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

The Solar System shows extreme nitrogen isotopic heterogeneity, with the 14N/15N ratio ranging from 440 in the Sun and Jupiter down to values as low as 50 in some grains within chondritic matrices. Remarkably, comets exhibit a universal 14N/15N ratio of 140, regardless of the cometary orbit type or the carrier in nitrogen. Evolved protoplanetary disks offer unique possibilities for studying the diversity of nitrogen reservoirs in contexts similar to the Solar System at the epoch of planet and comet formation. Using ALMA, Hily-Blant et al. present spatially-resolved measurements of HCN and its H13CN and HC15N isotopologues in the emblematic disk orbiting the 8 Myr-old TTauri star, TW Hya. The mean 14N/15N ratio measured over the disk in HCN is 225, which is significantly less than the value measured previously in CN by the authors in the same disk of 330. This confirms the existence of at least two reservoirs of nitrogen. Most significantly, the HC14N/HC15N ratio shows a radial gradient within the disk, which increases steadily from 120-180 at 20-30 au (i.e., at the distance of the formation zone of comets) to 340 at 45 au, where it joins the value measured in the local interstellar medium. The outward increase of the 14N/15N ratio in HCN may be explained, at least qualitatively, by a fractionation effect due to the opacity-sensitive photodissociation of N2. Overall, these results support a scenario in which an unfractionated, interstellar, nitrogen reservoir was dominated in the outer disk of the protosolar nebula, while fractionation was occurring closer to the star and, in particular, at the distance of comet formation.