  The isotopic ratio of nitrogen measured in primitive Solar System bodies
shows a broad range of values, the origin of which remains unknown. One key
question is whether these isotopic reservoirs of nitrogen predate the comet
formation stage or are posterior to it. Another central question is elucidating
the processes that can produce the observed variations in the 14N/15N isotopic
ratio. Disks that orbit pre-main-sequence stars provide unique opportunities
for observing the chemical content of analogs of the protosolar nebula and
therefore for building a comprehensive scenario for the origin of nitrogen in
the Solar System and in planet-forming disks. We present spectrally and
spatially resolved observations of the hyperfine structure of the 4-3
rotational transition of HCN and its main isotopologs H13CN and HC15N in the
disk orbiting the 8 Myr old TTauri star TW Hya. The spatially averaged
HCN/H13CN and HCN/HC15N abundance ratios are 86+/-4 and 223+/-21, respectively.
The latter value is significantly lower than the CN:C15N ratio 323+/-30 in this
disk and thus provides the first evidence that two isotopic reservoirs of
nitrogen are present in a disk at the stage of giant planet and comet
formation. Furthermore, we find clear evidence for an increase in the ratio of
HCN to HC15N with radius. The ratio in the outer disk is 339+/-28, in excellent
agreement with direct measurements in the local interstellar medium, and with
the bulk nitrogen isotopic ratio predicted from galactic evolution
calculations. In the comet formation region at r=20 au, the ratio is a factor
~3 lower, 121+/-11. This radial increase qualitatively agrees with the scenario
in which selective photodissociation of N 2 is the dominant fractionation
process. However, our isotopic ratios and kinetic temperature of the
HCN-emitting layers quantitatively disagree with models of nitrogen chemistry
in disks.
