Upper Limits on Radio Emission from the K2-18 System

TL;DR

The study addresses whether the M-dwarf host of K2-18 emits detectable radio signals from stellar magnetic activity or its planets. It employs VLA observations across $2-4$ GHz, $4.5-7.5$ GHz, and $8-10$ GHz (S, C, X bands) to search for both incoherent and coherent radio emission, finding no detections and deriving stringent upper limits on Stokes I and V, as well as on $L_ u$ and $L_R$ relative to $L_ ext{bol}$. The authors evaluate planetary auroral emission via radiometric Bode's law, constrain coronal gyrosynchrotron processes with the radio limits, and search for ECME bursts, all yielding non-detections but informative constraints on the system's magnetic environment. These results help contextualize transmission spectra of K2-18 b by limiting activity-induced contamination, while highlighting observational challenges such as favorable viewing angles, plasma densities, and orbital sampling for detecting planetary magnetosphere radio emission with current Earth-based instruments.

Abstract

Stellar and planetary magnetic fields play a crucial role in the habitability of a planet and the integrity of its atmosphere. The recently claimed detection of biosignatures, methane, carbon dioxide and dimethyl sulfide/disulfide, in the atmosphere of K2-18 b, a sub-Neptune orbiting an M dwarf star present an intriguing question regarding the stellar magnetic environment and the resistance of the planet's magnetosphere (if it exists) to erosion by magnetic activity from the host. To probe for radio emission from the system, we have conducted observations using the Karl G. Jansky Very Large Array (VLA) at S, C and X-bands (2-4, 4.5-7.5 and 8-10 GHz respectively) to search for coherent and incoherent radio emission. We detect no radio emission associated with incoherent emission mechanisms. We report $3σ$ Stokes I upper limits of $49.8\ μ\rm{Jybeam}^{-1}$ at S-band, $17.7\ μ\rm{Jybeam}^{-1}$at C-band and $18.0\ μ\rm{Jybeam}^{-1}$ at X-band and an upper limit of the ratio of the radio to the total bolometric luminosity of $\log L_\text{R}/\log L_\text{bol}<-8.8$. We have also searched for short duration bursts associated with coherent emission mechanisms at C and X-bands . No signals above a $3σ$ significance threshold are detected. Although no signals are detected our radio observations offer constraints, albeit limited, on the stellar magnetic environment supporting recent X-ray observations indicating K2-18 is a very faint emitter. Our results also contextualise any planetary transmission spectra by providing constraints on the activity level of the host.

Figures (3)

• Figure 1: Panel (a): S-band Stokes I image (top) and Stokes V (bottom). The Stokes I $1\sigma$ rms noise is $16.6\ \mu \text{Jybeam}^{-1}$ and the corresponding $1\sigma$ rms noise of the Stokes V image is $7.1\ \mu \text{Jybeam}^{-1}$. Panel(b) C-band Stokes I image (top) and Stokes V (bottom). The Stokes I $1\sigma$ rms noise is $5.9\ \mu \text{Jybeam}^{-1}$ and the corresponding $1\sigma$ rms noise of the Stokes V image is $5.0\ \mu \text{Jybeam}^{-1}$. Panel(c) X-band Stokes I image (top) and Stokes V (bottom). The Stokes I $1\sigma$ rms noise is $6.0\ \mu \text{Jybeam}^{-1}$ and the corresponding $1\sigma$ rms noise of the Stokes V image is $5.2\ \mu \text{Jybeam}^{-1}$. The black cross in the images indicates the position of K2-18. The restoring beam is shown in the filled white circle on the bottom left of each image. All the images are produced from observations conducted using the D-configuration of the VLA.
• Figure 2: C-band light curves of K2-18 for the observation conducted on 08/12/2023 using the VLA in its D-configuration . The visibilities are binned at a cadence of 1 minute. The broken line represents the mean values in the Stokes I and V at $\sim17.6\ \mu \text{Jy}$ and $\sim-0.8\ \mu \text{Jy}$, respectively. The rms noise $\sigma$ is $\sim35.4\ \mu \text{Jy}$ and $\sim34.8\ \mu \text{Jy}$ in the Stokes I and V and is represented by the shaded region with decreasing intensity corresponding to 1, 2 and $3\sigma$ respectively.
• Figure 3: X-band light curves of K2-18 for the observation conducted on 18/12/2023 using the VLA in its D-configuration . The visibilities are binned at a cadence of 1 minute. The broken line represents the mean values in the Stokes I and V at $\sim18.7\ \mu \text{Jy}$ and $\sim-0.6\ \mu \text{Jy}$, respectively. The rms noise $\sigma$ is $\sim40.6\ \mu \text{Jy}$ and $\sim50.8\ \mu \text{Jy}$ in the Stokes I and V and is represented by the shaded region with decreasing intensity corresponding to 1, 2 and $3\sigma$ respectively.