Far-Uv Spectroscopy of the Planet-Hosting Star Wasp-13: High-Energy Irradiance, Distance, Age, Planetary Mass-Loss Rate, and Circumstellar Environment

Abstract

Several transiting hot Jupiters orbit relatively inactive main-sequence stars. For some of those, the log R ′ HK 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 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 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 1.5×10 11 g s −1 . Despite the low log R ′ HK 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 value. Nevertheless, the large uncertainty in the Ca II ISM abundance does not allow us to firmly exclude the presence of circumstellar gas.

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

“…Table 6 of Pagano et al 2004). For WASP-18, we obtained a C IV/Si IV flux ratio of 1.87±0.35, while for WASP-13, the values reported by Fossati et al (2015b) lead to a flux ratio of 3.37±0.63. These line flux ratios give the ratios of the emission measures at the respective line formation temperatures, assuming thermodynamic equilibrium.…”

“…We also looked at whether the upper limit on the C I 1657 Å line flux provides meaningful constraints on the properties of WASP-18, but the lower limit on the C IV/C I ratio of 0.65 allows one to conclude just that WASP-18 is younger than 6 Gyr (see Eq. 2 of Fossati et al 2015b).…”

“…0.023±0.009 and 0.008 mag) to estimate the range of ISM column densities for H I, C I, C II, C III, C IV, Si IV, Mg II, Ca II, and Na I. Following the results and considerations of Fossati et al (2015bFossati et al ( , 2017, we derived the column densities employing the N HI -E(B − V) conversion 5 provided by Savage & Mathis (1979) and the ISM abundances and ionization mix given by Frisch & Slavin (2003, their model 2 - Table 5). The results are listed in Table 1.…”

“…1). To measure the fluxes of these features, we employed the procedure adopted for WASP-13 (Fossati et al 2015b). The wavelength regions considered for each feature are shown in Fig.…”

“…X-ray and Lyα fluxes are not available, and the stellar age does not correlate with the fluxes measured in the Xrays (corona) and in the FUV emission lines (chromosphere). To estimate the stellar XUV fluxes, we can only follow the procedure we previously adopted for WASP-13 (Fossati et al 2015b). We rescaled the solar irradiance reference spectrum (Woods et al 2009) to match the observed WASP-18 flux in the Si IV λ1394 feature, accounting for WASP-18's distance and radius.…”