A Radio Search for Star-Planet Interaction in TOI-540 and SPECULOOS-3

TL;DR

Targets deep centimeter-band radio searches for SPI in two nearby exoplanet systems (SPECULOOS-3 and TOI-540) using VLA and MeerKAT. The study reports non-detections, establishing 3σ upper limits that challenge SPI under sub-Alfvénic and stretch-and-break models given plausible stellar magnetic fields. By modeling ECMI flux across a range of planetary magnetic-field strengths, the work shows the observations could probe sub-Gauss planetary fields if SPI is beamed toward Earth, while geometry and phase coverage limit definitive conclusions. The results refine constraints on SPI occurrence around M-dwarf hosts and underscore the need for next-generation facilities (SKA/ngVLA) to achieve detections.

Abstract

We present the first targeted centimeter-band radio observations of two recently-discovered exoplanet systems that are prime candidates for magnetic star-planet interaction (SPI): TOI-540 and SPECULOOS-3. The targets were selected due to the small orbital separation of their known planets, as well as for indications of stellar magnetic activity, given that for SPI radio emission may be strongest when a sufficiently magnetized star hosts a close-in planet. The deep, multi-epoch Very Large Array (SPECULOOS-3) and MeerKAT (TOI-540) observations yield non-detections, with $3σ$ limits of $\lesssim 7.5$ $μ$Jy ($4-8$ GHz) and $\lesssim 30-80$ $μ$Jy ($0.8-1.7$ GHz), respectively. For SPECULOOS-3 b we rule out observable SPI for most of its orbit, while for TOI-540 b we sample a narrower range, around planetary transit. We model possible planetary magnetic field strength constraints for both systems, and conclude that our observations are sensitive enough to sample SPI emission in these systems if present and directed at us, even for a planetary field of only $\sim 1$ G.

Figures (7)

• Figure 1: Stellar rotation period ($P_{\rm rot}$) versus orbital semi-major axis ($a$) for all known transiting exoplanets with $a\le 1$ AU and $P_{\rm rot}\le 100$ d, from the NASA Exoplanet Archive. Color denotes stellar effective temperature ($T_{\mathrm{eff}}$) and marker size is proportional to the planet to stellar radius ratio ($R_p/R_s$). The two targets in this study, TOI-540 and SPECULOOS-3, are highlighted as the two systems in the archive with the smallest semi-major axes and shortest stellar rotation periods when choosing systems with known planetary radii. We note that the TRAPPIST-1 system is shown with the updated $P_{\rm rot} = 3.3$ d from luger_seven-planet_2017vida_frequent_2017.
• Figure 2: VLA C-band observations of SPECULOOS-3. The upper left panel is a 1-hr observation, while the remaining four observations are 4 hours each. The synthesized beam is shown in the bottom left corner of each panel, and the outlined beam in the center of each panel shows the Gaia proper-motion corrected coordinates of SPECULOOS-3 during each observation. Contours trace -3$\sigma$, 3$\sigma$, 5$\sigma$ and 7$\sigma$.
• Figure 3: Same as Figure \ref{['fig:speculoos_images']} but for MeerKAT L-band observations of TOI-540.
• Figure 4: Same as Figure \ref{['fig:speculoos_images']} but for the L-band VLA observation of SPECULOOS-3.
• Figure 5: Estimated radio flux density from SPI for the sub-Alfvénic interaction (blue) and stretch-and-break (red) scenarios. We show three different stellar magnetic field strength assumptions. The flat part of each curve represents the regime where the planets are effectively unmagnetized. For both targets our observations reach the sensitivity limits needed to infer a sub-Gauss planetary field if SPI is present during the observations. The dotted horizontal lines show our $3\sigma$ RMS non-detection sensitivity.