Hot Jupiter WASP-13b is Losing 100 Tons of Atmosphere per Second

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

Authors:

Fossati et al

Abstract:

Several transiting hot Jupiters orbit relatively inactive main-sequence stars. For some of those, the logR'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 logR'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 logR'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 >4 sigma lines belonging to three different ionization states of carbon (C1, C2, and C4) and the Si4 doublet at ~3 sigma. Using far-UV spectra of nearby early G-type stars of known age, we derive a C4/C1 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 C4 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 1.5x10^11 g s^-1. Despite the low logR'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 Ca2H&K lines indicates that the ISM absorption could be the origin of the low logR'HK value. Nevertheless, the large uncertainty in the Ca2 ISM abundance does not allow us to firmly exclude the presence of circumstellar gas.

The Dynamical Evolution of WASP-13b and WASP-32b

A Window on Exoplanet Dynamical Histories: Rossiter-McLaughlin Observations of WASP-13b and WASP-32b

Authors:

Brothwell et al

Abstract:

We present Rossiter-McLaughlin observations of WASP-13b and WASP-32b and determine the sky-projected angle between the normal of the planetary orbit and the stellar rotation axis (λ). WASP-13b and WASP-32b both have prograde orbits and are consistent with alignment with measured sky-projected angles of λ=8∘+13−12 and λ=−2∘+17−19, respectively.

Both WASP-13 and WASP-32 have Teff less than 6250K and therefore these systems support the general trend that aligned planetary systems are preferentially found orbiting cool host stars. A Lomb-Scargle periodogram analysis was carried out on archival SuperWASP data for both systems. A statistically significant stellar rotation period detection (above 99.9\% confidence) was identified for the WASP-32 system with Prot=11.6±1.0 days. This rotation period is in agreement with the predicted stellar rotation period calculated from the stellar radius, R⋆, and vsini if a stellar inclination of i⋆=90∘ is assumed. With the determined rotation period, the true 3D angle between the stellar rotation axis and the planetary orbit, ψ, was found to be ψ=11∘±14. We conclude with a discussion on the alignment of systems around cool host stars with Teff less than 6150K by calculating the tidal dissipation timescale. We find that systems with short tidal dissipation timescales are preferentially aligned and systems with long tidal dissipation timescales have a broad range of obliquities.