  We present the results of new spectral diagnostic investigations applied to
high-resolution long-slit spectra of the RW Aur bipolar jet obtained with
HST/STIS. The spectra include the forbidden doublets [O I] 6300,6363 \AA, [S
II] 6716,6731 \AA, and [N II] 6548, 6583 \AA that we utilized to determine
electron density, electron temperature, hydrogen ionisation fraction, total
hydrogen density, radial velocity and the mass outflow rate. We were able to
extract the parameters as far as 3".9 in the red- and 2".1 in the blueshifted
beam. The RW Aur jet appears to be the second densest outflow from a T Tauri
star studied so far, but its other properties are quite similar to those found
in other jets from young stars. The overall trend of the physical parameters
along the first few arcseconds of the RW Aur jet is similar to that of HH 30
and DG Tau and this can reflect analogies in the mechanisms operating in that
region, suggesting the same engine is accelerating the jets in the T Tauri
stars with outflows. Our study of the RW Aur jet indicates for the first time
that, despite the detected marked asymmetries in physical and kinematic
properties between the two lobes, the mass outflow rates in the two lobes are
similar. This appears to indicate that the central engine has constraining
symmetries on both sides of the system, and that the observed asymmetries are
probably due to different environmental conditions.
