High-resolution confirmation of an extended helium atmosphere around WASP-107b

Abstract

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 (10833 Å) revealed extended thermospheres in several exoplanets, opening a new window into evaporation. Aims. We aim at spectrally resolving the first helium absorption signature detected in the warm Saturn WASP-107b with HST/WFC3. Methods. We obtained one transit of WASP-107b with the high-resolution spectrograph CARMENES on the 3.5m telescope in Calar Alto. 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 done 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 filling half the Roche lobe and a large exospheric tail sustained by an escape rate of metastable helium on the order of 10 6 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 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, SPIRou or NIRPS) will deliver a statistical understanding of exoplanet thermospheres and exospheres via the helium triplet.

Search for helium in the upper atmosphere of the hot Jupiter WASP-127 b using Gemini/Phoenix

“…Flux relative to master out-of-transit spectrum The average precision of the transmission spectrum of WASP-127 b is 1.9% per wavelength bin. This level of uncertainty is comparable to the one obtained for one transit of WASP-107 b with the CARMENES spectrograph (Allart et al 2019). For comparison purposes, the number of measured spectra in both programs is similar, both WASP-127 and WASP-107 possess similar J magnitudes, but the Gemini South Telescope has a mirror size of 8.1 m, while CARMENES is installed on a 3.5 m telescope.…”

“…An inspection of the spectral ratio time series and the transmission spectrum does not reveal a strong planetary absorption in the He triplet. We computed the integrated flux inside the wavelength range [1083.280, 1083.355] nm, the same bandwidth as in (Allart et al 2019), and found an in-transit excess absorption consistent with null, 0.42% ± 0.36%, which corresponds to an upper limit of 0.87% at 90% confidence. In this project, we used a slit width of 0.25 arcsec (three pixels), but future projects with this instrument could benefit from a higher S/N if the widest slit width were used (0.34 arcsec, or four pixels).…”

Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)

“…To better constrain atmospheric escape, follow-up studies need to trace layers above those probed by sodium, using lighter gases such as hydrogen and helium. Additionally, WASP-76b is too far away from Earth to measure UV transit absorption signal in the stellar HI Lyman-α emission line (which is entirely absorbed by the interstellar medium), but there are good prospects for observations of escaping helium through high-resolution, near-infrared transit observations (Allart et al 2018(Allart et al , 2019Nortmann et al 2018;Salz et al 2018).…”

Mass determinations of the three mini-Neptunes transiting TOI-125

“…H-alpha, and other Balmer series lines, have been detected for several exoplanets, showing deep absorption features observed at high spectral resolution (Jensen et al 2012;Cauley et al 2017;Jensen et al 2018;Yan & Henning 2018). Likewise, the well known HeI triplet in the infrared (Seager & Sasselov 2000;Oklopčić & Hirata 2018;Oklopčić 2019), has also successfully detected exospheric absorption in other systems (Allart et al 2018;Nortmann et al 2018;Salz et al 2018;Allart et al 2019).…”