Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b

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Volume 701 (September 2025)

A&A, 701 (2025) A33

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Table 4

Retrieved parameters and species abundances of the best fit.

Parameters	Values
Metallicity (1σ)	$266_{- 61}^{+ 90}$ $266_{-61}^{+90}$
Metallicity (2σ)	$266_{- 104}^{+ 291}$ $266_{-104}^{+291}$
Metallicity (3σ)	≥ 129
C/O (1 σ)	≥ 7.1
C/O (2σ)	≥ 2.1
C/O (3 σ)	≥ 0.96
K_zz[cm² s⁻¹]	All within 1σ
Abundances at 1 mbar of best fit at 3σ
log₁₀[CH₄]	$- {0.3}_{- 1.7}^{+ 0.1}$ $-0.3_{-1.7}^{+0.1}$
log₁₀[CO₂]	$- {5.6}_{- 0.9}^{+ 4.9}$ $-5.6_{-0.9}^{+4.9}$
log₁₀[CO]	$- {2.2}_{- 0.6}^{+ 1.8}$ $-2.2_{-0.6}^{+1.8}$
log₁₀[H₂O]	$- {4.2}_{- 4.3}^{+ 3.5}$ $-4.2_{-4.3}^{+3.5}$
log10[NH₃]	$- {3.8}_{- 5.8}^{+ 0.6}$ $-3.8_{-5.8}^{+0.6}$
log₁₀[H₂CO]	$- {12.6}_{- 1.1}^{+ 5.9}$ $-12.6_{-1.1}^{+5.9}$
log₁₀[CH₃OH]	$- {9.6}_{- 4.0}^{+ 3.3}$ $-9.6_{-4.0}^{+3.3}$
log₁₀[CH₃CHO]	$- {11.3}_{- 1.8}^{+ 4.0}$ $-11.3_{-1.8}^{+4.0}$
log₁₀[C₂H₂]	$- {7.0}_{- 7.5}^{+ 4.0}$ $-7.0_{-7.5}^{+4.0}$
log₁₀[C₂H₄]	$- {9.6}_{- 1.1}^{+ 8.6}$ $-9.6_{-1.1}^{+8.6}$
log₁₀[C₂H₆]	$- {4.1}_{- 2.0}^{+ 2.2}$ $-4.1_{-2.0}^{+2.2}$
log₁₀[N₂]	$- {1.4}_{- 0.5}^{+ 0.3}$ $-1.4_{-0.5}^{+0.3}$

Notes. All values and their uncertainties are determined using FRECKLL and TauREx simulations compared to JWST data (Madhusudhan et al. 2023). The species abundance values are taken at 1 mbar, which is representative of the probed atmosphere.

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