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

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    Author
    Fossati, L. cc
    Koskinen, T.
    France, K. cc
    Cubillos, P. E. cc
    Haswell, C. A. cc
    Lanza, A. F. cc
    Pillitteri, I. cc
    Affiliation
    Univ Arizona, Lunar & Planetary Lab
    Issue Date
    2018-02-13
    Keywords
    planets and satellites: individual (WASP-18b)
    stars: activity
    stars: individual (WASP-18)
    ultraviolet: stars
    
    Metadata
    Show full item record
    Publisher
    IOP PUBLISHING LTD
    Citation
    Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System 2018, 155 (3):113 The Astronomical Journal
    Journal
    The Astronomical Journal
    Rights
    © 2018. The American Astronomical Society. All rights reserved.
    Collection Information
    This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu.
    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) approximate to 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(-20) M-J Gyr(-1). For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.
    ISSN
    1538-3881
    DOI
    10.3847/1538-3881/aaa891
    Version
    Final published version
    Sponsors
    NASA [NAS 5-26555, NAG5-13058]; NASA through Space Telescope Science Institute [13859]; STScI [HST-AR-10638.01-A]; ESO Telescopes at the La Silla Paranal Observatory [092.D-0587]; STFC [ST/P000584/1]; INAF; ASI through the ARIEL consortium
    Additional Links
    http://stacks.iop.org/1538-3881/155/i=3/a=113?key=crossref.95da8f47525cef293db3f90ac90672e4
    ae974a485f413a2113503eed53cd6c53
    10.3847/1538-3881/aaa891
    Scopus Count
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    UA Faculty Publications

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