Can 1D Radiative-equilibrium Models of Faculae Be Used for Calculating Contamination of Transmission Spectra?

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 analyzing 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 magnetoconvection 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 facular regions of different magnetic flux density. 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.