We present the first results of a global axisymmetric simulation of accretion on to rotating magnetized stars from a turbulent accretion disc, where the turbulence is driven by the magnetorotational instability (MRI). We observed that the angular momentum is transported outwards by the magnetic stress and accretion ...

We present the first results of a global axisymmetric simulation of accretion on to rotating magnetized stars from a turbulent accretion disc, where the turbulence is driven by the magnetorotational instability (MRI). We observed that the angular momentum is transported outwards by the magnetic stress and accretion ...

thin accretion discs boundary layms are presented for classical '1'l'auri stars, for various values of M* (0.8, l. OMO), R* (1 .6, '2.15, 4.3}~) and M ...

Bounds are placed on the growth rates of dynamical instability in shear flows in accretion discs and tori. Ft is shown that the critical layer lies in the flow even when the boundaries are free for flows in accretion discs, cylinders and isentropic, constant angular momentum tori (for which the ...

The dynamics of an axisymmetric stationary disc of accreting magnetofluid with finite conductivity around a rotating compact object is presented here. Along with the Maxwell equations and the generalized Ohm law, the basic equations governing the motion of a finitely conducting plasma in a curved space-time around a slowly rotating compact object are ...

We present seven separate Chandra observations of the 2001 superoutburst of WZ Sge. The high-energy outburst was dominated by intense EUV emission lines, which we interpret as boundary layer emission scattered into our line of sight in an accretion disc w...

The mass accretion rate of transonic spherical accretion flow onto compact objects such as black holes is known as the Bondi accretion rate, which is determined only by the density and the temperature of gas at the outer boundary. A rotating accretion flow has angular momentum, which modifies ...

The properties of external accretion disks (disks with a central source of angular momentum) are explored both analytically and numerically. An illustrative example of the effect of a disk of material around a binary star on the stellar separation is considered. A Greens-function-type solution is considered in which an initial ring of matter is put in orbit around the central ...

Magnetic interactions between a protostar and its accretion disc can induce warping in the disc and produce secular changes in the stellar spin direction, so that the spin axis may not always be perpendicular to the disc. This may help to explain the 7� misalignment between the ecliptic plane of the Solar system ...

We present seven separate Chandra observations of the 2001 superoutburst of WZ Sge. The high-energy outburst was dominated by intense EUV emission lines, which we interpret as boundary layer emission scattered into our line of sight in an accretion disc wind. The direct boundary layer emission was hidden from view, ...

Radial oscillations resulting from axisymmetric perturbations in viscous accretion disks surrounding

May 13, 1999... observations of quiescent discs to determine the local physical properties of their atmospheres, and resolve their spatial dependence. ...

Matter accreting onto black holes suffers a standing or oscillating shock wave in much of the parameter space. The post-shock region is hot, puffed up and reprocesses soft photons from a Keplerian disc to produce the characteristic hard tail of the spectrum of accretion discs. The post-shock torus is also the base ...

We calculate the accretion disc temperature profiles, disc luminosities and boundary layer luminosities for rapidly rotating neutron stars considering the full effect of general relativity. We compare the theoretical values of these quantities with their values inferred from EXOSAT data for four low mass X-ray ...

High Energy Astrophysics: Accretion Disks II 1/59 Accretion Disks II 1 The dynamical equations Astrophysics: Accretion Disks II 2/59 The simplest accretion disc model to construct is that of the thin disc on the azimuthal angle z #12;High Energy Astrophysics: ...

A steady-state model of the accretion disk around a white dwarf, including the boundary layer and the hot spot, has been constructed. Calculated distributions of radiation are compared with soft X-ray, ultraviolet, visual, and infrared observations of SS Cyg during outburst as well as with ultraviolet measurements of V 603 Aql and RR Pic. The results ...

Wind flows and collimated jets are believed to be a feature of a range of disc accreting systems. These include active galactic nuclei, T Tauri stars, X-ray binaries and cataclysmic variables. The observed collimation implies large-scale magnetic fields and it is known that dipole-symmetry fields of sufficient strength can channel wind flows emanating from ...

Chapitre IV Accretion Disc Outbursts: A New Version of an Old Model Jean�Marie HAMEURY, 1 Kristen inferred. In particular we show that outside�in outbursts are possible when a standard bimodal behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 #12; 86 Accretion Disc Outbursts: A New ...

In this letter we show that the observed gamma -ray luminosity can be produced by an accretion disc around a non-magnetic neutron star or low mass black hole in which the dominant dissipative process in the disc is due to magnetic fields. The shearing mot...

The presence of alpha-viscosity in the atmospheres of accretion discs is investigated. It results in the existence of coronae and/or winds. When discs are illuminated by the central X-ray sources, the mass loss may be a significant fraction of the total m...

We report results from calculations investigating stationary magnetic field configurations in accretion discs around magnetised neutron stars. Our strategy is to start with a very simple model and then progressively improve it, providing complementary insight into results obtained with large numerical simulations. In our first model, presented here, we ...

We model circumplanetary discs as accretion discs subject to the tidal forces of the central star. The tidal torques remove the disc angular momentum near the disc outer edge and permit the accreting disc gas to lose angular momentum at the rate ...

The accretion disc eclipse mapping method is an astrotomographic inversion technique that makes use of the information contained in eclipse light curves to probe the structure, the spectrum and the time evolution of accretion discs in cataclysmic variables. This paper presents examples of eclipse mapping results ...

Current theories on planetary formation suggest that the formation of giant planets is related to their quick migration towards the central star as a result of protoplanet-disc interactions on a time-scale of the order of 105yr, for objects of nearly 10 terrestrial masses. Such a time-scale should be smaller by an order of magnitude than that of gas ...

OUTFLOWS FROM DYNAMO-ACTIVE PROTOSTELLAR ACCRETION DISCS BRIGITTA VON REKOWSKI1 , AXEL BRANDENBURG2, Newcastle, UK Abstract. An axisymmetric model of a cool, dynamo-active accretion disc is applied by the disc dynamo driven by the magneto-rotational instability and some form of outflow is ...

It has long been recognized that the main obstacle to the accretion of gas on to supermassive black holes (SMBHs) is a large specific angular momentum. It is feared that the gas settles in a large-scale disc, and that accretion would then proceed too inefficiently to explain the masses of the observed SMBHs. Here we point out that, ...

Accretion discs are composed of ionized gas in motion around a central object. Sometimes, the disc is the source of powerful bipolar jets along its rotation axis. Theoretical models invoke the existence of a bipolar magnetic field crossing the disc and require two conditions to produce powerful jets: field lines ...

In this work, the roles of both the inflow kinematic conditions at the inner Lagrangian point L1 as an initial boundary condition and the gas compressibility, physical turbulent viscosity on an accretion disc's dynamics and structure were investigated via simulations of 3D SPH stationary accretion-disc models. ...

Adaptive spatial domains are currently used in smooth particle hydrodynamics (SPH) with the aim of performing better spatial interpolations, mainly for expanding or shock gas dynamics. In this work, we propose an SPH interpolating kernel reformulation, also suitable for treating free edge (FE) boundaries in the computational domain. Application to both inviscid and viscous ...

We analyse some properties of circumplanetary discs. Flow through such discs may provide most of the mass to gas giant planets, and such discs are likely sites for the formation of regular satellites. We model these discs as accretion discs subject to the tidal forces of ...

Hyper-accretion discs around black holes emit copious neutrinos and anti-neutrinos. A fraction of the emitted neutrinos convert to electron-positron plasma above the disc through the annihilation reaction $\

Galactic nuclei should contain a cluster of stars and compact objects in the vicinity of the central supermassive black hole due to stellar evolution, minor mergers and gravitational dynamical friction. By analogy with protoplanetary migration, nuclear cluster objects (NCOs) can migrate in the accretion discs that power active galactic nuclei (AGN) by ...

Emission-Line Variability and Implications for Reverberation Mapping: Ballantyne , D. R., .... Impact of reverberation in flared accretion discs on temporal ...

T Tauri stars (TTS) are low-mass pre-main-sequence stars that are accreting mass from the surrounding disc. The hotspots detected in some of them are probably heated by the release of gravitational energy in the accretion of the disc material on to the star. In this work we study the UV spectrum of the hotspot ...

In this study, a spectral analysis of the dwarf nova WX Hyi (which is a Cataclysmic Variables (CV)) is presented. The system was observed during the normal ourburst with the XMM-Newton observatory. X-ray bolometric flux value (EX<=12 keV) is estimated to be ~2.98�10-12 erg cm-2 s-1 and X-ray luminosity LX = 5�1031 erg/s. Elemental abundance was founded 3 factor of solar abundances. If the ...

We present the results of a broad-band spectral study of the transient low-mass X-ray binary Aql X-1 observed by the Suzaku and Rossi X-ray Timing Explorer satellites. The source was observed during its 2007 outburst in the high/soft (Banana) state and in the low/hard (Extreme Island) state. Both the Banana state and the Extreme Island state spectra are best described by a two-component model ...

We present the results of a broad-band spectral study of the transient low-mass X-ray binary Aql X-1 observed by the Suzaku and Rossi X-ray Timing Explorer satellites. The source was observed during its 2007 outburst in the high/soft (Banana) state and in the low/hard (Extreme Island) state. Both the Banana state and the Extreme Island state spectra are best described by a two-component model ...

Black holes grow by accreting matter from their surroundings. However, angular momentum provides an efficient natural barrier to accretion and so only the lowest angular momentum material will be available to feed the black holes. The standard subgrid model for black hole accretion in galaxy formation simulations - based on the ...

It is shown that some aspects of the accretion disc physics can be experimentally simulated with the use of an array of properly directed plasma jets created by intense laser beams. For the laser energy of 1 to 3 kJ, one can create a quasi-planar disc with the Reynolds number exceeding 104 and magnetic Reynolds number in the range of ...

Local hydromagnetic simulations of accretion-disc turbulence currently provide the most convincing evidence that the origin of turbulence in discs could be the Balbus-Hawley magnetorotational instability. The main results of such calculations are highlighted with particular emphasis on the generation of large-scale magnetic fields. Comparison with ...

We study the numerical convergence of hydrodynamical simulations of self-gravitating accretion discs, in which a simple cooling law is balanced by shock heating. It is well known that there exists a critical cooling time-scale for which shock heating can no longer compensate for the energy losses, at which point the disc fragments. The ...

We study the numerical convergence of hydrodynamical simulations of self-gravitating accretion discs, in which a simple cooling law is balanced by shock heating. It is well known that there exists a critical cooling time-scale for which shock heating can no longer compensate for the energy losses, at which point the disc fragments. The ...

We construct Super-Eddington Slim discs models around both stellar and supermassive black holes by allowing the formation of a porous layer with a reduced effective opacity. Unlike the standard scenario in which the discs become thick, super-Eddington discs remain slim. In addition, they accelerate a significant wind with a 'thick ...

Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, massive self-gravitating gaseous discs. With help of numerical simulations, we here consider whether the observed population of young stars could have originated from a large angle collision of two massive gaseous clouds at R ~= 1pc from SgrA*. In ...

We numerically study the long-term evolution of the accretion disc around the neutron star in a coplanar Be/X-ray binary with a short period and a moderate eccentricity. From three-dimensional smoothed particle hydrodynamics simulations, we find that the disc evolves through three distinct phases, each characterized by different ...

We report on the numerical discovery of quasi-periodic oscillations (QPOs) associated with accretion through a non-axisymmetric magnetic boundary layer in the unstable regime, when two ordered equatorial streams form and rotate synchronously at approximately the angular velocity of the inner disc. The streams hit the star's surface ...

The spin behaviour of a strongly magnetic star accreting from an internally disrupted disc is considered. The torques are calculated due to the interaction of the star with the disc and the accretion flow, together with the modified structure of the disc. The torques depend on the spin period ...

Slim accretion discs have a total luminosity of the order L/L(sub E) = m (proportional to) 1, where L(sub E) is the Eddington luminosity and m = M/M(sub c), where M(sub c) is a critical accretion rate, related to the Teddington one. The local stability pr...

We study the stability of disc accretion in the recurrent nova RS Ophiuchi. We construct a one-dimensional time-dependent model of the binary-disc system, which includes viscous heating and radiative cooling and a self-consistent treatment of the binary potential. We find that the extended accretion ...

We present optical spectroscopy of the microquasar SS433 covering a significant fraction of a precessional cycle of its jet axis. The components of the prominent stationary H? and H? lines are mainly identified as arising from three emitting regions: (i) a super-Eddington accretion disc wind, in the form of a broad component accounting for most of the mass ...

I present a model for acceleration of protons by the second-order Fermi process acting on randomly scrambled magnetic flux arches above an accretion disc. The accelerated protons collide with thermal protons in the disc, producing degraded energetic protons, charged and neutral pions, and neutrons. The pions produce gamma-rays by ...

The stress evolution process is taken into account in the linear stability analysis of standard thin accretion discs. We find that the growth rate of thermally unstable modes can decrease significantly owing to the stress delay, which may help to understand the quasi-periodic variability of GRS 1915+105. We also discuss the possible application of stress ...

The persistent black hole binary LMC X-3 is the best system on which to test theoretical models on the shape of the accretion disc spectrum. This is due to the combination of very low absorbing column density along this line of sight, which allows the shape of the disc emission to be constrained at low energies, and its mass ...

We consider the specific case of disc accretion for negligibly low viscosity and infinitely high electric conductivity. The key component in this model is the outflowing magnetized wind from the accretion disc, since this wind effectively carries away angular momentum of the accreting matter. ...

An approximation theory of the acceleration of particles in low-density shear flows is developed and is applied to a number of astrophysical flows including accretion disks, radial accretion, and convective turbulence. In the disk application, the theory suggests a mechanism by which the bulk of the accretion power may be converted to ...

We explore the role of X-ray photoevaporation in the evolution and dispersal of viscously evolving T Tauri discs. We show that the X-ray photoevaporation wind rates scale linearly with X-ray luminosity, such that the observed range of X-ray luminosities for solar-type T Tauri stars (1028-1031 erg s-1) gives rise to vigorous disc winds with rates of the ...

In this paper we analyze the behavior of the unusual dwarf nova EM Cyg using the data obtained in April-October, 2007, at the Vyhorlat observatory (Slovak Republic) and in September, 2006, at the Crimean Astrophysical Observatory (Ukraine). During our observations, EM Cyg has shown outbursts in every 15-40 days. Because on the light curves of EM Cyg a partial eclipse of the ...

We discuss transportation and redistribution of energy and angular momentum in the magnetic connection (MC) process and Blandford-Payne (BP) process. MC results in readjusting the interior viscous torque, and its effects are operative not only in but also beyond the MC region. The BP process is invoked to transfer the �excessive� angular momentum from an accretion ...

Gravitational instability plays an important role in driving gas accretion in massive protostellar discs. Particularly strong is the global gravitational instability which arises when the disc mass is of order 0.1 of the mass of the central star and has a characteristic spatial scale much greater than the disc's ...

In this paper we derive equations describing the dynamics and stationary configurations of a twisted fully relativistic thin accretion disc around a slowly rotating black hole. We assume that the inclination angle of the disc is small and that the standard relativistic generalization of the ? model of accretion ...

Young low-mass stars are characterized by ejection of collimated outflows and by circumstellar discs which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn remove the excess angular momentum from the ...

Young low-mass stars are characterized by ejection of collimated outflows and by circumstellar discs which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn remove the excess angular momentum from the ...

The broadened iron lines observed from accreting compact objects are most easily interpreted in terms of reflection onto the accretion disc of the hard X-ray photons emitted by the central source. In this context, such a broadness is due to the relativistic motion of the reflecting plasma, in the deep gravitational well of the compact ...

... Note that the four black discs inside the illusory square appear closer ... motion-related occlusion information as occurs in the accretion and deletion ...

... Additonal cues such as accretion/deletion of texture (not considered here) are ... a perceptually vivid, illusory white square in a field of black discs. ...

May 13, 1999 ... Astro-tomography also has applications over timescales longer or ... optical and IR reprocessing off the accretion disc and companion star. ...

Hyper-accretion discs around black holes emit copious neutrinos and anti-neutrinos. A fraction of the emitted neutrinos convert to electron-positron plasma above the disc through the annihilation reaction ?. This process may drive relativistic jets associated with gamma-ray bursts (GRB) explosions. We calculate the efficiency of energy ...

, Baltimore, MD 21250, USA e-mail: mukai@milkyway.gsfc.nasa.gov Received ???; accepted ??? ABSTRACT Context

turbulent viscocity in accretion discs," Astrophys. Letter & Comm. 34, 383-388 56. Abramowicz, M. A

... onto aluminum or steel discs previously sprayed with silicon oil. A spray mask size of 5 mm ... ...

Radial velocity studies using optical emission lines (from the accretion discs) have been performed for a few TOADs. However, when mass determinations ...

We present time-dependent models of the remnant accretion discs created during compact object mergers, focusing on the energy available from accretion at late times and the composition of the disc and its outflows. We calculate the dynamics near the outer edge of the disc, which contains the ...

String theory predicts the existence of extremely compact objects spinning faster than Kerr black holes. The spacetime exterior to such superspinars is described by Kerr naked singularity geometry breaking the black-hole limit on the internal angular momentum. We demonstrate that the conversion of Kerr superspinars into a near-extreme black hole due to an accretion ...

We study conditions for magnetocentrifugal jet launching from accretion discs around black holes, whereby large-scale magnetic field lines anchored in the disc may fling tenuous coronal gas outwards. We find that a disc around a critically spinning black hole may centrifugally launch a jet along the rotation axis.

This study inspects the light and radial-velocity curves of the eclipsing binary AV Del. In comparison with other studies already done, the study shows that the absolute elements, fundamental orbital and physical parameters of the system can be determined using the Wilson-Devinney code. Using these parameters, the configuration of the system is presented. Then, an accretion ...

-margin assemblages and accreted terranes in the northern Canadian Cordillera. Paleomagnetic data from Upper terranes to miogeoclinal assemblages in the northern Cordillera: Irreconcilable with paleomagnetic data of accreted terranes to miogeoclinal assemblages in the northern Cordillera: Irreconcilable with paleomagnetic

We investigate the evolution of a thin viscous disc surrounding a magnetic star, including the spin-down of the star by the magnetic torques it exerts on the disc. The transition from an accreting to a non-accreting state, and the change of the magnetic torque across the corotation radius rc are included in a ...

We investigate the evolution of a thin viscous disc surrounding a magnetic star, including the spin-down of the star by the magnetic torques it exerts on the disc. The transition from an accreting to a non-accreting state, and the change of the magnetic torque across the corotation radius rc are included in a ...

The main aim in the current survey is to suggest models of the development of structures, such as vortices and spirals, in accretion white dwarf's binaries. On the base of hydrodynamical analytical considerations it is applied numerical methods and simulations. It is suggested in the theoretical model the perturbation's parameters of the accretion flow, ...

We present the frequency-resolved energy spectra (FRS) of the low-mass X-ray binary dipper XB1323-619 during persistent emission in four different frequency bands using an archival XMM-Newton observation. FRS method helps to probe the inner zones of an accretion disc. XB1323-619 is an Atoll source and a type-I burster. We find that the FRS is well ...

A range of important astrophysical systems consist of a strongly magnetic star accreting material from a surrounding disc. These include young stars in T Tauri systems, magnetic neutron stars in X-ray binaries and strongly magnetic white dwarfs in some cataclysmic variables. In all these objects, the stellar magnetic field disrupts the inner part of the ...

In this paper we describe the application of the Smoothed Particle Hydrodynamics (SPH) method to the dynamics of protostellar discs, and in particular to the problem of characterising gravitational instabilities in such discs. Initially, we briefly describe the basic features of SPH, a Lagrangian mesh-free method of solving the equations of fluid dynamics ...

High-resolution two-dimensional magnetohydrodynamical simulations have been carried out to investigate the role of continuing infall of clumpy gas as a driver of turbulence in extended H I galactic discs. We have compared the responses of isothermal gas discs with sound speeds 4 and 8 km/s to infalling, condensed clouds. For mass ...

From observations collected with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope (CFHT) and with the NARVAL spectropolarimeter at the T�lescope Bernard Lyot (TBL), we report the detection of Zeeman signatures on the prototypical classical T Tauri star AA Tau, both in photospheric lines and accretion-powered emission lines. Using time series of ...

Highly nonstationary accretion onto a magnetized neutron star from a surrounding accretion disc is considered. Nonstationary accretion has been considered before in terms of droplets or rain or as a necessary consequence of instabilities in the accretion column flow. Here, the extreme of very ...

A model for emission of the hard photons of gamma bursts is presented. The model assumes accretion at nearly the Eddington limited rate onto a neutron star without magnetic a field. Initially soft photons are heated as they are compressed between the accreting matter and the star. A large electric field due to relatively small charge separation is required ...

In this paper, we study the structure of the broad emission-line regions (BLRs) of the well-known double-peaked emitter (an active galactic nucleus with broad double-peaked low-ionization emission lines) 3C 390.3. Besides the best-fitting results for the double-peaked broad optical Balmer lines of 3C 390.3 obtained from the theoretical disc model, we also try to find another ...

In this paper, we study the structure of the broad emission-line regions (BLRs) of the well-known double-peaked emitter (an active galactic nucleus with broad double-peaked low-ionization emission lines) 3C 390.3. Besides the best-fitting results for the double-peaked broad optical Balmer lines of 3C 390.3 obtained from the theoretical disc model, we also try to find another ...

The X-ray transient source XTE J1818-245 went through an outburst in 2005 that was observed during a multi-wavelength campaign from radio to soft ?-rays. We performed new optical observations with the ESO/NTT telescope at La Silla. The broad-band spectral energy distribution revealed that the outer parts of the accretion disc had to be irradiated by its ...

Dynamo�generated Turbulence and Outflows from Accretion Discs B y Axel Brandenburg Department scale magnetic fields. Comparison with mean�field dynamo theory is made. This theory is then used, in the case of the sun there is turbulent convection which leads to dynamo action. However, the strong shear

We report a time-lapse eclipse mapping analysis of B-band time-series of the nova-like variable UU Aqr along a typical stunted outburst in 2002 August. Disc asymmetries rotating in the prograde sense in the eclipse maps are interpreted as a precessing elliptical disc with enhanced emission at periastron. From the disc expansion ...

... in response to the motion of material points; accretive forces that act within the crystal lattice to drive the crystallization process; attachment forces ...

In this paper we investigate the dynamical behaviour of radiation-driven winds, specifically winds that arise when Compton scattering transfers momentum from the radiation field to the gas flow. Such winds occur during strong X-ray bursts from slowly accreting neutron stars, and also may be driven from the inner regions of a black hole or neutron star ...

Context. Interpretation of the X-ray spectra of X-ray binaries during their hard states requires a hot, optically thin medium. There are several accretion disc models that account for this aspect. However, none is designed to simultaneously explain powerful jets detected during these states. Aims: A new quasi-Keplerian hot accretion ...

We present a grid of models of accreting brown dwarf systems with circumstellar discs. The calculations involve a self-consistent solution of both vertical hydrostatic and radiative equilibrium along with a sophisticated treatment of dust sublimation. We have simulated observations of the spectral energy distributions and several broad-band photometric ...

Young Sun-like stars at the beginning of the pre-main sequence (PMS) evolution are surrounded by accretion discs and remnant protostellar envelopes. Photometric and spectroscopic variations of these stars are driven by interactions of the star with the disc. Time scales and wavelength dependence of the variability carry information on ...

Young Sun-like stars at the beginning of the pre-main-sequence (PMS) evolution are surrounded by accretion discs and remnant protostellar envelopes. Photometric and spectroscopic variations of these stars are driven by interactions of the star with the disc. Time-scales and wavelength dependence of the variability carry information on ...

It has been clear for many years now that the process of accretion on to a compact object is largely dominated by magnetic fields. The magnetic field channels the accretion flow onto strongly magnetized stars such as magnetic white dwarfs (cataclysmic variables), and neutron stars (accreting X-ray pulsars). Magnetic fields are likely ...

Accreting millisecond pulsars show significant variability of their pulse profiles, especially at low accretion rates. On the other hand, their X-ray spectra are remarkably similar with not much variability over the course of the outbursts. For the first time, we have discovered that during the 2008 outburst of SAX J1808.4-3658 a major pulse profile change ...

... dry? Who owns the new land? Does it make any difference if the land accretion ... Read the book. If you are a wetland scientist, it will certainly broaden ... ...

It was recently proposed that metal-rich white dwarfs (WDs) accrete their metals from compact discs of debris found to exist around more than a dozen of them. At the same time, elemental abundances measured in atmospheres of some WDs imply vigorous metal accretion at rates up to 1011 g s-1, far in excess of what can be supplied solely ...

It was recently proposed that metal-rich white dwarfs (WDs) accrete their metals from compact discs of debris found to exist around more than a dozen of them. At the same time, elemental abundances measured in atmospheres of some WDs imply vigorous metal accretion at rates up to 1011 g s-1, far in excess of what can be supplied solely ...

In a recent work by the first author, we show that accretion discs in binary systems could retrogradely precess by the same physics that causes the Earth to retrogradely precess [i.e. tidal torques by the Moon and the Sun (or a secondary star) on a tilted, spinning, non-spherical Earth (or e.g. a primary star surrounded by an accretion ...

TWO�PHASE PAIR CORONA MODEL FOR AGN: PHYSICAL MODELLING AND DIAGNOSTICS Juri Poutanen 1;2 , Roland of the two�phase accretion disc�corona models for active galactic nuclei are compared with ob� servations. We a hemisphere�corona. Keywords: accretion, accretion discs; galaxies: ...

A newborn star is encircled by a remnant disc of gas and dust. A fraction of the disc coalesces into planets. Another fraction spirals inward and accretes onto the star. Accreting gas not only produces observed ultraviolet radiation, but also drags along embedded planets, helping to explain otherwise mysterious ...

Context: .Satellite accretion events have been invoked for mimicking the internal secular evolutionary processes of bulge growth. However, N-body simulations of satellite accretions have paid little attention to the evolution of bulge photometric parameters, to the processes driving this evolution, and to the consistency of this evolution with ...

We investigate the properties of circumplanetary discs formed in three-dimensional, self-gravitating radiation hydrodynamical models of gas accretion by protoplanets. We determine disc sizes, scaleheights, and density and temperature profiles for different protoplanet masses, in solar nebulae of differing grain opacities. We find that ...

We investigate the behaviour of the accretion discs in the outbursts of the low-mass black hole X-ray binaries (BHXRBs), an overview of which we have presented previously. Almost all of the systems in which there are sufficient observations in the most-disc-dominated states show a variation in the disc luminosity ...

We analyse V-band photometry of the aperiodic variability in T CrB. By applying a simple idea of angular momentum transport in the accretion disc, we have developed a method to simulate the statistical distribution of flare durations with the assumption that the aperiodic variability is produced by turbulent elements in the disc. Both ...

We present high-resolution 3D smoothed particle hydrodynamics simulations of the formation and evolution of protostellar discs in a turbulent molecular cloud. Using a piecewise polytropic equation of state, we perform two sets of simulations. In both cases, we find that isolated systems undergo a fundamentally different evolution than members of binary or multiple systems. ...

We argue that the quiescent value of the viscosity parameter of the accretion disc in WZ Sge may be ?cold ~ 0.01, in agreement with estimates of ?cold for other dwarf novae. Assuming the white dwarf in WZ Sge to be magnetic, we show that, in quiescence, material close to the white dwarf can be propelled to larger radii, depleting the inner ...

... provide the context for a more detailed review of gear lubrication distinguishing, in particular, elastohydrodynamic and boundary lubrication regimes ...

We find long-term evolution of the ? class from the study of X-ray timing and spectral analysis of the Galactic microquasar GRS 1915+105 during two outburst activities, observed by the proportional counter array (PCA) and the High Energy X-ray Timing Experiment on-board Rossi X-ray Timing Explorer. The class is characterized by unusual periodic-like variation in intensity. With the passage of ...

Strong magnetic fields modify particle motion in the curved space-time of spinning black holes and change the stability conditions of circular orbits. We study conditions for magnetocentrifugal jet launching from accretion discs around black holes, whereby large-scale black hole lines anchored in the disc may fling tenuous coronal gas ...

May 27, 2010 ... OCEAN COLOR FROM SPACE: Western Boundary Currents ... shift between the low pigment waters of the currents and the higher concentrations in ...

We present a model of an outburst of the soft X-ray transient A0620-003. A two-dimensional time-dependent smoothed particle hydrodynamics scheme is used to simulate the evolution of the accretion disc through a complete outburst. The scheme includes the full tidal potential of the binary and a simple treatment of the thermal-viscous ...

Gas falling quasi-spherically onto a Schwarzschild black hole can form an inner thin accretion disc if its specific angular momentum, $l$, exceeds $\\lmin\\approx 0.75r_gc$, where $r_g$ is the Schwarzschild radius. The standard disc model assumes $l\\gg\\lmin$. We argue that in many black-hole sources the accretion ...

I calculate the specific angular momentum of mass accreted by a binary system embedded in the dense wind of a mass-losing asymptotic giant branch star. The accretion flow is of the Bondi-Hoyle-Lyttleton type. For most of the space of the relevant parameters the flow is basically an isothermal high Mach number accretion flow. I find ...

Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, massive self-gravitating gaseous discs. With help of numerical simulations, we here consider whether the observed population of young stars could have originated from a large angle collision of two massive gaseous clouds at R simeq 1 pc from Sgr ...

Understanding boundary layer heating is crucial in determining the thermal structure of the accreted envelope of a prenova white dwarf. The matched asymptotic expansion method was used to solve consistently for the structure of accretion disks transferring matter onto rotating white dwarfs. The fraction of ...

We present a new keplerian accretion disc solution, the so-called Jet Emitting Disc (JED hereafter), which is part of global self-consistent disc-jet MHD structure. In our framework, a large scale, organized vertical magnetic field is threading the JED giving birth, when conditions are met, to stationnary ...

The galactic black hole candidate Cygnus X-1 is observed to be in one of two X-ray spectral states: either the low/hard (low soft X-ray flux and a flat power-law tail) or high/soft (high blackbody-like soft X-ray flux and a steep power-law tail) state. The physical origin of these two states is unclear. We present here a model of an ionized accretion disc, ...

Quasars are the brilliant cores of remote galaxies, at the hearts of which lie supermassive black holes that can generate enough power to outshine the Sun a trillion times. These mighty power sources are fuelled by interstellar gas, thought to be sucked into the hole from a surrounding 'accretion disc'. A paper in this week's issue of the journal Nature, ...

We use a combination of a cosmological N-body simulation of the concordance ? cold dark matter paradigm and a semi-analytic model of galaxy formation to investigate the spin development of central supermassive black holes (BHs) and its relation to the BH host galaxy properties. In order to compute BH spins, we use the ? model of Shakura & Sunyaev and consider the King et al. warped ...

We introduce a prescription for the luminosity from accreting protostars into smoothed particle hydrodynamics simulation and apply the method to simulations of five primordial minihaloes generated from cosmological initial conditions. We find that accretion luminosity delays fragmentation within the haloes but does not prevent it. In haloes that slowly ...

Many compact sources appear to have a luminosity which is a fair fraction of the Eddington one, implying that slim accretion discs should be used in interpretations of e.g. quasi-periodic variability. Adopting a central mass M~1.4 M_solar, the radial instability structure of slim discs is examined in the accretion ...

We study the radial ionization structure at the surface of an X-ray illuminated accretion disc. We plot the expected iron K? line energy as a function of the Eddington ratio and of the distance of the emitting matter from the central source, for a non-rotating and a maximally rotating black hole. We compare the predicted disc line ...

Cataclysmic variables provide the most accessible view of accretion physics. They are the most numerous accretion powered X-ray sources, and there are thus many relatively bright, nearby examples. Known examples include several bright eclipsing systems that allow a quantitative determination of the accretion geometry. Perhaps most ...

We study the accretion of modified Chaplygin gas upon different types of black holes. Modified Chaplygin gas is one of the best candidates for a combined model of dark matter and dark energy. In addition, from a field theoretical point of view the modified Chaplygin gas model is equivalent to that of a scalar field having a self-interacting potential. We formulate the ...

In order to explain the main characteristics of the observed population of extrasolar planets and the giant planets in the Solar system, we need to get a clear understanding of which are the initial conditions that allowed their formation. To this end we develop a semi-analytical model for computing planetary systems formation based on the core instability model for the gas ...

The central engine that drives gamma ray burst (GRB) explosions may derive from the ability of electrons/positrons and nucleons to tap into the momentum and energy from the large neutrino luminosity emitted by an accretion disc surrounding a black hole. This transfer of momentum and energy occurs due to neutrino absorption, scattering and annihilation, and ...

. There is a class of dynamos where the field is caused by a velocity field which, in turn, is driven by the generated magnetic field. Numerical threedimensional simulations of this type of process are reviewed. This process is here discussed in connection with accretion discs, but we point out that similar mechanisms have also been suggested in the ...

We investigate the influence of radiative transport on the growth of the magnetorotational instability (MRI) in accretion discs. A general dispersion relation describing the growth of small disturbances on a homogeneous background shear flow is provided. It includes compressibility and radiative effects in the flux-limited diffusion approximation. All the ...

We perform hydrodynamical simulations of the accretion of pebbles and rocks on to protoplanets of a few hundred kilometres in radius, including two-way drag force coupling between particles and the protoplanetary disc gas. Particle streams interacting with the gas within the Hill sphere of the protoplanet spiral into a prograde circumplanetary ...

In Montgomery (2009a), we show that accretion discs in binary systems could retrogradely precess by tidal torques like the Moon and the Sun on a tilted, spinning, non-spherical Earth. In addition, we show that the state of matter and the geometrical shape of the celestial object could significantly affect the precessional value. For example, a Cataclysmic ...

We present results from modelling of quasi-simultaneous broad-band (radio through X-ray) observations of the Galactic stellar black hole (BH) transient X-ray binary (XRB) systems XTE J1118+480 and GX 339-4 using an irradiated disc + compact jet model. In addition to quantifying the physical properties of the jet, we have developed a new irradiated disc ...

Accretion disks around young stars produce excess infrared continuum associated with the disk and excess optical and ultraviolet continua associated with the boundary layer or 'hot spot' as material falls from the disk onto the stellar photosphere. When w...

Studies of the structure of mid-ocean ridges and the processes involved at these accreting plate boundaries have played an important role in the development of plate tectonics models over the last two decades. During this time our concept of the accretion...

The role of pull aparts and pushups in transcurrent systems, the rotation of faults and blocks within transcurrent fault systems, the role of accretion tectonics in plate boundary deformation, and power law creep behavior and the yielding at plate boundaries were investigated.

The outer parts of standard steady-state accretion discs around quasi-stellar objects (QSOs) are prone to self-gravity, and they might be expected to fragment into stars rather than feed the central black hole. Possible solutions to this well-known problem are examined with an emphasis on general dynamic constraints. Irradiation by the QSO is insufficient ...

Hyper-accreting discs occur in compact-object mergers and in collapsed cores of massive stars. They power the central engine of ?-ray bursts in most scenarios. We calculate the microphysical dissipation (the viscosity and resistivity) of plasma in these discs, and discuss the implications for their global structure and evolution. At ...

We present two-dimensional hydrodynamical simulations of slowly rotating gas that is under the influence of the gravity of a super massive black hole and is irradiated by a thin UV accretion disc and a spherical X-ray corona. We calculate the accretion luminosity of a system based on the accretion rate which is ...

The formation and evolution of the circumstellar disc in the collapsing molecular cloud with and without magnetic field is investigated from the pre-stellar stage resolving both the molecular cloud core and the protostar itself. In the collapsing cloud core, the first (adiabatic) core appears prior to the protostar formation. Reflecting the thermodynamics of the collapsing ...

In response to major changes in the mass accretion rate within the inner accretion flow, black hole binary transients undergo dramatic evolution in their X-ray timing and spectral behaviour during outbursts. In recent years a paradigm has arisen in which `soft' X-ray states are associated with an inner disc radius at, or very close to, ...

The extent to which the continuum spectral energy distributions of T Tauri stars from 0.2 to 10 microns can be explained by a simple model consisting of an active PMS star and active accretion disk is considered. The disk contributes both an IR excess due to accretion energy dissipation and stellar light reprocessing and an ultraviolet excess from the ...

A model of a thin polytropic disk accretion onto a polytropic star through a polytropic boundary layer is analyzed and a one-parameter series of models is calculated with the stellar angular velocity varying from small to somewhat supercritical values along the series. It is demonstrated that there is no limit to the amount of mass that can be ...

We present simulations of collapsing 100 M&sun; mass cores in the context of massive star formation. The effect of variable initial rotational and magnetic energies on the formation of massive stars is studied in detail. We focus on accretion rates and on the question under which conditions massive Keplerian discs can form in the very early ...

The standard general relativistic model of a razor-thin accretion disc around a black hole, developed by Novikov & Thorne (NT) in 1973, assumes the shear stress vanishes at the radius of the innermost stable circular orbit (ISCO) and that, outside the ISCO, the shear stress is produced by an effective turbulent viscosity. However, astrophysical ...

, together with discs mapped homeomorphically to meridians of R j 's. In this case we say f is a good map A into R j by coning to the center # # E in each meridian disc E of a 1�parameter family of meridian discs which foliate R j . We may assume that # intersects meridian discs e#ciently. Then we homotope f so

The origin of large scale magnetic fields in astrophysical rotators, and the conversion of gravitational energy into radiation near stars and compact objects via accretion have been subjects of active research for a half century. Magnetohydrodynamic turbulence makes both problems highly nonlinear, so both subjects have benefitted from numerical simulations.However, ...

We use the XMM-Newton EPIC-pn instrument in timing mode to extend spectral time-lag studies of hard state black hole X-ray binaries into the soft X-ray band. We show that variations of the disc blackbody emission substantially lead variations in the power-law emission, by tenths of a second on variability time-scales of seconds or longer. The large lags cannot be explained by ...

Abstract. Accretion disc turbulence is investigated in the framework of the shearing box approximation. The turbulence is either driven by the magneto-rotational instability or, in the non-magnetic case, by an explicit and artificial forcing term in the momentum equation. Unlike the magnetic case, where most of the dissipation occurs in the ...

I have investigated the effects of including a more realistic cooling law in SPH simulations of accretion discs. In order to improve the efficiency of the simulations, I have used Strang operator splitting, and I have developed a method of timestep control which guarantees the accuracy and stability of the simulations.

Detailed analysis of the lightcurve of CAL 87 clearly has shown that the high optical luminosity comes from the accretion disc rim and can only be explained by a severe thickening of the disc rim near the location where the accretion stream impinges. This area is irradiated by the X-rays where it faces the white ...

The continuum emission from the inner regions of Active Galactic Nuclei (AGN) is discussed. It is emphasized that a thermal UV component is a generic feature of accretion powered sources, and does not necessarily require a 'standard' disc; in particular, ...

Jan 24, 2001 ... The doughnut, called the Io torus, draws its raw material from ... called planetary accretion discs that can exist within the magnetic-field ...

We continue our study of weakly ionized protostellar accretion discs that are threaded by a large-scale magnetic field and power a centrifugally driven wind. It has been argued that there is already evidence in several protostellar systems that such a wind transports a significant fraction of the angular momentum from at least some part of the ...

Accretion onto black holes often proceeds via an accretion disc or a temporary disc-like pattern. Variability features, observed in the light curves of such objects, and theoretical models of accretion flows suggest that accretion discs are ...

We carry out a time-dependent numerical simulation where both the hydrodynamics and the radiative transfer are coupled together. We consider a two-component accretion flow in which the Keplerian disc is immersed inside an accreting low angular momentum flow (halo) around a black hole. The injected soft photons from the Keplerian ...

We carry out a time-dependent numerical simulation where both the hydrodynamics and the radiative transfer are coupled together. We consider a two-component accretion flow in which the Keplerian disc is immersed inside an accreting low angular momentum flow (halo) around a black hole. The injected soft photons from the Keplerian ...

Time-resolved eclipse spectroscopy of the nova-like variable UX UMa obtained with the HST/FOS on 1994 August and November is analysed with eclipse mapping techniques to produce spatially resolved spectra of its accretion disc and gas stream as a function of distance from the disc centre. The inner accretion ...

The effect of rapid accretion on a moderate-mass stellar model is studied. The surface boundary condition is examined for two modes of accretion - disk and direct impact. It is found that the effective temperature must be increased for disk accretion but not for direct-impact accretion. In the ...

We propose four snap-shot observations on the remarkable NLS1 J1633+4718 with the lowest soft X-ray excess temperature of 32eV. Our goals are to test the blackbody nature of this emission and to obtain high quality X-ray and UV data, which are essential for more quantitative testing of accretion disc models in AGN and, hopefully, shedding light on the ...

High-energy (E>2 keV) continuum flaring is detected in two narrow-line Seyfert 1 galaxies (I Zw 1 and NAB 0205+024), consistent with occurring in a hot corona distinct from the accretion disc. The flare in I Zw 1 is accompanied by an increase in the amount of gravitationally redshifted reflected emission coming from the accretion ...

We performed hydrodynamical simulations to investigate the formation and evolution of protostars and circumstellar discs from the pre-stellar cloud. As the initial state, we adopted the molecular cloud core with two non-dimensional parameters representing the thermal and rotational energies. With these parameters, we derived 17 models and calculated the cloud evolution �104 ...

We performed hydrodynamical simulations to investigate the formation and evolution of protostars and circumstellar discs from the pre-stellar cloud. As the initial state, we adopted the molecular cloud core with two non-dimensional parameters representing the thermal and rotational energies. With these parameters, we derived 17 models and calculated the cloud evolution �104 ...

We present results from the first three-dimensional radiation hydrodynamical calculations to follow the collapse of a molecular cloud core beyond the formation of the stellar core. We find that the energy released by the formation of the stellar core, within the optically thick first hydrostatic core, is comparable to the binding energy of the disc-like first core. This heats ...

Star-forming disc galaxies such as the Milky Way need to accrete >~1 Msolar of gas each year to sustain their star formation. This gas accretion is likely to come from the cooling of the hot corona, however it is still not clear how this process can take place. We present simulations supporting the idea that this cooling and the ...

Context. X-ray observations of unabsorbed active galactic nuclei provide an opportunity to explore the innermost regions of supermassive black hole accretion discs. Aims: Our goal in this paper is to investigate the central environment of a Seyfert 1.5 galaxy IRAS 05078+1626. Methods: We studied the time-averaged spectrum obtained with the EPIC and RGS ...

Young close binaries open central gaps in the surrounding circumbinary accretion disc, but the stellar components may still gain mass from gas crossing through the gap. It is not well understood how this process operates and how the stellar components are affected by such inflows. Our main goal is to investigate how gas accretion takes ...

We performed global three-dimensional magnetohydrodynamic simulations of accretion on to a star with magnetic field and other properties close to those observed in the classical T Tauri star BP Tau. We observed in the simulations that the disc is disrupted by the dipole component and matter flows towards the star in two funnel streams which form two ...

The active galactic nuclei(AGN)are the most luminous objects in the universe. Total power radiated by an AGN is greater than the total power emitted by all the stars in the host galaxy.The direct diagnostics of the conditions within AGNs come from radiations produced by a wide variety of mechanisms including high energy gamma ray processes atmospheric reprocessing dust emissions and ...

We assembled a sample of broad-line and narrow-line active galactic nuclei (AGNs) observed by ASCA, the excess variances of which have been determined. The central black hole masses in this sample can be obtained from the reverberation mapping method and the width of the H? emission line. Using the black hole masses and the bolometric luminosity, the Eddington ratio (the ratio of the ionizing ...

X-ray bursters are considered to be neutron stars with a weak magnetic field that are undergoing disk-type accretion. If the field is weak enough, the disk will reach to the neutron star surface; next to the surface, a narrow boundary layer will develop, girdling the star. Using available formulations for luminosity and for free particle motion, it is ...

The cartilaginous end plates define the anatomical boundaries of the disc, serve as an area of contact with the vertebral body, and act as a semi-peremeable membrane. The anatomic relationships that exist between the end plate and immediately adjacent str...

We present multi-epoch medium-resolution observations of two M4.5 candidate members in the halo of the �8 Myr ? Chamaeleontis open cluster. During six months of observations, both stars exhibited variations in their H? line profiles on time-scales of days to months, with at least one episode of substantial activity attributable to accretion from a circumstellar ...

Spectroscopy of X-ray emission lines emitted in accretion discs around supermassive black holes is one of the most powerful probes of the accretion flow physics and geometry, while also providing in principle observational constraints on the black hole spin.[...] We aim at determining the ultimate physical driver of the strength of ...

Both HerX-1 and SS433 may contain accretion disks slaved to a precessing companion star. If so, it is possible to bound the effective viscosity in these disks. The results, in terms of the disk parameter alpha, are lower bounds of 0.01 for HerX-1 and of 0.1 for SS433.

Two new elements, the Hybrid Accretion Disk (HAD) model and the Rosat survey of resolved X-ray sources and diffuse background, are combined in an attempt to resolve the long standing problem of the cosmic X-Ray Background (XRB), explaining the diffuse XRB...

The radial-azimuthal instability of a hot two-temperature accretion disc with advection is analyzed. After obtaining the dispersion equation, we find that advection and viscous force have an influence on the stability of acoustic modes, but not on the stability of thermal and viscous modes. We also find azimuthal perturbations to affect the stability of ...

We develop numerical models of accretions disks in cataclysmic variables (CVs), including and emphasizing the boundary layer region where the accretion disk meets the accreting white dwarf. We confine ourselves to solutions where the boundary layer region is vertically optically thick, and find ...

When gas accretes on to a black hole, at a rate either much less than or much greater than the Eddington rate, it is likely to do so in an `adiabatic' or radiatively inefficient manner. Under fluid (as opposed to magnetohydrodynamic) conditions, the disc should become convective and evolve toward a state of marginal instability. We model the resulting ...

We study numerically the interaction of the infalling gas stream and the rim of the accretion disc in cataclysmic variables. The simulations were performed with a smoothed particle hydrodynamics scheme with high spatial resolution. Parameters of the systems AM CVn, OY Car, DQ Her, U Gem and IP Peg were used for the simulations. The simulations cover a wide ...

We study spectral properties of the brightest GeV blazar 3C 454.3 using the Fermi data for 137 weeks of observations. We probe the behaviour of the source as a function of time at different brightness levels. The spectra in the GeV range can be well described by a wide underlying lognormal distribution with photon-photon absorption breaks produced by He II and H I recombination continua. We ...

Fully self consistent calculations of the spectra emitted by active galactic nuclei accretion disks are presented. The inner, radiation dominated regions of standard, alpha disks are treated. Incoherent Compton scattering and emission/absorption of radiation by hydrogen and helium are included in computing the vertical temperature structure within the disk and the emergent ...

We have studied the long-term variability of LMC X-3 in optical light curves spanning 6yr, in order to search for optical signatures which could confirm or refute the suggestion that the `modulation' is the result of accretion-rate variability rather than accretion-disc precession. We find that there is no stable period in the optical light curves, that ...

The authors consider the linear stability of a thin differentially rotating gaseous disc with reflecting boundaries against non-axisymmetric perturbations. They assume the unperturbed disc to be in centrifugal equilibrium with the gravitational attraction of a central mass M, and consider discs whose mass can be ...

Dark matter direct detection experiments need to know the local phase space density of dark matter fdm(r,v,t) in order to derive dark matter particle properties. To date, calculations for fdm(r,v,t) have been based on simulations that model the dark matter alone. Here we include the influence of the baryonic matter. We find that a star/gas disc at high redshift (z~1) causes ...

The flux of the [O III]?5007-� line is considered to be a good indicator of the bolometric emission of quasars. The observed continuum emission from the accretion disc should instead be strongly dependent on the inclination angle ? between the disc axis and the line of sight. Based on this, the equivalent width (EW) of [O III] should ...

Millisecond pulsars are believed to be old pulsars spun up by a surrounding accretion disc. Magnetic fields are thought to play a leading role in this, both by determining the location of the inner edge of the disc and by exerting an additional torque on the star (as a result of the interaction between the stellar magnetic field and ...

The present study examines the self-similar evolution of advection-dominated accretion flow (ADAF) in the presence of a toroidal magnetic field. In this research, it was assumed that angular momentum transport is due to viscous turbulence and the ?-prescription was used for the kinematic coefficient of viscosity. The flow does not have a good cooling efficiency and so a ...

Primordial matter mainly consists of hydrogen and helium with a small amount of lithium. Taking into account contributions to the opacity from hydrogen and helium only has been assumed to be sufficient so far. Lithium, however, influences the opacity indirectly through a change in chemical equilibrium due to absorption of atomic lithium and lithium hydride. The differences reach two orders of ...

Context. Spectroscopy of X-ray emission lines emitted from accretion discs around supermassive black holes is one of the most powerful probes of the accretion flow physics and geometry, while also providing in principle observational constraints on the black hole spin. Previous studies have suggested that relativistically broadened ...

Approximately half the luminosity of a typical cataclysmic variable may emerge as an optically thick component peaking in the EUV. Observations of this component are important for understanding the energetics and accretion rates of CV's in general, as well as for understanding the physics of the accretion process. The nature of the turbulent ...

We present the analysis of the morphology of the light curves of WZ Sge based on simultaneous multicolour highspeed photometry durings the superoutburst in 2001. Observations started around the middle of the main superoutburst and continued for several nights in each of the subsequent phases of its evolution. For the first time for WZ Sge in a superoutburst a significant difference between the ...

Astrophysical jets seem to occur in nearly all types of accreting objects, from supermassive black holes to young stellar objects. On the basis of x-ray binaries, a unified scenario describing the disc/jet coupling has evolved and been extended to many accreting objects. The only major exceptions are thought to be cataclysmic ...

We investigate whether a circumbinary gas disc can coalesce a supermassive black hole binary system in the centre of a galaxy. This is known to be problematic for a prograde disc. We show that in contrast, interaction with a retrograde circumbinary disc is considerably more effective in shrinking the binary because there are no orbital ...

Star formation is thought to be triggered by gravitational collapse of the dense cores of molecular clouds. Angular momentum conservation during the collapse results in the progressive increase of the centrifugal force, which eventually halts the inflow of material and leads to the development of a central mass surrounded by a disc. In the presence of an angular momentum ...

Protostars �0.1 Myr old are heavily obscured, but their circumstellar dust discs can be studied by millimetre interferometry that resolves out the obscuring envelope. Consistent estimates are made for the disc masses of Class 0 protostars, and these range over 7-660 MJup. A simple grain coagulation model reproduces the mass distributions of more evolved ...

We consider the interaction between a binary system (e.g. two supermassive black holes or two stars) and an external accretion disc with misaligned angular momentum. This situation occurs in galaxy merger events involving supermassive black holes, and in the formation of stellar mass binaries in star clusters. We work out the gravitational torque between ...