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

The line intensities of the fundamental rotational transitions of H13CN and HC15N were observed towards two prestellar cores, L183 and L1544, and lead to molecular isotopic ratios 140 ⩽ 14N/15N ⩽ 250 and 140 ⩽ 14N/15N ⩽ 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 15N-enriched compared to those carrying the amine functional group. A chemical origin for the differential 15N-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 15N-enrichments in cometary HCN and CN (that might represent a direct interstellar inheritance); and (iii) 15N-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.