Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 109, Issue 26, Pages 10233-10238Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1200017109
Keywords
astrochemistry; chemical kinetics
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Funding
- Agence Nationale de la Recherche [ANR-JC08_311018]
- Conseil Regional d'Aquitaine [20091102002]
- European Union [PERG03-GA-2008-230805]
- Centre National de la Recherche Scientifique (CNRS) interdisciplinary program Environnements Planetaires et Origines de la Vie
- Institut National des Sciences de l'Univers-CNRS national program Physique et Chimie du Milieu Interstellaire
- Observatoire Aquitain des Sciences de l'Univers
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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 N2H+. 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 NH3 should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.
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