We present spectrally resolved Herschel /HIFI observations of the young multiple system T Tau in atomic and molecular lines .
While CO , H _ { 2 } O , [ C ii ] , and SO lines trace the envelope and the outflowing gas up to velocities of 33 km s ^ { -1 } 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 ^ { \circ } \pm 5 ^ { \circ } ) .
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 .