Abstract
Highly collimated, supersonic Herbig-Haro (HH) jets have been recognized as an essential element of the star formation process. They show an emission-line spectrum dominated by optical red lines as [O I] lambda 6300, H alpha , [N II] lambda 6583, [S II] lambda lambda 6716,6731. Although it is widely accepted that such lines are produced in a gas excited by mild shocks, the detailed physical conditions of the plasma are still under debate. Observationally, the electron density n[e] is easily found from the [S II] doublet, but other crucial physical parameters, as the hydrogen ionization fraction x[e] (and hence the total density, n[H]), and the average temperature of the flow, are poorly known. Recently, we have elaborated a new spectroscopic diagnostic technique that has allowed us to find these quantities in a model-independent way. Starting with an estimate of n[e], the procedure combines the ratios [O I]/[N II] and [S II]/[O I] to derive the ionization fraction and an estimate of the electron temperature. One can then derive the average mass loss and momentum transfer rates in the jet beam. The procedure has been applied first to a sample of HH jets and bipolar outflows from T Tauri Stars, using moderate spatial resolution spectra taken from ground. The recent automation of the procedure has allowed us to analyze large high angular resolution ( <= 0''.1) images and spectra taken with the Hubble Space Telescope. From the latter we have derived for the first time detailed maps of the physical properties both along and across the flow. These investigations have identified a number of interesting features, among which the fact that HH jets are only partially ionized ( 0.02 < x[e] < 0.4), and that the ionization fraction appears to increase rapidly close to the source, to reach a plateau, and then decline gently along the beam.
