A combined optical/infrared spectral diagnostic analysis of the HH1 jet

Complete flux-calibrated spectra covering the spectral range from 6000 angstrom to 2.5 mu m have been obtained along the HH1 jet and analysed in order to explore the potential of a combined optical/near-IR diagnostic applied to jets from young stellar objects. The main physical parameters ( visual extinction, electron temperature and density, ionization fraction and total density) have been derived along the jet using various diagnostic line ratios. This multi-line analysis shows, in each spatially unresolved knot, the presence of zones at different excitation conditions, as expected from the cooling layers behind a shock front. In particular, a density stratification in the jet is evident from ratios of various lines of different critical density. We measure electron densities in the range 6 x 10(2) - 3 x 10(3) cm(-3) with the [S II] optical doublet lines, 4 x 10(3) - 10(4) cm(-3) with the near-IR [Fe II] lines, and 10(5) - 10(6) cm(-3) with optical [Fe II] and CaII lines. The electron temperature also shows variations, with values between 8000 - 11 000 K derived from optical/near-IR [Fe II] lines and 11 000 - 20 000 K from a combined diagnostic employing optical [O I] and [N II] lines. Thus [Fe II] lines originate in a cooling layer located at larger distances from the shock front than that generating the optical lines, where the compression is higher and the temperature is declining. The derived parameters were used to measure the mass flux along the jet, adopting different procedures, the advantages and limitations of which are discussed. The [Fe II] 1.64 mu m line luminosity turns out to be more suitable to measure (M) over dot(jet) than the optical lines, since it samples a fraction of the total mass flowing through a knot larger than the [O I] or [S II] lines. (M) over dot(jet) is high in the initial part of the flow (similar to 2.2 x 10(-7) M-. yr(-1)) but decreases by about an order of magnitude further out. Conversely, the mass flux associated with the warm molecular material is low, (M) over dot(H2) similar to 10(-9) M-. yr(-1), and does not show appreciable variations along the jet. We suggest that part of the mass flux in the external regions is not revealed in optical and IR lines because it is associated with a colder atomic component, which may be traced by the far-IR [O I] 63 mu m line. Finally, we find that the gas-phase abundance of refractory species, such as Fe, C, Ca, and Ni, is lower than the solar value, with the lowest values ( between 10 and 30% of solar) derived in the inner and densest regions. This suggests a significant fraction of dust grains may still be present in the jet beam, imposing constraints on the efficiency of grain destruction by multiple low-velocity shock events.