Modelling methanol and hydride formation in the JWST Ice Age era

¹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, 28850 Torrejón de Ardoz, Spain
² Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
³ Astrochemistry Laboratory, Code 691, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ Department of Physics, Catholic University of America, Washington, DC 20064, USA
⁵ Laboratoire d’astrophysique de Bordeaux, Univ.Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁶ Institute of Natural Sciences and Mathematics, Ural Federal University, 19 Mira Str., 620075 Ekaterinburg, Russia
⁷ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstrasse 1, 85748 Garching bei München, Germany
⁸ Department of Physics, University of Central Florida, Orlando, FL 32816, USA
⁹ Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay, 91405 Orsay, France
¹⁰ Physique des Interactions Ioniques et Moléculaires, CNRS, Aix Marseille Univ., 13397 Marseille, France
¹¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹² Transdisciplinary Research Area (TRA) ‘Matter’/Argelander-Institut für Astronomie, University of Bonn, Germany
¹³ Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁴ Institute of Astronomy of the RAS, Pyatnitskaya st. 48, 119017, Moscow
¹⁵ Department of Astronomy and Chemistry, University of Virginia, Charlottesville, VA 22904, USA
¹⁶ Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands
¹⁷ School of Physical Sciences, The Open University, Kents Hill, Milton Keynes MK7 6AA, UK
¹⁸ Center for Astrophysics | Harvard & Smithsonian, 160 Concord Avenue, Cambridge 02138 MA, USA
¹⁹ Physikalish-Meteorologisches Observatorium Davos und Weltstrahlungszentrum (PMOD/WRC), Dorfstrasse 33, 7260 Davos Dorf, Switzerland
²⁰ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

^★ Corresponding author; ijimenez@cab.inta-csic.es

Received: 27 September 2024
Accepted: 6 February 2025

Abstract

Context. Recent JWST observations have measured the ice chemical composition towards two highly extinguished background stars, NIR38 and J110621, in the Chamaeleon I molecular cloud. The observed excess of extinction on the long-wavelength side of the H₂O ice band at 3 μm has been attributed to a mixture of CH₃OH with ammonia hydrates NH₃·H₂O), which suggests that CH₃OH ice in this cloud could have formed in a water-rich environment with little CO depletion. Laboratory experiments and quantum chemical calculations suggest that CH₃OH could form via the grain surface reactions CH₃ + OH and/or C + H₂O in water-rich ices. However, no dedicated chemical modelling has been carried out thus far to test their efficiency. In addition, it remains unexplored how the efficiencies of the proposed mechanisms depend on the astrochemical code employed.

Aims. We modelled the ice chemistry in the Chamaeleon I cloud to establish the dominant formation processes of CH₃OH, CO, CO₂, and of the hydrides CH₄ and NH₃ (in addition to H₂O). By using a set of state-of-the-art astrochemical codes (MAGICKAL, MONACO, Nautilus, UCLCHEM, and KMC simulations), we can test the effects of the different code architectures (rate equation vs. stochastic codes) and of the assumed ice chemistry (diffusive vs. non-diffusive).

Methods. We consider a grid of models with different gas densities, dust temperatures, visual extinctions, and cloud-collapse length scales. In addition to the successive hydrogenation of CO, the codes’ chemical networks have been augmented to include the alternative processes for CH₃OH ice formation in water-rich environments (i.e. the reactions CH₃ + OH → CH₃OH and C + H₂O → H₂CO).

Results. Our models show that the JWST ice observations are better reproduced for gas densities ≥10⁵ cm⁻³ and collapse timescales ≥10⁵ yr. CH₃OH ice formation occurs predominantly (>99%) via CO hydrogenation. The contribution of reactions CH₃ + OH and C + H₂O is negligible. The CO₂ ice may form either via CO + OH or CO + O depending on the code. However, KMC simulations reveal that both mechanisms are efficient despite the low rate of the CO + O surface reaction. CH₄ is largely underproduced for all codes except for UCLCHEM, for which a higher amount of atomic C is available during the translucent cloud phase of the models. Large differences in the predicted abundances are found at very low dust temperatures (T_dust<12 K) between diffusive and non-diffusive chemistry codes. This is due to the fact that non-diffusive chemistry takes over diffusive chemistry at such low T_dust. This could explain the rather constant ice chemical composition found in Chamaeleon I and other dense cores despite the different visual extinctions probed.