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 \mathrm { { } ^ { 14 } N / ^ { 15 } N } ratios in common molecules during assembly of planetary systems .
We present ALMA observations of the \mathrm { H ^ { 13 } CN } and \mathrm { HC ^ { 15 } N } J = 3 - 2 lines at 0 ^ { \prime \prime } .5 angular resolution , toward a sample of six protoplanetary disks , selected to span a range of stellar and disk structure properties .
Adopting a typical \mathrm { { } ^ { 12 } C / ^ { 13 } C } ratio of 70 , we find comet-like \mathrm { { } ^ { 14 } N / ^ { 15 } N } 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 \mathrm { { } ^ { 14 } N / ^ { 15 } N } ratios in the sample .
One of the disks , V4046 Sgr , presents unusually bright HCN isotopologue emission , enabling us to model the radial profiles of \mathrm { H ^ { 13 } CN } and \mathrm { HC ^ { 15 } N } .
We find tentative evidence of an increasing \mathrm { { } ^ { 14 } N / ^ { 15 } N } ratio with radius , indicating that selective photodissociation in the inner disk is important in setting the \mathrm { { } ^ { 14 } N / ^ { 15 } N } ratio during planet formation .