Spectropolarimetric characterisation of exoplanet host stars in preparation of the $Ariel$ mission II. The magnetised wind environment of TOI-1860, DS Tuc A, and HD 63433

TL;DR

This study addresses how the magnetic activity of exoplanet host stars can impact the interpretation of exoplanet atmospheres in the Ariel mission by providing a detailed characterisation of their large-scale magnetic fields and wind environments. The authors perform a spectropolarimetric survey of 15 G–M stars using Neo-Narval, HARPSpol, and SPIRou, apply least-squares deconvolution to detect fields, and reconstruct surface magnetic maps with Zeeman-Doppler imaging for TOI-1860, DS Tuc A, and HD 63433, then drive 3D MHD wind simulations (AWSoM) using these maps to assess the wind conditions at known planets. They find 6 stars with detectable fields, reveal ZDI maps showing a mix of toroidal/poloidal and axisymmetric components (e.g., ⟨|B_V|⟩ ≈ 30–64 G), and show that the planetary orbits span sub- to super-Alfvénic regimes with TOI-1860 b mostly sub-Alfvénic while DS Tuc A b and HD 63433 b/c are trans- to super-Alfvénic; HD 63433 d exhibits increased sub-Alfvénic phases due to magnetic evolution. These results link stellar magnetic topology to wind structures and Alfvén surfaces, informing atmospheric modelling and the interpretation of Ariel observations, and motivate ongoing long-term spectropolarimetric monitoring of these targets. The framework integrates magnetic topology, wind dynamics, and planetary environments to quantify star–planet interactions and their impact on exoplanet atmospheres and observational signatures.

Abstract

We update the status of the spectropolarimetric campaign dedicated to characterise the magnetic field properties of a sample of known exoplanet-hosting stars included in the current target list of the $Ariel$ mission. We analysed spectropolarimetric data collected for 15 G-M type stars with Neo-Narval, HARPSpol, and SPIRou to assess the detectability of the large-scale magnetic field. For three stars we reconstructed the magnetic field topology and its temporal evolution via Zeeman-Doppler imaging (ZDI). Such reconstructions were then used to perform 3D MHD simulations of the stellar wind and environment impinging on the hosted exoplanets. We detected the magnetic field of six stars. Of these, we performed ZDI reconstructions for the first time of TOI-1860 and DS Tuc A, and for the second time of HD 63433, providing temporal information of its large-scale magnetic field. Consistently with previous results on young ($\rm\sim 50-100~Myr$) solar-like stars, the large-scale magnetic field is moderately strong (30-60 G on average) and complex, with a significant fraction of magnetic energy in the toroidal component and high-order poloidal components. We found the orbit of TOI-1860 b to be almost completely sub-Alfvénic, the orbits of DS Tuc A b and HD 63433 d to be trans-Alfvénic, and the orbits of HD 63433 b and c to be super-Alfvénic. We obtained marginal detections of the magnetic field for TOI-836 and TOI-2076, and detections for TOI-1136, but the number of observations is not sufficient for magnetic mapping. A magnetic star-planet connection can occur for most of TOI-1860 b's orbit. This can happen more sporadically for DS Tuc A b and HD 63433 c given the lower fraction of their orbit in the sub-Alfvénic regime. The orbit of HD 63433 c is nevertheless more sub-Alfvénic than previously simulated owing to the temporal evolution of the stellar magnetic field.

Figures (5)

• Figure 1: Joint search of differential rotation and equatorial rotation period for TOI-1860 (top) and HD 63433 (bottom). The panels illustrate the $\chi^2_r$ landscape over a grid of (P$_\mathrm{rot,eq}$,$d\Omega$) pairs, with the $1\sigma$ and $3\sigma$ contours. The best values are obtained by fitting a 2D paraboloid around the minimum, while their error bars are estimated from the projection of the $1\sigma$ contour on the respective axis Press1992.
• Figure 2: Reconstructed large-scale magnetic field maps of TOI-1860, DS Tuc A, and HD 63433 in flattened polar view. For completeness, we included our previous HD 63433 reconstruction using 2023 data Bellotti2024. From the left, the radial, azimuthal, and meridional components of the magnetic field vector are illustrated. Concentric circles represent different stellar latitudes: -30 $^{\circ}$, +30 $^{\circ}$, and +60 $^{\circ}$ (dashed lines), as well as the equator (solid line). The radial ticks are located at the rotational phases when the observations were collected. The rotational phases are computed with Eq. \ref{['eq:ephemeris']}. The colour bar indicates the polarity and strength (in G) of the magnetic field.
• Figure 3: Simulated stellar wind of TOI-1860, DS Tuc A, and HD 63433 in the $\rm x_\star-y_\star$ plane. The rotation axis lies along the positive $\rm z_\star$. The Alfvén surface is shown as a translucent surface and its intersection on the $\rm x_\star-y_\star$ plane is shown as a white curve. The colour bar indicates the total wind velocity. The orbits of the hosted exoplanets are also included as coloured ellipses. For HD 63433, we also show the simulations from the 2023 data analysed in Bellotti2024.
• Figure 4: Stellar wind conditions in the planetary frame as a function of stellar rotation phase. From the left, the columns refer to the TOI-1860, DS Tuc A, HD 63433 (2023), and HD 63433 (2024) systems. From the top, the panels show the wind density ($\rho$), relative velocity ($\rm\Delta u$), ram pressure ($\rm P_{ram}$), and Alfvén Mach number ($\rm M_A$). The sub-Alfvénic regime ($\rm M_A<1$) of the stellar wind is shown as a green shaded region. In the $\rm M_A$ panel of HD 63433, we also show the computation for planet d from 2023 data as a dashed line Bellotti2024.
• Figure 5: Time series of Stokes $V$ LSD profiles and the ZDI models for TOI-1860, DS Tuc A, and HD 63433. The observations are shown in black and the models in red. The numbers on the right indicate the rotational cycle computed from Eq. \ref{['eq:ephemeris']} using the first observation of an epoch as reference date for DS Tuc A and the median observation for TOI-1860 and HD 63433. The horizontal line represents the zero point of the profiles, which are shifted vertically based on their rotational phase for visualisation purposes.