  We present spectrally resolved Herschel/HIFI observations of the young
multiple system T Tau in atomic and molecular lines. While CO, H2O, [C II], and
SO lines trace the envelope and the outflowing gas up to velocities of 33 km/s
with respect to systemic, the CN 5-4 hyperfine structure lines at 566.7, 566.9
GHz show a narrow double-peaked profile centered at systemic velocity,
consistent with an origin in the outer region of the compact disk of T Tau N.
Disk modeling of the T Tau N disk with the thermo-chemical code ProDiMo
produces CN line fluxes and profiles consistent with the observed ones and
constrain the size of the gaseous disk (R_out = 110 (+10, -20) AU) and its
inclination (i = 25 \pm 5 degree). The model indicates that the CN lines
originate in a disk upper layer at 40-110 AU from the star, which is irradiated
by the stellar UV field and heated up to temperatures of 50-700 K. With respect
to previously observed CN 2-1 millimeter lines, the CN 5-4 lines appear to be
less affected by envelope emission, due to their larger critical density and
excitation temperature. Hence, high-J CN lines are a unique confusion-free
tracer of embedded disks, such as the disk of T Tau N.
