Hydromagnetic disk winds have great potential as a general theory for the production and collimation of astrophysical jets in both protostellar and black hole environments. We first review the analytic stationary theory of these outflows as well as recent numerical simulations of MHD disk winds. We then focus on simulations that we have done on winds from magnetized disks using the ZEUS 2-D code of Stone and Norman. We treat the Keplerian disk as a fixed platform throughout the simulations. We show that both stationary and episodic, jet-like outflows are driven from disks depending upon their magnetic structure and mass loss rates.

Approximate asymptotic solutions for rotating MHD winds are obtained analytically in terms of the first integrals of the motion. It is shown that the paraxial region of such winds is a line-shaped boundary layer which has, even at large distances, the structure of a pressure-supported current pinch. A necessary condition for cylindrically focused asymptotics to be possible is derived. A simplified model by which the asymptotic structure of such winds can be obtained in terms of general boundary conditions at the wind source is introduced. Results of semi numerical solutions of the model are reported. The model is analytically solved in the limit of very fast rotators, giving in this particular case an explicit and complete description of the wind outputs and asymptotic structure in terms of arbitrary boundary conditions at the wind source.

We present the results of numerical hydrodynamic models for the collimation of outflows from young stellar objects. We show that the presence of a toroidal environment can lead to efficient formation of jets and bipolar outflows from initially uncollimated central winds. The interaction between the wind and the environment leads to two types of collimation, one which is dominated by radiative cooling effects, and one which works when cooling is less efficient. We describe the two types of jets as they appear in the simulations and we suggest a description for the long term evolution of these structures in more realistic time-dependent wind sources.

Dense molecular jets cutting through dense molecular clouds are simulated here using a ZEUS-type hydrodynamics code extended with molecular physics. H2 ‘hot snapshots’ and CO ‘historical views’ are nicely modelled with overdense uniform jets. Attention is drawn to some remaining hot problems: observed jet knots are bow shaped, bipolar outflows can be highly asymmetric, some proper motions within jets are enigmatically low and H2 excitation can be exceedingly uniform.

We present the Hammer Jet, in which the nozzle introduces high velocity variations, as well as a strong ripping and spray action. Prominent jet bow shocks in H2 and CO emission lines are then produced. Wide tubular CO structures with concave bases show up. Proper motions are simulated. The asymmetries are modelled by jet break-out from a molecular core into a diffuse atomic environment.

The aim of this work is to show that, though 3D effects should be invoked to explain the less regular arc-like regions, the internal knotty emissions may be explained through a roughly axisymmetric pattern of oblique shocks.

To this purpose, numerical simulations of non-equilibrium, radiative, hydrodynamical jets in axisymmetric geometry have been performed. After showing that the internal reflecting waves, travelling in the beam of a stellar jet, steepen into oblique shocks, we verify that the optical emission arising from the shocked regions is in good agreement with observations.

Since the optical emission strongly depends on the physical and chemical parameters, (namely temperature and electron densities inside the emitting regions), special care is devoted to solving the energy transport equation.

The results of a detailed analysis of the linear properties, nonlinear growth, and saturation of asymmetric modes of the Kelvin-Helmholtz instability in cooling protostellar jet beams are summarized. In the linear regime, cooling can significantly alter the growth rate and wavelength of the most unstable mode in comparison to an adiabatic jet. In the nonlinear regime, sinusoidal oscillations at the maximum growth rate lead to distortions that will be observed as ‘wiggles’ or ‘kinks’ in the jet. Strong cooling behind shocks formed in the nonlinear regime can produce emission knots and filaments. In some cases, the modes grow until the jet is disrupted. Distortions in the surface of the jet drive shock spurs into the ambient gas, resulting in longitudinal acceleration. Rapid acceleration and entrainment of ambient gas is also observed if the jet is disrupted.

We discuss the non-linear evolution of Kelvin-Helmholtz instabilities in Herbig-Haro jets performing numerical simulations by means of a PPM hydro-code modified as to include non-equilibrium, optically thin, radiation losses and heating. In this paper we discuss in particular the effects of different functional dependences of heating on density. The results obtained show a weak dependency of the instability evolution on the different forms of the heating function, that is largely unknown, therefore the simple assumption of constant heating, adopted in previous papers on this matter, does not lead to severe limitations on the general applicability of the results to the astrophysical jets and, in particular, to the origin of the emission knots.

We discuss recent progress in analytic modeling of stellar wind bow shocks and colliding winds. For thin, radiative shocked layers in steady-state, the shape of the layer as well as its internal flux of mass and momentum are found from the conservation laws of mass, momentum and angular momentum. For the case that the shocked gas is well-mixed, the velocity distribution and mass column density of shocked material are also obtained. These solutions are extended to the problem of a jet bow shock, treated as a non-isotropic “wind” interacting with the ambient medium. We also examine the shell energetics for these simple analytic models. The constraint of conservation of momentum leads to an upper limit to the efficiency of thermalization and radiation of the pre-shock wind kinetic energy. Calculations are presented of this thermalization rate as a function of the input momentum rates of the pre-shock winds.

We report on Hubble Space Telescope imaging of eleven young stellar objects in the nearby Taurus molecular clouds. The high spatial resolution and stable point spread function of HST reveal important new details of the circumstellar nebulosity of these objects. Three sources (HH 30, FS Tau B, and DG Tau B) are resolved as compact bipolar nebulae without a directly visible star. In all three cases, jet widths near the sources are found to be 50 AU or less. Flattened disk structures are seen in absorption in HH 30 and FS Tau B, and in reflection about GM Aur. Extended envelope structures traced by scattered light are present in HL Tau, T Tau, DG Tau, and FS Tau. The jet in DG Tau exhibits a large opening angle and is already resolved into a bow-like structure less than 3″ from the star.

The IRAM interferometer has been used to map 3 high velocity outflows from Class 0 sources in SiO and CO rotational lines. The images show that such outflows are driven by highly collimated, dense, but optically invisible jets, through large travelling bow shocks. Precession is important in explaining some morphological and kinematic aspects. A significant part of the SiO emission is directly associated with the jet. Images of the molecular core surrounding L 1157 in CO isotopic lines show direct evidence for infall in a large, flattened disk.

For more evolved objects, “surveys” of mm emission from T Tauri stars provide evidence for circumstellar dust disks with relatively flat surface density distribution and typical radii 100 to 200 AU. Much larger gas disks (radii 800 to 1000 AU) have been detected and imaged in a few objects (2 singles and 2 binaries) through CO isotopomers, but in most sources 13CO is difficult to detect. The spectral line images provide unambiguous evidence for Keplerian rotation. In two cases (GG Tau and DM Tau), direct measurements of the gas mass (independently of the dust) has been made thanks to the detection of 7 molecular species. The molecules are found to be depleted by factors between 5 (for CO) to 100 (for H2CO). These results suggest that circumstellar disks consist in a small, dense, inner disk of outer radius ≃ 150AU (traced by the dust emission), surrounded by a much larger, less massive, outer disk (traced by the gas emission).

We are intensively studying low mass star formation with the radio telescopes at Nobeyama in Japan. Using both the Nobeyama 45 m dish equipped with a 2 × 2 array receiver and the Nobeyama Millimeter Array (NMA), we can cover a very wide spatial range from overall molecular clouds down to compact protoplanetary disks. With the 45 m dish we are investigating hierarchical structures of molecular clouds including star-forming cores. With NMA we are imaging disklike structures (i.e., envelopes, accretion disks, and protoplanetary disks) around protostars and T Tauri stars. Recently, we have completed our survey for dense disklike envelopes around eleven Class 0 & I protostars by NMA. In this paper, we will present our recent results of the disklike envelopes in addition to the previous NMA results of the disks around three T Tauri stars. On the basis of the data, we will discuss the evolution of the disklike structures (dense envelopes → tenuous ones → dispersing ones → accretion disks → protoplanetary ones), and propose a new scenario for the formation of low mass stars.

Outflows from low-mass young stellar objects are thought to draw upon the energy released by accretion onto T Tauri stars. I briefly summarize the evidence for this accretion and outline present estimates of mass accretion rates. Young stars show a very large range of accretion rates, and this has important implications for both mass ejection and for the structure of stellar magnetospheres which may truncate T Tauri disks.

We present preliminary results of an investigation into the radiative impact of FU Orionis outbursts on protostellar envelopes. In the thermal accretion disk instability model, the inner portion of the disk is inflated in such a way as to funnel most of the outburst luminosity along the poles of the system. A multidimensional radiative transfer code is employed to derive the thermal equilibrium structure of the surrounding protostellar envelope and the backheated accretion disk during outburst. The radiation emitted during outburst is modeled as a combination of a self-luminous accretion disk and a central point source. We compare the relative heating of circumstellar material due to (1) an isotropic point source and (2) an anisotropic point source in which the emitted radiation is confined to a 30° cone about the polar axis. The isotropic point source creates a spherical dust cavity, while the anisotropic source naturally results in a cavity which is hourglass-shaped. Repeated outbursts may ultimately be responsible for the large-scale polar cavities commonly inferred to exist around young stellar objects. We also show that due to the anisotropy of the radiation expected during outburst, disk annuli within a few au will be shielded from the radiation of the outburst.

The properties of Classical T Tauri (TTS) winds - their mass loss rate, temperature, origin, and energetics - are reviewed and interpreted in terms of the most recent observational evidence. TTS winds seem to have their origin in the disk and are powered by accretion. Most TTS have Ṁω ≤ 10–9Mʘ yr–1 and Ṁω/Ṁ ∼ 0.1, with T ∼ 104K.

Observational evidence suggests that disk accretion in low mass stars is mediated by the stellar magnetosphere in the inner few stellar radii of a star/accretion disk system. A strong stellar field appears to disrupt the inner disk and channel accreting material in a funnel flow toward the stellar surface. It is possible that coupling between the stellar magnetosphere and the inner accretion disk plays a profound role in the evolution of a forming star, regulating its spin, establishing its initial angular momentum, and launching the ubiquitous accretion-driven outflows which are the subject of this symposium. Although the evidence that forming low mass stars undergo magnetospherically mediated accretion is considerable, there are a growing number of challenges to this paradigm which require close observational scrutiny.

Arguments, based on analysis of the forbidden line emission, are summarized that point to two components of mass ejection in classical T Tauri stars. A low-speed wind originates from the accretion disk at between ∼ 0.05 AU and ≳3 AU from the star, and a high-speed flow, which becomes collimated into a jet within ≲ 100 AU, originates as a wind from either the star or the very inner part of the accretion disk. The [OI] λ5577 emission in the low-speed component also requires the presence of a warm disk corona, with an electron density of ∼ 107 cm–3 and a temperature of ∼ 8000 K. Additional indication of a warm disk corona comes from analysis of the central absorptions seen in the profiles of the hydrogen Balmer lines. The need to heat the disk corona implies that a substantial fraction of the energy released in the accretion of matter through the disk may be dissipated at the disk surface.

The thermodynamic structure of gas that is channeled by stellar magnetic fields onto a young (pre–main-sequence) star is presented. In this model, the star possesses a dipole magnetic field which disrupts the inner regions of a geometrically thin accretion disk and channels the inflowing gas onto the stellar surface, thereby forming an accretion funnel. The temperature and ionization degree of the inflowing gas is calculated by solving the heat equation coupled to statistical rate equations for hydrogen. It is found that for typical accretion rates of ∼ 10–7M⊘yr–1, temperatures of ∼ 7000 K and hydrogen ionization fractions (nH+/nH) of ∼ 10–2 can be attained in the funnel flow. The principal heat source is found to be adiabatic compression, and coolants include bremsstrahlung radiation as well as line emission from the Ca II and Mg II ions. The relatively hot and ionized funnel flow lead to observational signatures such as inverse P Cygni line profiles seen in upper Balmer and near-infrared lines. In addition, carbon monoxide bandhead emission may be an important tracer of the outer portions of the funnel flow.

The magnetic field strengths of several T Tauri stars are derived by measuring the width of unblended Fe I lines of high and low values of geff·λ2 using the autocorrelation function. The T Tauri stars were selected for their low values of v · sin i, and large strengths of the Ca II emission component. The derived magnetic field strength are 2.0 ± 0.6 kG and 2.6 ± 0.8 kG for the classical T Tauri stars Lk Ca 15 and T Tau, respectively. An upper limit of 0.6 ± 0.8 kG is found for the weak-line T Tauri star Lk Ca 16. The method is tested by analysing two non-magnetic main sequence stars, and a late-type star that is known to have a strong magnetic field.

We have observed at 5GHz the T Tau system with high resolution (≲0″.1) using the Multi-Element Radio Linked Interferometer (MERLIN) based at Jodrell Bank. Both the optical star (T Tau N) and its well-known infrared companion (T Tau S) were detected. The radio emission from T Tau S was found to be roughly extended in the direction of what is thought to be its outflow axis. More importantly we discovered that this radio emission split up into two spatially separated lobes of opposite helicity in the left and right-circular polarization channels. The circularly polarized lobes appear to straddle the star so that the “flow” and the “counterflow” were of opposite helicity. Such observations are the first direct evidence for the presence of magnetic fields in extended outflows that we are aware of. The radio flux appears to be due to gyrosynchrotron emission from mildly relativistic electrons (γ≈2–3). These electrons may have been accelerated in shocks close to the source. Using reasonable assumptions, the inferred magnetic fields strengths are surprisingly large (≳ several gauss) at distances of approximately 10-20 AU from their source. This is consistent with the magnetic fields being part of a collimated flow.

In this paper, I summarize the key properties of Class 0 protostars, discuss the observational evidence for a decline of outflow/inflow power with evolutionary stage, point out a possible connection with the initial conditions of fast protostellar collapse, and suggest a new collapse scenario in regions of induced, multiple star formation. The main thesis put forward here is that Class 0 objects have unusually powerful outflows because they accrete at a substantially higher rate than their Class I descendants.