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

摘要

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.Methane ice has been presumed to form via the sequential hydrogenation of carbon atoms on dust grains for many years, but now Qasim et al. have performed the experiment, with and without the presence of water. Methane forms more rapidly in the polar ice phase.