Elemental nitrogen partitioning in dense interstellar clouds
2012
Daranlot, Julien | Hincelin, Ugo | Bergeat, Astrid | Costes, Michel | Loison, Jean-Christophe | Wakelam, Valentine | Hickson, Kevin M.
Many chemical models of dense interstellar clouds predict that the majority of gas-phase elemental nitrogen should be present as N ₂, with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N ₂ is difficult to detect spectroscopically through infrared or millimeter-wavelength transitions. Therefore, its abundance is often inferred indirectly through its reaction product N ₂H ⁺. Two main formation mechanisms, each involving two radical-radical reactions, are the source of N ₂ in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N + CN reaction, down to 56 K. The measured rate constants for this reaction, and those recently determined for two other reactions implicated in N ₂ formation, are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of interstellar nitrogen present as N ₂ depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N ₂ formation are inefficient, we argue that N ₂ does not represent the main reservoir species for interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH ₃ should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.
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