In this paper we investigated the physical conditions in the beams of a number of well-known Herbig-Haro jets, using a diagnostic technique that allows one to derive the ionization fraction of the emitting gas directly from the observed spectra. The procedure uses ratios of the most commonly observed forbidden lines in HH jets, and is based on the fact that in the jet beam the ionization state of O and N can be assumed to be related to the ionization fraction of hydrogen through charge exchange (which is dominant), collisional ionization and radiative plus dielectronic recombination. We carefully checked our procedure to be consistent with the results of shock calculations, as those presented in HMR94, and find our results in good agreement. Here, we present the results for a sample of optical outflows, namely: HH 34, HH 46/47, HH 24G, HH 24C/E, HL Tau and HH 228 (Th 28). They can be summarized as follows.
1 - For all examined objects the gas in the beam is partially ionized ; the hydrogen ionization fraction typically ranges between 0.02 and 0.35, being higher for more excited and lighter jets. The diagnostic also indicates qualitatively that in regions of violent interaction between the beam and its surroundings, the ionization degree can be much higher, as in the faint section of the HH 46/47 jet.
2 - With the exception of the HL Tau jet the ionization fraction is generally observed to slowly decrease along the jet or along sections of it . As originally suggested by BCO95, the ionization state of the jet gas is probably produced in the acceleration region; then the gas slowly recombines traveling away from the source on temporal scales comparable to the travel time through the bright section of the jet. The HH 34 jet and the first section of the HH 46/47 jet are described by a single recombination curve of a conical flow model: here, internal shocks are likely to be too weak to appreciably contribute to the ionization. On the contrary, the HH 24E/C/G outflows show evidence of re-ionization events, and are better described by a series of recombination curves of different opening angles. We find best-fitting curves corresponding to opening angles of only a few degrees for most objects, in good agreement with the results of MRR91, and derive first estimates of the inclination angle for some flows. In the HL Tau jet the ionization fraction is observed to steadily increase along the flow. Here, however, could be `pumped' by a turbulent boundary layer, whose presence has been suggested on the basis of the H line profile.
3 - Where re-ionization episodes are present, the decay in is always observed downstream of the ionization jump . This raises new questions about the nature of shocks exciting the beam gas, because it contradicts the current paradigm which interprets the jet knots as internal bow shocks. The observed downstream decrease is more consistent with the flow passing through a shock front produced by, for example, Kelvin-Helmholtz instabilities arising in the interaction between the beam and its surrounding cocoon, or alternatively, when the jet collides with local nebular clumps. Bow shocks may be present as well in the jet beams, but they are required to be too weak to substantially alter the ionization state of the flowing gas.
4 - The average excitation temperature varies typically between 9000 and 12000 K. The limited spatial resolution of the investigated spectra does not allow us to study the post-shock cooling region in detail: the temperatures we derive are only a rough indication of the average excitation temperature of the forbidden lines emission region; taking this constraint into account, our results are not in contradiction with the prediction of radiative shock models (HMR94).
5 - The average total hydrogen density ranges between about 103 and a few 104 cm-3. Because of the difficulties to derive the shock compression factors, we do not attempt any correction to the derived values. This may lead to overestimates of the flow density, by a factor 3-5. Average mass loss and momentum transfer rates calculated under the assumption of constant density over the jet section are summarized in Table 3. We find that the momentum supply rates of the HH 34 jet and possibly of the HL Tau jet are large enough to drive their molecular outflows. This may not be the case for HH 46/47, where our estimate yields a momentum transfer rate of the optical jet less than one third of that associated with the molecular outflow.
6 - Our findings confirm that partial ionization is a dominant characteristic of the beams of HH jets, and we stress the importance of taking it into account in any jet model. Reliable calculations for beam shocks should consider that the fronts advance in a medium which could be already substantially ionized. This would greatly influence the cooling properties of the flow and the model predictions concerning line emission. In addition, partial ionization may introduce important differences in the modeling of magnetized jets, due to the effects introduced by collisions between charged particles and neutrals.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999