Searching for close-in planets around TWA 7 with SPIRou

TL;DR

This work investigates whether a close-in planet may reside around the young M-dwarf TWA 7 by combining SPIRou near-infrared spectropolarimetry with high-precision RV analysis. Using Zeeman-Doppler imaging on LSD Stokes I and V profiles plus photospheric temperature proxies, the authors map a predominantly poloidal, axisymmetric magnetic topology that evolves over 2019-2021, including a dipole strengthening from ~0.48 to ~0.69 kG. Radial-velocity analysis reveals a tentative planet signature at P_b ≈ 15.21 d with M_b sin i ≈ 12.5 M_Earth and a_b ≈ 0.092 au, though the signal sits near the data’s detection limit and could be affected by systematics; aliases at 20.8 and 30.4 d exist, and a 6.6 d modulation in He I is observed that may indicate star-planet interactions if a close-in planet is present. Overall, the study demonstrates SPIRou's capability to probe magnetic cycles in young stars and to place constraints on planet formation in inner disk regions, highlighting the need for longer, coordinated multi-instrument campaigns to confirm the planet and to explore dynamo activity in TWA 7.

Abstract

We outline in this paper observations of the young pre-main-sequence low-mass star TWA 7, hosting a debris disk and a distant planet. Using data collected with the near-infrared SPIRou spectropolarimeter / precision velocimeter at the Canada-France-Hawaii Telescope from early 2019 to mid 2021, we detected the magnetic field of TWA 7 from the circularly polarized Zeeman signatures and Zeeman broadening of atomic spectral lines, and the rotational modulation of its longitudinal component at the known stellar rotation period (of 5.012+-0.007 d). We then modeled the large-scale and small-scale magnetic properties of TWA 7 using Zeeman-Doppler imaging. We found that TWA 7 hosted a mainly poloidal field that significantly evolved from 2019 to 2021, the dipole component getting stronger (increasing from 0.5 kG in 2019 to 0.7 kG in 2021) and less inclined to the stellar rotation axis (from 22° in 2019 to 15° in 2021). We also analyzed the radial velocities of TWA 7 derived from the SPIRou data, and found a tentative planet signature at a period of 15.2 d (with aliases at 20.8 and 30.4 d), very close to the detection limit of our data and that would correspond to a 0.17 Mjup planet at a distance of 0.09 au if confirmed. We finally report modulation of the 1083 nm He I and 1282 nm Pa-beta lines of TWA 7 with a period of 6.6 d, different from the rotation period and potentially hinting at the presence of a close-in planet triggering star-planet interactions.

Figures (13)

• Figure 1: Longitudinal magnetic field $B_{\rm \ell}$ (top panel) and temperature variations $dT$ (bottom panel) of TWA 7 (red dots), and QP GPR fit to the data (cyan full line) with corresponding 68 percent confidence intervals (cyan dotted lines). The residuals, shown in the bottom plot of each panel, yield rms of 8.2 G and 2.9 K ($\hbox{$\chi^2_{\rm r}$}=0.67$ and 0.85). (March 1st 2019, 2020 and 2021 at noon correspond to x-values of $-$456, $-$90 and 275, respectively). A zoom on the 2021 data is shown in Fig. \ref{['fig:gpb2']}.
• Figure 2: Observed (thick black line) and modelled (thin red line) LSD Stokes $I$ (top row) and $V$ (bottom row) profiles of TWA 7, for seasons 2019, 2020 and 2021 (from left to right). Rotation cycles (counting from 0, 71, and 147, for seasons 2019, 2020, and 2021, respectively, see Table \ref{['tab:log']}) are indicated to the right of LSD profiles, and $\pm$1$\sigma$ error bars to the left of Stokes $V$ profiles.
• Figure 3: Reconstructed maps of the large-scale field of TWA 7 showing the radial, azimuthal and meridional components in spherical coordinates (left, middle and right columns, units in G), for season 2019, 2020 and 2021 (top to bottom rows, respectively). These maps, derived from the LSD Stokes $IV$ profiles of Fig. \ref{['fig:fit']} using ZDI, are displayed in a flattened polar projection down to latitude $-13$$^\circ$, with the north pole at the center and the equator depicted as a bold line. Outer ticks mark the phases of observations. Positive radial, azimuthal, and meridional fields point outwards, counterclockwise, and polewards, respectively.
• Figure 4: Raw (top), filtered (middle), and residual (bottom) RVs of TWA 7 (red dots) over the observing period. The top panel shows the MCMC fit to the data, including a QP GPR modeling of the activity and a tentative planet on a 15.21-d circular orbit (cyan). The middle panel shows the tentative planet RV signature (cyan) once activity is filtered out. The rms of the residuals is 2.8 m s$^{-1}$. A zoom on the 2021 data is shown in Fig. \ref{['fig:rv2']}.
• Figure 5: Left panel: periodogram of the raw (top), filtered (middle), and residual (bottom) RV data, including a tentative planet on a 15.21-d circular orbit in the MCMC modeling. Dashed vertical cyan lines trace the stellar rotation period and the orbital period of the tentative planet, while the dashed horizontal line indicates a 0.1% false-alarm probability Press92 in the periodogram of the RV data. No significant signal remains in the residual RVs at the main stellar and tentative planet periods (and their aliases). The orange curve depicts the periodogram of the window function. A version of this plot with frequencies on the x-axis is also provided in Fig. \ref{['fig:per2']}. Right panel: Stacked periodograms of the filtered RVs, as a function of the number of RV points included in the Fourier analysis, beginning from the first observation. The color scale indicates the logarithmic power in the periodogram. The vertical dashed line traces the orbital period of the tentative planet.