Can 1D radiative equilibrium models of faculae be used for calculating contamination of transmission spectra?

TL;DR

This study assesses the accuracy of 1D radiative equilibrium models in representing facular contrasts for stellar magnetic features, highlighting their limitations compared to 3D magnetohydrodynamic simulations, which impacts exoplanet transmission spectroscopy.

Contribution

It demonstrates that 1D models cannot reliably reproduce 3D facular contrasts, emphasizing the need for more realistic modeling in exoplanet atmosphere studies.

Findings

1D models fail to match 3D facular contrast wavelength dependence.

Simplified models may lead to biases in exoplanet atmospheric characterization.

3D MHD simulations provide more accurate stellar magnetic feature representations.

Abstract

The reliable characterization of planetary atmospheres with transmission spectroscopy requires realistic modeling of stellar magnetic features, since features that are attributable to an exoplanet atmosphere could instead stem from the host star's magnetic activity. Current retrieval algorithms for analysing transmission spectra rely on intensity contrasts of magnetic features from 1D radiative-convective models. However, magnetic features, especially faculae, are not fully captured by such simplified models. Here we investigate how well such 1D models can reproduce 3D facular contrasts, taking a G2V star as an example. We employ the well established radiative magnetohydrodynamic code MURaM to obtain three-dimensional simulations of the magneto-convection and photosphere harboring a local small-scale-dynamo. Simulations without additional vertical magnetic fields are taken to describe the quiet solar regions, while simulations with initially 100 G, 200 G and 300 G vertical magnetic fields are used to represent different magnetic activity levels. Subsequently, the spectra emergent from the MURaM cubes are calculated with the MPS-ATLAS radiative transfer code. We find that the wavelength dependence of facular contrast from 1D radiative-convective models cannot reproduce facular contrasts obtained from 3D modeling. This has far reaching consequences for exoplanet characterization using transmission spectroscopy, where accurate knowledge of the host star is essential for unbiased inferences of the planetary atmospheric properties.