Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation

International audienceThe isotopic ratio of nitrogen measured in primitive solar system bodies shows a large extent of values, the origin of which remain unknown. One key question is whether the isotopic reservoirs of nitrogen found in cosmomaterials are already present at, or posterior to, the comet formation stage. Another central question is to elucidate the processes capable of producing the observed variations of the 14 N/ 15 N isotopic ratio. Disks orbiting pre-main-sequence (TTauri) stars provide unique opportunities to observe the chemical content of analogs of the protosolar nebula and therefore to build a comprehensive scenario explaining the origin of nitrogen in the solar system and in planet-forming disks. With ALMA, it has become possible to measure isotopic ratios of nitrogen-bearing species in such environments. We present spectrally and spatially resolved observations of the hyperfine structure of the 4-3 rotational transition of HCN and its main isotopologues H13CN, and HC15N, in the disk orbiting the 8 Myr-old TTauri star TW Hya. The sensitivity allows the HCN:H13CN and HCN:HC15N abundance ratios to be measured directly, under minimal assumptions. The spatially averaged ratios are 86±4 and 223±21 respectively. The latter result, when compared with the directly measured CN/C15N ratio of 323±30 in this disk, provides clear-cut evidence that two isotopic reservoirs of nitrogen are present in this disk at the stage of giant-planet and comet formation. Furthermore, we find that the HCN/HC15N ratio increases from 121±11 to 339±28 as the radius increases from 20 to 45 au. This finding lends support to selective photo-dissociation of N2 being the dominant fractionation process