Electron densities and alkali atoms in exoplanet atmospheres

TL;DR

This study investigates alkali atom behavior in exoplanet atmospheres, focusing on electron densities and transit observations, revealing discrepancies between models and observations for HD 209458 b and better agreement for XO-2 b.

Contribution

It provides a detailed photochemical model of alkali atoms in exoplanet atmospheres and compares predictions with observations, highlighting the need for considering clouds or aerosols.

Findings

Photoionization leads to high electron densities in HD 209458 b's atmosphere.

Model overestimates K transit depth for HD 209458 b.

Good agreement between models and observations for XO-2 b.

Abstract

We describe a detailed study on the properties of alkali atoms in extrasolar giant planets, and specifically focus on their role in generating the atmospheric free electron densities, as well as their impact on the transit depth observations. We focus our study on the case of HD 209458 b, and we show that photoionization produces a large electron density in the middle atmosphere that is about two orders of magnitude larger than the density anticipated from thermal ionization. Our purely photochemical calculations though result in a much larger transit depth for K than observed for this planet. This result does not change even if the roles of molecular chemistry and excited state chemistry are considered for the alkali atoms. In contrast, the model results for the case of exoplanet XO-2 b are in good agreement with the available observations. Given these results we discuss other possible scenarios, such as changes in the elemental abundances, changes in the temperature profiles, and the possible presence of clouds, which could potentially explain the observed HD 209458 b alkali properties. We find that most of these scenarios can not explain the observations, with the exception of a heterogeneous source (i.e. clouds or aerosols) under specific conditions, but we also note the discrepancies among the available observations.