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

Abstract

Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436b absorbs over 50% of the stellar Ly$\alpha$ 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 HST COS G130M observations made during the planet's extended H I transit. These observations show no transit absorption in the C II 1334,1335 Å and Si III 1206 Å lines integrated over [-100, 100] km s${}^{-1}$, imposing 95% (2$\sigma$) 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$\alpha$. C${}^{+}$ is likely present in the outflow according to a simulation we carried out using a spherically-symmetric, photochemical-hydrodynamical model. This simulation predicts a $\sim$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 $1.6\times 10^9$ g s${}^{-1}$ ($3.1\times 10^9$ g s${}^{-1}$ for all species) for an upper atmosphere composed of hydrogen and helium.

7 pages, 4 figures, 1 table; accepted to ApJ Letters

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• The fundamentals of Lyman-alpha exoplanet transits
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• Transit Ly-$\alpha$ signatures of terrestrial planets in the habitable zones of M dwarfs
• Transit Visibility Zones of the Solar System Planets
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• H${}_2$ Fluorescence in M Dwarf Systems: A Stellar Origin
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• HST PanCET program: Non-detection of atmospheric escape in the warm Saturn-sized planet WASP-29 b
• Mysterious non-detection of HeI (23S) transit absorption of GJ436b
• sunset: A database of synthetic atmospheric-escape transmission spectra for nearly every transiting planet
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• Comparative Analysis of the Model for Exoplanet Atmosphere Outflow