Impact of occultations of stellar active regions on transmission spectra: Can occultation of a plage mimic the signature of a blue sky?

TL;DR

This study investigates how occultations of stellar active regions, like plages, can distort transmission spectra of transiting exoplanets, potentially mimicking atmospheric features such as blue sky scattering.

Contribution

It is the first to simulate and analyze the impact of occulted stellar plages on exoplanet transmission spectra, revealing significant effects that can bias atmospheric interpretations.

Findings

Occulted stellar plages can cause up to 10% errors in planet-to-star radius ratio at short wavelengths.

Such occultations can mimic atmospheric scattering signatures, affecting the interpretation of blue slopes in transmission spectra.

Multiple observations during transit are recommended to distinguish stellar activity effects from planetary atmospheric features.

Abstract

Transmission spectroscopy during planetary transits, which is based on the measurements of the variations of planet-to-star radius ratio as a function of wavelength, is a powerful technique to study the atmospheric properties of transiting planets. One of the main limitation of this technique is the effects of stellar activity, which up until now, have been taken into account only by assessing the effect of non-occulted stellar spots on the estimates of planet-to-star radius ratio. In this paper, we study, for the first time, the impact of the occultation of a stellar spot and plage on the transmission spectra of transiting exoplanets. We simulated this effect by generating a large number of transit light curves for different transiting planets, stellar spectral types, and for different wavelengths. Results of our simulations indicate that the anomalies inside the transit light curve can lead to a significant underestimation or overestimation of the planet-to-star radius ratio as a function of wavelength. At short wavelengths, the effect can reach to a difference of up to 10% in the planet-to-star radius ratio, mimicking the signature of light scattering in the planetary atmosphere. Atmospheric scattering has been proposed to interpret the increasing slopes of transmission spectra toward blue for exoplanets HD 189733b and GJ 3470b. Here we show that these signatures can be alternatively interpreted by the occultation of stellar plages. Results also suggest that the best strategy to identify and quantify the effects of stellar activities on the transmission spectrum of a planet is to perform several observations during the transit epoch at the same wavelength. This will allow for identifying the possible variations in transit depth as a function of time due to stellar activity variability.