Time-resolved eclipse spectroscopy of the novalike variable UX UMa obtained with the HST/FOS is analized with eclipse mapping techniques to produce spatially resolved spectra of its accretion disk and gas stream as a function of distance from disk center. The results reveal a noticeable asymmetry in the disk emission, with the disk side closest to the secondary star showing pronounced absorption bands in the UV and a Balmer jump in absorption. The spectrum of the infalling gas stream is noticeably different from the disk spectrum at the same radius suggesting that gas overflows through the impact point at disk rim and continues along the stream trajectory, producing distinct emission down to 0.1 RL1.

We discuss previous models of V Sge and, in light of our new phase resolved spectroscopy data, suggest a new model: a colliding wind.

The 3D transfer equation for polarized radiation is solved for several different model configurations. In the case of Rayleigh scattering the phasematrix is decomposed into a sum of matrix products to allow the use of operator splitting iteration schemes. For 30 slabs with constant extinction matrix and constant emission the emerging polarized fiuxe are calculated. We find that the optical depth and the scattering parameter can be fixed from observations even if only the fluxes are known. Specifying the extinction matrix our models can be applied to the inner regions of active galactic nuclei.

The winds associated with high states of non-magnetic (diskaccreting) cataclysmic variables are described and discussed. A quick summary of the basic phenomenology is given, and followed by a presentation of some of the more important recent developments in our understanding. The near-ubiquity of orbital-phase linked variability of the UV resonance lines (generally thought of as mainly wind-produced) is noted and its implications are considered. The impact of the much lower-thanexpected boundary layer luminosity upon mass loss rate determinations is also discussed. Current work on the role of radiation pressure (mediated by line opacity) is placed in context.

The structure of an accretion disk illuminated by X-rays is investigated. For high values of X-ray heating no static disk solution was found, while for low values of heating a corona was formed above the disk. In the high X-ray illumination case the only possible wind solution is the supersonic solution meaning the evaporation of the disk. Only a very narrow range of X- ray heating and critical point height yield the proper wind solution that agrees with the disk solution. Finally, a self-consistent model of accretion disk and a wind is presented.

The contributions that time-dependent multidimensional numerical simulations have made to our understanding of MHD winds from accretion disks are reviewed. Current simulations can be divided into four categories: (1) axisymmetric global models, (2) axisymmetric simulations of the wind only, (3) three-dimensional local models, and (4) threedimensional global models. Results from each category are discussed. Current results indicate that weakly magnetized disks are turbulent, and that this turbulence is responsible for dynamo action which amplifies the disk field. These effects may have important consequences for the production of MHD winds. We discuss the feasibility of fully three-dimensional global models that can capture these effects in the future.

A family of stationary solutions for magnetically driven outflows has been discovered by time-dependent, axisymmetric magnetohydrodynamic simulations of outflows from an accretion disk threaded by an inclined magnetic field. At the disk surface matter was given a small initial push with a speed less than the slow magnetosonic speed. In the simulations, the outflowing matter is observed to be accelerated by magnetic force and by centrifugal force up to velocities in excess of the fast magnetosonic speed and in excess of the escape speed from the central object.

Be star disks are formed by ejection of stellar matter from the surface of a B star rotating at almost critical velocity. In SPH simulations we find that most of the ejected particles fall back on the stellar surface but those with sufficient angular momentum are able to feed a disk-like structure. Owing to viscous interaction some particles are lifted to larger radii where they carry high angular momentum. Viscous forces also cause a thinning of the initially geometrically thick disk and the final accretion of most of the disk material. Different simulations show how the formation and the extension of the decretion disk depend on the ejection velocity, the viscous parameter α and on how long the source is active. After the outburst the disk thins out more and more, over a timescale much longer than the outburst time.

The simulations are compared to Hα observations of the Be star µ Cen.

We derive the temperature and density structure of the accretion disk of the dwarf nova U Gem in quiescence from 3D radiative line and continuum transport calculations of a differentially rotating disk.

Young binary star systems have associated gaseous disks that affect their evolution. Strong tidal forces of a binary give rise to gaps that separate circumstellar (CS) disks from the circumbinary (CB) disk. We discuss two important processes that involve binary/disk interactions. The first is eccentricity evolution of a binary embedded in a disk. The second is accretion of gas across the gap that separates CS and CB disks. These results may also have implications for the recently discovered planets.

Many, perhaps most, accreting T Tauri stars show evidence for magnetically dominated accretion flows near the stellar surface. I discuss the implications of this for the rotation rates and long-term photometric variability of T Tauri stars.

Hubble Space Telescope imaging of HH 30 has revealed this object to be a prototype young stellar object (YSO) accretion disk system. An optically thick circumstellar absorption disk, 450 AU in diameter, is seen extending perpendicular to highly collimated bipolar jets. The jet width is resolved in the transverse direction and increases with distance from the central star. The size and age of the disk compare well to the early solar nebula during planet formation. HH 30 is the first YSO disk ever observed at 14 AU resolution, and the first viewed close enough to edge-on that the vertical structure of the disk is clearly visible. These HST images of HH 30 offer the first-ever opportunity to directly measure the flaring profile of an astrophysical accretion disk. We have compared the images to model nebulosities calculated in both multiple and single scattering regimes, in the latter case via chi-squared minimization. This analysis constrains the circumstellar density distribution and grain scattering properties. A brief summary of the results is presented here; full details and references appear in Burrows et al., Ap.J. (1996).

The Herbig Ae/Be stars are optically visible pre-main sequence stars of intermediate mass (M* ≈ 3−8M⊙) and are thought to be the higher mass analogues of the T Tauri stars. While there is no doubt that classical T Tauri stars, i.e. those with EW(Hα) ≳ 10 Å, are surrounded by disks, it remains controversial as to whether this is the case with the equivalent Herbig Ae/Be stars. It has even been questioned whether the powerful winds that are ejected by Herbig Ae/Be stars are driven by accretion. To address these problems we have examined a large sample of these stars with the idea of using their forbidden line emission as an indirect diagnostic for the presence of disks. Striking similarities with the classical T Tauri stars are found. For example we have discovered evidence not only for a strong correlation between near-infrared colours and the equivalent width of the forbidden line emission but also that the forbidden line emission normally arises in a blueshifted outflow component. It has already been shown in the case of the classical T Tauri stars that the correlation of near-infrared colour with forbidden line equivalent width is due to a link between the accretion rate and the outflow rate. The virtually identical relationship seen in the case of the Herbig Ae/Be stars must then also have a similar origin. Our finding that the forbidden line emission in Herbig Ae/Be stars is normally blueshifted shows not only that it arises in an outflow but, as in the classical T Tauri stars, such an asymmetry in the velocity centre of the line must be caused by the obscuring effects of a disk. We find that the correlation seen in the classical T Tauri stars between the mass-loss rate and infrared excess can be extended, when we include the Herbig Ae/Be stars, to cover almost 5 orders of magnitude in stellar luminosity. Our observations therefore broaden the findings of earlier observers for low mass young stars and indicate the presence of circumstellar disks around the majority of Herbig Ae/Be stars with forbidden line emission. A corollary of our results is that the same outflow mechanism must operate in both the classical T Tauri stars and the Herbig Ae/Be stars with forbidden line emission.

We describe the appearance and significance of HH212, the most symmetric two-sided molecular hydrogen jet/counterjet system yet discovered. This prototype embedded protostellar H2 jet emanates from a low-luminosity isolated protostar in Orion. It exhibits matched pairs of knots and bow-shocks interpreted as arising from a time-variable source, which we take to indicate that protostellar accretion through the encircling disk is non-steady and pulsed.

The superb spatial resolution made possible by the WFPC2 imager on the Hubble Space Telescope has opened a new era in the study of shocks in stellar jets. This paper summarizes the results of imaging surveys of two of the brightest and best resolved stellar jets, HH 111 and HH 47. The new HST images show that the jets appear quite different in the Ha and [S II] lines; Ha concentrates in distinctive filaments while [S II] tends to be clumpier and more widespread. Filamentary Hα emission should trace the location of the shock fronts if the preshock gas is neutral, because a significant fraction of the incident H gas becomes excited collisionally before becoming ionized in moderate velocity shocks.

Both HH 111 and HH 47 exhibit multiple bow shocks along the channel of the jet, and in some cases we observe the classical double shock structure of a bow shock/Mach disk expected within the working surface of a jet. The jet material is considerably denser than that of the ambient gas. Ejection angles vary a few degrees with time, giving some jets a ‘wiggling’ appearance. However, motion along the jet appears to be radial; there is no convincing evidence for azimuthal motion in the radial velocity data. Multiple bow shocks observed along the jets probably arise from unsteady accretion of a protostellar disk onto the central object.

We present observations of the Orion Nebula made with the Hubble Space Telescope in which a number of stars are seen surrounded by dark silhouettes seen projected against the bright background HII region. We find a variety of morphologies, all consistent with thin circumstellar disks spanning a range of diameters (50 to 1000 AU) and inclination angles (0 to > 80 degrees). The silhouette intensity profiles cannot be fit by standard disk models in which the surface density follows a radial power law with an exponent in the range −0.75 to −1.5. Rather, the data are best fit by opaque inner disks with exponential edges, and we discuss possible physical origins of this apparent truncation. Masses in the range 6 × l026−4 × 1030 g (i.e., up to 0.002 M⊙) are determined for the disks by assuming that the faint light measured from them is background light transmitted through the disk. However, these are strict lower limits on the true disk masses, as most of this light can be accounted for by PSF blurring and scattering in the HST optical train; the present observations are in fact consistent with completely opaque disks. Central stars are seen directly in five of the silhouettes, while the presence of a star is inferred in the sixth, where small reflection nebulae are seen above and below the plane of the near edge-on disk. Optical and near-infrared stellar photometry is consistent with young (~ 1 Myr) low-mass (0.3−1.5 M⊙) stars, with several showing evidence for excess near-infrared emission from the disk inner edge. These direct imaging observations are discussed in the wider context of circumstellar disks in the Orion Nebula and Trapezium Cluster, additionally revealed as compact ionized nebulae (so-called “proplyds”) in the vicinity of the central OB stars, and via infrared (>2µm) excesses in stellar photometry. Overall, disks are found to be common in the cluster (>50% of all stars), implying that they can survive the rigours of life near massive stars.

Centrifugally driven winds from the surfaces of magnetized accretion disks are an attractive mechanism for removing the angular momentum of the accreted matter and of producing bipolar outflows and jets in compact astronomical objects. In this contribution, I first review steady–state disk–wind models that have been constructed for the different density regimes of circumstellar disks and comment on their expected stability. I then consider several nonsteady effects, including disk formation in molecular cloud-core collapse, magnetic flux transport through the disk, and the role of magnetic fields in the FU Orionis outburst phenomenon. I conclude with a discussion of some of the unique observational properties of disk-driven outflows in young stellar objects and in active galactic nuclei. These characteristics are a consequence of the highly stratified density and velocity structures of centrifugally driven outflows, their large momentum discharges (which result in the efficient uplifting of dust from the disk), and, in the case of molecular disks around lowluminosity objects, their comparatively low initial degrees of ionization (which can lead to rapid heating by ambipolar diffusion).

Angular momentum transport in protostellar disks can be achieved by the action of a large scale magnetic field that runs vertically through the disk. The magnetic field centrifugally drives material from the disk surfaces into a wind, initiating a bipolar outflow. One apparent difficulty for this model is that the conductivity of the disk is extremely low in the inner 0.1–10 AU of the disk, where grains are the dominant charge carriers. Near the midplane, charged grains are unable to drift through the neutral gas and there is negligible coupling between the magnetic field and the disk material.

However, the removal of angular momentum and acceleration of a wind by a magnetic field can still take place in the surface layers of the disk where the gas conductivity increases dramatically. Solutions to the multifluid MHD equations for the vertical structure of a disk at a particular radius are presented. Most of the disk material sits in hydrostatic equilibrium and does not interact with the magnetic field running vertically through it. Near the disk surfaces, the coupling between the magnetic field and disk material is sufficient to initiate an outflow from the disk surface.

A simplified axi-symmetric 2.5D radiative transfer model of a YSO envelope (HST-1, 177–341, O’Dell & Wen 1994) in the Orion Nebula (NGC1976) is computed assuming only a spherical distribution of gas with a powerlaw density function. The model fits recent Hubble Space Telescope data (Bally et al. 1996) very well on 1015 cm scales, suggesting that the morphology of the ‘proplyd’ objects is due to photoionization on this scale rather than a direct wind-wind interaction (Henney et al. 1996). The fact that a simple spherical distribution of gas fits the ionization observations so well also implies that somewhere between the scale of the YSO accretion disk and the observed ionization front radius the gas distribution has changed.

This paper discusses how high spatial and spectral resolution observations of bow-shock ‘bullets’ in Orion are challenging our understanding of molecular shock physics.