Monitoring the magnetospheric accretion of the classical T Tauri star DO Tau with SPIRou

Abstract

We present observations of the classical T Tauri star DO Tau collected with the near-infrared SPIRou spectropolarimeter and precision velocimeter at the Canada-France-Hawaii Telescope from early 2020 to late 2025. Circularly polarized Zeeman signatures were clearly detected at most epochs in the atomic spectral lines of DO Tau, yielding longitudinal magnetic fields of up to 280 G modulated with a period of 5.128+-0.002 d which we identified as the rotation period of DO Tau. Applying Zeeman-Doppler imaging to the SPIRou data recorded in 2021, 2024 and 2025, we found that DO Tau hosts an unusual large-scale magnetic field that is weaker, less poloidal, more inclined to the rotation axis, and varies more rapidly with time than those of previously studied T Tauri stars, possibly as a result of intense accretion between the inner disk and the stellar surface. The dipole component of this large-scale field of about 0.2-0.3 kG even flipped polarity toward the end of our observing campaign, making DO Tau the first T Tauri star for which a magnetic polarity reversal is reported. The magnetospheric gap surrounding the central star was quite compact, extending to ~1.6 Rstar (0.014 au) as a result of the strong accretion rate (log Mdot = -7.7 Msun/yr), with the inner accretion disk being warped by the tilted stellar magnetic field. Radial velocity variations suggest the presence of a close-in planet of a few Mjup or a density structure in the inner accretion disk at an orbital period of 21 d (corresponding to 0.12 au), which might be linked to the wiggle in the jet axis of DO Tau.

Figures (16)

• Figure 1: Longitudinal magnetic field $B_{\rm \ell}$ (red dots), and QP GPR fit to the data (full cyan line) with corresponding 68% confidence intervals (dotted cyan lines). The bottom panel zooms in on the 2025 data. The residuals, shown in the bottom plot of each panel, have an rms of 25 G ($\hbox{$\chi^2_{\rm r}$}=1.8$).
• Figure 2: Observed (thick black line) and modeled (thin red line) LSD Stokes $I$ (top row) and $V$ (bottom row) profiles of DO Tau in 2021, 2024, and 2025 (from left to right). Rotation cycles (counting from 60, 279, and 388 in 2021, 2024, and 2025, respectively, see Table \ref{['tab:log']}) are indicated to the right of the LSD profiles, and $\pm$1$\sigma$ error bars are shown to the left of the Stokes $V$ profiles.
• Figure 3: Reconstructed maps of the large-scale field of DO Tau showing the radial, azimuthal and meridional components in spherical coordinates (left, middle and right columns, units in G), for seasons 2021, 2024, and 2025 (top to bottom rows, respectively). These maps, derived from the LSD Stokes $I$ and $V$ profiles of Fig. \ref{['fig:fit']} using ZDI, are displayed in a flattened polar projection down to latitude $-45$$^\circ$, with the north pole at the center and the equator depicted as a bold line. Outer ticks mark the phases of observations. Positive radial, azimuthal, and meridional fields point outward, counterclockwise, and poleward, respectively.
• Figure 4: Raw (top), filtered (middle), and residual (bottom) LBL RVs of DO Tau (red dots) over the observing period. The top panel shows the MCMC fit to the data, including a QP GPR modeling of the activity and a candidate planet on a 21.14 d circular orbit (cyan). The middle panel shows the planet RV signature (cyan) after activity was filtered out. The rms of the residuals is 0.21 km s$^{-1}$. A zoom-in on the 2025 data is shown in Fig. \ref{['fig:rv2']}.
• Figure 5: Standard and stacked Lomb-Scargle periodograms of the RV data. Left panel: Periodogram of the raw (top), filtered (middle), and residual (bottom) LBL RV data, including a candidate planet on a 21.14 d circular orbit in the MCMC modeling. The dashed vertical cyan lines trace the stellar rotation period and the candidate planet orbital period, and the dashed horizontal lines indicate a 0.1% FAP level in the periodogram of the RV data. The orange curve depicts the periodogram of the window function. Right panel: Stacked periodograms of the filtered LBL RVs, as a function of the number of RV points included in the Fourier analysis. The color-scale indicates the logarithmic power in the periodogram. The vertical dashed line traces the candidate planet orbital period.