Isotopic Anomalies in Primitive Solar System Matter: Spin-state Dependent Fractionation of Nitrogen and Deuterium in Interstellar Clouds

TL;DR

This study models how spin-state dependent ion-molecule reactions in interstellar clouds can explain the diverse nitrogen and deuterium isotopic anomalies observed in primitive Solar System materials, supporting their interstellar origin.

Contribution

It introduces a new model considering H2 spin states to explain the wide range of isotopic fractionation in interstellar molecules, aligning with observed anomalies in meteorites.

Findings

Ammonia and related molecules show wide isotopic fractionation due to spin-state effects.

Nitriles like HCN and HNC have high 15N enrichment but not necessarily high D enrichment.

The model supports an interstellar origin for Solar System isotopic anomalies.

Abstract

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.