High-resolution confirmation of an extended helium atmosphere around WASP-107b
Observatoire astronomique de l’Université de Genève, Université de Genève, 51 chemin des Maillettes,
2 Centro Astronomico Hispano Aleman, Sierra de los filabres sn, Gérgal, Almería, Spain
3 Instituto de astrofísica de Andalucía (CSIC), Glorieta de la Astronomia sn, Granada Spain
4 Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
5 Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD, USA
6 Astrophysics Group, School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
7 Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
Accepted: 22 January 2019
Context. Probing the evaporation of exoplanet atmospheres is key to understanding the formation and evolution of exoplanetary systems. The main tracer of evaporation in the UV is the Lyman-α transition, which can reveal extended exospheres of neutral hydrogen. Recently, the near-infrared (NIR) metastable helium triplet (10 833 Å) revealed extended thermospheres in several exoplanets. This opens a new window into evaporation.
Aims. We aim at spectrally resolving the first helium absorption signature detected in the warm Saturn WASP-107b with the Wide Filed Camera 3 on board the Hubble Space Telescope (HST/WFC3).
Methods. We obtained one transit of WASP-107b with CARMENES installed on the 3.5 m telescope at the Calar Alto observatory.
Results. We detect an excess helium absorption signature of 5.54 ± 0.27% (20σ) in the planet rest frame during the transit. The detection is in agreement with the previous detection achieved with HST/WFC3. The signature shows an excess absorption in the blue part of the lines, suggesting that He I atoms are escaping from the atmosphere of WASP-107b. We interpret the time-series absorption spectra using the 3D EVE code. Our observations can be explained by combining an extended thermosphere that fills half of the Roche lobe and a large exospheric tail sustained by an escape rate of metastable helium of about 106 g s−1. In this scenario, however, the upper atmosphere needs to be subjected to a reduced photoionisation and radiation pressure from the star for the model to match the observations.
Conclusions. We confirm the presence of helium in the atmosphere of WASP-107b at high confidence. The helium feature is detected from space and from the ground. The ground-based high-resolution signal brings detailed information about the spatial and dynamical structure of the upper atmosphere, and simulations suggest that the He I signature of WASP-107b probes both its thermosphere and exosphere, establishing this signature as a robust probe of exoplanetary upper atmospheres. Surveys with NIR high-resolution spectrographs (e.g. CARMENES, the Spectromètre infrarouge (SPIRou), or the Near-Infrared Planet Searcher (NIRPS)) will deliver a statistical understanding of exoplanet thermospheres and exospheres through the helium triplet.
Key words: planets and satellites: atmospheres / planets and satellites: individual: WASP-107b / methods: observational / techniques: spectroscopic
© ESO 2019