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

• Published in: The Astronomical journal 2018-03, Vol.155 (3), p.113
• Main Authors: Fossati, L., Koskinen, T., France, K., Cubillos, P. E., Haswell, C. A., Lanza, A. F., Pillitteri, I.
• Format: Article
• Language: English
• Subjects: ABSORPTION; ACTIVITY LEVELS; Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Atmospheric models; COMPARATIVE EVALUATIONS; DETECTION; EMISSION; ENERGY SPECTRA; EXTREME ULTRAVIOLET RADIATION; Fluctuations; Hubble Space Telescope; INTERACTIONS; Interstellar matter; Interstellar medium; Irradiance; Jupiter; MASS TRANSFER; PLANETARY ATMOSPHERES; Planetary mass; Planetary orbits; Planets; RADIANT FLUX DENSITY; SATELLITES; SOLAR FLUX; Solar irradiance; SPACE; Space telescopes; Spectral energy distribution; STARS; Stellar activity; Stellar age; STELLAR WINDS; TELESCOPES; THERMAL MASS; Tidal interaction; ULTRAVIOLET SPECTRA; Upper atmosphere; X RADIATION
• ISSN: 0004-6256; 1538-3881; 1538-3881
• DOI: 10.3847/1538-3881/aaa891

WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age (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 −1 cm −2 . We employ the rescaled XUV solar fluxes to models of the planetary upper atmosphere, deriving an extremely low thermal mass-loss rate of 10 −20 M J Gyr −1 . For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.