# The GTC exoplanet transit spectroscopy survey X. Stellar spots versus Rayleigh scattering: the case of HAT-P-11b

## TL;DR

This study investigates how stellar spots can mimic Rayleigh scattering signals in exoplanet transmission spectra, using HAT-P-11b observations to distinguish stellar activity effects from atmospheric features.

## Contribution

It demonstrates that stellar spots can produce apparent spectral slopes and absorption features, emphasizing the importance of accounting for stellar activity in exoplanet atmosphere studies.

## Key findings

- Stellar spots can mimic Rayleigh scattering signatures in transmission spectra.
- The observed spectral slope was caused by unocculted stellar spots, not atmospheric Rayleigh scattering.
- Unocculted spots contributed to Na absorption features in the data.

## Abstract

Rayleigh scattering in a hydrogen-dominated exoplanet atmosphere can be detected from ground or space based telescopes, however, stellar activity in the form of spots can mimic Rayleigh scattering in the observed transmission spectrum. Quantifying this phenomena is key to our correct interpretation of exoplanet atmospheric properties. We obtained long-slit optical spectroscopy of two transits of HAT-P-11b with the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at Gran Telescopio Canarias (GTC) on August 30 2016 and September 25 2017. We integrated the spectrum of HAT-P-11 and one reference star in several spectroscopic channels across the $\lambda\sim$ 400-785 nm region, creating numerous light curves of the transits. We fit analytic transit curves to the data taking into account the systematic effects and red noise present in the time series in an effort to measure the change of the planet-to-star radius ratio ($R_\mathrm{p}/R_\mathrm{s}$) across wavelength. By fitting both transits together, we find a slope in the transmission spectrum showing an increase of the planetary radius towards blue wavelengths. A closer inspection to the transmission spectrum of the individual data sets reveals that the first transit presents this slope while the transmission spectrum of the second data set is flat. Additionally we detect hints of Na absorption in the first night, but not in the second. We conclude that the transmission spectrum slope and Na absorption excess found in the first transit observation are caused by unocculted stellar spots. Modeling the contribution of unocculted spots to reproduce the results of the first night we find a spot filling factor of $\delta=0.62^{+0.20}_{-0.17}$ and a spot-to-photosphere temperature difference of $\Delta T = 429^{+184}_{-299}$ K.