Ejection History of the IRAS 04166+2706 Molecular Jet

The high-velocity molecular jet driven by Class 0 protostar IRAS 04166+2706 exhibits a unique sawtooth velocity pattern. It consists of a series of well-aligned symmetric knots with similar averaged speeds, whose speeds at peaks of emission decrease roughly linearly away from the origin. Recent ALMA observations of knots R6 and B6 reveal kinematic behavior with expansion velocity increasing linearly from the axis to the edge. This pattern can be formed by a spherically expanding wind with axial density concentration. In this picture, the diverging velocity profile naturally possesses an increasing expansion velocity away from the axis, resulting in a tooth-like feature on the position-velocity diagram through projection. Such a geometric picture predicts a correspondence between the slopes of the teeth and the outflow inclination angles, and a single inclination angle of 52° of IRAS 04166+2706 can generally explain the whole pattern. Aided by numerical simulations in the framework of the unified wind model by Shang et al., the observed velocity pattern can indeed be generated. A proper geometrical distribution of the jet and wind material is essential to reconstruct the ejection history of the system.