Young Star’s 20-Day Cycle Reveals How Planets Form from Swirling Gas

Researchers investigating the dynamic interplay between accretion and stellar winds in young stars present a detailed analysis of RY Tauri, a classical T Tauri star exhibiting significant variability. Led by E. V. Babina, P. P. Petrov, and K. N. Grankin, with contributions from S. A. Artemenko, this study utilises 220 nights of spectroscopic and photometric data collected between 2013 and 2024 to reveal a crucial time lag between accretion and wind events. Their findings demonstrate that increases in accretion flow precede decreases in wind absorption by approximately two days, offering novel insight into the magnetospheric processes governing these young stellar objects. This research establishes a link between the star’s magnetic field geometry, the opening angle of its conical wind, and the observed fluctuations, suggesting RY Tauri operates in an unstable propeller mode driven by density variations within its accretion disc.

The research demonstrates a previously unobserved temporal relationship between these two phenomena, revealing a distinct sequence of events.

Analysis of the short-wavelength wing of the hydrogen-alpha emission line and the sodium resonance doublet provides crucial insights into the behaviour of gas flows around RY Tau. These spectral signatures vary on a timescale of approximately 20 days, indicating a dynamic and evolving system. Crucially, a consistent time lag has been identified when the dominant flow direction shifts: accretion onto the star demonstrably increases before a subsequent decrease in absorption within the line-of-sight wind, occurring after a two-day delay.

This precise timing suggests a direct physical connection between the inflow of material and the outflow of stellar wind. The study concludes that these spectral line profiles originate within magnetospheric accretion flows and a conical wind emanating from the boundary of the star’s magnetosphere. The observed time lag is attributed to the combined effect of the magnetic dipole’s tilt and the opening angle of the conical wind, providing a geometric explanation for the sequence of events.
Researchers propose that RY Tau operates in an unstable propeller mode, where fluctuations in accretion and wind are driven by density waves propagating through the accretion disk. Analysis focused on the short-wavelength wing of the Halpha emission line and the NaI resonance doublet to investigate variations in wind and accretion flows.

The research determined these flows change on a timescale of approximately 20 days, with a discernible time lag between changes in accretion and wind signatures. To quantify this relationship, the study employed a time series cross-correlation method using data series constructed from Heliocentric Julian Dates (HJD) paired with the equivalent width of the redshifted absorption in the D1 NaI resonance line (D1r) measured in Angstroms.
A second data series was created using HJD and the radiation flux (Fb) in the short-wavelength wing of the Halpha emission profile. Empty cells were included in the series to represent nights without observations, maintaining a consistent one-day step. Correlation coefficients were calculated by plotting Fb against D1r and then systematically shifting the D1r time series by one day relative to Fb.

This process was repeated in both directions to determine if changes in NaI lines preceded or followed changes in Halpha. The research anticipated a positive correlation, indicating stronger wind flux with stronger accretion flux, but acknowledged potential dispersion in the data. Observations were gathered in fragmented sets of 5-7 consecutive nights each month from September to March, limiting the viable time series shifts to ±4 days to maintain statistical significance.
The resulting correlation coefficients were plotted as a function of the shift, approximated by a third-degree polynomial, with 95% confidence intervals indicated. A maximum correlation was observed at a shift of -2 days, demonstrating that accretion changes precede wind changes, suggesting accretion drives the wind.

The error in the correlation coefficient was formally defined as p(1-r2)/(n-2), where r represents the correlation coefficient and n is the number of measurements. Further analysis revealed a maximum correlation coefficient of 0.69 at a shift of -2 days, remaining within the 99% confidence interval despite a reduced number of data points.

Accretion and Wind Dynamics Reveal Magnetospheric Interactions in RY Tau through observations of its circumstellar disk

Observations spanning 220 nights from 2013 to 2024 reveal that RY Tau’s brightness varies between V=9 and V=11 magnitudes. Analysis of the short-wavelength wing of the Hα emission line and the NaI resonance doublet demonstrates that wind and accretion flows exhibit variability on an approximate timescale of 20 days.

A consistent time lag is observed when the predominant flow direction shifts, with accretion initially increasing before a subsequent decrease in line-of-sight wind absorption after two days. These findings establish that spectral line profiles originate within magnetospheric accretion flows and a conical wind emanating from the star’s magnetosphere boundary.
The observed two-day time lag between accretion and wind events is attributed to the inclination of the magnetic dipole and the opening angle of the conical wind. RY Tau is considered to operate in an unstable propeller mode, where fluctuations in accretion and wind flows are driven by density variations within the accretion disk.

The study details how changes in accretion are registered before alterations in the wind, providing insight into the dynamic interplay between these phenomena. This research suggests that the observed time delay is not random, but rather a consequence of the star’s magnetic field geometry and the structure of the outflowing wind. Further investigation into the density waves within the accretion disk may clarify the mechanisms driving these observed fluctuations and the associated time lags.

Accretion-wind dynamics and magnetospheric geometry in RY Tau reveal complex interactions influencing its variability

Spectroscopic and photometric observations of the young star RY Tau, spanning eleven years, reveal a dynamic interplay between accretion and wind flows within its magnetosphere. Analysis of emission lines indicates that variations in these flows occur on timescales of approximately twenty days, with a consistent time lag observed between changes in accretion and wind absorption.

Specifically, increases in accretion are followed by decreases in wind absorption after a delay of two days. These findings support a model where spectral line profiles originate in magnetospheric accretion flows and a conical wind emanating from the boundary between the star’s magnetosphere and its surrounding disk.

The observed time lag is attributed to the geometry of the conical wind and the tilt of the star’s magnetic dipole, suggesting that the wind’s response to changes in accretion is not instantaneous but requires time to propagate outwards. RY Tau appears to operate in an unstable propeller mode, with fluctuations driven by density variations within the accretion disk.

The correlation between accretion and wind events is strong, with a correlation coefficient of 0.69 at a two-day time offset, further reinforcing the connection between these phenomena. The authors acknowledge that the variations in accretion and wind parameters do not align with the star’s rotational period, implying that the observed changes are not directly linked to the star’s rotation.

Future research could focus on more detailed modelling of the magnetospheric geometry and wind dynamics to refine the understanding of this interaction.