Abstract
Nitrogen is the fifth most abundant element in the Universe. In the interstellar medium, it has been thought to be mostly molecular (N2)1. However, N2 has no observable rotational or vibrational transitions, so its abundance in the interstellar medium remains poorly known. In comets, the N2 abundance is very low2,3, while the elemental nitrogen abundance is deficient with respect to the solar value. Moreover, large nitrogen isotopic anomalies are observed in meteorites and interstellar dust particles4. Here we report the N2H+ (and by inference the N2) abundance inside a cold dark molecular cloud. We find that only a small fraction of nitrogen in the gas phase is molecular, with most of it being atomic. Because the compositions of comets probably reflect those of dark clouds5, this result explains the low N2 abundance in comets. We argue that the elemental nitrogen abundance deficiency in comets can be understood if the atomic oxygen abundance is lower than predicted by present chemical models. Furthermore, the lack of molecular nitrogen in molecular clouds explains the nitrogen anomalies in meteorites and interstellar dust particles, as nitrogen fractionation is enhanced if gaseous nitrogen is atomic6.
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Acknowledgements
This work is supported by the National Science Foundation. S.M. is grateful to E. Herbst for discussions about the N2 formation reaction rates. S.M. also thanks E. Keto for discussions about the radiative transfer of N2H+ hyperfine structure lines. Author contributions S.M. and E.A.B. performed the chemical and radiative transfer modelling presented in this paper. S.M. wrote the paper. All authors discussed the results and commented on the manuscript.
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Supplementary Discussion
This discussion presents the chemical modeling performed in the paper into more details. It explains the time dependence of this model. It also describes the chains of reactions that lead to the formation of molecular nitrogen in the gas phase, and explain how a low oxygen abundance (with respect to current model predictions) could explain the elemental nitrogen deficiency observed in comets. Finally, the results of the present paper are compared with earlier studies. (PDF 29 kb)
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Maret, S., Bergin, E. & Lada, C. A low fraction of nitrogen in molecular form in a dark cloud. Nature 442, 425–427 (2006). https://doi.org/10.1038/nature04919
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DOI: https://doi.org/10.1038/nature04919
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