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

• Published in: The Astronomical journal 2022-05, Vol.163 (5), p.231
• Main Authors: Genest, Frédéric, Lafrenière, David, Boucher, Anne, Darveau-Bernier, Antoine, Doyon, René, Artigau, Étienne, Cook, Neil
• Format: Article
• Language: English
• Subjects: Astronomical models; Astronomy; Atmospheric models; Contamination; Exoplanet atmospheres; Exoplanets; Extrasolar planets; Gas giant planets; High resolution; Jupiter; K stars; Mitigation; Planetary systems; Rayleigh scattering; Red dwarf stars; Spectral resolution; Spectroscopy; Spectrum analysis; Starspots; Stellar activity; Stellar faculae; Stellar models; Terrestrial environments; Terrestrial planets; Transit; Transits; Transmission spectroscopy; Wavelengths
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/ac5e38

We present models designed to quantify the effects of stellar activity on exoplanet transit spectroscopy and atmospheric characterization at low ( R = 100) and high ( R = 100,000) spectral resolution. We study three model classes mirroring planetary system archetypes: a hot Jupiter around an early-K star (HD 189733 b); a mini-Neptune around an early-M dwarf (K2-18 b); and terrestrial planets around a late-M dwarf (TRAPPIST-1). We map photospheres with temperatures and radial velocities (RV) and integrate specific intensity stellar models. We obtain transit spectra affected by stellar contamination, the Rossiter–McLaughlin effect (RME), and center-to-limb variations (CLV). We find that, at low resolution, for later-type stars, planetary water features become difficult to distinguish from contamination. Many distributions of unocculted active regions can induce planetary-like features of similar amplitudes in the case of a late-M dwarf. Atmospheric characterization of planets around late-type stars will likely continue to suffer from degeneracy with stellar activity unless active regions' parameters can be constrained using additional information. For the early-K star, stellar contamination mostly manifests itself through a slope at optical wavelengths similar to Rayleigh scattering. In all cases, contamination induces offsets in measured planet radii. At high resolution, we show that we can determine the origin of H 2 O and CO detection signals and lift the degeneracy observed at low resolution, provided sufficient planet RV variation during transit and adequate correction for the RME and CLV when required. High-resolution spectroscopy may therefore help resolve issues arising from stellar contamination for favorable systems.