Nitrogen Isotopic Fractionation in Interstellar Ammonia

TL;DR

This study measures nitrogen isotopic ratios in ammonia within two nearby molecular clouds, revealing ratios that inform our understanding of nitrogen fractionation in the interstellar medium and its connection to solar system materials.

Contribution

First precise measurements of $^{14}$N/$^{15}$N ratios in ammonia in specific molecular clouds, enhancing understanding of nitrogen isotopic variation in the interstellar medium.

Findings

Barnard 1 ratio is about 334, between proto-solar and terrestrial values.

NGC 1333 results are consistent but less precise.

No evidence of high $^{15}$N enhancements seen in comets.

Abstract

Using the Green Bank Telescope (GBT), we have obtained accurate measurements of the $^{14}$N/$^{15}$N isotopic ratio in ammonia in two nearby cold, dense molecular clouds, Barnard~1 and NGC 1333. The $^{14}$N/$^{15}$N ratio in Barnard~1, $334 \pm 50$ (3$\sigma$), is particularly well constrained and falls in between the local interstellar medium/proto-solar value of $\sim 450$ and the terrestrial atmospheric value of 272. The NGC 1333 measurement is consistent with the Barnard~1 result, but has a larger uncertainty. We do not see evidence for the very high $^{15}$N enhancements seen in cometary CN. Sensitive observations of a larger, carefully selected sample of prestellar cores with varying temperatures and gas densities can significantly improve our understanding of the nitrogen fractionation in the local interstellar medium and its relation to the isotopic ratios measured in various solar system reservoirs.