# A decade of H$\alpha$ transits for HD 189733 b: stellar activity versus absorption in the extended atmosphere

## TL;DR

This study analyzes seven years of Hα transit observations of HD 189733 b, revealing variability influenced by stellar activity and suggesting the signals originate from the planet's extended atmosphere rather than stellar activity.

## Contribution

It provides a comprehensive analysis of long-term Hα transit data, linking stellar activity levels to atmospheric signals and modeling the contrast effect to distinguish planetary absorption.

## Key findings

- Hα transmission signals vary significantly over years.
- No clear correlation between stellar activity and Hα absorption strength.
- Planetary atmospheric signals likely dominate over stellar activity effects.

## Abstract

HD 189733 b is one of the most well-studied exoplanets due to its large transit depth and host star brightness. The focus on this object has produced a number of high-cadence transit observations using high-resolution optical spectrographs. Here we present an analysis of seven full H$\alpha$ transits of HD 189733 b using HARPS on the 3.6 meter La Silla telescope and HIRES on Keck I, taken over the course of nine years from 2006 to 2015. H$\alpha$ transmission signals are analyzed as a function of the stellar activity level, as measured using the normalized core flux of the Ca II H and K lines. We find strong variations in the strength of the H$\alpha$ transmission spectrum from epoch to epoch. However, there is no clear trend between the Ca II core emission and the strength of the in-transit H$\alpha$ signal, although the transit showing the largest absorption value also occurs when the star is the most active. We present simulations of the in-transit contrast effect and find that the planet must consistently transit active latitudes with very strong facular and plage emission regions in order to reproduce the observed line strengths. We also investigate the measured velocity centroids with models of planetary rotation and show that the small line profile velocities could be due to large velocities in the upper atmosphere of the planet. Overall, we find it more likely that the measured H$\alpha$ signals arise in the extended planetary atmosphere, although a better understanding of active region emission for active stars such as HD 189733 are needed.