2021A&A...649A..40D

Short-period gas giant exoplanets are susceptible to intense atmospheric escape due to their large scale heights and strong high-energy irradiation. This process is thought to occur ubiquitously, but to date we have only detected direct evidence of atmospheric escape in hot Jupiters and warm Neptunes. The latter planets are particularly more sensitive to escape-driven evolution as a result of their lower gravities with respect to Jupiter-sized planets. But the paucity of cases for intermediate, Saturn-sized exoplanets at varying levels of irradiation precludes a detailed understanding of the underlying physics in atmospheric escape of hot gas giants. Aiming to address this issue, our objectives here are to assess the high-energy environment of the warm (T_eq=970K) Saturn WASP-29 b and search for signatures of atmospheric escape. We used far-ultraviolet observations from the Hubble Space Telescope to analyze the flux time series of HI, CII, SiIII, SiIV, and NV during the transit of WASP-29 b. At 88pc, a large portion of the Lyman-α core of the K4V-type host WASP-29 is attenuated by interstellar medium absorption, limiting our ability to probe the escape of H at velocities between -84 and +35km/s. At 3σ confidence, we rule out any in-transit absorption of HI larger than 92% in the Lyman-α blue wing and 19% in the red wing. We found an in-transit flux decrease of 39%_–11%^+12% in the ground-state CII emission line at 1334.5Å. But due to this signal being significantly present in only one visit, it is difficult to attribute a planetary or stellar origin to the ground-state CII signal. We place 3σ absorption upper limits of 40, 49, and 24% on SiIII, SiIV, and for excited-state CII at 1335.7Å, respectively. Low activity levels and the faint X-ray luminosity suggest that WASP-29 is an old, inactive star. Nonetheless, an energy-limited approximation combined with the reconstructed EUV spectrum of the host suggests that the planet is losing its atmosphere at a relatively large rate of 4x10⁹g/s. The non-detection at Lyman-α could be partly explained by a low fraction of escaping neutral hydrogen, or by the state of fast radiative blow-out we infer from the reconstructed Lyman-α line.