Characterising M dwarf host stars of two candidate Hycean worlds

Abstract: Planetary systems orbiting M dwarf host stars are promising targets for atmospheric characterisation of low-mass exoplanets. Accurate characterisation of M dwarf hosts is important for detailed understanding of the planetary properties and physical processes, including potential habitability. Recent studies have identified several candidate Hycean planets orbiting nearby M dwarfs as promising targets in the search for habitability and life on exoplanets. In this study, we characterise two such M dwarf host stars, K2-18 and TOI-732. Using archival photometric and spectroscopic observations, we estimate their effective temperatures (T${\mathrm{eff}}$) and metallicities through high-resolution spectral analyses and ages through gyrochronology. We assess the stellar activity of the targets by analysing activity-sensitive chromospheric lines and X-ray luminosities. Additionally, we predict activity cycles based on measured rotation periods and utilise photometric data to estimate the current stellar activity phase. We find K2-18 to be 2.9-3.1 Gyr old with T${\mathrm{eff}}$ = 3645$\pm$52 K and metallicity of [Fe/H] = 0.10$\pm$0.12 dex, and TOI-732 to be older (6.7-8.6 Gyr), cooler (3213$\pm$92 K), and more metal-rich ([Fe/H] = 0.22$\pm$0.13 dex). Both stars exhibit relatively low activity making them favourable for atmospheric observations of their planets. The predicted activity cycle and analysis of available high-precision photometry for K2-18 suggest that it might have been near an activity minimum during recent JWST observations, though some residual activity may be expected at such minima. We predict potential activity levels for both targets to aid future observations, and highlight the importance of accurate characterisation of M dwarf host stars for exoplanet characterisation.