The ALMA Survey of 70$\mu$m Dark High-mass Clumps in Early Stages
  (ASHES) III. A Young Molecular Outflow Driven by a Decelerating Jet
  We present a spatio-kinematical analysis of the CO~($J$=2$\rightarrow$1) line
emission, observed with the Atacama Large Millimter/submillimter Array (ALMA),
of the outflow associated with the most massive core, ALMA1, in the 70 $\mu$m
dark clump G010.991$-$00.082. The position-velocity (P-V) diagram of the
molecular outflow exhibits a peculiar $\mathsf{S}$-shaped morphology that has
not been seen in any other star forming region. We propose a spatio-kinematical
model for the bipolar molecular outflow that consists of a decelerating
high-velocity component surrounded by a slower component whose velocity
increases with distance from the central source. The physical interpretation of
the model is in terms of a jet that decelerates as it entrains material from
the ambient medium, which has been predicted by calculations and numerical
simulations of molecular outflows in the past. One side of the outflow is
shorter and shows a stronger deceleration, suggesting that the medium through
which the jet moves is significantly inhomogeneous. The age of the outflow is
estimated to be $\tau$$\approx$1300 years, after correction for a mean
inclination of the system of $\approx$57$^{\circ}$.
