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We have observed the very low-mass Class 0 protostar IRAS 15398-3359 atscales ranging from 50 au to 1800 au, as part of the ALMA Large Program FAUST.We uncover a linear feature, visible in H2CO, SO, and C18O line emission, whichextends from the source along a direction almost perpendicular to the knownactive outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH furtherreveals an arc-like structure connected to the outer end of the linear featureand separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-likestructure is blue-shifted with respect to the systemic velocity. A velocitygradient of 1.2 km/s over 1200 au along the linear feature seen in the H2COemission connects the protostar and the arc-like structure kinematically. SO,SiO, and CH3OH are known to trace shocks, and we interpret the arc-likestructure as a relic shock region produced by an outflow previously launched byIRAS 15398-3359. The velocity gradient along the linear structure can beexplained as relic outflow motion. The origins of the newly observed arc-likestructure and extended linear feature are discussed in relation to turbulentmotions within the protostellar core and episodic accretion events during theearliest stage of protostellar evolution.

FAUST II. Discovery of a Secondary Outflow in IRAS 15398-3359: Variability in Outflow Direction during the Earliest Stage of Star Formation?