A Model of the H$\alpha$ and Na Transmission Spectrum of HD 189733b

TL;DR

This study develops a hydrostatic model of HD 189733b's upper atmosphere to interpret Hα and Na transmission spectra, revealing the influence of Lyα excitation and stellar LyC variability on observed transit depths.

Contribution

It introduces a detailed hydrostatic atmospheric model that links Hα and Na line formation to Lyα excitation and stellar LyC flux variability, advancing understanding of exoplanet atmospheric spectra.

Findings

Model spectra broadly agree with observations.

Lyα radiative excitation dominates H(2ℓ) population.

Hα transit depth variability linked to stellar LyC flux changes.

Abstract

This paper presents a detailed hydrostatic model of the upper atmosphere of HD 189733b, with the goal of constraining its temperature, particle densities, and radiation field over the pressure range $10^{-4}-10\, \mu \rm bar$, where the observed H$\alpha$ transmission spectrum is produced. The atomic hydrogen level population is computed including both collisional and radiative transition rates. The Ly$\alpha$ resonant scattering is computed using a Monte-Carlo simulation. The model transmission spectra are in broad agreement with the data. Excitation of the H(2$\ell$) population is mainly by Ly$\alpha$ radiative excitation due to the large Ly$\alpha$ intensity. The density of H(2$\ell$) is nearly flat over two decades in pressure, and is optically thick to H$\alpha$. Additional models computed for a range of the stellar Lyman continuum (LyC) flux suggest that the variability in H$\alpha$ transit depth may be due to the variability in the stellar LyC. Since metal lines provide the dominant cooling of this part of the atmosphere, the atmosphere structure is sensitive to the density of species such as Mg and Na which may themselves be constrained by observations. Since the H$\alpha$ and Na D lines have comparable absorption depths, we argue that the center of the Na D lines are also formed in the atomic layer where the H$\alpha$ line is formed.