We have numerically investigated the dust evolution in a protoplanetary accretion disk which is described by a time-dependent one-dimensional (radial) α-model. The coagulation of particles due to cohesive collisions is calculated by solving the non-linear Smoluchowski equation. The feedback of the dust evolution and the related opacity changes to the disk evolution is explicitly treated. Three different regimes during the evolution of the grain population can be identified.

We present recent results of time-resolved optical spectroscopy on two SW Sex stars, PG 0859+415 and BH Lyncis.

Stars in the earliest phases of their formation, i.e., those accreting the main component of their final mass, are deeply embedded within dense cores of dust and molecular material. Because of the high line-of-sight extinction and the large amount of circumstellar material, stellar emission is reprocessed by dust into long wavelength radiation, typically in the far-infrared and sub-millimeter bands. Consequently, the youngest sources are strong submillimeter continuum sources, and often undetectable as point sources in the near-infrared and optical. The most deeply embedded of these sources have been labelled “Class 0” sources by André, Ward-Thompson, & Barsony (1994), in an extension of the spectral energy distribution classification scheme first proposed by Adams, Lada, & Shu (1987).

Recently, it has become observationally evident that during a dwarf nova (DN) outburst, a significant portion of the gravitational energy accreted onto the white dwarf (WD) may not be radiated away instantly from a narrow boundary layer as predicted by the standard disk theory. Instead, it may be stored in the WD through various mechanisms and the radiative area may be much larger; thus the long-puzzling missing boundary layer may be accounted for when the response of the WD to the accretion is considered. The results from our group and collaborators on this aspect are outlined in the first part. A progress report on the development of a new numerical model forms the second.

We have studied spectra of the peculiar B[e] star HD 45677. Examination of the Balmer wings enabled us to determine a value of log g = 3.9 indicating a possibly luminosity class V. The weak Heɪ lines togetherwith the Si ɪɪ 4128 and 4130 Å doublet indicate a low rotation velocity in the order of 70 km s−1, which is much lower than the previously claimed value of 200 km s−1 by Swings & Allen (1971). We have examined high-resolution profiles of the strong He I line at 5876 Å and found on one hand that we can explain the variation of the profile by the motions of clouds, some of which accreted. The Balmer lines on the other hand show the presence of an accelerated wind plus absorption by a disk seen edge-on

An X-ray off state of CAL 83 has been observed in April 1996 with the ROSAT HRI three weeks after a normal on state. An upper limit for a linear decline time of the observed flux of ~20 days (and of an efolding decline time of ~6 days) is deduced. This decline may be due to the response of the white dwarf envelope to a temporary increased mass accretion rate giving rise to an envelope expansion. CAL 83 may resemble the recurrent supersoft LMC transient RX J0513.9–6951 with episodes of disappearance in X-rays.

We present a model for the large scale structure of FRIItype extragalactic radio sources. The sources are shown to grow selfsimilar and the dependence of the length of the source and the pressure inside the cocoon on the life time are calculated. The stability of the jets in such sources is investigated and the jet power at which FRII sources turn into FRIs is found to be in good agreement with observations.

The existence of winds from cataclysmic variables (CVs) has been known for some time, there are a variety of unresolved questions surrounding this phenomenon. These include: the origin of the apparent polar nature of the winds; the rate of mass loss in the winds and the associated driving mechanism; the origin for the characteristic shapes of the UV resonance line profiles, particularly the absorption component; and the apparent association between outburst state and the wind existence. Resolution of all these issues depends on understanding the dynamics of the wind. In this paper we describe one and two dimensional models for disk wind dynamics.

Using 1D analytic models we have explore the physical conditions necessary for the existence of a disk wind, and study the dependence of wind speed and mass loss rate on radius. This leads to a criterion which must be satisfied by the rate of increase in the radiation field with height above the disk surface and by the parameters describing the line radiation pressure force (c.f. Castor, Abbott and Klein, 1976).

The simulation of astrophysical radiative transfer problems (RTP) is a hard task. On the one hand, the high dimension of the computational domain (3 space, 2 ordinate and 1 frequency variables) calls for huge amounts of memory even for coarse discretisations. On the other hand, accretion discs show very localised features, making a high resolution in some regions of the disk necessary. Furthermore, there should be an estimate of the numerical errors to avoid physical misinterpretation.

Apart from the usage of parallel machines with their big memory to enable the computation of RTP’s, we developed a means to generate nearly optimal discretisation meshes. Combined with efficient error estimates, we can solve RTP’s to a very high guaranteed accuracy. These estimates, weighted residual a posteriori error estimates, can not only control global error quantities like the mean quadratic error or the error in the energy norm. They can be suited to nearly any error functional.

A natural site for water maser emission in AGNs is provided by dusty gas with properties characteristic of broad line region (BLR) clouds. Radiation shielding by dust in the clouds is critical for allowing molecular gas to exist ≤ 1 pc from the central engine. Thus, the innermost radius at which such masers appear should correspond to the grain sublimation radius rsub. We suggest a dynamical model in which the masing clouds are embedded within a magnetized accretion disk wind.

We have embarked on a general analysis of multi-component outflows from magnetized accretion disks in AGNs. In this scenario, dense diamagnetic clouds are embedded in a centrifugally driven wind and are uplifted from the disk surface by the dynamical drag of the wind as well as by the “melon seed” magnetic pressure gradient force. We follow the motion of the clouds above the disc surface, taking account of the radiation pressure force exerted by the central continuum for different spectral energy distributions and emission patterns. We study the evolution of the clouds as they accelerate through the wind, considering possible sources of confinement and estimating the cloud lifetimes against various disruptive processes. We then examine the spectral signatures of accelerated clouds in the context of this dynamical scheme and compute absorption line profiles as a function of viewing angle through the wind. We apply our results to the interpretation of the observed optical/UV spectra from BALQSOs and of the broad EUV and X-ray absorption features detected in a number of BL Lac objects, as well as to other AGNs in which radiative acceleration of clouds has been inferred (e.g., “warm absorbers”).

We present high-resolution radio observations of the second Galactic superluminal radio source GRO1655-40, which was detected as an X-ray transient on 1994 July 27. Our radio radio images reveal two components moving away from each other at an angular speed of 65±5 mas/day, corresponding to superluminal motion (υ/c = 1.4 ± 0.4) at the estimated distance of 3–5 kpc. The 12-day delay between the X-ray and radio outbursts suggests that the ejection of material at relativistic speeds occurs during a stable phase of accretion onto a black hole, which follows an unstable phase with a high accretion rate. A complete description and discussion of these observations can be found in Tingay et al 1995 (Nature, 374, pp 141–143).

With respect to the stability of accretion tori the linear stability of infinitely long cylinders is investigated taking into account the effect of viscosity. Critical Reynolds numbers as a function of the Mach number and the azimuthal wavenumber of the perturbation are calculated.

We present the results of an effort to model recent HST eclipse observations of the wind-formed C, ɪv 1550Å resonance line in the high-inclination nova-like variable UX UMa (Mason et al. 1995, Baptista et al. 1995). Within the framework of a simple kinematic model, in which the outflow is described as a rotating, biconical accretion disk wind (Knigge et al. 1995), we are able to reproduce not only the shapes and strengths of the observed line profiles both away from and during eclipse, but also most of the detailed behaviour of different parts of the line (blue wing, line centre and red wing) as a function of orbital phase (Figure 1).

The most important result of our modeling is that it strongly suggests the presence of a vertically extended (H ~ few × 10 RWD), relatively dense (ne ~ 1013 cm−3) and only slowly-outflowing (υpoloidal << υescape) transition region between the disk photosphere and the fast-moving wind. Thus the best current data already demand that the simplified picture of accretion disks and winds as quasi-independent physical phenomena should be replaced by a unified theory of accretion disk atmospheres. It may be important in this context that, barring strong contamination of the observed C ɪv wind line by an underlying disk-formed component, the evidence for rotation in the outflow is unambiguous: the observed line flux light curves clearly exhibit a rotational disturbance that is well matched by our simple model of a rotating disk wind.

The Lyman edge at 912Å is an important diagnostic region for studying quasi-stellar objects (QSOs). In particular, it reveals a great deal about the physical conditions within the atmospheres of accretion disks, a ubiquitous component of QSO theories. Observations of the Lyman edge regions of QSOs have shown scant evidence for the predicted features – few QSOs show the broad, partial Lyman edges expected to be common according to most theories. The FOS in its polarimetry mode has been instrumental in studying the Lyman edge region in QSOs. One of the most exciting discoveries is that the polarization rises, not at wavelengths longward of the Lyman edge, but at wavelengths well below it! Here we present the status of polarimetry observations at the Lyman edge and discuss their implications to accretions disks

Calculations of the spectrum resulting from accretion onto a massive black hole often make use of the “equipartition assumption” in order to estimate the magnetic field intensity. Thus, the mechanism for the dissipation of the magnetic field and the resulting dynamical influence on the gas have not been treated quantitatively, nor self-consistently. Here, we introduce an alternative approach for modelling the magnetic field dissipation from basic principles, using current ideas about turbulent fields and tearing mode instabilities.

We solved one-dimensional steady and axisymmetric magnetohydrodynamical (MHD) equations to study basic properties of astrophysical jets from accretion disks. We found that the mass flux of the jets depends on the poloidal magnetic field strength of the disk. The mass loss rates observed in T Tauri winds will constrain the physical quantities in the accretion disks.

We have performed 1D(1.5D) and 2D(2.5D) nonsteady MHD numerical simulations of astrophysical jets which are magnetically driven from Keplerian disks, in order to clarify the origin and structure of jets ejected from protostars and active galactic nuclei. The initial and boundary conditions are similar to those of 2D(2.5D) nonsteady MHD simulations of Shibata and Uchida (1986) and Matsumoto et al. (1996); there is initially a Keplerian disk with a nonrotating corona outside, both of which are penetrated by vertical magnetic fields. The subsequent interaction between the disk/corona and the vertical fields are studied as an initial value problem. Against the current belief that this kind of simulations show simply a transient jet caused by nonsteady interaction between the disk/corona and the magnetic field, we have found that the jets ejected from the disk in this way have the same properties of the steady magnetically driven jets that were investigated by using ID steady wind solution (Kudoh & Shibata 1995), even if the jets are not exactly in steady state.

We propose that a toroidal magnetic field is the main driving component of a protostellar jet. We discuss how the ram pressure of the jet may damp a Parker-like instability and provide a stable environment for a jet flow.

Since their first discovery in 1980, nearly 200 bipolar molecular outflows have now been observed. These outflows appear to be a ubiquitous phenomenon in the star formation process, although their origin remains unclear. Here, we adopt the thin-shell model of Shu, Ruden, Lada, & Lizano (1991, ApJL, 370, 31) in which bipolar molecular outflows are the swept-up ambient material by powerful winds from central young stellar objects. Besides explaining many observed features of bipolar sources, such as shape, velocity, and the typical extent, this model also naturally accounts for the so-called “Hubble law” of velocity field. However, it was criticized by Masson & Chernin (1992, ApJ, 387, L47) on the ground that much less mass is observed at the high velocity end of some bipolar outflows than expected from this thin-shell model, unless the wind and/or the ambient medium are extremely anisotropic. We show that the extreme anisotropy is in fact inevitable both in the wind and in the ambient medium, because of the presence of dynamically important magnetic fields.

The wind is anisotropic because it is magneto-centrifugally driven, and the frozen-in toroidal field tends to collimate the flow toward the rotation axis. This effect is studied using the well-known “method of characteristics” and we conclude that a MHD wind always has two components: a well-collimated jet and a wind-angle wind. It is the wide-angle wind component that will interact with the ambient medium and produce bipolar molecular outflows. The ambient core medium is anisotropic because it arises from lower-density, magnetically-supported molecular clouds through ambipolar diffusion. Using self-similarity technique, we model the “pivotal state” of star formation, which divides the core formation phase and the core collapse (accretion) phase. We find that pivotal states have toroid-like density contours in general, with lower-density regions around the magnetic axis (where matter drains more easily).

When these anisotropics induced by magnetic fields are taken into account properly, the main argument against the wind-angle wind-driven, thin-shell model for bipolar molecular outflows disappears. Indeed, we show that the empirical mass-velocity relation found by Masson & Chernin and others for bipolar outflows provides the best quantitative supporting evidence for the thin-shell theory!