An H-Alpha Transit of HD 189733b to Assess Stellar Activity Across Transit Chord Prior to JWST Observations

The field of exoplanets has expanded in recent years from detection of exoplanets to characterizations of their compositions and atmospheres. Transmission spectroscopy allows us to detect molecules in planetary atmospheres, however, it also is subject to contaminations from the stellar atmosphere and inhomogeneities in the stellar surface. Quantifying the extent of this contamination is challenging, but important, as features in stars can manifest as false atmospheric signals in planetary transmission spectra. This effect is exacerbated for planets with cooler host stars, because H2O in star spots can lead to apparent water features in the planet’s transmission spectrum. HD 189733b is a hot Jupiter around an active, cool star that shows evidence of stellar contamination in its transmission spectra. This is a benchmark planet with over 50 hours of observations scheduled for JWST, so it is important to understand the activity level of its host star, particularly around the planet’s transit chord and as close in time to JWST observations as feasible. We utilize high-resolution spectra centered around the Hα line from the Gran Telescopio Canarias’ MEGARA spectrograph to examine the activity level of HD 189733 during a transit, with observations before and after as a benchmark. By measuring deviations in the Hα depth and its equivalent width, we can see how active the star is during the transit, particularly in the region around the transit chord to determine the level of stellar contamination on future JWST observations of its transmission spectrum.