2006A&A...456..189P

We examine the conditions of the plasma along a sample of ``classical'' Herbig-Haro (HH) jets located in the Orion and Vela star forming regions, through combined optical-infrared spectral diagnostics. Our sample includes HH 111, HH 34, HH 83, HH 73, HH 24 C/E, HH 24 J, observed quasi-simultaneously and in the same manner at moderate spatial/spectral resolution. Once inter-calibrated, the obtained spectra cover a wide wavelength range from 0.6-2.5µm, including many transitions from regions of different excitation conditions. This allows us to probe the density and temperature stratification which characterises the cooling zones behind the shock fronts along the jet. From the line ratios we derive the variation of the visual extinction along the flow, the electron density and temperature (n_e and T_e), the hydrogen ionisation fraction x_e, and the total density n_H in the emission region of different lines. The knowledge of such parameters is essential for testing existing jet models and for planning follow-up high-angular resolution observations. From the diagnostics of optical forbidden lines we find, on average, that in the examined jets, in the region of optical emission, n_e varies between 50cm^–3 and 3x10³cm^–3, x_e ranges between 0.03 and 0.6, and the electron temperature T_e is ∼1.3x10⁴K in the HH 111 and HH 34 jets, while it appears to be higher (1.8x10⁴K on average) in the other examined jets. The electron density and temperature derived from [FeII] lines, turn out to be, respectively, higher and lower in comparison to those determined from optical lines, in agreement with the fact that the [FeII] lines arise in the more compressed gas located further from the shock front. An even denser component in the jets, with values of n_e up to 10⁶cm^–3 is detected using the ratio of calcium lines. The derived physical parameters are used to estimate the depletion onto dust grains of calcium and iron with respect to solar abundances. This turns out to be quite substantial, being between 70% and 0% for Ca and ∼90% for Fe. This leads us to suggest that the weak shocks present in the beams are not capable of completely destroying the ambient dust grains, confirming previous theoretical studies. We then derive the mass flux rates, {dot}(M)_jet, in the flows using two independent methods. Taking into account the filling factor of the emitting gas, {dot}(M)_jet is on average 5x10^–8 M_☉/yr. The associated linear momentum fluxes ({dot}(P)_jet=v_jet{dot}(M)_jet) are higher than, or of the same order as, those measured in the coaxial molecular flows, where present, suggesting that the flows are jet driven. Finally, we discuss differences between jets in our sample. In general, we find that higher ionisation and electron temperatures are associated with less dense jets. The comparison suggests that the shock mechanism exciting the knots along the flows has the same efficiency in all the examined objects, and the observed differences are consistent with the different densities, and hence cooling rates, found in the various flows.