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Arxiv: Modelling methanol and hydride formation in the JWST Ice Age era Published: 2/14/2025 2:44:05 PM Updated: 2/14/2025 2:44:05 PM

Paper abstract: (Abridged) JWST observations have measured the ice composition toward twohighly-extinguished field stars in the Chamaeleon I cloud. The observedextinction excess on the long-wavelength side of the H2O ice band at 3 micronhas been attributed to a mixture of CH3OH with ammonia hydrates, which suggeststhat CH3OH ice could have formed in a water-rich environment with little COdepletion. Laboratory experiments and quantum chemical calculations suggestthat CH3OH could form via the grain surface reactions CH3+OH and/or C+H2O inwater-rich ices. However, no dedicated chemical modelling has been carried outthus far to test their efficiency and dependence on the astrochemical codeemployed. We model the ice chemistry in the Chamaeleon I cloud using a set ofastrochemical codes (MAGICKAL, MONACO, Nautilus, UCLCHEM, and KMC simulations)to test the effects of the different code architectures and of the assumed icechemistry. Our models show that the JWST ice observations are better reproducedfor gas densities >1e5 cm-3 and collapse times >1e5 yr. CH3OH ice formspredominantly (>99%) via CO hydrogenation. The contribution of reactions CH3+OHand C+H2O, is negligible. The CO2 ice may form either via CO+OH or CO+Odepending on the code. However, KMC simulations reveal that both mechanisms areefficient despite the low rate constant of the CO+O surface reaction. CH4 islargely underproduced for all codes except for UCLCHEM, for which a higheramount of atomic C is available during the initial translucent cloud phase.Large differences in the ice abundances are found at Tdust<12 K betweendiffusive and non-diffusive chemistry codes. This is due to the fact thatnon-diffusive chemistry takes over diffusive chemistry at such low Tdust. Thiscould explain the rather constant ice chemical composition found in ChamaeleonI and other dense cores despite the different visual extinctions probed.