SALT observations of the chromospheric activity of transiting planet hosts: Mass-loss and star-planet interactions

Abstract

We measured the chromospheric activity of the four hot Jupiter hosts WASP-43, WASP-51/HAT-P-30, WASP-72 and WASP-103 to search for anomalous values caused by the close-in companions. The Mount Wilson CaIIH&KS-index was calculated for each star usingobservations taken with the Robert Stobie Spectrograph at the Southern African Large Tele-scope. The activity level of WASP-43 is anomalously high relative to its age and falls amongthe highest values of all known main-sequence stars. We found marginal evidence that theactivity of WASP-103 is also higher than expected from the system age. We suggest that forWASP-43 and WASP-103 star–planet interactions (SPI) may enhance the CaIIH&Kcoreemission. 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. Thiscan be attributed to circumstellar absorption due to planetary mass-loss, though absorption inthe interstellar medium may contribute. A quarter of known short-period planet hosts exhibitanomalously low activity levels, including systems with hot Jupiters and low-mass compan-ions. Since SPI can elevate and absorption can suppress the observed chromospheric activityof stars with close-in planets, their CaIIH & K activity levels are an unreliable age indicator.Systems where the activity is depressed by absorption from planetary mass-loss are key targetsfor examining planet compositions through transmission spectroscopy.