We review the events leading to the discovery of Herbig-Haro objects half a century ago, and the early efforts to understand the nature of these enigmatic objects. The recognition in the mid-seventies of the shocked nature of HH objects heralded a burst of observational and theoretical efforts, and further impetus was soon after provided by the discovery of high proper motions, and by detailed optical, infrared and ultraviolet spectroscopic studies. The recognition in the early eighties of HH jets was the starting point for the increasingly intense studies during the last 15 years which we discuss in this Symposium. In the second half of our review, we summarize the insights into the nature of HH jets provided by analyzing high resolution images obtained with the Hubble Space Telescope of two of the finest known HH jets, HH 47 and HH 111.

The jet emanating from the young stellar object, known as IRS5, located in the molecular cloud L1551 in Taurus has been imaged with the Hubble Space Telescope. The observations and a preliminary interpretation of them is presented. The relevant background history for this important object is briefly reviewed.

Recent observations with wide field-of-view CCDs have shown that over 20 Herbig-Haro flows extend for more than one parsec from their driving sources. We review the observed properties of these giant HH flows and discuss the physical consequences for star formation, and for the physics and chemistry of the surrounding interstellar medium.

The Orion Nebula Region has two different systems of objects classified as HH objects. The North System is associated with the H2 fingers seen in the infrared and is probably the result of Rayleigh-Taylor instabilities in shocked material moving into the near side of the giant molecular cloud OMC-1. The South System is associated with source(s) within the Trapezium cluster, with the shocked HH objects occuring where jets from pre-main sequence stars impinge on the neutral lid of material that lies across the front of the Orion Nebula. Such jets are different from those driving other HH objects in that these are passing through photoionized material and two of the Orion jets may have been detected.

While recent studies of Herbig-Haro (HH) objects have focused less on the details of their spectra than on the hydrodynamics of jets and their working surfaces, many open questions concerning these spectra remain. Attempts to quantitatively explain a wide range of lines for many HH objects point to discrepancies between theory and observation. Some lines (specifically [S II](6716+6731)) are much stronger than predicted by simple plane-shock and bow-shock models, while in general high ionization lines (e.g. lines of [O III], [Ne III] and [S III] in the optical and the [C IV] and [N V] in the ultra-violet) are much weaker than expected, pointing to difficulties with current models. On the other hand, examination of these lines has lent new insight into both the quality of our predictions and the nature of HH outflows. Examination of many Fe lines have demonstrated that our ability to estimate abundances from faint lines is surprisingly good (or surprisingly consistent). Position velocity diagrams have also been constructed (using forbidden emission lines), allowing outflows to be mapped to within 0.″3 arcseconds of the source star.

High-resolution long-slit spectroscopy of forbidden emission lines is used to investigate on a sub-arcsecond scale the spatial and kinematic properties and the physical conditions of the mass outflows from T Tauri stars in the immediate vicinity of the outflow source (microjets). Special attention is given to the case of DG Tau. The data permit us to distinguish physically different outflow components: (1) a high-velocity component (HVC) attributed to a fast jet, (2) a low-velocity component (LVC) attributed to gas entrained by the jet, and (3) a near-rest-velocity component (NRVC) attributed to a slow disk wind and/or disk corona.

An original spectroscopic diagnostic technique has been recently developed that allows to estimate in a model-independent way the ionization fraction xe and the average excitation temperature in the beam of Herbig-Haro jets (Bacciotti, Chiuderi and Oliva 1995). The procedure is based on the fact that in the low excitation conditions present in this region the ionization state of oxygen and nitrogen can be assumed to be regulated by charge exchange with atomic hydrogen. The application of this technique to long-slit spectra of several well-known stellar jets indicates that the hydrogen ionization fraction xe starts from 0.2-0.4 at the beginning of the flow and gently decreases along the whole jet or along sections of it as a result of time-dependent recombination. The average temperature stays almost constant, ranging from 4500 to 7000 K. The momentum transfer rates evaluated with the derived total number densities (nH ∼ 103–104 cm–3) give support to the picture in which the jet is responsible for the acceleration of a surrounding molecular outflow.

Since thermal radio jets can be observed with subarsecond angular resolution and are unaffected by dust absorption, they provide a useful tool to study collimated outflows very close to the young stars that produce them. Here, I review recent results on this area, giving emphasis to the study of thermal jets in the case of the quadrupolar outflows in L723 and HH 111.

This overview of recent observations of molecular hydrogen (H2) in outflows from young stars discusses the morphology, the excitation and the kinematics of the H2 gas in these flows. A comparison between the H2 and CO outflows will lead us to the conclusion that highly-collimated jets may drive the latter. We also discuss the mechanisms which can entrain ambient molecular gas into the jet flow.

Outflows arising from young stellar objects interact with their surrounding medium through different mechanisms, such as shocks, turbulence and /or entrainment. These two last mechanisms have recently begun to be fully developed in an effort to understand the coupling of the stellar jet gas (mostly atomic) with the molecular environment. Observationally a number of objects are being studied in the near infrared and millimeter wavelengths to map the warm (H2) and cold (CO) molecular gas, respectively. In this paper we discuss some of the properties in the near infrared of various embedded jets in the context of turbulent entrainment.

Full Iso-Lws spectral scans between about 45 to 190 μm of 17 individual HH objects in 7 star forming regions have revealed essentially only [O I] 63 μm line emission, implying that the Fircooling of these objects is totally dominated by this line alone. In this case, J-shock models can be used to determine the mass loss rates of the HH exciting sources. These mass loss rates are in reasonably good agreement with those estimated for the accompanying CO flows, providing first observational evidence that HH and molecular flows are driven by the same agent. The Lmech – Lbol relation, based on our results with the Lws, implies that young stellar objects of lower mass are loosing mass at relatively higher rates than their more massive counterparts.

Molecular outflows are intimately related to the highly collimated Herbig–Haro jets emanating from young stars. In consequence, the usual dynamical timescale significantly underestimates the true age of an outflow. If we correct for this factor, and assume an intrinsic outflow speed similar to that of the underlying jet, we predict that molecular outflows should have an overall extent of several parsecs, in accordance with recent results. It seems likely therefore that outflows are a major source of interstellar turbulence, and have a profound impact on the process of star formation.

Whilst interpretation of jet-like outflows is relatively straightforward, the origins of shell-like outflows, such as that from L 1551–IRS5, are less obvious. We discuss the current observational status of both types of flow, and hypothesize an evolutionary connection between them. A large and well-defined outflow sample is urgently required, to permit the establishment of an age-sequence; such a sample would also provide the basis for a proper investigation of outflow energetics and interaction with the ISM.

We present high angular resolution observations of the molecular outflow associated with the optical jet and HH objects of the HH111 system. Interferometric observations in the CO J =2–1 and J =1–0 lines of the high velocity bullets associated with HH111 are presented for the first time. The molecular gas in these high velocity clumps has a moderate kinetic temperature and a mass of a few 10–4 M⊙ per bullet. We favor the view that HH jets and CO bullets, which represent different manifestations of the same physical phenomena, are driving the low-velocity molecular outflow.

Chemical studies have a great potential to study the structure and evolution of the bipolar molecular outflows driven by young stellar objects. In this paper, we discuss some very recent mm-wave studies of L 1157, a bipolar molecular outflow driven by a Class 0 protostar. These observations are very useful to illustrate the chemical alterations produced by a violent highly-collimated outflow. Different molecular lines are observed to trace different components of the gas. Some molecules are abundant in the quiescent medium but are not observed in the shock (e.g. C3H2, N2H+, H13CO+, DCO+), whereas some otherwise rare molecules are very enhanced at the shocked region (e.g. SiO, CH3OH, H2CO, HCN, CN, SO, SO2). In addition, we have observed strong gradients in the chemical composition across the outflow blue lobe. We briefly discuss the chemistry of the most important molecules, devoting special attention to the species which are thought to be abundant in interstellar ice mantles.

This paper reviews various hydrodynamical models proposed for explaining bipolar molecular outflows from young stars. The different possibilities discussed are : wind-driven molecular shells, steady state filled flows, and jet-driven bow shocks. Particular emphasis is placed on comparisons between model predictions and observations of molecular flows. While it appears that jet-driven bow shocks are at the present time the most popular mechanism for explaining molecular outflows, other models have interesting characteristics that might be relevant for at least some objects.

Shock waves light up the jets, winds and outflows around YSOs and diagnose the physical conditions and processes resident in these regions. This paper discusses the differences between the jet/wind shock and the ambient shock, between C shocks and J shocks, and between the shocks produced by pure jets and by collimated wide angle winds. Basic shock physics is briefly reviewed, with a special focus on the temperature structure in shocks and the Wardle instability of C shocks. Application is made to the origin of shocked H2 emission and to H2O masers.

The bow shocks which are observed in jets associated with low-mass star formation probably have C-shock characteristics over at least part of their extent. We show that ion-neutral streaming in C-type shocks can give rise to the release of significant amounts of silicon from the refractory cores of dust grains and hence to the formation of SiO in the gas phase. The computed profiles of SiO emission lines are found to be qualitatively similar to those observed in molecular outflow regions such as L1448. The question of the validity of the stationary-state assumption, adopted in all previous studies of C-shocks in the interstellar medium, is addressed. Time-dependent calculations of the evolution of a C-type shock propagating in low-density gas suggests that steady state is unlikely to be attained under the dynamical conditions in outflows.

Stars form from dense pockets of gas that have undergone gravitational collapse, and young stars are still embedded in this molecular material. There is no lack of evidence that the outflows from these stars, manifested in the form of Herbig-Haro objects and jets, interact dynamically with the dense gas either in the form of shock excitation or the imparting of momentum; molecular outflows, H2 emission, and perhaps also CO and SiO bullets and jets, are tracers of this phenomenon. However there are a set of radio observations of molecules showing emission near HH objects that are remarkable for their apparent lack of signatures of such dynamic interaction. A likely cause of this ‘quiescent’ emission is a radiative interaction between the HH shock system emission and ambient gas.

We review the theory of x-winds in young stellar objects (YSOs). In particular, we consider how a model where the central star does not corotate with the inner edge of the accretion disk may help to explain the enhanced emission of X-rays from embedded protostars. We argue, however, that the departure from corotation is not large, so a mathematical formulation that treats the long-term average state as steady and axisymmetric represents a useful approximation. Magnetocentrifugally driven x-winds of this description collimate into jets, and their interactions with the surrounding molecular cloud cores of YSOs yield bipolar molecular outflows.

Formation of jets in low-mass protostellar objects and young pre-main sequence stars is ultimately related to the existence of some gaseous disk around a rapidly rotating central object. This configuration has deep parallels to extragalactic systems such as radio galaxies and quasars. Rotating black holes are still thought to be the prime-mover behind the activity detected in centers of galaxies, while, in the case of protostellar jets, rapidly rotating stars and disks are responsible for the ejection of bipolar outflows. In both cases, magnetic fields are invoked for the acceleration, the collimation and propagation of these outflows. The ultimate rooting of these fields is still under debate. We discuss models where winds injected into rapidly rotating magnetospheres of the central object drive the outflows. From these considerations it follows that the jets of young stellar objects can only be produced magnetically and that their progagation is determined by their magnetic properties. Such jets have low Mach numbers ≃ 2 and their instabilities are dominated by the pinch mode. Knots closest to the source are attributed to compression by the time-dependent pinches. Multiple bow shocks occur on longer time-scales (a few thousand years) and are attributed to variations in the magnetospheric structure of the star, or the disk.