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

Research output: Contribution to journalJournal article – Annual report year: 2019Researchpeer-review


  • Author: Sing, David K.

    Johns Hopkins University, United States

  • Author: Lavvas, Panayotis

    Centre National de la Recherche Scientifique, France

  • Author: Ballester, Gilda E.

    University of Arizona, United States

  • Author: des Etangs, Alain Lecavelier

    Centre National de la Recherche Scientifique, France

  • Author: Marley, Mark S.

    National Aeronautics and Space Administration, United States

  • Author: Nikolov, Nikolay

    Johns Hopkins University, United States

  • Author: Ben-Jaffel, Lotfi

    Centre National de la Recherche Scientifique, France

  • Author: Bourrier, Vincent

    University of Geneva, Switzerland

  • Author: Buchhave, Lars A.

    Astrophysics and Atmospheric Physics, National Space Institute, Technical University of Denmark, Elektrovej, 2800, Kgs. Lyngby, Denmark

  • Author: Deming, Drake L.

    University of Maryland, College Park, United States

  • Author: Ehrenreich, David

    University of Geneva, Switzerland

  • Author: Mikal-Evans, Thomas

    Massachusetts Institute of Technology, United States

  • Author: Kataria, Tiffany

    National Aeronautics and Space Administration, United States

  • Author: Lewis, Nikole K.

    Cornell University, United States

  • Author: Lopez-Morales, Mercedes

    Harvard-Smithsonian Center for Astrophysics, United States

  • Author: Muñoz, Antonio García

    Technical University of Berlin, Germany

  • Author: Henry, Gregory W.

    Tennessee State University, United States

  • Author: Sanz-Forcada, Jorge

    CSIC, Spain

  • Author: Spake, Jessica J.

    University of Exeter, United Kingdom

  • Author: Wakeford, Hannah R.

    Space Telescope Science Institute, United States

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We present Hubble Space Telescope (HST) near-ultraviolet (NUV) transits of the hot Jupiter WASP-121b, acquired as part of the PanCET program. Time-series spectra during two transit events were used to measure the transmission spectra between 2280 and 3070 Å at a resolution of 30,000. Using HST data from 61 Space Telescope Imaging Spectrograph visits, we show that data from HST's Pointing Control System can be used to decorrelate the instrument systematic errors (jitter decorrelation), which we used to fit the WASP-121b light curves. The NUV spectra show very strong absorption features, with the NUV white light curve found to be larger than the average optical and near-infrared value at 6σ confidence. We identify and spectrally resolve absorption from the Mg ii doublet in the planetary exosphere at a 5.9σ confidence level. The Mg ii doublet is observed to reach altitudes of R pl/R star = 0.284 ± 0.037 for the 2796 Å line and 0.242 ± 0.0431 for the 2804 Å line, which exceeds the Roche lobe size as viewed in transit geometry (R eqRL/R star = 0.158). We also detect and resolve strong features of the Fe ii UV1 and UV2 multiplets, and observe the lines reaching altitudes of R pl/R star ≈ 0.3. At these high altitudes, the atmospheric Mg ii and Fe ii gas is not gravitationally bound to the planet, and these ionized species may be hydrodynamically escaping or could be magnetically confined. Refractory Mg and Fe atoms at high altitudes also indicate that these species are not trapped into condensate clouds at depth, which places constraints on the deep interior temperature.

Original languageEnglish
Article number91
JournalAstrophysical Journal
Issue number2
Number of pages16
Publication statusPublished - 2019
CitationsWeb of Science® Times Cited: No match on DOI

    Research areas

  • Planets and satellites: atmospheres, Stars: individual (WASP-121), Techniques: photometric, Techniques: spectroscopic

ID: 189491976