Nitrogen Isotopic Fractionation in Interstellar Ammonia
  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.
