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HD 189733 b is one of the most well-studied exoplanets due to its largetransit depth and host star brightness. The focus on this object has produced anumber of high-cadence transit observations using high-resolution opticalspectrographs. Here we present an analysis of seven full H$\alpha$ transits ofHD 189733 b using HARPS on the 3.6 meter La Silla telescope and HIRES on KeckI, 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 findstrong variations in the strength of the H$\alpha$ transmission spectrum fromepoch to epoch. However, there is no clear trend between the Ca II coreemission and the strength of the in-transit H$\alpha$ signal, although thetransit showing the largest absorption value also occurs when the star is themost active. We present simulations of the in-transit contrast effect and findthat the planet must consistently transit active latitudes with very strongfacular and plage emission regions in order to reproduce the observed linestrengths. We also investigate the measured velocity centroids with models ofplanetary rotation and show that the small line profile velocities could be dueto large velocities in the upper atmosphere of the planet. Overall, we find itmore likely that the measured H$\alpha$ signals arise in the extended planetaryatmosphere, although a better understanding of active region emission foractive stars such as HD 189733 are needed.

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