Do highly variable 15N-enrichments in Solar System reflect different routes of interstellar N isotopic fractionation?
Large nitrogen (N) isotopic variations are observed in our Solar System (14N/15N ~ 50 to 450), with most of the objects being 15N-enriched compared to the presolar nebula (14N/15N = 442). The variation of the nitrogen isotopic composition in Solar System objects is most likely caused by a variety of effects, including nucleosynthetic origin, photochemical self-shielding in the solar nebula, spallation reactions caused by the irradiation of the young Sun, and low temperature isotope exchanges. The absence of large 15N-enrichments across the Galaxy and the small fractionation effects predicted by standard gas-phase chemical models have weakened so far the hypothesis of a preserved (low temperature) interstellar chemistry to explain the 15N-enrichments observed in primitive solar cosmomaterials. The absence of a direct correlation between D and 15N-enrichments in organics from primitive cosmomaterials has also been interpreted as the lack of remnant interstellar chemistry for N isotopologues.
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. The present work (i) brings new observational constraints on nitrogen isotopic fractionation in gas phase and (ii) puts a new perspective on the actively debated and long questioning issue of the origin of the 15N-enrichments observed in primitive cosmomaterials as compared to the protosolar nebula. This various points will be discussed during the seminar.