Shaping the envelope of the asymptotic giant branch star W43A with a
  collimated fast jet
  One of the major puzzles in the study of stellar evolution is the formation
process of bipolar and multi-polar planetary nebulae. There is growing
consensus that collimated jets create cavities with dense walls in the
slowly-expanding (10--20 ~km~s$^{-1}$) envelope ejected in previous
evolutionary phases, leading to the observed morphologies. However, the
launching of the jet and the way it interacts with the circumstellar material
to create such asymmetric morphologies have remained poorly known. Here we
present for the first time CO emission from the asymptotic giant branch star
W43A that traces the whole stream of a jet, from the vicinity of its driving
stellar system out to the regions where it shapes the circumstellar envelope.
We found that the jet has a launch velocity of 175~km~s$^{-1}$ and decelerates
to a velocity of 130~km~s$^{-1}$ as it interacts with circumstellar material.
The continuum emission reveals a bipolar shell with a compact bright dot in the
centre that pinpoints the location of the driving source of the jet. The
kinematical ages of the jet and the bipolar shell are equal,
$\tau$$\sim$60~years, indicating that they were created simultaneously,
probably by a common underlying mechanism, and in an extremely short time.
These results provide key initial conditions for the theoretical models that
aim to explain the formation of bipolar morphologies in the circumstellar
envelopes of low and intermediate mass stars.
