SIMBAD references

In this paper we discuss the possible mechanisms of formation ot the nodular structure of Herbig-Haro jets. The available spectral observations of the linear jet section in the HH 34 and the HH 111 complexes are reviewed, in order to diagnostic the temperature and ionization state of the emitting material in a way as model-independent as possible. The values we find by means of an original although straightforward diagnostic procedure (T∼6000K, x=n_H+^_/nH_∼0.1, in both cases), are similar to those empirically determined by Bruegel et al. (1981ApJS...47..117B) and Boehm & Solf (1990ApJ...348..297B) for the compact object HH 7. Our results lead to a value of the momentum rate in these flows comparable to the rates observed in high-speed neutral winds (Lizano et al. 1988ApJ...328..763L), giving support to the view according to which the bright optical jet actually identifies the axial portion of the neutral flow, and marks the location where the matter attains its maximum velocity and an observable amount of ionization. The ionization derived from the observations is likely to be a remnant of the heating and the excitation occurred in the accelerating region, where the wind is probably focused by the passage through a nozzle. The recombination time scale appropriate for the rather low electron densities in the jet is found to be close to the crossing time of the visible portion of the jet. Combining kinematical and energetic considerations we show that physical conditions consistent with the observed jet's emission can be obtained through "soft" compressions of the central portion of the flow, caused by 'damped' Kelvin-Helmoltz instabilities generated at the jet-ambient contact discontinuity. These compressions concentrate in smaller volumes the already available internal energy that is eventually radiated in optical and IR lines, unlike the weak shocks that actually increase the internal energy content by transforming the bulk kinetic energy into thermal random motions. A picture of this kind seems capable of resolving the long-standing problem of conciling the high supersonic velocity of the ionized material and the low excitation nature of the emission, and, at the same time, provides a very natural explanation for a number of observational constraints, among which the increase of the [SII]/Hα ratio along the jet and the disappearance of the optical emission after a characteristic scale length.