Hubble PanCET: an extended upper atmosphere of neutral hydrogen around the warm Neptune GJ 3470b

Abstract

GJ 3470b is a warm Neptune transiting a M dwarf star at the edge of the evaporation desert. It offers the possibility to investigate how low-mass, close-in exoplanets evolve under the irradiation from their host stars. We observed three transits of GJ 3470b in the Lyman-α line with the Hubble Space Telescope (HST) as part of the Panchromatic Comparative Exoplanet Treasury (PanCET) program. Absorption signatures are detected with similar properties in all three independent epochs, with absorption depths of 35±7% in the blue wing of the line, and 23±5% in the red wing. The repeatability of these signatures, their phasing with the planet transit, and the radial velocity of the absorbing gas, allow us to conclude that there is an extended upper atmosphere of neutral hydrogen around GJ 3470 b. We determine from our observations the stellar radiation pressure and XUV irradiation from GJ 3470 and use them to perform numerical simulations of GJ 3470b's upper atmosphere with the EVaporating Exoplanets (EVE) code. The unusual redshifted signature can be explained by the damping wings of dense layers of neutral hydrogen extending beyond the Roche lobe and elongated in the direction of the planet motion. This structure could correspond to a shocked layer of planetary material formed by the collision of the expanding thermosphere with the wind of the star. The blueshifted signature is well explained by neutral hydrogen atoms escaping at rates of about 10 10 g s −1 , blown away from the star by its strong radiation pressure, and quickly photoionized, resulting in a smaller exosphere than that of the warm Neptune GJ 436b. The stronger escape from GJ 3470b, however, may have led to the loss of about 4-35% of its current mass over its ∼2 Gyr lifetime.

MOVES III. Simultaneous X-ray and ultraviolet observations unveiling the variable environment of the hot Jupiter HD 189733b

“…Lecavelier des Etangs et al 2004, Owen & Jackson 2012. This scenario is strengthened by the recent observations of warm Neptunes at the border of the desert, on the verge of (Kulow et al 2014, Bourrier et al 2015, Lavie et al 2017 or undergoing (Bourrier et al 2018b) considerable mass loss. Because they survive more extreme conditions than lower-mass gaseous exoplanets, hot Jupiters are particularly interesting targets to study star-planet interactions.…”

“…In this case, absorption should increase toward low velocities in the planet rest frame (see e.g. Bourrier et al 2018b), whereas no absorption is detected in Visit B or D between ∼30 and 70 km s −1 (Fig. 16).…”

“…calstis corrects the final 1D spectra for airglow contamination, but it is recommended to treat with caution the regions where the airglow is stronger than the stellar flux (e.g. Bourrier et al 2017Bourrier et al , 2018b. The strength and position of the airglow varies with the epoch of observation, and after preliminary analyses of the Lyman-α spectra we identified the wavelength windows shown in Fig.…”

“…2). We sampled the posterior distributions of the master spectra parameters using emcee, and derived their best-fit values and uncer- tainties using the same method as in Bourrier et al (2018b).…”

The Hubble Space Telescope PanCET Program: Exospheric Mg ii and Fe ii in the Near-ultraviolet Transmission Spectrum of WASP-121b Using Jitter Decorrelation

“…The transit depth reaches ∼50% at Ly-α, showing H i extending well beyond the Roche lobe and surviving photo-ionization Lavie et al 2017). Similarly, the warm Neptune GJ 3470b also shows an extended upper atmosphere of H i (Bourrier et al 2018), with transit absorption depths reaching 35%.…”