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 14N/15N ratios in common molecules during the assembly of planetary
systems. We present ALMA observations of the H13CN and HC15N 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
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.
