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

TL;DR

This study investigates the evaporating atmosphere of GJ436b, focusing on hydrogen, carbon, and silicon, using ultraviolet transit spectroscopy and modeling to understand atmospheric escape and composition.

Contribution

It provides new observational limits on C II and Si III escape and introduces a photochemical-hydrodynamical model predicting escape rates and ion presence.

Findings

No significant transit absorption detected in C II and Si III lines.

Model predicts a 2% C II transit consistent with observations.

Hydrogen escape rate estimated at 1.6×10^9 g/s.

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 \AA\ and Si III 1206 \AA\ 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 $1.6\times10^{9}$ g s$^{-1}$ ($3.1\times10^{9}$ g s$^{-1}$ for all species) for an upper atmosphere composed of hydrogen and helium.