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Far-UV spectroscopy of the planet-hosting star WASP-13: High-energy irradiance, distance, age, planetary mass-loss rate, and circumstellar environment

Fossati, L.; France, K.; Koskinen, T.; Juvan, I. G.; Haswell, C. A. and Lendl, M. (2015). Far-UV spectroscopy of the planet-hosting star WASP-13: High-energy irradiance, distance, age, planetary mass-loss rate, and circumstellar environment. The Astrophysical Journal, 815(2) p. 118.

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Several transiting hot Jupiters orbit relatively inactive main-sequence stars. For some of those, the {log}{R}{HK}\prime activity parameter lies below the basal level (-5.1). Two explanations have been proposed so far: (i) the planet affects the stellar dynamo, (ii) the {log}{R}{HK}\prime measurements are biased by extrinsic absorption, either by the interstellar medium (ISM) or by material local to the system. We present here Hubble Space Telescope/COS far-UV spectra of WASP-13, which hosts an inflated hot Jupiter and has a measured {log}{R}{HK}\prime value (-5.26), well below the basal level. From the star's spectral energy distribution we obtain an extinction E(B - V) = 0.045 ± 0.025 mag and a distance d = 232 ± 8 pc. We detect at ≳4σ lines belonging to three different ionization states of carbon (C i, C ii, and C iv) and the Si iv doublet at ˜3σ. Using far-UV spectra of nearby early G-type stars of known age, we derive a C iv/C i flux ratio-age relation, from which we estimate WASP-13's age to be 5.1 ± 2.0 Gyr. We rescale the solar irradiance reference spectrum to match the flux of the C iv 1548 doublet. By integrating the rescaled solar spectrum, we obtain an XUV flux at 1 AU of 5.4 erg s-1 cm-2. We use a detailed model of the planet's upper atmosphere, deriving a mass-loss rate of 1.5 × 1011 g s-1. Despite the low {log}{R}{HK}\prime value, the star shows a far-UV spectrum typical of middle-aged solar-type stars, pointing toward the presence of significant extrinsic absorption. The analysis of a high-resolution spectrum of the Ca ii H&K lines indicates that the ISM absorption could be the origin of the low {log}{R}{HK}\prime value. Nevertheless, the large uncertainty in the Ca ii ISM abundance does not allow us to firmly exclude the presence of circumstellar gas.
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 #13859.

Item Type: Journal Item
Copyright Holders: 2015 The American Astronomical Society
ISSN: 1538-4357
Project Funding Details:
Funded Project NameProject IDFunding Body
Astronomy Consolidated GrantST/L000776/1The Open University (OU)
Keywords: ISM: abundances; planets and satellites: individual (WASP-13b) ; stars: activity; stars: individual (WASP-13) ; ultraviolet: stars
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Physical Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Item ID: 46918
Depositing User: Carole Haswell
Date Deposited: 02 Aug 2016 15:18
Last Modified: 08 Dec 2018 13:34
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