An Experimental Study of the Surface Formation of Methane in Interstellar Molecular Clouds


Visualization of the two experiments highlighted in this study. (Left) The simultaneous deposition of C- and H-atoms on a 10 K carbonaceous surface is shown. (Right) The addition of H2O molecules is illustrated. Note that the formation of carbonaceous layers is due to the high sticking of C-atoms and available flux. The angles of deposition are arbitrarily displayed.

Methane is one of the simplest stable molecules that is both abundant and widely distributed across space. It is thought to have partial origin from interstellar molecular clouds, which are near the beginning of the star formation cycle.

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. Yet, this has not been investigated under controlled laboratory conditions, as carbon-atom chemistry of interstellar ice analogues has not been experimentally realized.

In this study, 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 right 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 2 times greater 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, i.e., when C- and H-atoms simultaneously accrete with O-atoms on dust grains. For the first time, 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.

D. Qasim, G. Fedoseev, K.-J. Chuang, J. He, S. Ioppolo, E.F. van Dishoeck, H. Linnartz
(Submitted on 6 Apr 2020)
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2004.02506 [astro-ph.SR] (or arXiv:2004.02506v1 [astro-ph.SR] for this version)
Submission history
From: Danna Qasim
[v1] Mon, 6 Apr 2020 09:19:19 UTC (1,452 KB)
https://arxiv.org/abs/2004.02506
Astrobiology, Astrochemistry

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