The Hubble PanCET program: an extensive search for metallic ions in the exosphere of GJ 436 b

¹ Observatoire astronomique de l’Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
e-mail: leonardo.dossantos@unige.ch
² Institut d’Astrophysique de Paris, CNRS, UMR 7095 & Sorbonne Universités UPMC Paris 6, 98bis bd Arago, 75014 Paris, France
³ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
⁵ Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD, USA
⁶ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
⁷ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
⁸ Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany
⁹ Center of Excellence in Information Systems, Tennessee State University, Nashville, TN 37209, USA
¹⁰ NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹¹ Groupe de Spectrométrie Moléculaire et Atmosphérique, Université de Reims, CNRS UMR, Champagne-Ardenne 7331, Reims, France
¹² Department of Astronomy and Carl Sagan Institute, Cornell University, 122 Sciences Drive, 14853 Ithaca, NY, USA
¹³ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 37-241, Cambridge, MA 02139, USA
¹⁴ Centro de Astrobiología (CSIC-INTA), 28692 Villanueva de la Cañada, Madrid, Spain
¹⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 11 April 2019
Accepted: 10 July 2019

Abstract

Context. The quiet M2.5 star GJ 436 hosts a warm Neptune that displays an extended atmosphere that dwarfs its own host star. Predictions of atmospheric escape in such planets state that H atoms escape from the upper atmosphere in a collisional regime and that the flow can drag heavier atoms to the upper atmosphere. It is unclear, however, what astrophysical mechanisms drive the process.

Aims. Our objective is to leverage the extensive coverage of observations of the far-ultraviolet (FUV) spectrum of GJ 436 obtained with the Cosmic Origins Spectrograph (COS) to search for signals of metallic ions in the upper atmosphere of GJ 436 b, as well as study the activity-induced variability of the star.

Methods. We analyzed flux time-series of species present in the FUV spectrum of GJ 436 and successfully performed geocoronal contamination removal in the COS Lyman-α profiles obtained near the Earth’s night-side.

Results. GJ 436 displays flaring events with a rate of ~10 d⁻¹. There is evidence for a possibly long-lived active region or longitude that modulates the FUV metallic lines of the star with amplitudes up to 20%. Despite the strong geocoronal contamination in the COS spectra, we detected in-transit excess absorption signals of ~50 and ~30% in the blue and red wings, respectively, of the Lyman-α line. We rule out a wide range of excess absorption levels in the metallic lines of the star during transit.

Conclusions. The large atmospheric loss of GJ 436 b observed in Lyman-α transmission spectra is stable over the timescale of a few years, and the red wing signal supports the presence of a variable hydrogen absorption source besides the stable exosphere. The previously claimed in-transit absorption in the Si III line is likely an artifact resulting from the stellar magnetic cycle. The non-detection of metallic ions in absorption could indicate that the escape is not hydrodynamic or that the atmospheric mixing is not efficient in dragging metals high enough for sublimation to produce a detectable escape rate of ions to the exosphere.