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We present models designed to quantify the effects of stellar activity onexoplanet transit spectroscopy and atmospheric characterization at low (R =100) and high (R = 100,000) spectral resolution. We study three model classesmirroring planetary system archetypes: a hot Jupiter around an early-K star (HD189733 b); a mini-Neptune around an early-M dwarf (K2-18 b); and terrestrialplanets around a late M dwarf (TRAPPIST-1). We map photospheres withtemperatures and radial velocities (RV) and integrate specific intensitystellar models. We obtain transit spectra affected by stellar contamination,the Rossiter--McLaughlin effect (RME), and center-to-limb variations (CLV). Wefind that, at low resolution, for later-type stars, planetary water featuresbecome difficult to distinguish from contamination. Many distributions ofunocculted active regions can induce planetary-like features of similaramplitudes in the case of a late M dwarf. Atmospheric characterization ofplanets around late-type stars will likely continue to suffer from degeneracywith stellar activity unless active regions' parameters can be constrainedusing additional information. For the early-K star, stellar contaminationmostly manifests itself through a slope at optical wavelengths similar toRayleigh scattering. In all cases, contamination induces offsets in measuredplanet radii. At high resolution, we show that we can determine the origin of$\text{H}_2$O and CO detection signals and lift the degeneracy observed at lowresolution, provided sufficient planet RV variation during transit and adequatecorrection for the RME and CLV when required. High-resolution spectroscopy maytherefore help resolve issues arising from stellar contamination for favorablesystems.

On the Effect of Stellar Activity on Low-resolution Transit Spectroscopy and the Use of High Resolution as Mitigation