An experimental study of the surface formation of methane in interstellar molecular clouds

Abstract

Methane is one of the simplest stable molecules that is both abundant and widely distributed across space. Observational surveys of CH4 ice towards low- and high-mass young stellar objects showed that much of the CH4 is expected to be formed by the hydrogenation of C on dust grains, and that CH4 ice is strongly correlated with solid H2O. However, this has not been investigated under controlled laboratory conditions. Here, we successfully demonstrate with a C-atom beam implemented in an ultrahigh vacuum apparatus the formation of CH4 ice in two separate co-deposition experiments: C + H on a 10 K surface to mimic CH4 formation directly before H2O ice is formed on the dust grain, and C + H + H2O on a 10 K surface to mimic CH4 formed simultaneously with H2O ice. We confirm that CH4 can be formed by the reaction of atomic C and H, and that the CH4 formation rate is twice as high when CH4 is formed within a H2O-rich ice. This is in agreement with the observational finding that interstellar CH4 and H2O form together in the polar ice phase. The conditions that lead to interstellar CH4 (and CD4) ice formation are reported, and can be incorporated into astrochemical models to further constrain CH4 chemistry in the interstellar medium and in other regions where CH4 is inherited.

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Fig. 1: Visualization of the two experiments highlighted in this study.
Fig. 2: RAIRS spectra of CH4 formation acquired after co-deposition in two different experiments, in which only the selected feature of interest is shown (that is, CH4 ν4 mode).

Data availability

The data that support the figures and table within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This research benefited from financial support by the Dutch Astrochemistry Network II (DANII). Further support includes a VICI grant from NWO (the Netherlands Organization for Scientific Research) and A-ERC grant 291141 CHEMPLAN. Funding by NOVA (the Netherlands Research School for Astronomy) is acknowledged. D.Q. acknowledges J. Bouwman and E. Fayolle for stimulating discussions. S.I. recognizes the Royal Society for financial support and the Holland Research School for Molecular Chemistry (HRSMC) for a travel grant.

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D.Q. performed the experiments and wrote the manuscript. D.Q. and G.F. designed the experiments and analysed the data. K.-J.C. helped with column density measurements and error calculations. J.H. and S.I. provided insights on the surface formation mechanism. E.F.v.D. and H.L. generously assisted with the astrochemical implications. H.L. initiated the project. All authors participated in discussion of the experiments, analysis and interpretation of the results and shaping of the manuscript.

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Correspondence to D. Qasim.

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Peer review information Nature Astronomy thanks Martin McCoustra and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Qasim, D., Fedoseev, G., Chuang, K. et al. An experimental study of the surface formation of methane in interstellar molecular clouds. Nat Astron 4, 781–785 (2020). https://doi.org/10.1038/s41550-020-1054-y

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