Direct evidence of multiple reservoirs of volatile nitrogen in a protosolar nebula analogue

Isotopic ratios are keys to understanding the origin and early evolution of the solar system in the context of Galactic nucleosynthesis. The large range of measured N-14/N-15 isotopic ratios in the solar system reflects distinct reservoirs of nitrogen whose origins remain to be determined. We have directly measured a (CN)-N-14/(CN)-N-15 abundance ratio of 323 +/- 30 in the disk orbiting the nearby young star TW Hya. This value, which is in good agreement with nitrogen isotopic ratios measured for prestellar cores, likely reflects the primary present-day reservoir of nitrogen in the solar neighbourhood. These results support models invoking novae as primary N-15 sources as well as outward migration of the Sun over its lifetime, and suggest that comets sampled a secondary, N-15-rich reservoir during solar system formation.