The kinematics of the HH 399 jet in the Trifid Nebula

We have studied the kinematics of the HH 399 jet, which was recently discov ered in the Trifid Nebula. The data were obtained in the [N II] 6583 Angstr om line with a scanning Fabry-Perot interferometer (TAURUS-2) and in the [S II] 6717, 6731 Angstrom lines with an echelle spectrograph. The analysis o f the radial velocities measured along the jet indicates that its axis lies almost perpendicular to the line of sight. We resolve two kinematical comp onents along the jet that differ in radial velocity by about 15 km s(-1), a nd we identify the bow shock associated with the jet. We find that the brig htest kinematic component has an electron density similar to the surroundin g interstellar medium (100-340 cm(-3)), while the second velocity component has much higher densities (1000-7500 cm(-3)). We discuss different models that can explain the observational data. The more suitable ones identify th e second velocity component with the jet material and imply a jet-to-ambien t density ratio eta = n(j)/n(amb) ranging from 8 to up to 70. However, the lack of theoretical work on the kinematics of jets immersed in a fully ioni zed medium complicates the interpretation of HH 399.