A search for radio emission from exoplanets around evolved stars

TL;DR

This study explores the potential for detecting radio emissions from exoplanets orbiting evolved stars using LOFAR, focusing on larger orbital distances and magnetic field strengths, despite non-detections in initial observations.

Contribution

It demonstrates the feasibility of using LOFAR to search for radio emissions from exoplanets around evolved stars and provides upper limits on flux densities at 150 MHz.

Findings

No radio emission detected from the three observed systems.

Established tight upper limits on flux density at 150 MHz.

Highlighted LOFAR's potential for future exoplanetary radio searches.

Abstract

The majority of searches for radio emission from exoplanets have to date focused on short period planets, i.e., the so-called hot Jupiter type planets. However, these planets are likely to be tidally locked to their host stars and may not generate sufficiently strong magnetic fields to emit electron cyclotron maser emission at the low frequencies used in observations (typically >150 MHz). In comparison, the large mass-loss rates of evolved stars could enable exoplanets at larger orbital distances to emit detectable radio emission. Here, we first show that the large ionized mass-loss rates of certain evolved stars relative to the solar value could make them detectable with the Low Frequency Array (LOFAR) at 150 MHz ($\lambda$ = 2 m), provided they have surface magnetic field strengths >50 G. We then report radio observations of three long period (>1 au) planets that orbit the evolved stars $\beta$ Gem, $\iota$ Dra, and $\beta$ UMi using LOFAR at 150 MHz. We do not detect radio emission from any system but place tight 3$\sigma$ upper limits of 0.98, 0.87, and 0.57 mJy on the flux density at 150 MHz for $\beta$ Gem, $\iota$ Dra, and $\beta$ UMi, respectively. Despite our non-detections these stringent upper limits highlight the potential of LOFAR as a tool to search for exoplanetary radio emission at meter wavelengths.