Jets from young stars represent one of the most striking signposts of star
formation. The phenomenon has been researched for over two decades and there is
now general agreement that such jets are generated as a by-product of
accretion; most likely by the accretion disk itself. Thus they mimic what
occurs in more exotic objects such as active galactic nuclei and micro-quasars.
The precise mechanism for their production however remains a mystery. To a
large degree, progress is hampered observationally by the embedded nature of
many jet sources as well as a lack of spatial resolution: Crude estimates, as
well as more sophisticated models, nevertheless suggest that jets are
accelerated and focused on scales of a few AU at most.
It is only in the past few years however that we have begun to probe such
scales in detail using classical T Tauri stars as touchstones. Application of
adaptive optics, data provided by the HST, use of specialised techniques such
as spectro-astrometry, and the development of spectral diagnostic tools, are
beginning to reveal conditions in the jet launch zone. This has helped
enormously to constrain models. Further improvements in the quality of the
observational data are expected when the new generation of interferometers come
on-line. Here we review some of the most dramatic findings in this area since
Protostars and Planets~IV including indications for jet rotation, i.e. that
they transport angular momentum. We will also show how measurements, such as
those of width and the velocity field close to the source, suggest jets are
initially launched as warm magneto-centrifugal disk winds. (abridged)Comment: 14 pages, 8 figures, contributed chapter for Planets and Protostars V
meeting (October 2005
