Medium Stars with Big Attitudes: Testing the Effect of Stellar Activity in the Atmospheric Signals of Plants Transiting F-type Stars

While our ability to observe and characterize exoplanet atmospheres has grown immensely in the past decade, stellar activity continues to present a major barrier towards a clear understanding of transmission spectra. Notably, there have been three exoplanets around F- type stars (WASP-121 b, WASP-79 b, and WASP-103 b) with quiet photometric curves but radial velocity (RV) plots that indicate activity, revealing the potential presence of faculæ. In this work, we explore how faculæ can present a source of contamination in transmission spectra of F-type stars by modifying SOAP 2.0, a code that simulates the effects of active regions on stars. We compare the activity indicators (i.e. bisector span, full width at half maximum) observed in these three F stars to the values modeled in SOAP 2.0 to determine if faculæ can cause the activity observed while not appearing in the star’s photometry. We find that it is plausible for widespread, hot faculæ to be present within observations of WASP-121 b, in contrast to prior studies that have rejected significant stellar activity as a source of contamination. We then explore the wide applicability of our method to other transiting exoplanet systems, focusing specifically on the case of TOI-1266—an M-dwarf with two planetary companions. Although stellar activity is historically not considered for transmission spectra around F stars, our results highlight a potential source of contamination that must be taken into account for other observations of transmission spectra in exoplanets around F stars. Further, we present a method that future studies can use to assess the likelihood that faculæ are contaminating transit observations—which can be applied to any system with F stars or smaller.