Axisymmetric High Spot Coverage on Exoplanet Host HD 189733 A

Abstract Transmission spectroscopy is a powerful tool to study exoplanet atmospheres, which can be affected by the ability of stellar photospheric heterogeneity to mimic or mask exoplanetary spectral signatures. The canonical HD 189733 system provides a textbook example of this spectroscopic discrepancy with features that have been variously interpreted as signatures of scattering by haze in the planetary atmosphere or unocculted spots on the stellar disk. Here, we leverage three archival data sets from the Hubble Space Telescope to directly infer the covering fraction of HD 189733 A and explore the evidence for photospheric heterogeneity in the out-of-transit spectra. We model the stellar spectrum using one to three spectral components in a nested-sampling framework, finding that the two-component model (photosphere and spot) is preferred for all data sets. We find photospheric and spot temperatures of 5295 ± 41 43 and 3222 ± 116 100 K, respectively, which are consistent across data sets. The spot covering fraction is large and varies between 38% ± 4% and 47% ± 3%. Combined with time-domain insights from Transiting Exoplanet Survey Satellite data revealing HD 189733 A's 1.4% peak-to-peak variability, our findings imply that the majority of the spots must be distributed axisymmetrically, e.g., in a densely filled latitudinal band or at the poles. More work with complementary data sets is necessary to investigate those possible arrangements.