Effect of stellar spot-only spectral lines in high-resolution transit spectroscopy
We investigate how a representative single spectral line present only in a stellar spot can introduce distortions in high-resolution planetary absorption spectra. Specifically, we assess whether such spot-induced features may resemble planetary atmospheric absorption and the observational or stellar conditions under which they become significant. Using the SOAPv4 code, we simulated transit observations of an atmosphere-less planet across a star with a spot containing an additional absorption line compared to the photosphere's spectrum. Two systems were modelled: a close-in Jupiter-sized planet and a distant Earth-sized planet transiting a Sun-sized star. We varied the stellar rotation period and the spot size, and ensured that the planet transits the spot. Spectral lines were idealised Gaussians to isolate the impact of the spot-specific feature. We show that a single stellar spot line can generate spurious features in the absorption spectrum. These features contaminate the absorption spectrum in ways that may lead to misinterpretation if not accounted for. For Jupiter-sized planets with short transits, the features are mainly emission-like, but can nonetheless dampen a potential true atmospheric absorption signature. For Earth-sized planets with long transits, the distortion is mainly absorption-like and could be interpreted as planetary absorption. These results highlight the importance of accounting for stellar heterogeneity and activity in the analysis of high-resolution absorption spectra.