We use Smoothed Particle Hydrodynamics (SPH) to numerically investigate gap clearing around a protoplanetary body in a selfgravitating gaseous disk. It has been suggested that proto-Jupiter regulated it’s own growth by clearing a gap around itself and thus halting accretion.

A self-consistent procedure is outlined for determining the thermal structure of gas inflowing along magnetic field lines of a young stellar object. A young pre-main-sequence star (e.g., a classical T Tauri star) is assumed to possess a dipole magnetic field that disrupts a geometrically thin accretion disk and channels the incoming gas toward the stellar surface, leading to the formation of a pair of accretion funnels that terminate in shocks at high stellar latitudes. The heat equation is solved together with the rate equations for hydrogen, and the main physical processes that heat and cool the gas are identified. In particular, in the case of T Tauri stars, it is found that adiabatic compression is the principal heat source and that the Ca II and Mg II ions act as a powerful thermostat that regulates the gas temperature. The ionization state of the gas in the radiation field of the stellar photosphere and of the accretion shocks is found in this case to be controlled by Balmer continuum photons. The implications of these calculations to the observational signatures of accreting YSOs (e.g., their near-infrared hydrogen and CO overtone line emission) are discussed.

A systematic difference in alignment between the central black hole spin and the angular momentum of the accreting gas may help to explain several differences found in the optical and UV HST spectra of radio loud and radio quiet AGN.

The idea that a significant fraction of line emission in AGN comes directly from an accretion disk is attractive for several reasons – not least the obvious one of establishing the black hole paradigm. We performed a few consistency tests for the hypothesis that both FE Kα and Hα are emitted by an accretion disk.

The main results of this preliminary investigation are as follows. FE Kα is systematically much broader than Hα at FWHM, but the widths of the two lines are not correlated. For each of the 9 objects with FE Kα profile most consistent with disk emission, we computed the possible disk contribution on the basis of a weak field model. Although the Hα profiles are not consistent with disk model emission, rotational motion could still be the main broadening mechanism. We then compared disk inclinations deduced from FE Kα model profile fitting to the ones we derived from model fits of Hα. The inclination values are consistent. Finally, we found that a simple illumination model can produce both disk FE Kα and Hα emission, if a small fraction of continuum is scattered toward the disk to yield ionization parameter ξ > 10−2 ergs s−1 cm.

We conclude that the bulk FE Kα cannot be associated to the Broad Line Region. Albeit several radio-quiet Seyfert 1 objects show FE Kα profiles consistent with emission from the inner part of an illuminated accretion disk, the general situation remains unclear, as fits to line profiles require an unsatisfactory large range in model parameters like emissivity and inner and outer radii.

Recently, a substantial population of Blazar-like sources exhibiting sharp near-infrared cutoffs and hence exceedingly reddened optical to near-infrared colours (Bj – K ≿ 6) have been discovered in a complete sample of flat spectrum radio sources (Drinkwater et al. 1996). We argue that such “red” colours cannot be due to a synchrotron radiation mechanism alone.

Nonlinear growth of the Parker instability (PI) and the Balbus & Hawley instability (BHI) in accretion disks have been studied by local three-dimensional magnetohydrodynamic (MHD) simulations. In high-β disks (β = Pgas/Pmag > 1), the PI has only minor effects on the saturation level of BHI. In low β disks (β ≤ 1), the disk stays in a low-β state because magnetic flux cannot escape fast enough to convert the disk into a high-β state. We found that even in low-β disk the BHI generates fluctuating magnetic fields. The effective magnetic viscosity αB(= –⟨BrBΦ/4πP0⟩) is O(0.1) when β ~ 1.

The evolution of the inner disk region of magnetized accreting systems is studied by means of numerical simulations. Two different initial magnetic field topologies are studied: a pure dipole field which threads the disk continuously and a dipole field excluded from the disk by surface currents. When present, the Balbus-Hawley (BH) instability provides efficient angular momentum transport, and accretion is equatorial. Otherwise, magnetic coupling between the disk and star surfaces as the dominant mechanism for transport and nonequatorial accretion results. In all of the simulations, low density, collimated disk-driven winds lead to mass loss.

Smoothed Particle Hydrodynamics (SPH) is now seen as a numerical scheme well suited to the study of accretion disks. SPH simulations have been conducted of cataclysmic variable disks (Lubow 1991, Murray 1996, Armitage and Livio 1996), galactic disks (Artymowicz and Lubow 1989), and protostellar disks (Artymowicz and Lubow 1994). It is therefore important to test the technique against theory and other numerical results to obtain an estimate of the accuracy and reliability of SPH in this context. Previously SPH has been tested against standard stationary and time-dependent results of viscous thin disk theory (Murray 1996). Strictly these tests relate to disks where ‘viscous’ terms dominate pressure terms in the equations of motion.

In this paper we describe tests of the code more appropriate for hot disks where pressure forces are relatively more important than viscosity. Specifically we consider the form of the spiral density waves that can be excited in a disk by a perturbing gravitational potential. Very low mass perturbing bodies excite linear spiral waves which redistribute angular momentum in the disk. For increasingly massive perturbers, the disk response becomes nonlinear and eventually shocks form. In the standard formulation of SPH, an artificial viscosity term is added to the SPH equations to improve shock capture. This is equivalent to introducing a fixed ratio of shear to bulk viscosity into the equations of motion. In Eulerian schemes, artificial viscosity has been discarded in favour of other more accurate, less dissipative schemes for resolving shocks. The continued use of artificial viscosity in SPH has become a source of ‘friction’ between numericists. The simulations described here demonstrate the scheme’s ability to resolve spiral shocks, and show that SPH is a valuable tool for probing the structure of tidally perturbed accretion disks.

Classical nova outbursts are thermonuclear explosions on the surfaces of the white dwarfs in cataclysmic variables. The explosion heats the surface layers of the white dwarf, which are expected to cool on a timescale of a hundred years. The hot white dwarf should have two obvious effects on the system.

1. (1) It will heat the surface of the accretion disc and secondary star, increasing the overall luminosity of the system.

2. (2) By irradiating the surface of the secondary star it may bloat it and drive more mass transfer, thus again increasing the overall luminosity.

Non force-free cylindrical equilibrium of the magnetically driven relativistic MHD flows is investigated as a model of astrophysical jets/winds. This work is a generalization of the previous work Nitta (1995). We consider the situation that the magnetic field lines emanated from the central object are rotating, and the frozen plasmas are magnetically accelerated outward and construct a self-organized magnetosphere due to the Lorentz force and the inertial force. Recent observations revealed that many objects have a hybrid structure of collimated jet and non-collimated wind regardless with species of the central object, e.g. YSO’s, stars, pulsars or AGN’s. In order to explain these structures, we present a model that the magnetosphere has a hybrid asymptotic structure: the cylindrical jet and the conical wind fill the infinitely distant polar region and the lower latitude region, respectively. The asymptotically conical wind was investigated in the previous paper (Nitta 1994). Here we investigate the asymptotically cylindrical jet at ‘finite cylindrical radius’ from the rotational axis. In this case, the inertial force is important for the force balance because a fraction of the flows having finite angular momentum converges to finite cylindrical radius.

We present UKIRT infrared images of the X-ray binary Cygnus X-3. We address the possibility of extended infrared emission and show that it could be either warm circumstellar material or a star near the binary’s line of sight.

We examine the two-dimensional structure of m = 1 modes in disks around white dwarfs in close binary systems. We find that the odd modes (warping modes) as well as even modes (eccentric modes) are confined to the outermost part of the disk. The period of the fundamental mode is of a few percent of the binary period, and is insensitive to the parity of the mode. These modes naturally explain the superhump periods of SU UMa stars.

We present preliminary results of a time-dependent, three-dimensional hydrodynamic simulation of LMC-X4, an HMXB known to be undergoing RLOF. The simulation is initialized with the collapsed companion embedded in the undisturbed primary wind. The primary is in contact with the Roche surface, although no tidal stream or accretion disk is initialed; they are allowed to form independently.

Several features of general interest to disk-fed HMXBs are apparent in the simulation. First, the primary immediately develops a compressed-wind disk in the orbital plane. This may be a natural result in most disk-fed HMXBs. Fora circularized system in an orbit close enough for RLOF to take place, we may expect the primary to be in corotation. The surface velocity may then be a significant fraction of the breakup velocity, leading to a compressed-wind disk.

We present calculations of the structure of magnetized winds of classical T Tauri stars (TTSs) in an axisymmetric, stationary, polytropic approximation in a given magnetosphere (i.e. we do not selfconsistently take into account the influence of the flow dynamics on the structure of the underlying magnetosphere). According to the widely accepted model of CTTSs the magnetosphere is dominated by a strong stellar magnetic field (B* ≃ 1 kG) which may be of dipolar type. The behaviour of the magnetic flux-tube function yields valuable constraints on the wind solutions.

We present some initial results of our numerical, 2D hydrodynamical models of line driven flows from the accretion disk in cataclysmic variables. We assume the disk radiation pressure pushes out the isothermal material from a flat, geometrically thin, Keplerian disk.

We calculate the disk radiation field using the surface brightness of a standard “α disk” (Shakura & Sunyaev 1973). We do not include a bright boundary layer in the calculations. We approximate the total radiative line acceleration, adopting the formalism due to Castor, Abbott, & Klein (1975). We use our generalized 2D version of their force multiplier. The multiplier is still described by two parameters representing the number of lines and the ratio of optically thin to optically thick lines. The main modification of the original CAK force multiplier is in the depth parameter, which is now a function of the gradients of two velocity components instead of the single velocity gradient as in the ID case.

We investigate how the disk structure and mass loss rate depend on the disk and central star luminosity, and boundary conditions such as the disk density.

We find that transonic flows from disks do not relax toward steady states. However, their time averaged properties become constant after some time. Our models show that most of mass loss originates from close to the central star – a few stellar radii. Models without a central star radiation field produce flows more vertical than models in which one is present. However, other global, time averaged properties of flows such as the total wind mass, the wind mass loss rate, and velocity are similar. The ratio between the wind mass loss and disk accretion rate increases rapidly with the accrection rate.

The CV PQ Gem is thought by many to be the first truly ‘Intermediate’ Polar in that it displays characteristics typical of the synchronous and high magnetic field strength (B ~10–230MG) AM Her systems as well as those of the asynchronous (and much weaker magnetic field strength) Intermediate Polar systems. Here we present the results of quasi-simultaneous optical photopolarimetric and EUVE observations.

Two-dimensional hydrodynamical simulations of mass transfer in short period Algol type binaries were performed using the numerical code VH−1. This code uses the Piecewise Parabolic Method with a Lagrangian Remap. Our version of the code also accounts for radiative cooling processes. The purpose of performing the simulations was to study the Hα emission from circumstellar gas in the Algols. Using observational evidence from the literature to constrain the gas stream properties, hydrodynamical maps of the Hα emissivity in the two systems β Per (P = 2.87d) and TT Hya (P = 6.95d) were made in both Cartesian and velocity coordinates from the simulation data. The velocity maps were then compared to Doppler tomograms constructed from observed Hα line emission in these systems. Since the tomograms cannot be directly transformed to maps of emission in spatial coordinates, the simulated Cartesian maps enable us to interpret the dynamical processes which produce the features observed in the Doppler tomograms. We find that the simulations produce asymmetric accretion structures with many features similar to those found in the Doppler tomograms of Algol systems.

The transient luminous soft X-ray source RXJ0513.9–6951 (Schaeidt et al., 1993, A&A 270, L9) is a high-mass-transfer binary system (Cowley et al., 1993, ApJ 418, L63; Pakull et al., 1993, A&A 278, L39) with a probable orbital period of 0.76 days (Crampton et al., 1996, ApJ 456, 320). Here, we summarize the results of a quasi-simultaneous optical and X-ray monitoring (see Fig. 1). The sudden decrease of the optical flux, the accompanying reddening, and the turn-on in the soft X-ray band can be quantitatively described by variations in the irradiation of the accretion disk by the hot central star (Reinsch et al., 1996, A&A 309, L11). In this simple model, we consider a white dwarf with nuclear burning of accreted matter (van den Heuvel et al., 1992, A&A 262, 97), surrounded by a flat standard disk. In the optical high state, accretion at near-Eddington rate occurs and the white dwarf photospheric radius must be considerably expanded causing an enhanced illumination of the disk and the secondary. In the optical low state, the photosphere shrinks in response to a temporarily slightly reduced mass-transfer rate. At the same time, the effective temperature increases, and the soft X-ray flux becomes detectable with ROSAT. This model does not depend on the particular cause for the drop in the accretion rate and can describe the optical/ X-ray variability with the total luminosity changing by less than 20 %.

The light curve of V1974 Cyg shows two distinct periodicities. The shorter periodicity is clearly the orbital period of the binary system. We show that the longer variation has similar features to permanent superhumps. This result indicates the existence of an accretion disk in the system no later than 30 months after the nova outburst. We used the precessing disk model of the superhump phenomenon and previous results in order to estimate that the white dwarf mass is at the range 0.75 – 1.07 M⊙.

The nova-like variable BZ Cam (also called 0623+71) is an unusual cataclysmic variable star in that it resides in a nebula, discovered by Ellis, Grayson, & Bond (1984) and designated EGB 4. It is more unusual in that its optical spectrum reveals a wind from its accretion disk. This was discovered serendipitously during a radial velocity study to find orbital period by Thorstensen, Thomas, & Patterson (1993; see also Patterson et al. 1996), who found intermittent P Cygni profiles in the He I λ 5876 Å and Hα emission lines.

The 4.2-m William Herschel Telescope and its ISIS spectrograph were used to obtain time-resolved spectra of these lines with 0.4 Å/pixel dispersion and 30-second time resolution. These new spectra have about 5 times the spectral resolution and 24 times the time resolution of those of Thorstensen et al. (1993). These observations were obtained on the nights of 1995 February 6 – 8 UT. The blue arm of ISIS was centered on the He I λ 5876 Å; the red arm covered Hα. Typical signal-to-noise ratio was 18 per resolution element.