2012ApJ...757L..11W

Organic material found in meteorites and interplanetary dust particles is enriched in D and ¹⁵N. This is consistent with the idea that the functional groups carrying these isotopic anomalies, nitriles and amines, were formed by ion-molecule chemistry in the protosolar nebula. Theoretical models of interstellar fractionation at low temperatures predict large enrichments in both D and ¹⁵N and can account for the largest isotopic enrichments measured in carbonaceous meteorites. However, more recent measurements have shown that, in some primitive samples, a large ¹⁵N enrichment does not correlate with one in D, and that some D-enriched primitive material displays little, if any, ¹⁵N enrichment. By considering the spin-state dependence in ion-molecule reactions involving the ortho and para forms of H₂, we show that ammonia and related molecules can exhibit such a wide range of fractionation for both ¹⁵N and D in dense cloud cores. We also show that while the nitriles, HCN and HNC, contain the greatest ¹⁵N enrichment, this is not expected to correlate with extreme D enrichment. These calculations therefore support the view that solar system ¹⁵N and D isotopic anomalies have an interstellar heritage. We also compare our results to existing astronomical observations and briefly discuss future tests of this model.