  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 \mu m line for the
first time. Within the inner 2" the blueshifted jet is brighter, whereas beyond
~ 5" the redshifted counterjet dominates the [Fe II] emission. The innermost
blueshifted knot is spectrally resolved to have a large line width of ~ 110
km/s, while the innermost redshifted knot appears spectrally unresolved. The
velocity ratio of the jet to the counterjet is ~ 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.
