Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System
- Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz (Austria)
- Lunar and Planetary Laboratory, University of Arizona, 1629 East University Boulevard, Tucson, AZ 85721-0092 (United States)
- Laboratory for Atmospheric and Space Physics, University of Colorado, 600 UCB, Boulder, CO 80309 (United States)
- Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA (United Kingdom)
- INAF—Osservatorio Astrofisico di Catania, Via S. Sofia 78, I-95123 Catania (Italy)
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 logR{sub HK}{sup ′} activity parameter lies slightly below the basal level; there is no significant time-variability in the logR{sub HK}{sup ′} 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{sup −1} cm{sup −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{sup −20} M {sub J} Gyr{sup −1}. For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.
- OSTI ID:
- 22897413
- Journal Information:
- Astronomical Journal (New York, N.Y. Online), Vol. 155, Issue 3; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 1538-3881
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COSMOLOGY AND ASTRONOMY
ABSORPTION
ACTIVITY LEVELS
COMPARATIVE EVALUATIONS
DETECTION
EMISSION
ENERGY SPECTRA
EXTREME ULTRAVIOLET RADIATION
INTERACTIONS
MASS TRANSFER
PLANETARY ATMOSPHERES
RADIANT FLUX DENSITY
SATELLITES
SOLAR FLUX
SPACE
STARS
STELLAR WINDS
TELESCOPES
THERMAL MASS
ULTRAVIOLET SPECTRA
X RADIATION