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 $^{14}$N/$^{15}$N isotopic ratios in the solar system reflects distinct reservoirs of nitrogen whose origins remain to be determined. We have directly measured a C$^{14}$N/C$^{15}$N abundance ratio of 323$\pm$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 $^{15}$N sources as well as outward migration of the Sun over its lifetime, and suggest that comets sampled a secondary, $^{15}$N-rich reservoir during solar system formation.