Evaluating Star-Planet Interactions with Zeeman Doppler Imaging: Case Study in YZ Ceti

TL;DR

This study uses Zeeman-Doppler Imaging to map the large-scale magnetic topology of the M-dwarf YZ Ceti and tests magnetic star-planet interaction (SPI) scenarios for its innermost planet b. By extrapolating the measured field with a PFSS model and coupling it to a Weber-Davis wind, the authors predict the energy available for planet-induced electron cyclotron maser emissions and compare it to observed radio bursts, finding partial support for SPI while highlighting that the field evolution and small-scale structure can significantly affect predictions. The updated field strength (average ~225 G; peak ~560 G) lowers the predicted radio flux relative to earlier works unless the planetary dipole field is correspondingly stronger (≈15–22 G), or the stellar field is effectively enhanced by small-scale components, which may be suppressed in ZDI. Importantly, the analysis links specific polarized radio epochs to magnetic footpoints on the stellar surface, showing a recurring geometry that strengthens the case for SPI but also underscoring the need for time-resolved magnetic maps to robustly interpret radio variability over multi-year baselines.

Abstract

The recent detections of radio emission from the nearby exoplanet host, YZ Ceti, suggest that the star is possibly interacting with its rocky innermost planet. These radio emissions are characterized by strong circular polarization, and appear to repeat within consistent orbital phase windows dictated by the orbital position of YZ Ceti b. If confirmed, this interaction would provide a first means to concretely assess the magnetic field of a close-in rocky exoplanet. This kind of magnetic star-planet interaction (SPI) should depend on both the exoplanetary orbit, and the geometry of the stellar magnetic field. In this article, we report measurements of the large-scale magnetic field topology of the star YZ Ceti for the first time, and interpret the cumulative radio data sets in that context to evaluate the plausibility of magnetic SPIs. We find evidence both against and in support of the SPI hypothesis, but crucially that the measured magnetic field does not rule out SPI scenarios. However, clear evaluation of these possibilities requires more accurate assessments of the magnetic field evolution across time. We additionally suggest that YZ Ceti may be exhibiting planet-induced flaring potentially triggered by exoplanet crossings of the Alfvén surface as the planet orbit approaches the stellar magnetic equator, and YZ Ceti b experiences dramatic shifts in the ambient field, its polarity, and connectivity to the host star.

Figures (6)

• Figure 1: Time series of Stokes $V$ LSD profiles of YZ Ceti. Black clurves indicate the observations and red lines represent the ZDI models for P$_\mathrm{rot}=68.4$ d, $v_\mathrm{eq}\sin(i)=0.1$ km s$^{-1}$, $i=60^\circ$, and $\mathrm{d}\Omega=0.0$ rad d$^{-1}$. The horizontal line represents the zero point of the profiles, which are shifted vertically based on their rotational phase for visualization purposes.
• Figure 2: Reconstructed ZDI map of the large-scale magnetic field of YZ Ceti. 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 color bar indicates the polarity and strength (in G) of the magnetic field.
• Figure 3: Predicted strengths for planet-induced radio bursts from YZ Ceti require a potentially strong planetary field for YZ Ceti b to match detections with VLA (Epochs A and B). The three curves compare the previous result from Pineda2023NatAs...7..569P to this work utilizing the new magnetic field measurements of YZ Ceti. The top trend (purple) further examines the implied predictions if the measured radial magnetic field were scaled by a factor of 2. VLA sensitivity is shown for 3 min integrations.
• Figure 4: Illustrations of 3D prograde orbit for YZ Ceti b aligned with stellar rotational axis, showing strength of magnetic field in plane of sky (left) and in plane of orbit (right); the same color bar for the field is used in both panels. Left - system orientation as viewed from Earth. When the planet is in front of the star (darker black trace) the magnetic field lines connecting the exoplanet and the star can trace to visible surface footpoints. Right - system viewed from above, perpendicular to orbit plane, with the location of the Alfvén surface indicated in white. The orbit is typically in the sub-Alfvénic regime (within white contour), but may intersect the Alfvén surface. Along the magnetic equator, close to the wind current sheet where the radial field switches polarity, the weakening magnetic field pushes the Alfvén surface inward cutting a wedge across the planet orbit. The location of this equatorial wedges rotates with the stellar magnetic field, here shown at the initial ZDI epoch.
• Figure 5: A diagram of the radio epochs (see Table \ref{['tab:radio']}), with durations arranged as arcs according to the orbital phase of the exoplanet YZ Ceti b. Longer arcs correspond to Pineda2023NatAs...7..569P and shorter arcs to Trigilio2023arXiv230500809T. The blue dot shows the planet position at inferior conjunction. The circularly polarized radio detections are clustered in phase just in the lower right, denoted by different colors. Each arc shows both the monitoring length of those epochs, with burst durations as filled arcs (e.g., blue/orange subarcs). Unpolarized detections are shaded black, with non-detections shown in gray.