Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System*

Abstract

WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age (<1 Gyr), the star presents an anomalously low stellar activity level: the measured log R ′ HK activity parameter lies slightly below the basal level; there is no significant time-variability in the log R ′ HK value; there is no detection of the star in the X-rays. We present results of far-UV observations of WASP-18 obtained with COS on board of Hubble Space Telescope aimed at explaining this anomaly. From the star's spectral energy distribution, we infer the extinction (E(B − V) ≈ 0.01 mag) and then the interstellar medium (ISM) column density for a number of ions, concluding that ISM absorption is not the origin of the anomaly. We measure the flux of the four stellar emission features detected in the COS spectrum (C II, C III, C IV, Si IV). Comparing the C II/C IV flux ratio measured for WASP-18 with that derived from spectra of nearby stars with known age, we see that the far-UV spectrum of WASP-18 resembles that of old (>5 Gyr), inactive stars, in stark contrast with its young age. We conclude that WASP-18 has an intrinsically low activity level, possibly caused by star-planet tidal interaction, as suggested by previous studies. Re-scaling the solar irradiance reference spectrum to match the flux of the Si IV line, yields an XUV integrated flux at the planet orbit of 10.2 erg s . For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.

A Chandra/LETGS Survey of Main-sequence Stars

“…However, there are also cases where the exoplanet seems to be inhibiting stellar activity. The WASP-18 system seems to be such a case, with P orb = 0.96 days and with stellar X-ray and UV emission both anomalously low (Pillitteri et al 2014b;Fossati et al 2018). For systems such as this, tidal effects of the planet may actually be adversely affecting the stellar dynamo.…”

Ca II H&K stellar activity parameter: a proxy for extreme ultraviolet stellar fluxes

“…Other works have reconstructed stellar EUV spectra by scaling the solar UV spectrum to match measurements of high-energy far-ultraviolet (FUV) emission lines (e.g. Fossati et al 2015Fossati et al , 2018a. X-ray and FUV measurements have also been combined via the differential emission measure (e.g.…”

“…Ribas et al 2005). Furthermore, this method may in some cases lead to incorrect conclusions, such as for the planet-hosting star WASP-18, which is a fast rotator but has an extremely low activity level that is believed to be dampened by tidal interactions with the massive close-in planet (Fossati et al 2018a).…”

Supermassive hot Jupiters provide more favourable conditions for the generation of radio emission via the cyclotron maser instability – a case study based on Tau Bootis b

“…For planets more massive than Jupiter, like Tau Bootis b, the planetary gravity keeps the atmosphere strongly bound. In some cases, this will lead to a hydrostatic rather than hydrodynamic upper atmosphere (for example, WASP-18b, is one of these cases; Fossati et al 2018), so that radio emission could be generated and may escape, a situation which is comparable with the known solar system conditions for e.g. Earth.…”