Correcting the effect of stellar spots on ARIEL transmission spectra II.The limb darkening effect

TL;DR

This study enhances the correction of stellar spot effects on exoplanet transmission spectra by incorporating limb darkening, demonstrating its importance for accurate spectral retrieval in active star systems.

Contribution

It introduces a method that accounts for limb darkening in modeling stellar spots, improving the accuracy of transmission spectrum correction for transiting exoplanets.

Findings

Limb darkening significantly affects spot parameter estimation.

Correct modeling allows reliable spectral retrieval for spots near the stellar center.

Method performs well with accurate stellar parameters and instrument simulations.

Abstract

This paper is part of an effort to correct the transmission spectra of a transiting planet orbiting an active star. In Paper I (Cracchiolo et al. 2020) we have demonstrated a methodology to minimize the potential bias induced by unocculted star spots on the transmission spectrum, assuming a spot model parameterized by filling factor and temperature. In this work we introduce the limb darkening effect, therefore the position of the spot in the stellar disk and the impact parameter of the transiting planet now play a key role. The method is tested on simulations of planetary transits of three representative kinds of planetary systems, at ARIEL resolution. We find that a realistic treatment of the limb darkening is required to reliably estimate both the spots parameters and the transmission spectrum of the transiting planet. Furthermore, we show that the influence of the spots onthe retrieval of the planetary transmission spectrum is significant for spots close to the center of the star, covering a fraction greater than 0.05 and with a temperature contrast greater than 500 K, and that for these cases our method can confidently extract the transmission spectrum and the impact parameter of the transiting planet for both cases of occulted and not occulted spots, provided that we have an accurate characterization of the stellar parameters and a reliable simulator of the instrument performances.