Nitrogen Fractionation in Protoplanetary Disks from the H13CN/HC15N Ratio

Abstract

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 assembly of planetary systems. We present ALMA observations of the H13CN and HC15N J = 3 − 2 lines at 000.5 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 5/6 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.