‑correlation analysis. Shifting the D1r series relative to Fb by ±5 days, they found the strongest positive correlation when D1r leads Fb by roughly 2 days (correlation coefficient ≈0.6, significant at the 99 % confidence level). In other words, an increase in the accretion signature is followed, after about two days, by a decrease in wind absorption (i.e., an increase in the blue‑wing flux). The reverse shift yields a weaker or negative correlation, supporting the “accretion‑first, wind‑later” scenario.

Physically, the authors interpret this lag as a consequence of the star operating in an unstable propeller regime. In this mode, the stellar magnetosphere rotates faster than the Keplerian flow at the truncation radius, so material that reaches the magnetospheric boundary is preferentially expelled in a conical wind rather than accreted. Density waves propagating through the inner disk temporarily enhance the mass loading onto the magnetic field lines, boosting the accretion stream (higher D1r). The increased loading reduces the efficiency of the magnetic propeller, leading to a brief suppression of the wind (lower wind column density, higher Fb). The ~2‑day delay corresponds to the travel time of material from the truncation radius (≈10 ± 4 R⊙, inferred from the maximum red‑shifted velocity of ≈250 km s⁻¹) to the region where the conical wind is launched and becomes optically thick in Hα.

The derived magnetospheric radius (Rm≈10 R⊙) is comparable to the corotation radius, consistent with the star being near the propeller threshold. The tilt of the magnetic dipole relative to the rotation axis is assumed to be small, while the large inclination of the system ensures that both accretion and wind signatures are observable simultaneously. The authors also note that the observed variability does not reflect the stellar rotation period; during continuous 6‑day observing windows (≈two rotations) the line profiles remain essentially unchanged, indicating axial symmetry of the flows on short timescales.

The paper situates its findings within the broader context of CTTS studies. Earlier works reported quasi‑periodic photometric variations (≈377 d) possibly linked to orbiting dust clumps, a 23‑day UV Mg I line modulation, and a 21.6‑day Hα flux periodicity attributed to density streams in the disk wind at ~0.2 AU. The present 20‑day accretion–wind cycle and the 2‑day lag add a new layer of dynamical behavior, directly linking inner‑disk density perturbations to magnetospheric processes. The authors argue that the conical wind model (Romanova et al. 2009) and the unstable propeller framework (e.g., Lii et al. 2014) together explain the observed phenomenology.

In conclusion, the study provides compelling observational evidence that RY Tau’s accretion and wind activities are causally linked with a measurable time lag, governed by the geometry of the tilted magnetosphere and the opening angle of the conical wind. The results support the view that RY Tau operates in an unstable propeller mode, with disk density waves driving episodic enhancements of accretion that temporarily suppress the wind. The work demonstrates the power of long‑term, high‑resolution monitoring combined with quantitative time‑series analysis to uncover causal relationships in young stellar objects. Future work suggested includes denser temporal coverage, multi‑line diagnostics (e.g., He I 5876 Å, Ca II IR triplet), and 3‑D magnetohydrodynamic simulations to refine the physical parameters of the propeller transition and the wind launching region.