  We present an analysis of the gas physics at the base of jets from five T
Tauri stars based on high angular resolution optical spectra, using the Hubble
Space Telescope Imaging Spectrograph (HST/STIS). The spectra refer to a region
within 100 AU of the star, i.e. where the collimation of the jet has just taken
place. We form PV images of the line ratios to get a global picture of the flow
excitation. We then apply a specialised diagnostic technique to find the
electron density, ionisation fraction, electron temperature and total density.
Our results are in the form of PV maps of the obtained quantities, in which the
gas behaviour is resolved as a function of both radial velocity and distance
from the jet axis. They highlight a number of interesting physical features of
the jet collimation region, including regions of extremely high density,
asymmetries with respect to the axis, and possible shock signatures. Finally,
we estimate the jet mass and angular momentum outflow rates, both of which are
fundamental parameters in constraining models of accretion/ejection structures,
particularily if the parameters can be determined close to the jet footpoint.
Comparing mass flow rates for cases where the latter is available in the
literature (i.e. DG Tau, RW Aur and CW Tau) reveals a mass
ejection-to-accretion ratio of 0.01 - 0.07. Finally, where possible (i.e. DG
Tau and CW Tau), both mass and angular momentum outflow rates have been
resolved into higher and lower velocity jet material. For the clearer case of
DG Tau, this revealed that the more collimated higher velocity component plays
a dominant role in mass and angular momentum transport.
