Ultraviolet C II and Si III Transit Spectroscopy and Modeling of the Evaporating Atmosphere of GJ436b

R. O.Parke Loyd, T. T. Koskinen, Kevin France, Christian Schneider, Seth Redfield

Research output: Contribution to journalArticlepeer-review

26 Scopus citations


Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436b absorbs over 50% of the stellar Ly emission during transit. Given the planet's atmospheric composition and energy-limited escape rate, this hydrogen outflow is expected to entrain heavier atoms such as C and O. We searched for C and Si in the escaping atmosphere of GJ436b using far-ultraviolet Hubble Space Telescope COS G130M observations made during the planet's extended H i transit. These observations show no transit absorption in the C ii 1334,1335 and Si iii 1206 lines integrated over [-100, 100] km s-1, imposing 95% (2σ) upper limits of 14% (C ii) and 60% (Si iii) depth on the transit of an opaque disk and 22% (C ii) and 49% (Si iii) depth on an extended highly asymmetric transit similar to that of H i Ly. C+ is likely present in the outflow according to a simulation we carried out using a spherically symmetric photochemical-hydrodynamical model. This simulation predicts an ∼2% transit over the integrated bandpass, consistent with the data. At line center, we predict the C ii transit depth to be as high as 19%. Our model predicts a neutral hydrogen escape rate of g s-1 (g s-1 for all species) for an upper atmosphere composed of hydrogen and helium.

Original languageEnglish (US)
Article numberL17
JournalAstrophysical Journal Letters
Issue number2
StatePublished - Jan 10 2017


  • planets and satellites: atmospheres
  • planets and satellites: individual (GJ436b)
  • planetstar interactions

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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