Nature 442, 425-427 (27 July 2006) | doi:10.1038/nature04919; Received 27 February 2006; Accepted 16 May 2006

A low fraction of nitrogen in molecular form in a dark cloud

S. Maret1, E. A. Bergin1 and C. J. Lada2

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.

  1. Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, Michigan 48109-1042, USA
  2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

Correspondence to: S. Maret1 Correspondence and requests for materials should be addressed to S.M. (Email: smaret@umich.edu).


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