TY - JOUR
T1 - Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System
AU - Fossati, L.
AU - Koskinen, T.
AU - France, K.
AU - Cubillos, P. E.
AU - Haswell, C. A.
AU - Lanza, A. F.
AU - Pillitteri, I.
N1 - Funding Information:
This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained from MAST at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program No. 13859, to which support was provided by NASA through a grant from the Space Telescope Science Institute. Support for StarCAT was provided by grant HST-AR-10638.01-A from STScI and grant NAG5-13058 from NASA. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 092.D-0587. This work has made use of public databases hosted by SIMBAD and VizieR, both maintained by CDS, Strasbourg, France. We thank the anonymous referee for the very useful and constructive comments. C.H. is supported by STFC under grant ST/P000584/1. I.P. acknowledges support from INAF and ASI through the ARIEL consortium.
Funding Information:
This work is based on observations made with the NASA/ ESA Hubble Space Telescope, obtained from MAST at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program No. 13859, to which support was provided by NASA through a grant from the Space Telescope Science Institute. Support for StarCAT was provided by grant HST-AR-10638.01-A from STScI and grant NAG5-13058 from NASA. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 092.D-0587. This work has made use of public databases hosted by SIMBAD and VizieR, both maintained by CDS, Strasbourg, France. We thank the anonymous referee for the very useful and constructive comments. C.H. is supported by STFC under grant ST/P000584/1. I.P. acknowledges support from INAF and ASI through the ARIEL consortium. Facilities: HST (COS, STIS), VLT (UVES).
Publisher Copyright:
© 2018. The American Astronomical Society. All rights reserved.
PY - 2018/3
Y1 - 2018/3
N2 - 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 RHK′ activity parameter lies slightly below the basal level; there is no significant time-variability in the log RHK′ 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-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 10-20MJ Gyr-1. For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.
AB - 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 RHK′ activity parameter lies slightly below the basal level; there is no significant time-variability in the log RHK′ 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-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 10-20MJ Gyr-1. For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.
KW - planets and satellites: individual (WASP-18b)
KW - stars: activity
KW - stars: individual (WASP-18)
KW - ultraviolet: stars
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UR - http://www.scopus.com/inward/citedby.url?scp=85043511944&partnerID=8YFLogxK
U2 - 10.3847/1538-3881/aaa891
DO - 10.3847/1538-3881/aaa891
M3 - Article
AN - SCOPUS:85043511944
VL - 155
JO - Astronomical Journal
JF - Astronomical Journal
SN - 0004-6256
IS - 3
M1 - 113
ER -