FS Tau B is one of the few T Tauri stars that possess a jet and a counterjet as well as an optically-visible cavity wall .
We obtained images and spectra of its jet-cavity system in the near-infrared H and K bands using Subaru/IRCS and detected the jet and the counterjet in the [ Fe ii ] 1.644 µm line for the first time .
Within the inner 2″ the blueshifted jet is brighter , whereas beyond \sim 5 \arcsec the redshifted counterjet dominates the [ Fe ii ] emission .
The innermost blueshifted knot is spectrally resolved to have a large line width of \sim 110 km s ^ { -1 } , while the innermost redshifted knot appears spectrally unresolved .
The velocity ratio of the jet to the counterjet is \sim 1.34 , which suggests that FS Tau B is driving an asymmetric jet , similar to those found in several T Tauri Stars .
Combining with optical observations in the literature , we showed that the blueshifted jet has lower density and higher excitation than the redshifted counterjet .
We suggest that the asymmetry in brightness and velocity is the manifestation of a bipolar outflow driving at different mass-loss rates , while maintaining balance of linear momentum .
A full explanation to the asymmetry in the FS Tau B system awaits detail modeling and further investigation of the kinematic structure of the wind-associated cavity walls .