  Young stars are associated with prominent outflows of molecular gas. The
ejection of gas via these outflows is believed to remove angular momentum from
the protostellar system, thus permitting young stars to grow by accretion of
material from the protostellar disk. The underlying mechanism for outflow
ejection is not yet understood, but is believed to be closely linked to the
protostellar disk. Assorted scenarios have been proposed to explain
protostellar outflows; the main difference between these models is the region
where acceleration of material takes place: close to the protostar itself
('X-wind', or stellar wind), in a larger region throughout the protostellar
disk (disk wind), or at the interface between. Because of the limits of
observational studies, outflow launching regions have so far only been probed
by indirect extrapolation. Here we report observations of carbon monoxide
toward the outflow associated with the TMC1A protostellar system. These data
show that gas is ejected from a region extending up to a radial distance of 25
astronomical units from the central protostar, and that angular momentum is
removed from an extended region of the disk. This demonstrates that the
outflowing gas is launched by an extended disk wind from a Keplerian disk.
Hence, we rule out X-wind and stellar wind launching scenarios as the source of
the emission on the scales we observe.
