2005ApJ...621L..33O

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 N₂and CO, R_BE, is taken to be between 0.5 and 0.75. This Letter is the first experimental report of the desorption of CO and N₂from 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 N₂desorbs at lower temperatures than CO, up to 50% of the N₂molecules 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_BEis 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.