Non-Local Thermodynamic Equilibrium Transmission Spectrum Modelling of HD209458b

TL;DR

This paper develops a NLTE radiative transfer model for exoplanet transmission spectra, applied to HD209458b, revealing significant NLTE effects on spectral line strengths and improving the interpretation of atmospheric observations.

Contribution

We adapt the Cloudy code to model NLTE effects in exoplanet atmospheres, providing a versatile framework for analyzing transmission spectra across diverse planetary conditions.

Findings

NLTE effects can cause up to 40% stronger absorption in spectral lines.

The model successfully reproduces observed features like He I 10830 and Na I D lines.

NLTE modeling enhances understanding of exoplanet atmospheric composition.

Abstract

Context - Exoplanetary upper atmospheres are low density environments where radiative processes can compete with collisional ones and introduce non-local thermodynamic equilibrium (NLTE) effects into transmission spectra. Aims - We develop a NLTE radiative transfer framework capable of modelling exoplanetary transmission spectra over a wide range of planetary properties. Methods - We adapt the NLTE spectral synthesis code Cloudy to produce an atmospheric structure and atomic transmission spectrum in both NLTE and local thermodynamic equilibrium (LTE) for the hot Jupiter HD209458b, given a published T-P profile and assuming solar metallicity. Selected spectral features, including H$\alpha$, Na I D, He I $\lambda$10830, Fe I & II ultra-violet (UV) bands, and C, O and Si UV lines, are compared with literature observations and models where available. The strength of NLTE effects are measured for individual spectral lines to identify which features are most strongly affected. Results - The developed modelling framework computing NLTE synthetic spectra reproduces literature results for the He I $\lambda$10830 triplet, the Na I D lines, and the forest of Fe I lines in the optical. Individual spectral lines in the NLTE spectrum exhibit up to 40 % stronger absorption relative to the LTE spectrum.