FAUST II. Discovery of a Secondary Outflow in IRAS 15398-3359:
  Variability in Outflow Direction during the Earliest Stage of Star Formation?
  We have observed the very low-mass Class 0 protostar IRAS 15398-3359 at
scales 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, which
extends from the source along a direction almost perpendicular to the known
active outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH further
reveals an arc-like structure connected to the outer end of the linear feature
and separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-like
structure is blue-shifted with respect to the systemic velocity. A velocity
gradient of 1.2 km/s over 1200 au along the linear feature seen in the H2CO
emission connects the protostar and the arc-like structure kinematically. SO,
SiO, and CH3OH are known to trace shocks, and we interpret the arc-like
structure as a relic shock region produced by an outflow previously launched by
IRAS 15398-3359. The velocity gradient along the linear structure can be
explained as relic outflow motion. The origins of the newly observed arc-like
structure and extended linear feature are discussed in relation to turbulent
motions within the protostellar core and episodic accretion events during the
earliest stage of protostellar evolution.
