Correcting the effect of stellar spots on ARIEL transmission spectra

TL;DR

This paper presents a method to model and correct the effects of stellar spots on ARIEL's planetary transmission spectra, improving the accuracy of atmospheric characterization by accounting for stellar activity.

Contribution

It introduces a novel approach to model stellar spectra with spots using out-of-transit data and corrects transit depth variations, enhancing spectral analysis accuracy.

Findings

Successfully recovers spot parameters from out-of-transit spectra.

Confidently retrieves planetary transmission spectra within 3.3% uncertainty.

Identifies biases caused by occulted spots, especially in bright targets.

Abstract

The goal of this study is to assess the impact of the stellar spots on the extraction of the planetary transmission spectra observed by ARIEL. We develop a method to model the stellar spectrum of a star in the presence of spots by using the out-of-transit observations. It is based on a chi squared minimization procedure of the out-of-transit spectrum on a grid of stellar spectra with different sizes and temperatures of the spots. The approach allows us also to study the temporal evolution of the spots when comparing stellar spectra observed at different epochs. We also present a method to correct the transit depth variations due to non-occulted stellar spots and estimate the error we introduce if we apply the same correction to crossings over the stellar spots. The method is tested on three types of stellar targets that ARIEL will observe in its 4-years mission lifetime. In all the explored cases, the approach allows to reliably recover the spot parameters (size and temperature) from out-of-transit observations and, for non-occulted spots, to confidently recover the planetary atmosphere transmission spectrum within the noise level(with average uncertainty of at most 3.3% of the planetary signal). Conversely, we find systematic biases in the inferred planetary spectra due to the occulted spots, with measurable effects for the brightest targets especially for more contrasted spots.