2017ApJ...836...30G

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 ¹⁴N/¹⁵N ratios in common molecules during the assembly of planetary systems. We present ALMA observations of the H¹³CN and HC¹⁵NJ=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 ¹²C/¹³C ratio of 70, we find comet-like ¹⁴N/¹⁵N ratios 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 ¹⁴N/¹⁵N ratios in the sample. One of the disks, V4046 Sgr, presents unusually bright HCN isotopologue emission, enabling us to model the radial profiles of H¹³CN and HC¹⁵N. We find tentative evidence of an increasing ¹⁴N/¹⁵N ratio with radius, indicating that selective photodissociation in the inner disk is important in setting the ¹⁴N/¹⁵N ratio during planet formation.