Ionization and density along the beams of Herbig-Haro jets

Physical properties of several well-known HerbigHaro jets are investigated using an improved version of the spectroscopic diagnostic technique originally developed by Bacciotti et al. (1995). The procedure allows one to derive in a model-independent way the hydrogen ionization fraction in regions of low excitation. The ionization fraction, the electron and gas density, and the average excitation temperature are derived for various positions along the flows. We find that the hydrogen ionization fraction, with typical initial values of 20 – 30 %, generally decreases along the whole jet or along parts of the flow, following well-defined recombination laws. These results are consistent with the idea that the gas is initially ionized in the jet acceleration region, and then slowly recombines while traveling away from the source. If shocks along the jet beam are present, they can at most have a minor contribution to the ionization of the gas, as apparent in HH 34 and in the first 4500 of the HH 46/47 jet, where the ionization fraction decreases almost monotonically. In the jets in which re-ionization episodes occur (i.e. HH 24C/E and HH 24G), the ionization fraction suddenly increases and then gently decays downstream of the re-ionization event. Both findings apparently disfavour a mini-bow shock interpretation for the production of the ionization of the beam. The total densities derived from the ratio between the electron density and the ionization fraction range from about 103 to a few 104 cm 3. Without applying a correction for shock compression, the average mass loss rate varies from 3.8 10 8 (in the HL Tau jet) to 1.2 10 6 M yr 1 (in HH24 G), while momentum supply rates vary between 1.6 10 5 (in the HL Tau jet) and 3.1 10 4 M yr 1 km s 1 (in HH 24G). Taking shock compression into account, these values may be reduced by a factor 3 – 5.