HCN is a commonly observed molecule in Solar System bodies and in interstellar environments .
Its abundance with respect to CN is a proposed tracer of UV exposure .
HCN is also frequently used to probe the thermal history of objects , by measuring its degree of nitrogen fractionation .
To address the utility of HCN as a probe of disks , we present ALMA observations of CN , HCN , H ^ { 13 } CN and HC ^ { 15 } N toward the protoplanetary disk around Herbig Ae star MWC480 , and of CN and HCN toward the disk around T Tauri star DM Tau .
Emission from all molecules is clearly detected and spatially resolved , including the first detection of HC ^ { 15 } N in a disk .
Toward MWC 480 , CN emission extends radially more than 1 " exterior to the observed cut-off of HCN emission .
Quantitative modeling further reveals very different radial abundance profiles for CN and HCN , with best-fit outer cut-off radii of > 300 AU and 110 \pm 10 AU , respectively .
This result is in agreement with model predictions of efficient HCN photodissociation into CN in the outer-part of the disk where the vertical gas and dust column densities are low .
No such difference in CN and HCN emission profiles are observed toward DM Tau , suggestive of different photochemical structures in Herbig Ae and T Tauri disks .
We use the HCN isotopologue data toward the MWC 480 disk to provide the first measurement of the \mathrm { { } ^ { 14 } N / ^ { 15 } N } ratio in a disk .
We find a low disk averaged \mathrm { { } ^ { 14 } N / ^ { 15 } N } ratio of 200 \pm 100 , comparable to what is observed in cloud cores and comets , demonstrating interstellar inheritance and/or efficient nitrogen fractionation in this disk .