Volume 605, September 2017
|Number of page(s)||7|
|Published online||14 September 2017|
The long egress of GJ 436b’s giant exosphere
1 Observatoire de l’Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
2 University of Bern, Space Research and Planetary Sciences, Sidlerstrasse 5, 3012 Bern, Switzerland
3 CNRS, Institut d’Astrophysique de Paris, UMR 7095, 98bis boulevard Arago, 75014 Paris, France
4 Dept. of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
5 Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
Received: 9 June 2017
Accepted: 19 August 2017
The M dwarf GJ 436 hosts a transiting warm Neptune known to experience atmospheric escape. Previous observations revealed the presence of a giant hydrogen exosphere transiting the star for more than 5 h, and absorbing up to 56% of the flux in the blue wing of the stellar Lyman-α line of neutral hydrogen (H i Lyα). The unexpected size of this comet-like exosphere prevented observing the full transit of its tail. In this Letter, we present new Lyα observations of GJ 436 obtained with the Space Telescope Imaging Spectrograph (STIS) instrument onboard the Hubble Space Telescope. The stability of the Lyα line over six years allowed us to combine these new observations with archival data sets, substantially expanding the coverage of the exospheric transit. Hydrogen atoms in the tail of the exospheric cloud keep occulting the star for 10–25 h after the transit of the planet, remarkably confirming a previous prediction based on 3D numerical simulations with the EVaporating Exoplanet code (EVE). This result strengthens the interpretation that the exosphere of GJ 436b is shaped by both radiative braking and charge exchanges with the stellar wind. We further report flux decreases of 15 ± 2% and 47 ± 10% in the red wing of the Lyα line and in the line of ionised silicon (Si iii). Despite some temporal variability possibly linked with stellar activity, these two signals occur during the exospheric transit and could be of planetary origin. Follow-up observations will be required to assess the possibility that the redshifted Lyα and Si iii absorption signatures arise from interactions between the exospheric flow and the magnetic field of the star.
Key words: planets and satellites: atmospheres / planets and satellites: gaseous planets / planets and satellites: individual: GJ 436
© ESO, 2017
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.