Zooming into the Collimation Zone in a Massive Protostellar Jet
  Protostellar jets have a fundamental role at the earliest evolution of
protostars of all masses. In the case of low-mass (<8 Msun) protostars, strong
observational evidence exists that the launching and collimation is due to the
X- and/or disk-wind mechanisms. In these models, it is the protostar/disk
system that creates all the necessary conditions to launch and collimate the
jets near the protostar via strong magnetic fields. The origin of jets from
more massive protostars has been investigated much less, in part because of the
difficulty of resolving the collimation zone in these more distant objects.
Here we present the highest angular resolution observations of a jet powered by
a massive protostar, the Cep A HW2 radio jet. We imaged the radio emission at
projected distances of only ~20 au from the protostar, resolving the innermost
100 au of a massive protostellar jet for the first time. The morphology of the
radio jet emission in this massive object is very different than what is
usually observed in jets from low-mass protostars. We found that the outflowing
material in HW2 has two components: a wide-angle wind launched from the
protostar/disk system, and a highly collimated jet starting at 20-30 au from
the protostar. We discuss two possible scenarios: an extension of the classical
disk-wind to a massive protostar, or external collimation of a wide-angle wind.
These results have important consequences for our understanding of how stars of
different masses are formed.
