SALT observations of the Chromospheric Activity of Transiting Planet Hosts: Mass Loss and Star Planet Interactions

TL;DR

This study measures chromospheric activity in hot Jupiter host stars, revealing anomalies likely caused by star-planet interactions or planetary mass loss, which impact age estimates and planetary atmosphere studies.

Contribution

It provides new observational evidence linking star-planet interactions and planetary mass loss to chromospheric activity variations in hot Jupiter systems.

Findings

WASP-43 shows unusually high activity levels.

WASP-103 may have elevated activity due to star-planet interactions.

WASP-51/HAT-P-30 and WASP-72 exhibit low activity, possibly due to planetary mass loss absorption.

Abstract

We measured the chromospheric activity of the four hot Jupiter hosts WASP-43, WASP-51/HAT-P-30, WASP-72 & WASP-103 to search for anomalous values caused by the close-in companions. The Mount Wilson Ca II H&K S-index was calculated for each star using observations taken with the Robert Stobie Spectrograph at the Southern African Large Telescope. The activity level of WASP-43 is anomalously high relative to its age and falls among the highest values of all known main sequence stars. We found marginal evidence that the activity of WASP-103 is also higher than expected from the system age. We suggest that for WASP-43 and WASP-103 star-planet interactions (SPI) may enhance the Ca II H&K core emission. The activity levels of WASP-51/HAT-P-30 and WASP-72 are anomalously low, with the latter falling below the basal envelope for both main sequence and evolved stars. This can be attributed to circumstellar absorption due to planetary mass loss, though absorption in the ISM may contribute. A quarter of known short period planet hosts exhibit anomalously low activity levels, including systems with hot Jupiters and low mass companions. Since SPI can elevate and absorption can suppress the observed chromospheric activity of stars with close-in planets, their Ca II H&K activity levels are an unreliable age indicator. Systems where the activity is depressed by absorption from planetary mass loss are key targets for examining planet compositions through transmission spectroscopy.