Segregation effect and N 2 binding energy reduction in CO-N 2 systems adsorbed on water ice substrates

Abstract : Context. CO and N 2 are two abundant species in molecular clouds. CO molecules are heavily depleted from the gas phase towards the centre of pre-stellar cores, whereas N 2 maintains a high gas phase abundance. For example, in the molecular cloud L183, CO is depleted by a factor of ≈400 in its centre with respect to the outer regions of the cloud, whereas N 2 is only depleted by a factor of ≈20. The reason for this difference is not yet clear, since CO and N 2 have identical masses, similar sticking properties, and a relatively close energy of adsorption. Aims. We present a study of the CON 2 system in sub-monolayer regimes, with the aim to measure, analyse and elucidate how the adsorption energy of the two species varies with coverage, with much attention to the case where CO is more abundant than N 2. Methods. Experiments were carried out using the ultra-high vacuum (UHV) setup called VENUS. Sub-monolayers of either pure 13 CO or pure 15 N 2 and 13 CO: 15 N 2 mixtures were deposited on compact amorphous solid water ice, and crystalline water ice. Temperature-programmed desorption experiments, monitored by mass spectrometry, are used to analyse the distributions of binding energies of 13 CO and 15 N 2 when adsorbed together in different proportions. Results. The distribution of binding energies of pure species varies from 990 K to 1630 K for 13 CO, and from 890 K to 1430 K for 15 N 2. When a CO:N 2 mixture is deposited, the 15 N 2 binding energy distribution is strongly affected by the presence of 13 CO, whereas the adsorption energy of CO is unaltered. Conclusions. Whatever types of water ice substrate we used, the N 2 effective binding energy was significantly lowered by the presence of CO molecules. We discuss the possible impact of this finding in the context of pre-stellar cores.
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T. Nguyen, S. Baouche, E. Congiu, S. Diana, L. Pagani, et al.. Segregation effect and N 2 binding energy reduction in CO-N 2 systems adsorbed on water ice substrates. Astronomy and Astrophysics - A&A, EDP Sciences, 2018, 619, pp.A111. ⟨10.1051/0004-6361/201832774⟩. ⟨hal-01947971⟩

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