Is FS Tau B Driving an Asymmetric Jet?

TL;DR

This study presents near-infrared imaging and spectroscopy of FS Tau B, revealing an asymmetric jet with differing brightness, velocity, and excitation properties, indicating unequal mass-loss rates in its bipolar outflow.

Contribution

First detection of the jet and counterjet in [Fe II] emission for FS Tau B, demonstrating its asymmetric nature and providing detailed kinematic and excitation properties.

Findings

The jet and counterjet are asymmetric in brightness and velocity.

The velocity ratio of jet to counterjet is approximately 1.34.

The blueshifted jet has lower density and higher excitation than the redshifted counterjet.

Abstract

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.