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Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436babsorbs over 50% of the stellar Ly$\alpha$ emission during transit. Given theplanet's atmospheric composition and energy-limited escape rate, this hydrogenoutflow is expected to entrain heavier atoms such as C and O. We searched for Cand Si in the escaping atmosphere of GJ436b using far-ultraviolet HST COS G130Mobservations made during the planet's extended H I transit. These observationsshow no transit absorption in the C II 1334,1335 \AA\ and Si III 1206 \AA\lines integrated over [-100, 100] km s$^{-1}$, imposing 95% (2$\sigma$) upperlimits of 14% (C II) and 60% (Si III) depth on the transit of an opaque diskand 22% (C II) and 49% (Si III) depth on an extended, highly asymmetric transitsimilar to that of H I Ly$\alpha$. C$^+$ is likely present in the outflowaccording to a simulation we carried out using a spherically-symmetric,photochemical-hydrodynamical model. This simulation predicts a $\sim$2% transitover the integrated bandpass, consistent with the data. At line center, wepredict the C II transit depth to be as high as 19%. Our model predicts aneutral hydrogen escape rate of $1.6\times10^{9}$ g s$^{-1}$ ($3.1\times10^{9}$g s$^{-1}$ for all species) for an upper atmosphere composed of hydrogen andhelium.

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