The 15N-enrichment in dark clouds and Solar System objects

The line intensities of the fundamental rotational transitions of (HCN)-C-13 and (HCN)-N-15 were observed towards two prestellar cores, L183 and L1544, and lead to molecular isotopic ratios 140 <= N-14/N-15 <= 250 and 140 <= N-14/N-15 <= 360, respectively. The range of values reflect genuine spatial variations within the cores. A comprehensive analysis of the available measurements of the nitrogen isotopic ratio in prestellar cores show that molecules carrying the nitrile functional group appear to be systematically N-15-enriched compared to those carrying the amine functional group. A chemical origin for the differential N-15-enhancement between nitrile- and amine-bearing interstellar molecules is proposed. This sheds new light on several observations of Solar System objects: (i) the similar N isotopic fractionation in Jupiter's NH3 and solar wind N+; (ii) the N-15-enrichments in cometary HCN and CN (that might represent a direct interstellar inheritance); and (iii) N-15-enrichments observed in organics in primitive cosmomaterials. The large variations in the isotopic composition of N-bearing molecules in Solar System objects might then simply reflect the different interstellar N reservoirs from which they are originating. (C) 2013 Elsevier Inc. All rights reserved.