Optical properties of potential condensates in exoplanetary atmospheres

TL;DR

This paper introduces a new open-source Mie scattering code, LX-MIE, and compiles optical properties of 32 condensate species for exoplanetary atmospheres, enabling improved modeling of cloud effects in transmission spectra.

Contribution

The paper presents LX-MIE capable of handling large size parameters and provides a comprehensive database of refractive indices for 32 condensates, with simple fit functions for extinction efficiencies.

Findings

LX-MIE validated against classical codes.

Refractive indices compiled for 32 condensates.

Extinction efficiency fits achieve ~10% error in key wavebands.

Abstract

The prevalence of clouds in currently observable exoplanetary atmospheres motivates the compilation and calculation of their optical properties. First, we present a new open-source Mie scattering code known as LX-MIE, which is able to consider large size parameters ($\sim 10^7$) using a single computational treatment. We validate LX-MIE against the classical MIEV0 code as well as previous studies. Second, we embark on an expanded survey of the published literature for both the real and imaginary components of the refractive indices of 32 condensate species. As much as possible, we rely on experimental measurements of the refractive indices and resort to obtaining the real from the imaginary component (or vice versa), via the Kramers-Kronig relation, only in the absence of data. We use these refractive indices as input for LX-MIE to compute the absorption, scattering and extinction efficiencies of all 32 condensate species. Finally, we use a three-parameter function to provide convenient fits to the shape of the extinction efficiency curve. We show that the errors associated with these simple fits in the Wide Field Camera 3 (WFC3), J, H and K wavebands are $\sim 10\%$. These fits allow for the extinction cross section or opacity of the condensate species to be easily included in retrieval analyses of transmission spectra. We discuss prospects for future experimental work. The compilation of the optical constants and LX-MIE are publicly available as part of the open-source Exoclime Simulation Platform (http://www.exoclime.org).