HST and TESS Point to a Highly Spotted Photosphere for HD189733A

Transmission spectra can be affected by stellar photospheric heterogeneities that alter transit depths. The HD189733 system provides a clear example of this challenge, with features variously interpreted as signatures of scattering by haze in the planetary atmosphere or unocculted starspots. Here, we use three Hubble datasets to directly infer the spot covering fraction of HD189733A and explore the evidence for photospheric heterogeneity in the out-of-transit spectra. We model the stellar spectrum using 1–3 spectral components, finding that a two-component model (photosphere and spot) is preferred for all datasets. We find photospheric and spot temperatures of ~5295 K and ~3222 K, respectively, which are consistent across datasets. The spot covering fraction is large and varies between ~38% and ~47%. Combined with time-domain insights from TESS data revealing HD189733A'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. This work underscores the importance of considering axisymmetric spot distributions in analyses of transmission spectra and the unique role that TESS plays in this effort.