Water/Hycean Worlds & Oceanography

Characterising M Dwarf Host Stars Of Two Candidate Hycean Worlds

By Keith Cowing
Status Report
astro-ph.EP
March 27, 2025
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Characterising M Dwarf Host Stars Of Two Candidate Hycean Worlds
Left panel: The rotation period-effective temperature distribution for M dwarfs across open clusters. Each data point is colour-coded according to its corresponding cluster. The spectral type is indicated on a second y-axis. The filled circles are coloured according to their corresponding cluster membership: Pleiades (age:0.12 Gyr) in blue; Praesepe (age: 0.67 Gyr) in green; NGC 6811 (age: 1 Gyr) in purple; NGC 752 (age: 1.4 Gyr) in pink; Ruprecht 147 (age: 2.7 Gyr) in yellow; M67 (age: 4 Gyr) in cyan. K2-18 and TOI 732 are shown as magenta and blue star symbols, respectively. Right panel: Log-log plot of age and rotation period for M dwarfs in the dataset from Engle & Guinan 2023 (shown in black circles). The red colour-coded data points represent cluster members (as reported by Curtis et al. 2020 and Godoy-Rivera et al. 2021) spectral types ranging from M2 to M6, as indicated by the colour bar. A third-degree polynomial best fit (Eqn. 5) to the dataset from Engle & Guinan 2023 is shown as an orange line. The grey shaded area represents the ±1𝜎 confidence interval around the fitted model. The magenta and blue vertical lines show the estimated age ranges as given in Table 4 for K2-18 and TOI-732, respectively. — astro-ph.EP

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 (Teff) 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 Teff = 3645±52 K and metallicity of [Fe/H] = 0.10±0.12 dex, and TOI-732 to be older (6.7-8.6 Gyr), cooler (3213±92 K), and more metal-rich ([Fe/H] = 0.22±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.

Lalitha Sairam, Nikku Madhusudhan

Comments: 18 pages, 15 figures, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2503.19908 [astro-ph.EP] (or arXiv:2503.19908v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2503.19908
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Submission history
From: Sairam Lalitha
[v1] Tue, 25 Mar 2025 17:59:14 UTC (2,672 KB)
https://arxiv.org/abs/2503.19908
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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