Competition between CO and N₂ desorption from interstellar ices

Öberg, K. I.; van Broekhuizen, F.; Fraser, H. J.; Bisschop, S. E.; van Dishoeck, E. F. and Schlemmer, S. (2005). Competition between CO and N₂ desorption from interstellar ices. Astrophysical Journal Letters, 621(1) L33-L36.

DOI: https://doi.org/10.1086/428901

Abstract

Millimeter observations of pre- and protostellar cores show that the abundances of the gas-phase tracer molecules, C¹⁸O and N₂H⁺, anticorrelate with each other and often exhibit “holes” where the density is greatest. These results are reasonably reproduced by astrochemical models, provided that the ratio between the binding energies of N₂ and CO, R_BE, is R taken to be between 0.5 and 0.75. This Letter is the first experimental report of the desorption of CO and N₂ from layered and mixed ices at temperatures relevant to dense cores, studied under ultrahigh vacuum laboratory conditions using temperature programmed desorption. From control experiments with pure ices, R_BE = 0.923 ± 0.003, given E_b (N₂ -N₂) = 790 ± 25 K and E_b(CO-CO) = 855 ± 25 K. In mixed (CO:N₂ = 1:1) and layered (CO above or below N₂) ice systems, both molecules become mobile within the ice matrix at temperatures as low as 20 K and appear miscible. Consequently, although a fraction of the deposited N₂ desorbs at lower temperatures than CO, up to 50% of the N₂ molecules leave the surface as the CO itself desorbs, a process not included in existing gas-grain models. This codesorption suggests that for a fraction of the frozen-out molecules, R_BE is unity. The relative difference between the CO and N₂ binding energies as derived from these experiments is therefore significantly less than that currently adopted in astrochemical models.

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