No X-rays from WASP-18. Implications for its age, activity, and the influence of its massive hot Jupiter

TL;DR

This study reports a non-detection of X-ray emission from WASP-18, a star with a massive hot Jupiter, suggesting the planet may suppress stellar magnetic activity, challenging expectations based on the star's estimated age.

Contribution

It provides evidence that a massive hot Jupiter can significantly inhibit stellar magnetic activity, impacting our understanding of star-planet interactions.

Findings

WASP-18 shows no detectable X-ray emission despite its estimated age.

The star's X-ray luminosity limit is over 100 times lower than expected.

The massive planet may disrupt the star's magnetic dynamo.

Abstract

About 20% out of the $>1000$ known exoplanets are Jupiter analogs orbiting very close to their parent stars. It is still under debate to what detectable level such hot Jupiters possibly affect the activity of the host stars through tidal or magnetic star-planet interaction. In this paper we report on an 87 ks Chandra observation of the hot Jupiter hosting star WASP-18. This system is composed of an F6 type star and a hot Jupiter of mass $10.4 M_{Jup}$ orbiting in less than 20 hr around the parent star. On the basis of an isochrone fitting, WASP-18 is thought to be 600 Myr old and within the range of uncertainty of 0.5-2 Gyr. The star is not detected in X-rays down to a luminosity limit of $4\times10^{26}$ erg/s, more than two orders of magnitude lower than expected for a star of this age and mass. This value proves an unusual lack of activity for a star with estimated age around 600 Myr. We argue that the massive planet can play a crucial role in disrupting the stellar magnetic dynamo created within its thin convective layers. Another additional 212 X-ray sources are detected in the Chandra image. We list them and briefly discuss their nature.