Chasing star-planet magnetic interactions: the case of Kepler-78

Abstract: Observational evidence of star-planet magnetic interactions (SPMI) in compact exo-systems have been looked for in the past decades. Indeed planets in close-in orbit can be magnetically connected to their host star, and channel Alfv\'en waves carrying large amounts of energy towards the central star. The strength and temporal modulation of SPMIs are primarily set by the magnetic topology of the host star and the orbital characteristics of the planet. As a result, SPMI signals can be modulated over the rotational period of the star, the orbital period of the planet, or a complex combination of the two. The detection of SPMI thus have to rely on multiple-epochs and multiple-wavelengths observational campaigns. We present a new method to characterize SPMIs and apply it to Kepler-78, a late G star with a super-Earth on an 8.5 hours orbit. We model the corona of Kepler-78 using the large-scale magnetic topology of the star observed with Zeeman-Doppler-Imaging. We show that the closeness of Kepler-78b allows the interaction to channel energy flux densities up to a few kW m$^{-2}$ towards the central star. We show that this flux is large enough to be detectable in classical activity tracers such as H$\alpha$. It is nonetheless too weak to explain the modulation observed by \citet{Moutou2016}. We furthermore demonstrate how to predict the temporal modulation of SPMI signals in observed systems such as Kepler-78. The methodology presented here thus paves the road towards denser, specific observational campaigns that would allow a proper identification of SPMIs in compact star-planet systems.

• The paper quantifies magnetic energy channeling through 3D MHD simulations and Zeeman-Doppler Imaging, finding flux densities up to several kW m⁻².
• The paper highlights significant temporal modulation of SPMI signals driven by the interplay between stellar rotation and the planet’s ultra-short orbit.
• The paper refines detection techniques by linking SPMI variability to observable spectral activity, enhancing exoplanet magnetic characterization.

Analysis of Star-Planet Magnetic Interactions in the Kepler-78 System

This paper examines the star-planet magnetic interactions (SPMI) within the context of the compact star-planet system of Kepler-78. Utilizing a comprehensive approach that integrates observational data and advanced modeling techniques, the study aims to quantify the magnetic energy channeled between the ultra-short period planet, Kepler-78b, and its host star, a late G-type star, over an 8.5-hour orbit.

Key Findings and Methodology

A pivotal aspect of SPMI is its potential to generate detectable signatures via energy channeling through Alfvén waves from the planet to the star. The methodology underpinning this investigation leverages Zeeman-Doppler Imaging (ZDI) to characterize the star's magnetic field topology. This data serves as input to 3D magnetohydrodynamic (MHD) simulations, complemented by a potential-field source surface (PFSS) extrapolation, which reveal the complex magnetized environment surrounding the planet.

1. Energy Flux and Detection Prospects: The energy flux densities calculated range up to several kW m$^-2$, suggesting potential observability in classical stellar activity tracers such as Hα lines. The study meticulously cross-references these modeled fluctuations with prior observed modulations in the spectral lines of Kepler-78.
2. Temporal Variation Analysis: The simulation unveils significant modulation in SPMI signals, linked to the interplay of the star's rotational and the planet's orbital dynamics. This variability is a crucial factor in differentiating SPMI from intrinsic stellar magnetic activity.
3. SPMI Signal Modulation: The results imply that emission signals from SPMIs are susceptible to substantial temporal modulation dictated by the stellar rotation and the planetary orbit. This is methodically analyzed, providing groundwork for informing observational campaign timing to maximize detection probability.

Implications and Future Directions

The implications of these identified SPMI-induced signatures extend into multiple domains:

• Stellar and Planetary Characterization: The feasibility of sensing planetary magnetic fields across substantial interplanetary distances provides a novel avenue for indirect measurement of exoplanetary magnetic properties, particularly for close-in planets subjected to strong tidal and magnetic influences.
• Astrophysical Processes Exploration: Understanding SPMIs contributes to broader astrophysical knowledge concerning magnetosphere-ionosphere interactions, akin to the Jupiter-Io system on a cosmic scale, and may influence models of stellar wind dynamics and magnetic field evolution.
• Refining Detection Techniques: The insights regarding signal modulation and timescale highlight the need for concerted, multi-phase observational strategies that account for both orbital and rotational dynamics, enhancing the precision of SPMI detection methodologies.

By elucidating these interactions in the Kepler-78 system, the paper lays a foundation for future research aimed at systematically identifying and analyzing SPMIs across diverse exoplanetary environments. The authors advocate for expansive, targeted observational initiatives to discern SPMI signatures from broader stellar activity—a pursuit that stands to refine our capacity to explore magnetic phenomena in extraterrestrial systems.