Nitrogen Fractionation in Protoplanetary Disks from the H13CN/HC15N Ratio

Nitrogen fractionation is commonly used to assess the thermal history of solar system volatiles. With ALMA it is for the first time possible to directly measure N-14/N-15 ratios in common molecules during the assembly of planetary systems. We present ALMA observations of the (HCN)-C-13 and (HCN)-N-15 J = 3 - 2 lines at 0.5 angular resolution, toward a sample of six protoplanetary disks, selected to span a range of stellar and disk structure properties. Adopting a typical C-12/C-13 ratio of 70, we find comet-like N-14 (15)Nratios of 80-160 in five of the disks (3 T Tauri and 2 Herbig Ae disks) and lack constraints for one of the T Tauri disks (IM Lup). There are no systematic differences between T Tauri and Herbig Ae disks, or between full and transition disks within the sample. In addition, no correlation is observed between disk-averaged D/H and N-14 N-15 ratios in the sample. One of the disks, V4046 Sgr, presents unusually bright HCN isotopologue emission, enabling us to model the radial profiles of (HCN)-C-13 and (HCN)-N-15. We find tentative evidence of an increasing N-14/N-15 ratio with radius, indicating that selective photodissociation in the inner disk is important in setting the N-14/N-15 ratio during planet formation.