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\section{Introduction}   In many areas of astrophysics massive central objects accrete mass and angular momentum from a disk and at the same time they eject a highly collimated jet. This is seen for central objects as massive as AGN or as light as (proto) brown dwarfs. For the most massive and most compact objects like AGN or accreting neutron stars the jets reach relativistic energies while the velocities are significantly lower in young stellar systems.   Young, low-mass stars that actively accrete from a circum-stellar disk are called classical T Tauri stars. The slowest velocities are observed in molecular lines with typical line shifts of only a few km~s$^{-1}$ \citep{http://adsabs.harvard.edu/abs/2008ApJ...676..472B}. \citep{2008ApJ...676..472B}.  These molecular outflows have wide opening angles around 90\degree \citep[e.g.][]{http://adsabs.harvard.edu/abs/2013A%26A...557A.110S} \citep[e.g.][]{2013A&A...557A.110S}  and are presumably launched from the disk. Faster components are seen in optical emission lines like H$\alpha$ or in forbidden emission lines such as [\ion{O}{1}] or [\ion{S}{2}]. \citet{http://adsabs.harvard.edu/abs/2000ApJ...537L..49B} \citet{2000ApJ...537L..49B}  observed the jet from the CTTS \object{DG Tau} with seven long-slit exposures of \emph{HST}/STIS to resolve the kinematic structure of the jet both along and perpendicular to the jet axis. They find that the faster jet components are better collimated and propose an ``onion'' scenario, where the fastest jet components make up the innermost layer and the surrounding layers have lower velocities the further away from the jet axis they are. The fastest components seen in the optical emission lines are typically 200-300~km~s$^{-1}$ \citep{http://adsabs.harvard.edu/abs/2004Ap%26SS.292..651B,http://adsabs.harvard.edu/abs/2008ApJ...689.1112C,http://adsabs.harvard.edu/abs/2013A%26A...550L...1S}. \citep{2004Ap&SS.292..651B,2008ApJ...689.1112C,2013A&A...550L...1S}.