2002ApJ...569L.133C

We describe a mechanism for enhanced nitrogen isotope fractionation in dense molecular gas where most of the molecules containing carbon and oxygen have condensed on grains but where N₂ remains in the gas. The lack of hydroxl molecules prevents the recycling of N atoms into N₂, and the nitrogen eventually becomes atomic. Ammonia is formed efficiently under these conditions and rapidly accretes as ice. We find that a significant fraction of the total nitrogen is ultimately present as solid NH₃. This interstellar ammonia is enhanced in ¹⁵N with ¹⁵NH₃/¹⁴NH₃ almost 80% higher than the cosmic ¹⁵N/¹⁴N ratio. It is possible that a large part of the nitrogen available to the early solar system was highly fractionated ammonia ice and hence that the ¹⁵N enhancements of primitive solar system material and the depletion of N₂ in comets are concomitant. Other implications of this theory for observations of dense molecular material and the nitrogen inventory available to the protosolar nebula are briefly discussed.