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Sample records for accretion disc 2d

  1. How do accretion discs break?

    NASA Astrophysics Data System (ADS)

    Dogan, Suzan

    2016-07-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. In this study, we first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. We compare the disc precession torque with the disc viscous torque to determine whether the disc should warp or break. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. To check our analytical findings, we perform 3D hydrodynamical numerical simulations using the PHANTOM smoothed particle hydrodynamics code, and confirm that disc breaking is widespread and enhances accretion on to the central object. For some inclinations, the disc goes through strong Kozai cycles. Disc breaking promotes markedly enhanced and variable accretion and potentially produces high-energy particles or radiation through shocks. This would have significant implications for all binary systems: e.g. accretion outbursts in X-ray binaries and fuelling supermassive black hole (SMBH) binaries. The behaviour we have discussed in this work is relevant to a variety of astrophysical systems, for example X-ray binaries, where the disc plane may be tilted by radiation warping, SMBH binaries, where accretion of misaligned gas can create effectively random inclinations and protostellar binaries, where a disc may be misaligned by a variety of effects such as binary capture/exchange, accretion after binary formation.

  2. Broken discs: warp propagation in accretion discs

    NASA Astrophysics Data System (ADS)

    Nixon, Christopher J.; King, Andrew R.

    2012-04-01

    We simulate the viscous evolution of an accretion disc around a spinning black hole. In general, any such disc is misaligned, and warped by the Lense-Thirring effect. Unlike previous studies, we use effective viscosities constrained to be consistent with the internal fluid dynamics of the disc. We find that non-linear fluid effects, which reduce the effective viscosities in warped regions, can promote breaking of the disc into two distinct planes. This occurs when the Shakura & Sunyaev dimensionless viscosity parameter α is ≲0.3 and the initial angle of misalignment between the disc and hole is ≳45°. The break can be a long-lived feature, propagating outwards in the disc on the usual alignment time-scale, after which the disc is fully co-aligned or counter-aligned with the hole. Such a break in the disc may be significant in systems where we know the inclination of the outer accretion disc to the line of sight, such as some X-ray binaries: the inner disc, and so any jets, may be noticeably misaligned with respect to the orbital plane.

  3. Counter-rotating accretion discs

    NASA Astrophysics Data System (ADS)

    Dyda, S.; Lovelace, R. V. E.; Ustyugova, G. V.; Romanova, M. M.; Koldoba, A. V.

    2015-01-01

    Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud on to the surface of an existing corotating disc or from the counter-rotating gas moving radially inwards to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc centre. We discuss high-resolution axisymmetric hydrodynamic simulations of viscous counter-rotating discs for the cases where the two components are vertically separated and radially separated. The viscosity is described by an isotropic α-viscosity including all terms in the viscous stress tensor. For the vertically separated components, a shear layer forms between them and the middle part of this layer free-falls to the disc centre. The accretion rates are increased by factors of ˜102-104 over that for a conventional disc rotating in one direction with the same viscosity. The vertical width of the shear layer and the accretion rate are strongly dependent on the viscosity and the mass fraction of the counter-rotating gas. In the case of radially separated components where the inner disc corotates and the outer disc rotates in the opposite direction, a gap between the two components opens and closes quasi-periodically. The accretion rates are ≳25 times larger than those for a disc rotating in one direction with the same viscosity.

  4. Black hole accretion disc impacts

    NASA Astrophysics Data System (ADS)

    Pihajoki, P.

    2016-04-01

    We present an analytic model for computing the luminosity and spectral evolution of flares caused by a supermassive black hole impacting the accretion disc of another supermassive black hole. Our model includes photon diffusion, emission from optically thin regions and relativistic corrections to the observed spectrum and time-scales. We test the observability of the impact scenario with a simulated population of quasars hosting supermassive black hole binaries. The results indicate that for a moderate binary mass ratio of 0.3, and impact distances of 100 primary Schwarzschild radii, the accretion disc impacts can be expected to equal or exceed the host quasar in brightness at observed wavelength λ = 510 nm up to z = 0.6. We conclude that accretion disc impacts may function as an independent probe for supermassive black hole binaries. We release the code used for computing the model light curves to the community.

  5. Strongly magnetized accretion discs require poloidal flux

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg; Armitage, Philip J.; Simon, Jacob B.; Begelman, Mitchell C.

    2016-08-01

    Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

  6. Strongly magnetized accretion discs require poloidal flux

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg; Armitage, Philip J.; Simon, Jacob B.; Begelman, Mitchell C.

    2016-05-01

    Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

  7. Floquet analysis in accretion disc dynamics

    NASA Astrophysics Data System (ADS)

    Tamburini, F.; Bianchini, A.

    2002-01-01

    Floquet analysis is proposed to analyze the evolution of exponentially growing modes of the local instabilities in accretion discs of CVs induced by perturbations in the velocity field. Both the stability of the disc and the deviation of the Reynolds number are described by the Floquet exponents μj of the perturbations, which represent Landau's modes of the fluid.

  8. Accretion Discs Show Their True Colours

    NASA Astrophysics Data System (ADS)

    2008-07-01

    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, partly based on observations collected with ESO's Very Large Telescope, verifies a long-standing prediction about the intensely luminous radiation emitted by these accretion discs. Uncovering the disc ESO PR Photo 21/08 Uncovering the inner disc "Astronomers were puzzled by the fact that the best models of these discs couldn't quite be reconciled with some of the observations, in particular, with the fact that these discs did not appear as blue as they should be," explains lead-author Makoto Kishimoto. Such a discrepancy could be the signal that there was something very wrong with the models. With his colleagues, he investigated this discrepancy by studying the polarised light from six quasars. This enabled them to demonstrate that the disc spectrum is as blue as predicted. "The crucial observational difficulty here has been that the disc is surrounded by a much larger torus containing hot dust, whose light partly outshines that of the disc," says Kishimoto. "Because the light coming from the disc is scattered in the disc vicinity and thus polarised, by observing only polarised light from the quasars, one can uncover the buried light from the disc." In a similar way that a fisherman would wear polarised sunglasses to help get rid of the glare from the water surface and allow him to see more clearly under the water, the filter on the telescope allowed the astronomers to see beyond surrounding clouds of dust and gas to the blue colour of the disc in infrared light. The observations were done with the FORS and ISAAC instruments on one of the 8.2-m Unit Telescopes of ESO's Very Large Telescope, located in the Atacama

  9. Magnetically driven accretion in protoplanetary discs

    NASA Astrophysics Data System (ADS)

    Simon, Jacob B.; Lesur, Geoffroy; Kunz, Matthew W.; Armitage, Philip J.

    2015-11-01

    We characterize magnetically driven accretion at radii between 1 and 100 au in protoplanetary discs, using a series of local non-ideal magnetohydrodynamic (MHD) simulations. The simulations assume a minimum mass solar nebula (MMSN) disc that is threaded by a net vertical magnetic field of specified strength. Confirming previous results, we find that the Hall effect has only a modest impact on accretion at 30 au, and essentially none at 100 au. At 1-10 au the Hall effect introduces a pronounced bimodality in the accretion process, with vertical magnetic fields aligned to the disc rotation supporting a strong laminar Maxwell stress that is absent if the field is anti-aligned. In the anti-aligned case, we instead find evidence for bursts of turbulent stress at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear instability. The presence or absence of these bursts depends upon the details of the adopted chemical model, which suggests that appreciable regions of actual protoplanetary discs might lie close to the borderline between laminar and turbulent behaviour. Given the number of important control parameters that have already been identified in MHD models, quantitative predictions for disc structure in terms of only radius and accretion rate appear to be difficult. Instead, we identify robust qualitative tests of magnetically driven accretion. These include the presence of turbulence in the outer disc, independent of the orientation of the vertical magnetic fields, and a Hall-mediated bimodality in turbulent properties extending from the region of thermal ionization to 10 au.

  10. Massive star formation by accretion. I. Disc accretion

    NASA Astrophysics Data System (ADS)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.

    2016-01-01

    Context. Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the Hertzsprung-Russell (HR) diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. Aims: We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the HR diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. Methods: We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. We compare them with previously published equivalent models. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the accretion history of most of the intermediate-mass stars. Results: In the numerical computation of the time derivative of the entropy, some treatment leads to an artificial loss of entropy and thus reduces the inflation that the accreting star undergoes along the birthline. In the case of cold disc accretion, the existence of a significant swelling during the accretion phase, which leads to radii ≳ 100 R⊙ and brings the star back to the red part of the HR diagram, depends sensitively on the initial conditions. For an accretion rate of 10-3M⊙ yr-1, only models starting from a core with a significant radiative region evolve back to the red part of the HR diagram. We also obtain that, in order to reproduce the observed upper envelope of pre-MS stars in the HR diagram with an accretion law deduced from the observed mass outflows in ultra-compact HII regions, the fraction of the

  11. Retrograde binaries of massive black holes in circumbinary accretion discs

    NASA Astrophysics Data System (ADS)

    Amaro-Seoane, Pau; Maureira-Fredes, Cristián; Dotti, Massimo; Colpi, Monica

    2016-06-01

    Context. We explore the hardening of a massive black hole binary embedded in a circumbinary gas disc under a specific circumstance: when the binary and the gas are coplanar and the gas is counter-rotating. The binary has unequal mass and the interaction of the gas with the lighter secondary black hole is the main cause of the braking torque on the binary that shrinks with time. The secondary black hole, revolving in the direction opposite to the gas, experiences a drag from gas-dynamical friction and from direct accretion of part of it. Aims: In this paper, using two-dimensional (2D) hydrodynamical grid simulations we investigate the effect of changing the accretion prescriptions on the dynamics of the secondary black hole, which in turn affect the binary hardening and eccentricity evolution. Methods: We find that realistic accretion prescriptions lead to results that differ from those inferred assuming accretion of all the gas within the Roche Lobe of the secondary black hole. Results: When considering gas accretion within the gravitational influence radius of the secondary black hole (which is smaller than the Roche Lobe radius) to better describe gas inflows, the shrinking of the binary is slower. In addition, in this case, a smaller amount of accreted mass is required to reduce the binary separation by the same amount. Different accretion prescriptions result in different discs' surface densities, which alter the black hole's dynamics back. Full 3D Smoothed-particle hydrodynamics realizations of a number of representative cases, run over a shorter interval of time, validate the general trends observed in the less computationally demanding 2D simulations. Conclusions: Initially circular black hole binaries increase their eccentricity only slightly, which then oscillates around small values (<0.1) while they harden. By contrast, initially eccentric binaries become more and more eccentric. A semi-analytical model describing the black hole's dynamics under

  12. Collective excitations in 2D hard-disc fluid.

    PubMed

    Huerta, Adrian; Bryk, Taras; Trokhymchuk, Andrij

    2015-07-01

    Collective dynamics of a two-dimensional (2D) hard-disc fluid was studied by molecular dynamics simulations in the range of packing fractions that covers states up to the freezing. Some striking features concerning collective excitations in this system were observed. In particular, the short-wavelength shear waves while being absent at low packing fractions were observed in the range of high packing fractions, just before the freezing transition in a 2D hard-disc fluid. In contrast, the so-called "positive sound dispersion" typically observed in dense Lennard-Jones-like fluids, was not detected for the 2D hard-disc fluid. The ratio of specific heats in the 2D hard-disc fluid shows a monotonic increase with density approaching the freezing, resembling in this way the similar behavior in the vicinity of the Widom line in the case of supercritical fluids. PMID:25595625

  13. Tearing up a misaligned accretion disc with a binary companion

    NASA Astrophysics Data System (ADS)

    Doğan, Suzan; Nixon, Chris; King, Andrew; Price, Daniel J.

    2015-05-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. We calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. We run hydrodynamical simulations to check these results, and confirm that disc breaking is widespread and generally enhances accretion on to the central object. This applies in many cases of astrophysical accretion, e.g. supermassive black hole binaries and X-ray binaries.

  14. Tearing up the disc: misaligned accretion on to a binary

    NASA Astrophysics Data System (ADS)

    Nixon, Chris; King, Andrew; Price, Daniel

    2013-09-01

    In a recent paper, we have shown that the evolution of a misaligned disc around a spinning black hole can result in tearing the disc into many distinct planes. Tearing discs with random orientations produce direct dynamical accretion on to the hole in ≈70 per cent of all cases. Here, we examine the evolution of a misaligned disc around a binary system. We show that these discs are susceptible to tearing for almost all inclinations. We also show that tearing of the disc can result in a significant acceleration of the disc evolution and subsequent accretion on to the binary - by factors up to 104 times that of a coplanar prograde disc with otherwise identical parameters. This provides a promising mechanism for driving mergers of supermassive black hole (SMBH) binaries on time-scales much shorter than a Hubble time. Disc tearing also suggests new observational signatures of accreting SMBH binaries and other systems such as protostellar binaries.

  15. Bulk Comptonization by turbulence in accretion discs

    NASA Astrophysics Data System (ADS)

    Kaufman, J.; Blaes, O. M.

    2016-06-01

    Radiation pressure dominated accretion discs around compact objects may have turbulent velocities that greatly exceed the electron thermal velocities within the disc. Bulk Comptonization by the turbulence may therefore dominate over thermal Comptonization in determining the emergent spectrum. Bulk Comptonization by divergenceless turbulence is due to radiation viscous dissipation only. It can be treated as thermal Comptonization by solving the Kompaneets equation with an equivalent `wave' temperature, which is a weighted sum over the power present at each scale in the turbulent cascade. Bulk Comptonization by turbulence with non-zero divergence is due to both pressure work and radiation viscous dissipation. Pressure work has negligible effect on photon spectra in the limit of optically thin turbulence, and in this limit radiation viscous dissipation alone can be treated as thermal Comptonization with a temperature equivalent to the full turbulent power. In the limit of extremely optically thick turbulence, radiation viscous dissipation is suppressed, and the evolution of local photon spectra can be understood in terms of compression and expansion of the strongly coupled photon and gas fluids. We discuss the consequences of these effects for self-consistently resolving and interpreting turbulent Comptonization in spectral calculations in radiation magnetohydrodynamic simulations of high luminosity accretion flows.

  16. Magnetised accretion discs in Kerr spacetimes

    NASA Astrophysics Data System (ADS)

    Ranea-Sandoval, Ignacio F.; García, Federico

    2015-01-01

    Context. Observational data from X-ray binary systems provide strong evidence of astronomical objects that are too massive and compact to be explained as neutron or hybrid stars. When these systems are in the thermal (high/soft) state, they emit mainly in the 0.1-5 keV energy range. This emission can be explained by thin accretion discs that formed around compact objects like black holes. The profile of the fluorescent iron line is useful to obtain insight into the nature of the compact object. General relativity does not ensure that a black hole must form after the complete gravitational collapse of very massive stars, and other theoretical models such as naked singularities cannot be discarded. The cosmic censorship conjecture was proposed by Penrose to avoid these possibilities and is yet to be proven. Aims: We study the effect caused by external magnetic fields on the observed thermal spectra and iron line profiles of thin accretion discs formed around Kerr black holes and naked singularities. We aim to provide a tool that can be used to estimate the presence of magnetic fields in the neighbourhood of a compact object and to probe the cosmic censorship conjecture in these particular astrophysical environments. Methods: We developed a numerical scheme able to calculate thermal spectra of magnetised Page-Thorne accretion discs formed around rotating black holes and naked singularities as seen by an arbitrary distant observer. We incorporated two different magnetic field configurations: uniform and dipolar, using a perturbative scheme in the coupling constant between matter and magnetic field strength. Under the same assumptions, we obtained observed synthetic line profiles of the 6.4 keV fluorescent iron line. Results: We show that an external magnetic field produces potentially observable modifications on the thermal energy spectrum and the fluorescent iron line profile. Thermal energy spectra of naked singularities are harder and brighter than those from black

  17. On the stability of elliptical vortices in accretion discs

    NASA Astrophysics Data System (ADS)

    Lesur, G.; Papaloizou, J. C. B.

    2009-04-01

    Context: The existence of large-scale and long-lived 2D vortices in accretion discs has been debated for more than a decade. They appear spontaneously in several 2D disc simulations and they are known to accelerate planetesimal formation through a dust trapping process. In some cases, these vortices may even lead to an efficient way to transport angular momentum in protoplanetary discs when MHD instabilities are inoperative. However, the issue of the stability of these structures to the imposition of 3D disturbances is still not fully understood, and it casts doubts on their long term survival Aims: We present new results on the 3D stability of elliptical vortices embedded in accretion discs, based on a linear analysis and several non-linear simulations. Methods: We introduce a simple steady 2D vortex model which is a non-linear solution of the equations of motion, and we show that its core is made of elliptical streamlines. We then derive the linearised equations governing the 3D perturbations in the core of this vortex, and we show that they can be reduced to a Floquet problem. We solve this problem numerically in the astrophysical regime, including a simplified model to take into account vertical stratification effects. We present several analytical limits for which the mechanism responsible for instability can be explained. Finally, we compare the results of the linear analysis to some high resolution numerical simulations obtained with spectral and finite difference methods. A discussion is provided, emphasising the astrophysical consequences of our findings for the dynamics of vortices. Results: We show that most anticyclonic vortices are unstable due to a resonance between the turnover time and the local epicyclic oscillation period. A small linearly stable domain is found for vortex cores with an aspect-ratio of around 5. However, our simulations show that it is only the vortex core that is stable, with the instability still appearing on the vortex boundary

  18. Spiral shocks in a solar-size accretion disc

    NASA Astrophysics Data System (ADS)

    Harlaftis, E. T.; Steeghs, D.

    Accretion discs are fundamental in understanding astrophysical phenomena such as AGNs, novae outbursts and star formation. In interacting binaries, a compact star accretes matter from a donor star through an accretion disc. The outburst origin (disc or secondary star) and the mechanism for the angular momentum transport of the disc material (`viscosity') are still controversial subjects. The rarely-observed rise to outburst may hold the key to a better understanding. Imaging of the Balmer and He{I} emission lines of the dwarf nova IP Peg, during such a rise to outburst, shows a two-arm spiral pattern on the accretion disc around the white dwarf and provides the first convincing observational evidence for spiral waves in a stellar accretion disc (Steeghs, Harlaftis, Horne, 1997, Nature, submitted). Recent observations during the recent November 1996 outburst (Harlaftis, Steeghs, Horne, Martin, ApJ, 1997, in preparation) also demonstrate spiral arms in high-ionization lines such as HeII and the Bowen fluorescence lines which suggests that the spiral pattern may provide an efficient mechanism for trasport of angular momentum out of the disc through spiral shocks. We discuss the origin and location of the spiral arms. The tidal interaction of the secondary star with the enlarged (0.6 Rodot) outburst disc can raise such spiral waves in the outer disc. Comparison and implications for theories of spiral galaxies and planet formation is briefly outlined.

  19. Accretion disc viscosity: a limit on the anisotropy

    NASA Astrophysics Data System (ADS)

    Nixon, Chris

    2015-07-01

    Observations of warped discs can give insight into the nature of angular momentum transport in accretion discs. Only a few objects are known to show strong periodicity on long time-scales, but when such periodicity is present it is often attributed to precession of the accretion disc. The X-ray binary Hercules X-1/HZ Herculis (Her X-1) is one of the best examples of such periodicity and has been linked to disc precession since it was first observed. By using the current best-fitting models to Her X-1, which invoke precession driven by radiation warping, I place a constraint on the effective viscosities that act in a warped disc. These effective viscosities almost certainly arise due to turbulence induced by the magnetorotational instability. The constraints derived here are in agreement with analytical and numerical investigations into the nature of magnetohydrodynamic disc turbulence, but at odds with some recent global simulations.

  20. Materietransport in Akkretionsscheiben %t Transport of matter in accretion discs

    NASA Astrophysics Data System (ADS)

    Keller, Christof Martin

    2003-07-01

    Time-scales that need to be considered in time-dependent computations of accretion discs are many orders of magnitude larger than stable time-step sizes of common numerical codes. Therefore, theoretical investigation of these objects is severely limited by present-day computational resources, unless more efficient algorithms are found. Due to large differences in the underlying physics of cosmic accretion discs, algorithms need to be adjusted to the particular problem. During the course of this thesis, several algorithms have been implemented and tested. One of the implemented splitting-methods could efficiently be employed to 1D-simulations of supersonic accretion flows onto black holes. Another splitting method and a pressure correction scheme were applied to simulate two-dimensional protostellar accretion flows, which have been investigated more elaborately in this thesis. With these methods, performance in simulating protostellar discs could be improved in at least some cases. Numerical simulations of flow-structures in protostellar discs could thus be conducted and compared to higher order analytical approximations. Disc models using an α-description of the viscosity produced meridional flow-structures that have already been observed by several authors. Unlike flow-structures resulting from stationary one-zone-approximations, meridional flows exhibit outward directed velocities in the midplane of the disc. Test cases showed, that meridional flows can play an important role in the mixing processes of protostellar disc material that is reflected in the composition of cometary and meteorite material.

  1. Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency

    NASA Astrophysics Data System (ADS)

    Sakurai, Y.; Vorobyov, E. I.; Hosokawa, T.; Yoshida, N.; Omukai, K.; Yorke, H. W.

    2016-06-01

    The formation of supermassive stars (SMSs) is a potential pathway to seed supermassive black holes in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper, we study the evolution of an accreting SMS and its UV emissivity with realistic variable accretion from a circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the protostellar accretion until the stellar mass exceeds 104 M⊙. The disc fragments by gravitational instability, creating many clumps that migrate inward to fall on to the star. The resulting accretion history is highly time-dependent: short episodic accretion bursts are followed by longer quiescent phases. We show that the disc for the direct collapse model is more unstable and generates greater variability than normal Pop III cases. Next, we conduct a stellar evolution calculation using the obtained accretion history. Our results show that, regardless of the variable accretion, the stellar radius monotonically increases with almost constant effective temperature at Teff ≃ 5000 K as the stellar mass increases. The resulting UV feedback is too weak to hinder accretion due to the low flux of stellar UV photons. The insensitivity of stellar evolution to variable accretion is attributed to the fact that time-scales of variability, ≲103 yr, are too short to affect the stellar structure. We argue that this evolution will continue until the SMS collapses to produce a black hole by the general relativistic instability after the mass reaches ≳105 M⊙.

  2. Cloudy intergalactic accretion flows in the outer discs of galaxies

    NASA Astrophysics Data System (ADS)

    Sánchez-Salcedo, F. J.; Santillán, A.; Franco, J.

    2007-02-01

    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 accretion rates of ˜0.6 M⊙ yr -1, the turbulent motions in the outer disc become slightly faster than transonic. We suggest that the rain of compact high velocity clouds on the disc not only can fuel the Milky Way with fresh material but is a potential source of random motions in outer regions of H I discs.

  3. Thin accretion discs are stabilized by a strong magnetic field

    NASA Astrophysics Data System (ADS)

    Sądowski, Aleksander

    2016-07-01

    By studying three-dimensional, radiative, global simulations of sub-Eddington, geometrically thin (H/R ≈ 0.15) black hole accretion flows we show that thin discs which are dominated by magnetic pressure are stable against thermal instability. Such discs are thicker than predicted by the standard model and show significant amount of dissipation inside the marginally stable orbit. Radiation released in this region, however, does not escape to infinity but is advected into the black hole. We find that the resulting accretion efficiency (5.5 ± 0.5 per cent for the simulated 0.8dot{M}_Edd disc) is very close to the predicted by the standard model (5.7 per cent).

  4. Structure of evolving Accretion Discs and their Implications to the Formation of Planetary Cores

    NASA Astrophysics Data System (ADS)

    Bitsch, Bertram; Morbidelli, A.; Crida, A.; Lega, E.

    2013-10-01

    Two features in a protoplanetary disc can have profound effects on planet formation. The first feature is "pressure bumps", i.e. local maxima in the gas surface density distribution that can arise e.g. at the inner edge of the dead zone. Pressure bumps stop the inward migration of small bodies undergoing gas drag (Brauer et al., 2008), promote the onset of the streaming instability (Johansen and Youdin, 2007), help the accretion of planetary embryos by the pebble-accretion process (Lambrechts and Johansen, 2012) and stop inward type-I migration by the planet-trap mechanism (Masset et al., 2006). The second feature is "scale height bumps", that originate from opacity transitions. The regions of the disc that are shadowed, where H/r decreases with r, allow planetary cores to migrate outwards due to entropy gradient effects (Paardekooper and Mellema (2006), Baruteau and Masset (2008)), until they reach the local minimum of the H/r profile (Bitsch et al. 2013). Thus, it is important to model the existence and the location of these structures in realistic protoplanetary discs. The structure of the disc is dependent on the mass-flux (accretion rate) through the disc, which determines the evolution of the density profile. This mass-flux changes in time, as the whole disc gets accreted onto the central star. We will show using 2D hydrodynamical models how the change of the accretion rate affects the disc structure and how this will change the sweet-spots for saving planetary cores from too rapid inward migration. We will focus here on "scale height bumps" in the disc that will change the alpha-viscosity and consequently the gas surface density (as the mass-flux is constant through the disc). Therefore the formation of pressure bumps is possible, whose prominence and effects on migration will be investigated in detail. This will give important indications of where and when in the disc the cores of giant planets and thus giant planets can form.

  5. Accretion of the Moon from non-canonical discs.

    PubMed

    Salmon, J; Canup, R M

    2014-09-13

    Impacts that leave the Earth-Moon system with a large excess in angular momentum have recently been advocated as a means of generating a protolunar disc with a composition that is nearly identical to that of the Earth's mantle. We here investigate the accretion of the Moon from discs generated by such 'non-canonical' impacts, which are typically more compact than discs produced by canonical impacts and have a higher fraction of their mass initially located inside the Roche limit. Our model predicts a similar overall accretional history for both canonical and non-canonical discs, with the Moon forming in three consecutive steps over hundreds of years. However, we find that, to yield a lunar-mass Moon, the more compact non-canonical discs must initially be more massive than implied by prior estimates, and only a few of the discs produced by impact simulations to date appear to meet this condition. Non-canonical impacts require that capture of the Moon into the evection resonance with the Sun reduced the Earth-Moon angular momentum by a factor of 2 or more. We find that the Moon's semi-major axis at the end of its accretion is approximately 7R⊕, which is comparable to the location of the evection resonance for a post-impact Earth with a 2.5 h rotation period in the absence of a disc. Thus, the dynamics of the Moon's assembly may directly affect its ability to be captured into the resonance. PMID:25114307

  6. Accretion of the Moon from non-canonical discs

    PubMed Central

    Salmon, J.; Canup, R. M

    2014-01-01

    Impacts that leave the Earth–Moon system with a large excess in angular momentum have recently been advocated as a means of generating a protolunar disc with a composition that is nearly identical to that of the Earth's mantle. We here investigate the accretion of the Moon from discs generated by such ‘non-canonical’ impacts, which are typically more compact than discs produced by canonical impacts and have a higher fraction of their mass initially located inside the Roche limit. Our model predicts a similar overall accretional history for both canonical and non-canonical discs, with the Moon forming in three consecutive steps over hundreds of years. However, we find that, to yield a lunar-mass Moon, the more compact non-canonical discs must initially be more massive than implied by prior estimates, and only a few of the discs produced by impact simulations to date appear to meet this condition. Non-canonical impacts require that capture of the Moon into the evection resonance with the Sun reduced the Earth–Moon angular momentum by a factor of 2 or more. We find that the Moon's semi-major axis at the end of its accretion is approximately 7R⊕, which is comparable to the location of the evection resonance for a post-impact Earth with a 2.5 h rotation period in the absence of a disc. Thus, the dynamics of the Moon's assembly may directly affect its ability to be captured into the resonance. PMID:25114307

  7. Slim accretion discs with different viscosity prescriptions

    NASA Astrophysics Data System (ADS)

    Szuszkiewicz, Ewa

    1990-05-01

    The variability of X-ray sources powered by accretion may be connected to thermal instabilities in the innermost parts of slim disks. The time-scales of variability predicted by the theory with the standard alpha-viscosity prescription agree with those observed in a wide range of sources. The amplitudes (3-4 orders of magnitude in luiminosity) are correctly predicted for X-ray transient sources, but in general are too big for quasars, Seyferts, galactic blackhole candidates and LMXBs. It is shown that a slight modification of the viscosity prescription can offer a much better agreement with observations.

  8. Ubiquitous equatorial accretion disc winds in black hole soft states

    NASA Astrophysics Data System (ADS)

    Ponti, G.; Fender, R. P.; Begelman, M. C.; Dunn, R. J. H.; Neilsen, J.; Coriat, M.

    2012-05-01

    High-resolution spectra of Galactic black holes (GBHs) reveal the presence of highly ionized absorbers. In one GBH, accreting close to the Eddington limit for more than a decade, a powerful accretion disc wind is observed to be present in softer X-ray states and it has been suggested that it can carry away enough mass and energy to quench the radio jet. Here we report that these winds, which may have mass outflow rates of the order of the inner accretion rate or higher, are a ubiquitous component of the jet-free soft states of all GBHs. We furthermore demonstrate that these winds have an equatorial geometry with opening angles of few tens of degrees, and so are only observed in sources in which the disc is inclined at a large angle to the line of sight. The decrease in Fe XXV/Fe XXVI line ratio with Compton temperature, observed in the soft state, suggests a link between higher wind ionization and harder spectral shapes. Although the physical interaction between the wind, accretion flow and jet is still not fully understood, the mass flux and power of these winds and their presence ubiquitously during the soft X-ray states suggest they are fundamental components of the accretion phenomenon.

  9. Dust dynamics in 2D gravito-turbulent discs

    NASA Astrophysics Data System (ADS)

    Shi, Ji-Ming; Zhu, Zhaohuan; Stone, James M.; Chiang, Eugene

    2016-06-01

    The dynamics of solid bodies in protoplanetary discs are subject to the properties of any underlying gas turbulence. Turbulence driven by disc self-gravity shows features distinct from those driven by the magnetorotational instability (MRI). We study the dynamics of solids in gravito-turbulent discs with two-dimensional (in the disc plane), hybrid (particle and gas) simulations. Gravito-turbulent discs can exhibit stronger gravitational stirring than MRI-active discs, resulting in greater radial diffusion and larger eccentricities and relative speeds for large particles (those with dimensionless stopping times tstopΩ > 1, where Ω is the orbital frequency). The agglomeration of large particles into planetesimals by pairwise collisions is therefore disfavoured in gravito-turbulent discs. However, the relative speeds of intermediate-size particles (tstopΩ ˜ 1) are significantly reduced as such particles are collected by gas drag and gas gravity into coherent filament-like structures with densities high enough to trigger gravitational collapse. First-generation planetesimals may form via gravitational instability of dust in marginally gravitationally unstable gas discs.

  10. Arrangement of discs in 2d binary assemblies

    NASA Astrophysics Data System (ADS)

    Gervois, A.; Annic, C.; Lemaitre, J.; Ammi, M.; Oger, L.; Troadec, J.-P.

    1995-02-01

    We study arrangements of the two species of discs in binary assemblies at an intermediate scale. Small discs rearrange along large ones in clusters whose mass and compactness are analyzed with the tools of percolation. The assemblies are generated analogically on an air table or numerically from RSA or Powell algorithms. At a given packing fraction, an infinite cluster of small discs exists above a critical composition; a phenomenological expression for this threshold is proposed. Like in usual percolation problems, the number of inner links in a cluster is a linear function of its mass, with a slope depending both on the packing fraction, the composition of the mixture and the building procedure. An approximate expression is derived for it.

  11. Eclipsing the innermost accretion disc regions in AGN

    NASA Astrophysics Data System (ADS)

    Sanfrutos, M.; Miniutti, G.; Dovčiak, M.; Agís-González, B.

    2016-05-01

    Variable X-ray absorption has been observed in active galactic nuclei (AGN) on several time scales. Observations allow us to identify the absorber with clouds associated either with the clumpy torus (parsec scales, long timescales) or with the broad line region (BLR) (short timescales). In the latter, the cloud size has been estimated to be of the order of few gravitational radii from the observed absorption variability. Such small cloud sizes are comparable to the X-ray emitting regions so that a detailed modeling of occultation events in AGN has the potential of enabling us to infer accurately the geometry of the system. We have developed a relativistic X-ray spectral model for occultation events and we present here theoretical predictions on the different observables that can be inferred by studying X-ray eclipses in simulated XMM-Newton data. These include the size of the X-ray emitting regions as well as more fundamental parameters such as the black hole spin and the system inclination. We find that absorption varies as a function of the energy range and that its maximum takes place when the approaching part of the accretion disc is covered. Therefore we study the hard-to-soft (H / S) ratio light curves produced during an eclipse and use them to characterise the properties of the inner accretion disc in a new model-independent way.

  12. An accurate geometric distance to the compact binary SS Cygni vindicates accretion disc theory.

    PubMed

    Miller-Jones, J C A; Sivakoff, G R; Knigge, C; Körding, E G; Templeton, M; Waagen, E O

    2013-05-24

    Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 ± 12 parsecs measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source substantially closer at 114 ± 2 parsecs. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects. PMID:23704566

  13. Turbulence in Accretion Discs. The Global Baroclinic Instability

    NASA Astrophysics Data System (ADS)

    Klahr, Hubert; Bodenheimer, Peter

    The transport of angular momentum away from the central object is a sufficient condition for a protoplanetary disk to accrete matter onto the star and spin it down. Magnetic fields cannot be of importance for this process in a large part of the cold and dusty disk where the planets supposedly form. Our new hypothesis on the angular momentum transport based on radiation hydro simulations is as follows: We present the global baroclinic instability as a source for vigorous turbulence leading to angular momentum transport in Keplerian accretion disks. We show by analytical considerations and three-dimensional radiation hydro simulations that, in particular, protoplanetary disks have a negative radial entropy gradient, which makes them baroclinic. Two-dimensional numerical simulations show that this baroclinic flow is unstable and produces turbulence. These findings are currently tested for numerical effects by performing barotropic simulations which show that imposed turbulence rapidly decays. The turbulence in baroclinic disks draws energy from the background shear, transports angular momentum outward and creates a radially inward bound accretion of matter, thus forming a self consistent process. Gravitational energy is transformed into turbulent kinetic energy, which is then dissipated, as in the classical accretion paradigm. We measure accretion rates in 2D and 3D simulations of dot M= - 10-9 to -10-7 Msolar yr-1 and viscosity parameters of α = 10-4 - 10-2, which fit perfectly together and agree reasonably with observations. The turbulence creates pressure waves, Rossby waves, and vortices in the (r-φ) plane of the disk. We demonstrate in a global simulation that these vortices tend to form out of little background noise and to be long-lasting features, which have already been suggested to lead to the formation of planets.

  14. Accretion-disc precession in UX Ursae Majoris

    NASA Astrophysics Data System (ADS)

    de Miguel, E.; Patterson, J.; Cejudo, D.; Ulowetz, J.; Jones, J. L.; Boardman, J.; Barret, D.; Koff, R.; Stein, W.; Campbell, T.; Vanmunster, T.; Menzies, K.; Slauson, D.; Goff, W.; Roberts, G.; Morelle, E.; Dvorak, S.; Hambsch, F.-J.; Starkey, D.; Collins, D.; Costello, M.; Cook, M. J.; Oksanen, A.; Lemay, D.; Cook, L. M.; Ogmen, Y.; Richmond, M.; Kemp, J.

    2016-04-01

    We report the results of a long campaign of time series photometry on the nova-like variable UX Ursae Majoris during 2015. It spanned 150 nights, with ˜ 1800 h of coverage on 121 separate nights. The star was in its normal `high state' near magnitude V = 13, with slow waves in the light curve and eclipses every 4.72 h. Remarkably, the star also showed a nearly sinusoidal signal with a full amplitude of 0.44 mag and a period of 3.680 ± 0.007 d. We interpret this as the signature of a retrograde precession (wobble) of the accretion disc. The same period is manifest as a ±33 s wobble in the timings of mid-eclipse, indicating that the disc's centre of light moves with this period. The star also showed strong `negative superhumps' at frequencies ωorb + N and 2ωorb + N, where ωorb and N are, respectively, the orbital and precession frequencies. It is possible that these powerful signals have been present, unsuspected, throughout the more than 60 yr of previous photometric studies.

  15. Energy flows in thick accretion discs and their consequences for black hole feedback

    NASA Astrophysics Data System (ADS)

    Sądowski, Aleksander; Lasota, Jean-Pierre; Abramowicz, Marek A.; Narayan, Ramesh

    2016-03-01

    We study energy flows in geometrically thick accretion discs, both optically thick and thin, using general relativistic, three-dimensional simulations of black hole accretion flows. We find that for non-rotating black holes the efficiency of the total feedback from thick accretion discs is 3 per cent - roughly half of the thin disc efficiency. This amount of energy is ultimately distributed between outflow and radiation, the latter scaling weakly with the accretion rate for super-critical accretion rates, and returned to the interstellar medium. Accretion on to rotating black holes is more efficient because of the additional extraction of rotational energy. However, the jet component is collimated and likely to interact only weakly with the environment, whereas the outflow and radiation components cover a wide solid angle.

  16. HERO - A 3D general relativistic radiative post-processor for accretion discs around black holes

    NASA Astrophysics Data System (ADS)

    Zhu, Yucong; Narayan, Ramesh; Sadowski, Aleksander; Psaltis, Dimitrios

    2015-08-01

    HERO (Hybrid Evaluator for Radiative Objects) is a 3D general relativistic radiative transfer code which has been tailored to the problem of analysing radiation from simulations of relativistic accretion discs around black holes. HERO is designed to be used as a post-processor. Given some fixed fluid structure for the disc (i.e. density and velocity as a function of position from a hydrodynamic or magnetohydrodynamic simulation), the code obtains a self-consistent solution for the radiation field and for the gas temperatures using the condition of radiative equilibrium. The novel aspect of HERO is that it combines two techniques: (1) a short-characteristics (SC) solver that quickly converges to a self-consistent disc temperature and radiation field, with (2) a long-characteristics (LC) solver that provides a more accurate solution for the radiation near the photosphere and in the optically thin regions. By combining these two techniques, we gain both the computational speed of SC and the high accuracy of LC. We present tests of HERO on a range of 1D, 2D, and 3D problems in flat space and show that the results agree well with both analytical and benchmark solutions. We also test the ability of the code to handle relativistic problems in curved space. Finally, we discuss the important topic of ray defects, a major limitation of the SC method, and describe our strategy for minimizing the induced error.

  17. Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems

    NASA Astrophysics Data System (ADS)

    Moss, D.; Sokoloff, D.; Suleimanov, V.

    2016-04-01

    Context. Magnetic fields are important for accretion disc structure. Magnetic fields in a disc system may be transported with the accreted matter. They can be associated with either the central body and/or jet, and be fossil or dynamo excited in situ. Aims: We consider dynamo excitation of magnetic fields in accretion discs of accreting binary systems in an attempt to clarify possible configurations of dynamo generated magnetic fields. We first model the entire disc with realistic radial extent and thickness using an alpha-quenching non-linearity. We then study the simultaneous effect of feedback from the Lorentz force from the dynamo-generated field. Methods: We perform numerical simulations in the framework of a relatively simple mean-field model which allows the generation of global magnetic configurations. Results: We explore a range of possibilities for the dynamo number, and find quadrupolar-type solutions with irregular temporal oscillations that might be compared to observed rapid luminosity fluctuations. The dipolar symmetry models with Rα< 0 have lobes of strong toroidal field adjacent to the rotation axis that could be relevant to jet launching phenomena. Conclusions: We have explored and extended the solutions known for thin accretion discs.

  18. Inhomogeneous accretion discs and the soft states of black hole X-ray binaries

    NASA Astrophysics Data System (ADS)

    Dexter, Jason; Quataert, Eliot

    2012-10-01

    Observations of black hole binaries (BHBs) have established a rich phenomenology of X-ray states. The soft states range from the low variability, accretion disc dominated thermal (TD) state to the higher variability, non-thermal steep power law (SPL) state. The disc component in all states is typically modelled with standard thin disc accretion theory. However, this theory is inconsistent with optical/UV spectral, variability and gravitational microlensing observations of active galactic nuclei (AGNs), the supermassive analogues of BHBs. An inhomogeneous disc (ID) model with large (≃0.4 dex) temperature fluctuations in each radial annulus can qualitatively explain all of these AGN observations. The inhomogeneity may be a consequence of instabilities in radiation-dominated discs, and therefore may be present in BHBs as well. We show that ID models can explain many features of the TD and SPL states of BHBs. The observed relationships between spectral hardness, disc fraction and rms variability amplitude in BHBs are reproduced with temperature fluctuations similar to those inferred in AGNs, suggesting a unified picture of luminous accretion discs across orders of magnitude in black hole mass. This picture can be tested with spectral fitting of ID models, X-ray polarization observations and radiation magnetohydrodynamic simulations. If BHB accretion discs are indeed inhomogeneous, only the most disc-dominated states (disc fraction ≳0.95) can be used to robustly infer black hole spin using current continuum fitting methods.

  19. Accretion disc atmospheres and winds in low-mass X-ray binaries

    NASA Astrophysics Data System (ADS)

    Díaz Trigo, M.; Boirin, L.

    2016-05-01

    In the last decade, X-ray spectroscopy has enabled a wealth of discoveries of photoionised absorbers in X-ray binaries. Studies of such accretion disc atmospheres and winds are of fundamental importance to understand accretion processes and possible feedback mechanisms to the environment. In this work, we review the current observational state and theoretical understanding of accretion disc atmospheres and winds in low-mass X-ray binaries, focusing on the wind launching mechanisms and on the dependence on accretion state. We conclude with issues that deserve particular attention.

  20. Magnetised accretion discs in Kerr spacetimes. II. Hot spots

    NASA Astrophysics Data System (ADS)

    García, Federico; Ranea-Sandoval, Ignacio F.; Johannsen, Tim

    2016-03-01

    Context. Quasi-periodic variability has been observed in a number of X-ray binaries that harbor black hole candidates. In general relativity, black holes are uniquely described by the Kerr metric and, according to the cosmic censorship conjecture, curvature singularities always have to be clothed by an event horizon. Aims: In this paper, we study the observed light curves that arise from orbiting hotspots in thin accretion discs around Kerr black holes and naked singularities, and the effect introduced by the presence of an external magnetic field. Methods: We employ a ray-tracing algorithm to calculate the light curves and power spectra of these hot spots as seen by a distant observer for uniform and dipolar magnetic field configurations, assuming a weak coupling between the magnetic field and the disc matter. Results: We show that the presence of an external dipolar magnetic field leads to potentially observable modifications of these light curves for both Kerr black holes and naked singularities, while an external uniform magnetic field has practically no effect. In particular, we demonstrate that the emission from a hotspot, which is orbiting near the innermost stable circular orbit of a naked singularity in a dipolar magnetic field, can be significantly harder than the emission of the same hotspot in the absence of this type of magnetic field. Conclusions: The comparison of our model with observational data may allow us to study the geometry of magnetic fields around compact objects and to test the cosmic censorship conjecture in conjunction with other observables, such as thermal continuum spectra and iron line profiles.

  1. Magneto centrifugal winds from accretion discs around black hole binaries

    NASA Astrophysics Data System (ADS)

    Chakravorty, S.; Petrucci, P.; Ferreira, J.; Henri, G.

    2015-07-01

    X-ray observations of black hole X-ray binaries (BHBs) suggest that disc winds occur in the softer (disk-dominated) states of the outburst and are less prominent or absent in the harder (power-law dominated) states, which are more characterized by radio-loud jets. We investigate the presence/absence and physical characteristics of disk winds in BHBs through the use of the magneto-hydrodynamic (MHD) solutions of Ferreira (1997). These models treat accretion and ejection self-consistently within a self-similar ansatz that allows to solve the full set of dynamical MHD equations without neglecting any term. As a consequence the ejection efficiency is not a free parameter but depends on the global structure of the disk. By testing different sets of solutions with varying disk aspect ratio and ejection efficiency, we attempt to reproduce the observed state dependent prevalence of the winds. With no a priori theoretical assumption about the state of the black hole, we recover this observed bias of the winds for the softer states. In this talk I shall detail the methods employed by us, followed by the results.

  2. The growth of planets by pebble accretion in evolving protoplanetary discs

    NASA Astrophysics Data System (ADS)

    Bitsch, Bertram; Lambrechts, Michiel; Johansen, Anders

    2015-10-01

    The formation of planets depends on the underlying protoplanetary disc structure, which in turn influences both the accretion and migration rates of embedded planets. The disc itself evolves on time scales of several Myr, during which both temperature and density profiles change as matter accretes onto the central star. Here we used a detailed model of an evolving disc to determine the growth of planets by pebble accretion and their migration through the disc. Cores that reach their pebble isolation mass accrete gas to finally form giant planets with extensive gas envelopes, while planets that do not reach pebble isolation mass are stranded as ice giants and ice planets containing only minor amounts of gas in their envelopes. Unlike earlier population synthesis models, our model works without any artificial reductions in migration speed and for protoplanetary discs with gas and dust column densities similar to those inferred from observations. We find that in our nominal disc model, the emergence of planetary embryos preferably should occur after approximately 2 Myr in order to not exclusively form gas giants, but also ice giants and smaller planets. The high pebble accretion rates ensure that critical core masses for gas accretion can be reached at all orbital distances. Gas giant planets nevertheless experience significant reduction in semi-major axes by migration. Considering instead planetesimal accretion for planetary growth, we show that formation time scales are too long to compete with the migration time scales and the dissipation time of the protoplanetary disc. All in all, we find that pebble accretion overcomes many of the challenges in the formation of ice and gas giants in evolving protoplanetary discs. Appendices are available in electronic form at http://www.aanda.org

  3. Constraints on the temperature inhomogeneity in quasar accretion discs from the ultraviolet-optical spectral variability

    NASA Astrophysics Data System (ADS)

    Kokubo, Mitsuru

    2015-05-01

    The physical mechanisms of the quasar ultraviolet (UV)-optical variability are not well understood despite the long history of observations. Recently, Dexter & Agol presented a model of quasar UV-optical variability, which assumes large local temperature fluctuations in the quasar accretion discs. This inhomogeneous accretion disc model is claimed to describe not only the single-band variability amplitude, but also microlensing size constraints and the quasar composite spectral shape. In this work, we examine the validity of the inhomogeneous accretion disc model in the light of quasar UV-optical spectral variability by using five-band multi-epoch light curves for nearly 9 000 quasars in the Sloan Digital Sky Survey (SDSS) Stripe 82 region. By comparing the values of the intrinsic scatter σint of the two-band magnitude-magnitude plots for the SDSS quasar light curves and for the simulated light curves, we show that Dexter & Agol's inhomogeneous accretion disc model cannot explain the tight inter-band correlation often observed in the SDSS quasar light curves. This result leads us to conclude that the local temperature fluctuations in the accretion discs are not the main driver of the several years' UV-optical variability of quasars, and consequently, that the assumption that the quasar accretion discs have large localized temperature fluctuations is not preferred from the viewpoint of the UV-optical spectral variability.

  4. Magnetohydrodynamic Accretion Around Supermassive Black Holes : Short-Length Disc for Stronger Field

    NASA Astrophysics Data System (ADS)

    Biswas, Ritabrata

    2016-07-01

    Thin accretion flow, i.e., geometrically thin accretion disc was first studied by Shakura and Sunyaev. Relativistic fluid flows around a black hole produce enormous energy on the cost of permanent lost of the gravitational potential due to the fall into a infinitely sloped gravitational well or to be specific, into a space time singularity. This energy is actually observed in different wavelengths and we specify the source as Active Galactic Nuclei, quasars, Gamma-ray burst sources etc. Eventually, two popular kind of accretion disc models are there. The first one is advection dominated, known as geometrically thin optically thick accretion disc. The other is geometrically thick but optically thin as it does not capture photons inside! The jets formed by accretion phenomena are still not well explained. Size of the accretion disc, power of the jets can be powered by magnetic fields generated by the ionized particles of the accretion flow. We show the exact dependency of the disc size upon the magnetic field present along with the quantity of the central gravitating mass.

  5. Conditions for Circumstellar Disc Formation II: Effects of Initial Cloud Stability and Mass Accretion Rate

    NASA Astrophysics Data System (ADS)

    Machida, Masahiro N.; Matsumoto, Tomoaki; Inutsuka, Shu-ichiro

    2016-09-01

    Disc formation in strongly magnetized cloud cores is investigated using a three-dimensional magnetohydrodynamic simulation with a focus on the effects of the initial cloud stability and the mass accretion rate. The initial cloud stability greatly alters the disc formation process even for prestellar clouds with the same mass-to-flux ratio. A high mass accretion rate onto the disc-forming region is realized in initially unstable clouds, and a large angular momentum is introduced into the circumstellar region in a short time. The region around the protostar has both a thin infalling envelope and a weak magnetic field, which both weaken the effect of magnetic braking. The growth of the rotation-supported disc is promoted in such unstable clouds. Conversely, clouds in an initially near-equilibrium state show lower accretion rates of mass and angular momentum. The angular momentum is transported to the outer envelope before protostar formation. After protostar formation, the circumstellar region has a thick infalling envelope and a strong magnetic field that effectively brake the disc. As a result, disc formation is suppressed when the initial cloud is in a nearly stable state. The density distribution of the initial cloud also affects the disc formation process. Disc growth strongly depends on the initial conditions when the prestellar cloud has a uniform density, whereas there is no significant difference in the disc formation process in prestellar clouds with nonuniform densities.

  6. Star-disc interaction in galactic nuclei: orbits and rates of accreted stars

    NASA Astrophysics Data System (ADS)

    Kennedy, Gareth F.; Meiron, Yohai; Shukirgaliyev, Bekdaulet; Panamarev, Taras; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-07-01

    We examine the effect of an accretion disc on the orbits of stars in the central star cluster surrounding a central massive black hole by performing a suite of 39 high-accuracy direct N-body simulations using state-of-the art software and accelerator hardware, with particle numbers up to 128k. The primary focus is on the accretion rate of stars by the black hole (equivalent to their tidal disruption rate for black holes in the small to medium mass range) and the eccentricity distribution of these stars. Our simulations vary not only the particle number, but disc model (two models examined), spatial resolution at the centre (characterized by the numerical accretion radius) and softening length. The large parameter range and physically realistic modelling allow us for the first time to confidently extrapolate these results to real galactic centres. While in a real galactic centre both particle number and accretion radius differ by a few orders of magnitude from our models, which are constrained by numerical capability, we find that the stellar accretion rate converges for models with N ≥ 32k. The eccentricity distribution of accreted stars, however, does not converge. We find that there are two competing effects at work when improving the resolution: larger particle number leads to a smaller fraction of stars accreted on nearly circular orbits, while higher spatial resolution increases this fraction. We scale our simulations to some nearby galaxies and find that the expected boost in stellar accretion (or tidal disruption, which could be observed as X-ray flares) in the presence of a gas disc is about a factor of 10. Even with this boost, the accretion of mass from stars is still a factor of ∼100 slower than the accretion of gas from the disc. Thus, it seems accretion of stars is not a major contributor to black hole mass growth.

  7. Star-disc interaction in galactic nuclei: orbits and rates of accreted stars

    NASA Astrophysics Data System (ADS)

    Kennedy, Gareth F.; Meiron, Yohai; Shukirgaliyev, Bekdaulet; Panamarev, Taras; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-07-01

    We examine the effect of an accretion disc on the orbits of stars in the central star cluster surrounding a central massive black hole by performing a suite of 39 high-accuracy direct N-body simulations using state-of-the art software and accelerator hardware, with particle numbers up to 128k. The primary focus is on the accretion rate of stars by the black hole (equivalent to their tidal disruption rate for black holes in the small to medium mass range) and the eccentricity distribution of these stars. Our simulations vary not only the particle number, but disc model (two models examined), spatial resolution at the centre (characterized by the numerical accretion radius) and softening length. The large parameter range and physically realistic modelling allow us for the first time to confidently extrapolate these results to real galactic centres. While in a real galactic centre both particle number and accretion radius differ by a few orders of magnitude from our models, which are constrained by numerical capability, we find that the stellar accretion rate converges for models with N ≥ 32k. The eccentricity distribution of accreted stars, however, does not converge. We find that there are two competing effects at work when improving the resolution: larger particle number leads to a smaller fraction of stars accreted on nearly circular orbits, while higher spatial resolution increases this fraction. We scale our simulations to some nearby galaxies and find that the expected boost in stellar accretion (or tidal disruption, which could be observed as X-ray flares) in the presence of a gas disc is about a factor of 10. Even with this boost, the accretion of mass from stars is still a factor of ˜100 slower than the accretion of gas from the disc. Thus, it seems accretion of stars is not a major contributor to black hole mass growth.

  8. The Role of Physical Viscosity in Accretion Disc Dynamics in Close Binaries and AGN

    NASA Astrophysics Data System (ADS)

    Lanzafame, G.

    2008-04-01

    The role of turbulent physical viscosity is here considered as far as an accretion disc is concerned both in close binaries (CB) and around massive black holes in active galactic nuclei (AGN). The study has been performed via SPH simulations of disc models. Physical viscosity has been considered according to the Shakura and Sunjaev α prescription. Results show that physical viscosity supports and favours accretion disc formation in low compressibility models. Spiral shocks in the radial flux develop only in some high compressibility models. Physical viscosity efficiently supports mass, angular momentum and heat radial transport towards the compact primary star as well as the radial disc spread. Results show that compressibility-viscosity domains exist, where turbulent physical viscosity supports the accretion disc formation. A role also played by the injection kinematics at the inner Lagrangian point L1 is also found. A grid of physically viscous 3D SPH, axially symmetric, accretion disc simulations around black holes (BH) in AGN, have also been performed, paying also attention to the role of the specific angular momentum λ as an initial boundary condition at the disc outer edge. A shock front usually develops, according to assigned outer edge initial and boundary conditions, mainly due to the centrifugal barrier. Pairs of (λ, α) values exist, determining radial periodical oscillations in the shock front. Periodical outflows can develop from the subsonic post shock region close to the BH in some cases.

  9. The truncated and evolving inner accretion disc of the black hole GX 339-4

    NASA Astrophysics Data System (ADS)

    Plant, D. S.; Fender, R. P.; Ponti, G.; Muñoz-Darias, T.; Coriat, M.

    2015-01-01

    The nature of accretion onto stellar mass black holes in the low/hard state remains unresolved, with some evidence suggesting that the inner accretion disc is truncated and replaced by a hot flow. However, the detection of relativistic broadened Fe emission lines, even at relatively low luminosities, seems to require an accretion disc extending fully to its innermost stable circular orbit. Modelling such features is, however, highly susceptible to degeneracies, which could easily bias any interpretation. We present the first systematic study of the Fe line region to track how the inner accretion disc evolves in the low/hard state of the black hole GX 339-4. Our four observations display increased broadening of the Fe line over two magnitudes in luminosity, which we use to track any variation of the disc inner radius. We find that the disc extends closer to the black hole at higher luminosities, but is consistent with being truncated throughout the entire low/hard state, a result which renders black hole spin estimates inaccurate at these stages of the outburst. Furthermore, we show that the evolution of our spectral inner disc radius estimates corresponds very closely to the trend of the break frequency in Fourier power spectra, supporting the interpretation of a truncated and evolving disc in the hard state.

  10. Comparisons and connections between mean field dynamo theory and accretion disc theory

    NASA Astrophysics Data System (ADS)

    Blackman, E. G.

    2010-01-01

    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, understanding the key principles and practical modeling of observations warrants testable semi-analytic mean field theories that distill the essential physics. Mean field dynamo (MFD) theory and alpha-viscosity accretion disc theory exemplify this pursuit. That the latter is a mean field theory is not always made explicit but the combination of turbulence and global symmetry imply such. The more commonly explicit presentation of assumptions in 20th century textbook MFDT has exposed it to arguably more widespread criticism than incurred by 20th century alpha-accretion theory despite complementary weaknesses. In the 21st century however, MFDT has experienced a breakthrough with a dynamical saturation theory that consistently agrees with simulations. Such has not yet occurred in accretion disc theory, though progress is emerging. Ironically however, for accretion engines, MFDT and accretion theory are presently two artificially uncoupled pieces of what should be a single coupled theory. Large scale fields and accretion flows are dynamically intertwined because large scale fields likely play a key role in angular momentum transport. I discuss and synthesize aspects of recent progress in MFDT and accretion disc theory to suggest why the two likely conspire in a unified theory.

  11. Suppression of the accretion rate in thin discs around binary black holes.

    NASA Astrophysics Data System (ADS)

    Ragusa, Enrico; Lodato, Giuseppe; Price, Daniel J.

    2016-05-01

    We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with H/R ≳ 0.1, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs, in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on H/R) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of systems accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed after binary coalescence, thus influencing also the evolutionary path both of the binary and of the host galaxy. Our results, being scale-free, are also applicable to equal mass, circular binaries of stellar mass black holes, such as the progenitor of the recently discovered gravitational wave source GW150914.

  12. Suppression of the accretion rate in thin discs around binary black holes

    NASA Astrophysics Data System (ADS)

    Ragusa, Enrico; Lodato, Giuseppe; Price, Daniel J.

    2016-08-01

    We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with H/R ≳ 0.1, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs; in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on H/R) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of system accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed after binary coalescence, thus influencing also the evolutionary path both of the binary and of the host galaxy. Our results, being scale-free, are also applicable to equal-mass, circular binaries of stellar mass black holes, such as the progenitor of the recently discovered gravitational wave source GW150914.

  13. An accretion disc instability induced by a temperature sensitive α parameter

    NASA Astrophysics Data System (ADS)

    Potter, William J.; Balbus, Steven A.

    2014-06-01

    In the standard thin-disc formalism, the dimensionless α parameter is usually assumed to be constant. However, there are good theoretical reasons for believing, as well as evidence from simulations, that α is dependent on intrinsic disc properties. In this paper, we analyse the conditions for the stability of a thin accretion disc in which α is a function of the magnetic Prandtl number, the ratio of collisional viscosity to resistivity. In the inner disc, where the free electron opacity and radiation viscosity dominate, the disc is unstable if α is proportional to the magnetic Prandtl number with an exponent >0.5. This is within the range of values for the power-law index found in magnetohydrodynamic simulations with simple energetics. We calculate the evolution of the unstable disc within the α-formalism and show that the physically accessible solutions form a limit cycle, analogous to the behaviour seen in recurrent dwarf novae. It is noteworthy that the time-dependent global behaviour of the instability results in cyclic heating of the inner section of the disc, when parameters appropriate for an X-ray binary system are used. We calculate a model spectrum of the disc in the flaring and quiescent states and show that the behaviour is compatible with X-ray observations of the thermal accretion disc in flaring X-ray binary systems.

  14. Revealing the accretion disc corona in Mrk 335 with multi-epoch X-ray spectroscopy

    NASA Astrophysics Data System (ADS)

    Keek, L.; Ballantyne, D. R.

    2016-03-01

    Active galactic nuclei host an accretion disc with an X-ray producing corona around a supermassive black hole. In bright sources, such as the Seyfert 1 galaxy Mrk 335, reflection of the coronal emission off the accretion disc has been observed. Reflection produces spectral features such as an Fe Kα emission line, which allow for properties of the inner accretion disc and the corona to be constrained. We perform a multi-epoch spectral analysis of all XMM-Newton, Suzaku, and NuSTAR observations of Mrk 335, and we optimize our fitting procedure to unveil correlations between the Eddington ratio and the spectral parameters. We find that the disc's ionization parameter correlates strongly with the Eddington ratio: the inner disc is more strongly ionized at higher flux. The slope of the correlation is less steep than previously predicted. Furthermore, the cut-off of the power-law spectrum increases in energy with the Eddington ratio, whereas the reflection fraction exhibits a decrease. We interpret this behaviour as geometrical changes of the corona as a function of the accretion rate. Below ˜10 per cent of the Eddington limit, the compact and optically thick corona is located close to the inner disc, whereas at higher accretion rates the corona is likely optically thin and extends vertically further away from the disc surface. Furthermore, we find a soft excess that consists of two components. In addition to a contribution from reflection in low ionization states, a second component is present that traces the overall flux.

  15. A viscosity prescription for a self-gravitating accretion disc

    NASA Technical Reports Server (NTRS)

    Lin, D. N. C.; Pringle, J. E.

    1987-01-01

    A model for treating the transfer of angular momentum within a gaseous differentially rotating disc subject to gravitational instability is discussed in terms of an effective kinematic viscosity. It is assumed that even when matter in the disc is subject to self-gravitation, the instability does not necessarily lead directly to condensation of parts of the disc into self-gravitating bodies. Conditions under which the present model permits a similarity solution are discussed, and it is shown that the general solution tends to the similarity solution at large times.

  16. Nickel-rich outflows from accretion discs formed by the accretion-induced collapse of white dwarfs

    NASA Astrophysics Data System (ADS)

    Metzger, B. D.; Piro, A. L.; Quataert, E.

    2009-07-01

    A white dwarf (WD) approaching the Chandrasekhar mass may in several circumstances undergo accretion-induced collapse (AIC) to a neutron star (NS) before a thermonuclear explosion ensues. It has generally been assumed that such an AIC does not produce a detectable supernova (SN). If, however, the progenitor WD is rapidly rotating (as may be expected due to its prior accretion), a centrifugally supported disc forms around the NS upon collapse. We calculate the subsequent evolution of this accretion disc and its nuclear composition using time-dependent height-integrated simulations with initial conditions taken from the AIC calculations of Dessart and collaborators. Soon after its formation, the disc is cooled by neutrinos and its composition is driven neutron rich (electron fraction Ye ~ 0.1) by electron captures. However, as the disc viscously spreads, it is irradiated by neutrinos from the central proto-NS, which dramatically alters its neutron-to-proton ratio. We find that electron neutrino captures increase Ye to ~0.5 by the time that weak interactions in the disc freeze out. Because the disc becomes radiatively inefficient and begins forming α-particles soon after freeze out, powerful winds blow away most of the disc's remaining mass. These Ye ~ 0.5 outflows synthesize up to a few times 10-2Msolar in 56Ni. As a result, AIC may be accompanied by a radioactively powered SN-like transient that peaks on a time-scale of ~1 d. Since few intermediate mass elements are likely synthesized, these nickel-rich explosions should be spectroscopically distinct from other SNe. The time-scale, velocity and composition of the AIC transient can be modified if the disc wind sweeps up a ~0.1Msolar remnant disc created by a WD-WD merger; such an `enshrouded' AIC may account for sub-luminous, sub-Chandrasekhar Type I SNe. Optical transient surveys such as the Panoramic Survey Telescope and Rapid Response System and the Palomar Transient Factory should detect a few AIC transients per

  17. Spiral-driven accretion in protoplanetary discs. II. Self-similar solutions

    NASA Astrophysics Data System (ADS)

    Hennebelle, Patrick; Lesur, Geoffroy; Fromang, Sébastien

    2016-04-01

    Context. Accretion discs are ubiquitous in the Universe, and it is crucial to understand how angular momentum and mass are radially transported in these objects. Aims: Here, we study the role played by non-linear spiral patterns within hydrodynamical and non-self-gravitating accretion discs assuming that external disturbances such as infall onto the disc may trigger them. Methods: To do so, we computed self-similar solutions that describe discs in which a spiral wave propagates. These solutions present shocks and critical sonic points that were analyzed. Results: We calculated the wave structure for all allowed temperatures and for several spiral shocks. In particular, we inferred the angle of the spiral pattern, the stress it exerts on the disc, and the associated flux of mass and angular momentum as a function of temperature. We quantified the rate of angular momentum transport by means of the dimensionless α parameter. For the thickest disc we considered (corresponding to h/r values of about one-third), we found values of α as high as 0.1 that scaled with the temperature T such that α ∝ T3 / 2 ∝ (h/r)3. The spiral angle scales with the temperature as arctan(r/h). Conclusions: These solutions suggests that perturbations occurring at disc outer boundaries, such as perturbations due to infall motions, can propagate deep inside the disc and therefore should not be ignored, even when considering small radii.

  18. Precession and accretion in circumbinary discs: the case of HD 104237

    NASA Astrophysics Data System (ADS)

    Dunhill, A. C.; Cuadra, J.; Dougados, C.

    2015-04-01

    We present the results of smoothed particle hydrodynamics (SPH) simulations of the disc around the young, eccentric stellar binary HD 104237. We find that the binary clears out a large cavity in the disc, driving a significant eccentricity at the cavity edge. This then precesses around the binary at a rate of dot{\\varpi } = 0.48°Tb^{-1}, which for HD 104237 corresponds to a precession period of 40 years. We find that the accretion pattern into the cavity and on to the binary changes with this precession, resulting in a periodic accretion variability driven purely by the physical parameters of the binary and its orbit. For each star we find that this results in order of magnitude changes in the accretion rate. We also find that the accretion variability allows the primary to accrete gas at a higher rate than the secondary for approximately half of each precession period. Using a large number of three-body integrations of test particles orbiting different binaries, we find good agreement between the precession rate of a test particle and our SPH disc precession. These rates also agree very well with the precession rates predicted by the analytic theory of Leung & Lee, showing that their prescription can be accurately used to predict long-term accretion variability time-scales for eccentric binaries accreting from a disc. We discuss the implications of our result, and suggest that this process provides a viable way of preserving unequal-mass ratios in accreting eccentric binaries in both the stellar and supermassive black hole regimes.

  19. Infrared accretion disc mapping of the dwarf nova V2051 Ophiuchi in outburst and in quiescence

    NASA Astrophysics Data System (ADS)

    Wojcikiewicz, E.; Baptista, R.

    2014-10-01

    Dwarf novae are compact binaries where a late-type star (the secondary) fills its Roche lobe and transfers matter to a companion white dwarf (the primary) via an accretion disc. They show outbursts which recur on timescales of weeks to years, where the accretion disc brightens by factors 20 to 100 either due to a thermal-viscous instability in the disc (DI model) or to a burst of enhanced mass-transfer from the secondary (MTI model). We report time-series of fast photometry of the dwarf nova V2051 Oph in the J and H bands, obtained with the CAMIV at the 1.6 m telescope of Observatório Pico dos Dias/Brazil, during the decline of an outburst in 2005 June, and in 2008 when the object was in quiescence. We modeled the ellipsoidal variations caused by the secondary to infer its contribution to the J and H fluxes, and fitted stellar atmosphere models to find a photometric parallatic distance of d = (111± 14)pc. Front-back brightness asymmetries in J and H-band eclipse maps along the decline from the 2005 outburst suggest that the accretion disc had a non-negligible opening angle which decreased as the disc cooled down. The time evolution of the disc radial temperature distribution along the outburst decline shows a cooling wave which accelerates as is travels inwards - in contradiction to a basic prediction from the DI model.

  20. Accretion and magnetic field morphology around Class 0 stage protostellar discs

    NASA Astrophysics Data System (ADS)

    Seifried, D.; Banerjee, R.; Pudritz, R. E.; Klessen, R. S.

    2015-01-01

    We analyse simulations of turbulent, magnetized molecular cloud cores focusing on the formation of Class 0 stage protostellar discs and the physical conditions in their surroundings. We show that for a wide range of initial conditions Keplerian discs are formed in the Class 0 stage already. In particular, we show that even subsonic turbulent motions reduce the magnetic braking efficiency sufficiently in order to allow rotationally supported discs to form. We therefore suggest that already during the Class 0 stage the fraction of Keplerian discs is significantly higher than 50 per cent, consistent with recent observational trends but significantly higher than predictions based on simulations with misaligned magnetic fields, demonstrating the importance of turbulent motions for the formation of Keplerian discs. We show that the accretion of mass and angular momentum in the surroundings of protostellar discs occurs in a highly anisotropic manner, by means of a few narrow accretion channels. The magnetic field structure in the vicinity of the discs is highly disordered, revealing field reversals up to distances of 1000 au. These findings demonstrate that as soon as even mild turbulent motions are included, the classical disc formation scenario of a coherently rotating environment and a well-ordered magnetic field breaks down. Hence, it is highly questionable to assess the magnetic braking efficiency based on non-turbulent collapse simulation. We strongly suggest that, in addition to the global magnetic field properties, the small-scale accretion flow and detailed magnetic field structure have to be considered in order to assess the likelihood of Keplerian discs to be present.

  1. Self-similar evolution of self-gravitating viscous accretion discs

    NASA Astrophysics Data System (ADS)

    Illenseer, Tobias F.; Duschl, Wolfgang J.

    2015-06-01

    A new one-dimensional, dynamical model is proposed for geometrically thin, self-gravitating viscous accretion discs. The vertically integrated equations are simplified using the slow accretion limit and the monopole approximation with a time-dependent central point mass to account for self-gravity and accretion. It is shown that the system of partial differential equations can be reduced to a single non-linear advection diffusion equation which describes the time evolution of angular velocity. In order to solve the equation, three different turbulent viscosity prescriptions are considered. It is shown that for these parametrizations the differential equation allows for similarity transformations depending only on a single non-dimensional parameter. A detailed analysis of the similarity solutions reveals that this parameter is the initial power-law exponent of the angular velocity distribution at large radii. The radial dependence of the self-similar solutions is in most cases given by broken power laws. At small radii, the rotation law always becomes Keplerian with respect to the current central point mass. In the outer regions, the power-law exponent of the rotation law deviates from the Keplerian value and approaches asymptotically the value determined by the initial condition. It is shown that accretion discs with flatter rotation laws at large radii yield higher accretion rates. The methods are applied to self-gravitating accretion discs in active galactic nuclei. Fully self-gravitating discs are found to evolve faster than nearly Keplerian discs. The implications on supermassive black hole formation and Quasar evolution are discussed.

  2. Exact time-dependent solutions for the thin accretion disc equation: boundary conditions at finite radius

    NASA Astrophysics Data System (ADS)

    Tanaka, Takamitsu

    2011-01-01

    We discuss Green's function solutions of the equation for a geometrically thin, axisymmetric Keplerian accretion disc with a viscosity prescription ν∝Rn. The mathematical problem was solved by Lynden-Bell & Pringle for the special cases with boundary conditions of zero-viscous torque and zero mass flow at the disc centre. While it has been widely established that the observational appearance of astrophysical discs depends on the physical size of the central object(s), exact time-dependent solutions with boundary conditions imposed at finite radius have not been published for a general value of the power-law index n. We derive exact Green's function solutions that satisfy either a zero-torque or a zero-flux condition at a non-zero inner boundary Rin > 0, for an arbitrary initial surface density profile. Whereas the viscously dissipated power diverges at the disc centre for the previously known solutions with Rin= 0, the new solutions with Rin > 0 have finite expressions for the disc luminosity that agree, in the limit t→∞, with standard expressions for steady-state disc luminosities. The new solutions are applicable to the evolution of the innermost regions of thin accretion discs.

  3. Efficiency of gas cooling and accretion at the disc-corona interface

    NASA Astrophysics Data System (ADS)

    Armillotta, L.; Fraternali, F.; Marinacci, F.

    2016-08-01

    In star-forming galaxies, stellar feedback can have a dual effect on the circumgalactic medium both suppressing and stimulating gas accretion. The trigger of gas accretion can be caused by disc material ejected into the halo in the form of fountain clouds and by its interaction with the surrounding hot corona. Indeed, at the disc-corona interface, the mixing between the cold/metal-rich disc gas (T ≲ 104 K) and the hot coronal gas (T ≳ 106 K) can dramatically reduce the cooling time of a portion of the corona and produce its condensation and accretion. We studied the interaction between fountain clouds and corona in different galactic environments through parsec-scale hydrodynamical simulations, including the presence of thermal conduction, a key mechanism that influences gas condensation. Our simulations showed that the coronal gas condensation strongly depends on the galactic environment, in particular it is less efficient for increasing virial temperature/mass of the haloes where galaxies reside and it is fully ineffective for objects with virial masses larger than 1013M⊙. This result implies that the coronal gas cools down quickly in haloes with low-intermediate virial mass (Mvir ≲ 3 × 1012M⊙) but the ability to cool the corona decreases going from late-type to early-type disc galaxies, potentially leading to the switching off of accretion and the quenching of star formation in massive systems.

  4. Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources

    NASA Astrophysics Data System (ADS)

    Sutton, Andrew D.; Done, Chris; Roberts, Timothy P.

    2014-11-01

    Although attempts have been made to constrain the stellar types of optical counterparts to ultraluminous X-ray sources (ULXs), the detection of optical variability instead suggests that they may be dominated by reprocessed emission from X-rays which irradiate the outer accretion disc. Here, we report results from a combined X-ray and optical spectral study of a sample of ULXs, which were selected for having broadened disc-like X-ray spectra and known optical counterparts. We simultaneously fit optical and X-ray data from ULXs with a new spectral model of emission from an irradiated, colour-temperature-corrected accretion disc around a black hole, with a central Comptonizing corona. We find that the ULXs require reprocessing fractions of ˜10-3, which is similar to sub-Eddington thermal dominant state black hole binaries (BHBs), but less than has been reported for ULXs with soft ultraluminous X-ray spectra. We suggest that the reprocessing fraction may be due to the opposing effects of self-shielding in a geometrically thick supercritical accretion disc and reflection from far above the central black hole by optically thin material ejected in a natal super-Eddington wind. Then, the higher reprocessing fractions reported for ULXs with wind-dominated X-ray spectra may be due to enhanced scattering on to the outer disc via the stronger wind in these objects. Alternatively, the accretion discs in these ULXs may not be particularly geometrically thick, rather they may be similar in this regard to the thermal dominant state BHBs.

  5. Three-dimensional simulations of supercritical black hole accretion discs - luminosities, photon trapping and variability

    NASA Astrophysics Data System (ADS)

    Sądowski, Aleksander; Narayan, Ramesh

    2016-03-01

    We present a set of four three-dimensional, general relativistic, radiation magnetohydrodynamical simulations of black hole accretion at supercritical mass accretion rates, dot{M} > dot{M}_Edd. We use these simulations to study how disc properties are modified when we vary the black hole mass, the black hole spin, or the mass accretion rate. In the case of a non-rotating black hole, we find that the total efficiency is of the order of 3 per cent dot{M} c^2, approximately a factor of 2 less than the efficiency of a standard thin accretion disc. The radiation flux in the funnel along the axis is highly super-Eddington, but only a small fraction of the energy released by accretion escapes in this region. The bulk of the 3 per cent dot{M} c^2 of energy emerges farther out in the disc, either in the form of photospheric emission or as a wind. In the case of a black hole with a spin parameter of 0.7, we find a larger efficiency of about 8 per cent dot{M} c^2. By comparing the relative importance of advective and diffusive radiation transport, we show that photon trapping is effective near the equatorial plane. However, near the disc surface, vertical transport of radiation by diffusion dominates. We compare the properties of our fiducial three-dimensional run with those of an equivalent two-dimensional axisymmetric model with a mean-field dynamo. The latter simulation runs nearly 100 times faster than the three-dimensional simulation, and gives very similar results for time-averaged properties of the accretion flow, but does not reproduce the time-variability.

  6. VizieR Online Data Catalog: Wind-driving protostellar accretion discs (Salmeron+, 2011)

    NASA Astrophysics Data System (ADS)

    Salmeron, R.; Konigl, A.; Wardle, M.

    2011-10-01

    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 disc. We model this situation by considering a radially localized disc model in which the matter is everywhere well coupled to the field and the wind is the main repository of excess angular momentum. We consider stationary configurations in which magnetic diffusivity counters the shearing and advection of the magnetic field lines. In Wardle & Koenigl (1997, ASP Conf. Ser., 121, 561) we analysed the disc structure in the hydrostatic approximation (vertical motions neglected inside the disc) and presented exact disc/wind solutions for the ambipolar diffusivity regime. In Koenigl, Salmeron & Wardle (Paper I, 2010MNRAS.401..479K) we generalized the hydrostatic analysis to the Hall and Ohm diffusivity domains and used it to identify the disc parameter sub-regimes in which viable solutions with distinct physical properties can be expected to occur. In this paper we test the results of Paper I by deriving full numerical solutions (integrated through the sonic critical surface) of the disc equations in the Hall domain. (1 data file).

  7. Magneto centrifugal winds from accretion discs around black hole binaries

    NASA Astrophysics Data System (ADS)

    Chakravorty, S.; Petrucci, P.-O.; Ferreira, J.; Henri, G.; Belmont, R.; Clavel, M.; Corbel, S.; Rodriguez, J.; Coriat, M.; Drappeau, S.; Malzac, J.

    2016-05-01

    We want to test if self-similar magneto-hydrodynamic (MHD) accretion-ejection models can explain the observational results for accretion disk winds in BHBs. In our models, the density at the base of the outflow from the accretion disk is not a free parameter but is determined by solving the full set of dynamical MHD equations without neglecting any physical term. Different MHD solutions were generated for different values of (a) the disk aspect ratio (ǎrepsilon) and (b) the ejection efficiency (p). We generated two kinds of MHD solutions depending on the absence (cold solution) or presence (warm solution) of heating at the disk surface. The cold MHD solutions are found to be inadequate to account for winds due to their low ejection efficiency. The warm solutions can have sufficiently high values of p (\\gtrsim 0.1) which is required to explain the observed physical quantities in the wind. The heating (required at the disk surface for the warm solutions) could be due to the illumination which would be more efficient in the Soft state. We found that in the Hard state a range of ionisation parameter is thermodynamically unstable, which makes it impossible to have any wind at all, in the Hard state. Our results would suggest that a thermo-magnetic process is required to explain winds in BHBs.

  8. Accretion disc time lag distributions: applying CREAM to simulated AGN light curves

    NASA Astrophysics Data System (ADS)

    Starkey, D. A.; Horne, Keith; Villforth, C.

    2016-02-01

    Active galactic nuclei (AGN) vary in their brightness across all wavelengths. Moreover, longer wavelength ultraviolet-optical continuum light curves appear to be delayed with respect to shorter wavelength light curves. A simple way to model these delays is by assuming thermal reprocessing of a variable point source (a lamp post) by a blackbody accretion disc. We introduce a new method, CREAM (Continuum REprocessed AGN Markov Chain Monte Carlo), that models continuum variations using this lamp post model. The disc light curves lag the lamp post emission with a time delay distribution sensitive to the disc temperature-radius profile and inclination. We test CREAM's ability to recover both inclination and product of black hole mass and accretion rate {Mdot{M}}, and show that the code is also able to infer the shape of the driving light curve. CREAM is applied to synthetic light curves expected from 1000 s exposures of a 17th magnitude AGN with a 2-m telescope in Sloan g and i bands with Signal-to-Noise Ratio (SNR) of 500-900 depending on the filter and lunar phase. We also test CREAM on poorer quality g and i light curves with SNR = 100. We find in the high-SNR case that CREAM can recover the accretion disc inclination to within an uncertainty of 5° and an {Mdot{M}} to within 0.04 dex.

  9. Outflows from accretion discs formed in neutron star mergers: effect of black hole spin

    NASA Astrophysics Data System (ADS)

    Fernández, Rodrigo; Kasen, Daniel; Metzger, Brian D.; Quataert, Eliot

    2015-01-01

    The accretion disc that forms after a neutron star merger is a source of neutron-rich ejecta. The ejected material contributes to a radioactively powered electromagnetic transient, with properties that depend sensitively on the composition of the outflow. Here, we investigate how the spin of the black hole (BH) remnant influences mass ejection on the thermal and viscous time-scales. We carry out two-dimensional, time-dependent hydrodynamic simulations of merger remnant accretion discs including viscous angular momentum transport and approximate neutrino self-irradiation. The gravity of the spinning BH is included via a pseudo-Newtonian potential. We find that a disc around a spinning BH ejects more mass, up to a factor of several, relative to the non-spinning case. The enhanced mass-loss is due to energy release by accretion occurring deeper in the gravitational potential, raising the disc temperature and hence the rate of viscous heating in regions where neutrino cooling is ineffective. The mean electron fraction of the outflow increases moderately with BH spin due to a highly irradiated (though not neutrino-driven) wind component. While the bulk of the ejecta is still very neutron-rich, thus generating heavy r-process elements, the leading edge of the wind contains a small amount of Lanthanide-free material. This component can give rise to an ≲1 d blue optical `bump' in a kilonova light curve, even in the case of prompt BH formation, which may facilitate its detection.

  10. A test of truncation in the accretion discs of X-ray Binaries.

    NASA Astrophysics Data System (ADS)

    Eckersall, A.

    2016-06-01

    The truncated-disc model is generally used to help explain the change between the soft and hard states in X-ray Binaries, where the standard accretion disc is truncated in the inner regions and replaced by a radiatively inefficient accretion flow. There is still disagreement though in the extent of this truncation, particularly in at what point truncation begins. Here we analyze XMM EPIC-pn spectra in both the soft and hard states for a number of galactic XRBs, along with RGS data and the latest absorption and emission models to get an independent fit for the ISM column densities for each source. Specifically, we assume the 'canonical' model where the luminous accretion disc extends down to the innermost stable orbit at 6r_g, and construct a spectral model accounting for thermal, reflection and Compton processes ensuring consistent geometrical properties of the models. Rather than attempting to infer the inner disc location from spectral fitting and/or reflection models, we instead attempt a direct test of whether a consistent model will fit assuming no truncation. We discuss the implications for emission models of XRBs.

  11. Accretion disc mapping of the shortest period eclipsing binary SDSS J0926+36

    NASA Astrophysics Data System (ADS)

    Schlindwein, W.; Baptista, R.

    2014-10-01

    AM CVn stars are ultracompact binaries (P_{orb}< 65 min) where a hydrogen-deficient low-mass, degenerate donor star overfills its Roche lobe and transfers matter to a companion white dwarf via an accretion disc. SDSS J0926+36 is currently the only eclipsing AM CVn star and also the shortest period eclipsing binary known. Its light curve displays deep (˜ 2 mag) eclipses every 28.3 min, which last for ˜ 2 min, as well as ˜ 2 mag amplitude outbursts every ˜ 100-200 d. Superhumps were seen in its quiescent light curve in some occasions, probably as a reminiscence of a (in some cases undetected) previous outburst. Its eclipsing nature allows a unique opportunity to disentangle the emission from several different light sources, and to map the surface brightness distribution of its hydrogen-deficient accretion disc with the aid of maximum entropy eclipse mapping techniques. Here we report the eclipse mapping analysis of optical light curves of SDSS J0926+36, collected with the 2.4 m Liverpool Robotic Telescope, covering 20 orbits of the binary over 5 nights of observations between 2012 February and March. The object was in quiescence at all runs. Our data show no evidence of superhumps nor of orbital modulation due to anisotropic emission from a bright spot at disc rim. Accordingly, the average out-of-eclipse flux level is consistent with that of the superhump-subtracted previous light curves. We combined all runs to obtain an orbital light curve of improved S/N. The corresponding eclipse map shows a compact source at disc centre (T_{b}simeq 17000 K), a faint, cool accretion disc (˜ 4000 K) plus enhanced emission along the gas stream (˜ 6000 K) beyond the impact point at the outer disc rim, suggesting the occurrence of gas stream overflow at that epoch.

  12. Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons

    NASA Astrophysics Data System (ADS)

    Rosenblatt, Pascal; Charnoz, Sébastien; Dunseath, Kevin M.; Terao-Dunseath, Mariko; Trinh, Antony; Hyodo, Ryuki; Genda, Hidenori; Toupin, Stéven

    2016-08-01

    Phobos and Deimos, the two small satellites of Mars, are thought either to be asteroids captured by the planet or to have formed in a disc of debris surrounding Mars following a giant impact. Both scenarios, however, have been unable to account for the current Mars system. Here we use numerical simulations to suggest that Phobos and Deimos accreted from the outer portion of a debris disc formed after a giant impact on Mars. In our simulations, larger moons form from material in the denser inner disc and migrate outwards due to gravitational interactions with the disc. The resulting orbital resonances spread outwards and gather dispersed outer disc debris, facilitating accretion into two satellites of sizes similar to Phobos and Deimos. The larger inner moons fall back to Mars after about 5 million years due to the tidal pull of the planet, after which the two outer satellites evolve into Phobos- and Deimos-like orbits. The proposed scenario can explain why Mars has two small satellites instead of one large moon. Our model predicts that Phobos and Deimos are composed of a mixture of material from Mars and the impactor.

  13. Integrated accretion disc angular momentum removal and astrophysical jet acceleration mechanism

    NASA Astrophysics Data System (ADS)

    Bellan, P. M.

    2016-06-01

    Ions and neutrals in the weakly ionized plasma of an accretion disc are tightly bound because of the high ion-neutral collision frequency. A cluster of a statistically large number of ions and neutrals behaves as a fluid element having the charge of the ions and the mass of the neutrals. This fluid element is effectively a metaparticle having such an extremely small charge-to-mass ratio that its cyclotron frequency can be of the order of the Kepler angular frequency. In this case, metaparticles with a critical charge-to-mass ratio can have zero canonical angular momentum. Zero canonical angular momentum metaparticles experience no centrifugal force and spiral inwards towards the central body. Accumulation of these inward spiralling metaparticles near the central body produces radially and axially outward electric fields. The axially outward electric field drives an out-of-plane poloidal electric current along arched poloidal flux surfaces in the highly ionized volume outside the disc. This out-of-plane current and its associated magnetic field produce forces that drive bidirectional astrophysical jets flowing normal to and away from the disc. The poloidal electric current circuit removes angular momentum from the accreting mass and deposits this removed angular momentum at near infinite radius in the disc plane. The disc region is an electric power source (E\\cdot J <0) while the jet region is an electric power sink (E\\cdot J>0).

  14. Lithium and oxygen in globular cluster dwarfs and the early disc accretion scenario

    NASA Astrophysics Data System (ADS)

    Salaris, Maurizio; Cassisi, Santi

    2014-06-01

    A new scenario - early disc accretion - has recently been proposed to explain the discovery of multiple stellar populations in Galactic globular clusters. According to this model, the existence of well defined (anti-)correlations amongst light element abundances (i.e. C, N, O, Na) in the photospheres of stars belonging to the same cluster (and the associated helium enrichment) is caused by accretion of the ejecta of short-lived interacting massive binary systems (and single fast rotating massive stars) on fully convective pre-main sequence low- and very low-mass stars, during the early stages of the cluster evolution. In a previous paper we applied this scenario to the cluster NGC 2808, showing how the knowledge of the He abundance of its triple main sequence can constrain both the He abundance of the accreted matter and the accretion efficiency. Here we have investigated the constraints provided by considering simultaneously the observed spread of lithium and oxygen (and when possible also sodium) abundances for samples of turn-off stars in NGC 6752, NGC 6121 (M4), and NGC 104 (47Tuc), and the helium abundance of their multiple main sequences. These observations provide a very powerful test of the accretion scenario, because the observed O, Li, and He abundance distributions at the turn-off can be used to constrain the composition (and mass) of the accreted matter and the timescales of the polluting stars. In the case of NGC 6752, we could not find a physically consistent solution. If early disc accretion does happen, observations point towards accretion of gas with a non-negligible Li abundance, contrary to the expectations for the ejecta of the "natural" polluters in this scenario. For M4, spectroscopic errors are too large compared to the intrinsic spread, to constrain the properties of the accreted matter. As for 47Tuc, we could find a physically consistent solution for the abundances of He and O (and Na) in the accreted gas and predict the abundances of these

  15. HEROIC: 3D general relativistic radiative post-processor with comptonization for black hole accretion discs

    NASA Astrophysics Data System (ADS)

    Narayan, Ramesh; Zhu, Yucong; Psaltis, Dimitrios; Saḑowski, Aleksander

    2016-03-01

    We describe Hybrid Evaluator for Radiative Objects Including Comptonization (HEROIC), an upgraded version of the relativistic radiative post-processor code HERO described in a previous paper, but which now Includes Comptonization. HEROIC models Comptonization via the Kompaneets equation, using a quadratic approximation for the source function in a short characteristics radiation solver. It employs a simple form of accelerated lambda iteration to handle regions of high scattering opacity. In addition to solving for the radiation field, HEROIC also solves for the gas temperature by applying the condition of radiative equilibrium. We present benchmarks and tests of the Comptonization module in HEROIC with simple 1D and 3D scattering problems. We also test the ability of the code to handle various relativistic effects using model atmospheres and accretion flows in a black hole space-time. We present two applications of HEROIC to general relativistic magnetohydrodynamics simulations of accretion discs. One application is to a thin accretion disc around a black hole. We find that the gas below the photosphere in the multidimensional HEROIC solution is nearly isothermal, quite different from previous solutions based on 1D plane parallel atmospheres. The second application is to a geometrically thick radiation-dominated accretion disc accreting at 11 times the Eddington rate. Here, the multidimensional HEROIC solution shows that, for observers who are on axis and look down the polar funnel, the isotropic equivalent luminosity could be more than 10 times the Eddington limit, even though the spectrum might still look thermal and show no signs of relativistic beaming.

  16. A transition in circumbinary accretion discs at a binary mass ratio of 1:25

    NASA Astrophysics Data System (ADS)

    D'Orazio, Daniel J.; Haiman, Zoltán; Duffell, Paul; MacFadyen, Andrew; Farris, Brian

    2016-07-01

    We study circumbinary accretion discs in the framework of the restricted three-body problem (R3Bp) and via numerically solving the height-integrated equations of viscous hydrodynamics. Varying the mass ratio of the binary, we find a pronounced change in the behaviour of the disc near mass ratio q ≡ Ms/Mp ˜ 0.04. For mass ratios above q = 0.04, solutions for the hydrodynamic flow transition from steady, to strongly fluctuating; a narrow annular gap in the surface density around the secondary's orbit changes to a hollow central cavity; and a spatial symmetry is lost, resulting in a lopsided disc. This phase transition is coincident with the mass ratio above which stable orbits do not exist around the L4 and L5 equilibrium points of the R3Bp. Using the DISCO code, we find that for thin discs, for which a gap or cavity can remain open, the mass ratio of the transition is relatively insensitive to disc viscosity and pressure. The q = 0.04 transition has relevance for the evolution of massive black hole binary+disc systems at the centres of galactic nuclei, as well as for young stellar binaries and possibly planets around brown dwarfs.

  17. Evolution of accretion discs around a kerr black hole using extended magnetohydrodynamics

    NASA Astrophysics Data System (ADS)

    Foucart, Francois; Chandra, Mani; Gammie, Charles F.; Quataert, Eliot

    2016-02-01

    Black holes accreting well below the Eddington rate are believed to have geometrically thick, optically thin, rotationally supported accretion discs in which the Coulomb mean free path is large compared to GM/c2. In such an environment, the disc evolution may differ significantly from ideal magnetohydrodynamic (MHD) predictions. We present non-ideal global axisymmetric simulations of geometrically thick discs around a rotating black hole. The simulations are carried out using a new code GRIM, which evolves a covariant extended magnetohydrodynamics model derived by treating non-ideal effects as a perturbation of ideal MHD. Non-ideal effects are modelled through heat conduction along magnetic field lines, and a difference between the pressure parallel and perpendicular to the field lines. The model relies on an effective collisionality in the disc from wave-particle scattering and velocity-space (mirror and firehose) instabilities. We find that the pressure anisotropy grows to match the magnetic pressure, at which point it saturates due to the mirror instability. The pressure anisotropy produces outward angular momentum transport with a magnitude comparable to that of MHD turbulence in the disc, and a significant increase in the temperature in the wall of the jet. We also find that, at least in our axisymmetric simulations, conduction has a small effect on the disc evolution because (1) the heat flux is constrained to be parallel to the field and the field is close to perpendicular to temperature gradients, and (2) the heat flux is choked by an increase in effective collisionality associated with the mirror instability.

  18. The imprint of satellite accretion on the chemical and dynamical properties of disc galaxies

    NASA Astrophysics Data System (ADS)

    Ruiz-Lara, T.; Few, C. G.; Gibson, B. K.; Pérez, I.; Florido, E.; Minchev, I.; Sánchez-Blázquez, P.

    2016-02-01

    Aims: We study the effects of the cosmological assembly history on the chemical and dynamical properties of the discs of spiral galaxies as a function of radius. Methods: We made use of the simulated Milky Way mass, fully-cosmological discs from Ramses Disc Environment Study (RaDES). We analysed their assembly history by examining the proximity of satellites to the galactic disc, instead of their merger trees, to better gauge which satellites impact the disc. We presented stellar age and metallicity profiles, age-metallicity relation (AMR), age-velocity dispersion relation (AVR), and stellar age distribution (SAD) in several radial bins for the simulated galaxies. Results: Assembly histories can be divided into three different stages: i) a merger dominated phase, when a large number of mergers with mass ratios of ~1:1 take place (lasting ~3.2 ± 0.4 Gyr on average); ii) a quieter phase, when ~1:10 mergers take place (lasting ~4.4 ± 2.0 Gyr); and iii) a secular phase where the few mergers that take place have mass ratios below 1:100, which do not affect the disc properties (lasting ~5.5 ± 2.0 Gyr). The first two phases are able to kinematically heat the disc and produce a disc that is chemically mixed over its entire radial extension. Phase 2 ends with a final merger event (at time tjump) marking the onset of important radial differences in the AMR, AVR, and SAD. Conclusions: Inverted AMR trends in the outer parts of discs, for stars younger than tjump, are found as the combined effect of radial motions and star formation in satellites temporarily located in these outer parts. U-shaped stellar age profiles change to an old plateau (~10 Gyr) in the outer discs for the entire RaDES sample. This shape is a consequence of inside-out growth of the disc, radial motions of disc stars (inwards and outwards), and the accretion of old stars from satellites. We see comparable age profiles even when ignoring the influence of stellar migration due to the presence of early in

  19. Modelling accretion disc and stellar wind interactions: the case of Sgr A*

    NASA Astrophysics Data System (ADS)

    Christie, I. M.; Petropoulou, M.; Mimica, P.; Giannios, D.

    2016-07-01

    Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ˜108 cm s-1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 1033 erg s-1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of dot{M}_w= 10^{-7} M_{⊙} yr^{-1}, nd = 105 cm-3, and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ˜3000 gravitational radii from the supermassive black hole.

  20. Infalling clouds on to supermassive black hole binaries - I. Formation of discs, accretion and gas dynamics

    NASA Astrophysics Data System (ADS)

    Goicovic, F. G.; Cuadra, J.; Sesana, A.; Stasyszyn, F.; Amaro-Seoane, P.; Tanaka, T. L.

    2016-01-01

    There is compelling evidence that most - if not all - galaxies harbour a supermassive black hole (SMBH) at their nucleus; hence binaries of these massive objects are an inevitable product of the hierarchical evolution of structures in the Universe, and represent an important but thus-far elusive phase of galaxy evolution. Gas accretion via a circumbinary disc is thought to be important for the dynamical evolution of SMBH binaries, as well as in producing luminous emission that can be used to infer their properties. One plausible source of the gaseous fuel is clumps of gas formed due to turbulence and gravitational instabilities in the interstellar medium, that later fall towards and interact with the binary. In this context, we model numerically the evolution of turbulent clouds in near-radial infall on to equal-mass SMBH binaries, using a modified version of the SPH (smoothed particle hydrodynamics) code GADGET-3. We present a total of 12 simulations that explore different possible pericentre distances and relative inclinations, and show that the formation of circumbinary discs and discs around each SMBH (`mini-discs') depend on those parameters. We also study the dynamics of the formed discs, and the variability of the feeding rate on to the SMBHs in the different configurations.

  1. Kinematics and excitation of the molecular hydrogen accretion disc in NGC 1275

    NASA Astrophysics Data System (ADS)

    Scharwächter, J.; McGregor, P. J.; Dopita, M. A.; Beck, T. L.

    2013-03-01

    We report the results of high spatial and spectral resolution integral-field spectroscopy of the central ˜3 × 3 arcsec2 of the active galaxy NGC 1275 (Perseus A), based on observations with the Near-infrared Integral Field Spectrograph (NIFS) and the ALTitude conjugate Adaptive optics for the InfraRed (ALTAIR) adaptive-optics system on the Gemini North telescope. The circum-nuclear disc in the inner R ˜ 50 pc of NGC 1275 is seen in both the H2 and [Fe II] lines. The disc is interpreted as the outer part of a collisionally excited turbulent accretion disc. The kinematic major axis of the disc at a position angle of 68° is oriented perpendicular to the radio jet. A streamer-like feature to the south-west of the disc, detected in H2 but not in [Fe II], is discussed as one of possibly several molecular streamers, presumably falling into the nuclear region. Indications of an ionization structure within the disc are deduced from the He I and Brγ emission lines, which may partially originate from the inner portions of the accretion disc. The kinematics of these two lines agrees with the signature of the circum-nuclear disc, but both lines display a larger central velocity dispersion than the H2 line. The ro-vibrational H2 transitions from the core of NGC 1275 are indicative of thermal excitation caused by shocks and agree with excitation temperatures of ˜1360 and ˜4290 K for the lower and higher energy H2 transitions, respectively. The data suggest X-ray heating as the dominant excitation mechanism of [Fe II] emission in the core, while fast shocks are a possible alternative. The [Fe II] lines indicate an electron density of ˜4000 cm-3. The H2 disc is modelled using simulated NIFS data cubes of H2 emission from inclined discs in Keplerian rotation around a central mass. Assuming a disc inclination of 45° ± 10°, the best-fitting models imply a central mass of (8 + 7 - 2) × 108 M⊙. Taken as a black hole mass estimate, this value is larger than previous

  2. X-Ray Evidence for the Accretion Disc-Outflow Connection in 3C 111

    NASA Technical Reports Server (NTRS)

    Tombesi, Frank; Sambruna, R. M.; Reeves, J. N.; Reynolds, C. S.; Braito, V.

    2011-01-01

    We present the spectral analysis of three Suzaku X-ray Imaging Spectrometer observations of 3C III requested to monitor the predicted variability of its ultrafast outflow on approximately 7 d time-scales. We detect an ionized iron emission line in the first observation and a blueshifted absorption line in the second, when the flux is approximately 30 per cent higher. The location of the material is constrained at less than 0.006 pc from the variability. Detailed modelling supports an identification with ionized reflection off the accretion disc at approximately 20-100rg from the black hole and a highly ionized and massive ultrafast outflow with velocity approximately 0.1c, respectively. The outflow is most probably accelerated by radiation pressure, but additional magnetic thrust cannot be excluded. The measured high outflow rate and mechanical energy support the claims that disc outflows may have a significant feedback role. This work provides the first direct evidence for an accretion disc-outflow connection in a radio-loud active galactic nucleus, possibly linked also to the jet activity.

  3. Emission line eclipse mapping of velocity fields in dwarf nova accretion discs

    NASA Astrophysics Data System (ADS)

    Makita, M.; Mineshige, S.

    2002-01-01

    We propose a new method, emission-line eclipse mapping, to map the velocity fields in an accretion disc. We apply the usual eclipse mapping technique to the light curves at each of 12-24 wavelengths across the line center to map the region with same line-of-sight velocity, from which we are able to plot the rotational velocity as a function of radius on the assumption of axisymmetric disc. We calculate time changes of the emission line profiles, assuming Keplerian rotation fields (vvarphi propto r-1/2) and the emissivity distribution of j propto r-3/2, and reconstruct emissivity profiles. The results show typically a `two-eye' pattern for high line-of-sight velocities and we can recover the relation, vvarphi propto d-1/2, where d is the separation of two lq eyes.'

  4. Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

    NASA Astrophysics Data System (ADS)

    Chattopadhyay, Indranil; Kumar, Rajiv

    2016-04-01

    We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion-ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von-Zeipel surfaces and projected the jet equations of motion onto the streamline and solved them simultaneously with the accretion disc equations of motion. We found that steady shock cannot exist beyond α ≲ 0.06 in the general relativistic prescription, but is lower if massloss is considered too. We showed that for fixed outer boundary, the shock moves closer to the horizon with increasing viscosity parameter. The mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock. The jet terminal speed increases with stronger shocks, quantitatively speaking, the terminal speed of jets vj∞ > 0.1 if rsh < 20rg. The maximum of the outflow rate obtained in the general relativistic regime is less than 6% of the mass accretion rate.

  5. Flickering of accreting white dwarfs: the remarkable amplitude-flux relation and disc viscosity

    NASA Astrophysics Data System (ADS)

    Zamanov, R. K.; Boeva, S.; Latev, G.; Sokoloski, J. L.; Stoyanov, K. A.; Genkov, V.; Tsvetkova, S. V.; Tomov, T.; Antov, A.; Bode, M. F.

    2016-03-01

    We analyse optical photometric data of short term variability (flickering) of accreting white dwarfs in cataclysmic variables (KR Aur, MV Lyr, V794 Aql, TT Ari, V425 Cas), recurrent novae (RS Oph and T CrB) and jet-ejecting symbiotic stars (CH Cyg and MWC 560). We find that the amplitude-flux relationship is visible over four orders of magnitude, in the range of fluxes from 1029 to 1033 erg s-1 Å-1, as a `statistically perfect correlation with correlation coefficient 0.96 and p-value ˜10-28. In the above range, the amplitude of variability for any of our 9 objects is proportional to the flux level with (almost) one and the same factor of proportionality for all nine accreting white dwarfs with ΔF = 0.36(±0.05)Fav, σrms = 0.086(±0.011)Fav, and σrms/ΔF = 0.24 ± 0.02. Overall, our results indicate that the viscosity in the accretion discs is practically the same for all nine objects in our sample, in the mass accretion rate range 2 × 10-11 - 2 × 10-7 M⊙ yr-1.

  6. Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

    NASA Astrophysics Data System (ADS)

    Chattopadhyay, Indranil; Kumar, Rajiv

    2016-07-01

    We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion-ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von Zeipel surfaces and projected the jet equations of motion on to the streamline and solved them simultaneously with the accretion disc equations of motion. We found that steady shock cannot exist beyond α ≳ 0.06 in the general relativistic prescription, but is lower if mass-loss is considered too. We showed that for fixed outer boundary, the shock moves closer to the horizon with increasing viscosity parameter. The mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock location. The jet terminal speed increases with stronger shocks; quantitatively speaking, the terminal speed of jets vj∞ > 0.1 if rsh < 20rg. The maximum of the outflow rate obtained in the general relativistic regime is less than 6 per cent of the mass accretion rate.

  7. Powerful radiative jets in supercritical accretion discs around non-spinning black holes

    NASA Astrophysics Data System (ADS)

    Sądowski, Aleksander; Narayan, Ramesh

    2015-11-01

    We describe a set of simulations of supercritical accretion on to a non-rotating supermassive black hole (BH). The accretion flow takes the form of a geometrically thick disc with twin low-density funnels around the rotation axis. For accretion rates {gtrsim } 10 dot{M}_Edd, there is sufficient gas in the funnel to make this region optically thick. Radiation from the disc first flows into the funnel, after which it accelerates the optically thick funnel gas along the axis. The resulting jet is baryon loaded and has a terminal density-weighted velocity ≈0.3c. Much of the radiative luminosity is converted into kinetic energy by the time the escaping gas becomes optically thin. These jets are not powered by BHrotation or magnetic driving, but purely by radiation. Their characteristic beaming angle is ˜0.2 rad. For an observer viewing down the axis, the isotropic equivalent luminosity of total energy is as much as 1048 erg s- 1 for a 107 M⊙ BH accreting at 103 Eddington. Therefore, energetically, the simulated jets are consistent with observations of the most powerful tidal disruption events, e.g. Swift J1644. The jet velocity is, however, too low to match the Lorentz factor γ > 2 inferred in J1644. There is no such conflict in the case of other tidal disruption events. Since favourably oriented observers see isotropic equivalent luminosities that are highly super-Eddington, the simulated models can explain observations of ultraluminous X-ray sources, at least in terms of luminosity and energetics, without requiring intermediate-mass BHs.

  8. X-ray variability of SS 433: effects of the supercritical accretion disc

    NASA Astrophysics Data System (ADS)

    Atapin, Kirill; Fabrika, Sergei; Medvedev, Aleksei; Vinokurov, Alexander

    2015-01-01

    We study a stochastic variability of SS 433 in the 10-4-5 × 10-2 Hz frequency range based on RXTE data, and on simultaneous observations with RXTE and optical telescopes. We find that the cross-correlation functions and power spectra depend drastically on the precession phase of the supercritical accretion disc. When the wind funnel of the disc is maximally open to the observer, a flat part emerges in the power spectrum; a break is observed at the frequency 1.7 × 10-3 Hz, with a power-law index β ≈ 1.67 at higher frequencies. The soft emission forming mostly in the jets lags behind the hard and optical emission. When the observer does not see the funnel and jets (the `edge-on' disc), the power spectrum is described by a single power-law with β ≈ 1.34 and no correlations between X-ray ranges are detected. We investigated two mechanisms to explain the observed variability at the open disc phase: (1) reflection of radiation at the funnel wall (X-rays and optical) and (2) the gas cooling in the jets (X-rays only). The X-ray variability is determined by the contribution of both mechanisms; however, the contribution of the jets is much higher. We found that the funnel size is (2-2.5) × 1012 cm, and the opening angle is ϑf ˜ 50°. X-ray jets may consist of three fractions with different densities: 8 × 1013, 3 × 1013 and 5 × 1011 cm-3, with most of the jet's mass falling within the latter fraction. We suppose that revealed flat part in the power spectrum may be related to an abrupt change in the disc structure and viscous time-scale at the spherization radius, because the accretion disc becomes thick at this radius, h/r ˜ 1. The extent of the flat spectrum depends on the variation of viscosity at the spherization radius.

  9. Collisionless kinetic regimes for quasi-stationary axisymmetric accretion disc plasmas

    SciTech Connect

    Cremaschini, C.; Tessarotto, M.

    2012-08-15

    This paper is concerned with the kinetic treatment of quasi-stationary axisymmetric collisionless accretion disc plasmas. The conditions of validity of the kinetic description for non-relativistic magnetized and gravitationally bound plasmas of this type are discussed. A classification of the possible collisionless plasma regimes which can arise in these systems is proposed, which can apply to accretion discs around both stellar-mass compact objects and galactic-center black holes. Two different classifications are determined, which are referred to, respectively, as energy-based and magnetic field-based classifications. Different regimes are pointed out for each plasma species, depending both on the relative magnitudes of kinetic and potential energies and the magnitude of the magnetic field. It is shown that in all cases, there can be quasi-stationary Maxwellian-like solutions of the Vlasov equation. The perturbative approach outlined here permits unique analytical determination of the functional form for the distribution function consistent, in each kinetic regime, with the explicit inclusion of finite Larmor radius-diamagnetic and/or energy-correction effects.

  10. Modelling accretion disc and stellar wind interactions: the case of Sgr A*

    PubMed Central

    Christie, I. M.; Petropoulou, M.; Mimica, P.; Giannios, D.

    2016-01-01

    Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin–Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ∼108 cm s−1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 1033 erg s−1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{upgreek} \\usepackage{mathrsfs} \\setlength{\\oddsidemargin}{-69pt} \\begin{document} }{}$\\dot{M}_{\\rm w}= 10^{-7} \\,\\mathrm{M}_{\\odot }\\, {\\rm yr}^{-1}$\\end{document}, nd = 105 cm−3, and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ∼3000 gravitational radii from the supermassive black hole. PMID:27279781

  11. Mass-loss from advective accretion disc around rotating black holes

    NASA Astrophysics Data System (ADS)

    Aktar, Ramiz; Das, Santabrata; Nandi, Anuj

    2015-11-01

    We examine the properties of the outflowing matter from an advective accretion disc around a spinning black hole. During accretion, rotating matter experiences centrifugal pressure-supported shock transition that effectively produces a virtual barrier around the black hole in the form of post-shock corona (hereafter PSC). Due to shock compression, PSC becomes hot and dense that eventually deflects a part of the inflowing matter as bipolar outflows because of the presence of extra thermal gradient force. In our approach, we study the outflow properties in terms of the inflow parameters, namely specific energy (E) and specific angular momentum (λ) considering the realistic outflow geometry around the rotating black holes. We find that spin of the black hole (ak) plays an important role in deciding the outflow rate R_{dot{m}} (ratio of mass flux of outflow to inflow); in particular, R_{dot{m}} is directly correlated with ak for the same set of inflow parameters. It is found that a large range of the inflow parameters allows global accretion-ejection solutions, and the effective area of the parameter space (E, λ) with and without outflow decreases with black hole spin (ak). We compute the maximum outflow rate (R^{max}_{dot{m}}) as a function of black hole spin (ak) and observe that R^{max}_{dot{m}} weakly depends on ak that lies in the range ˜10-18 per cent of the inflow rate for the adiabatic index (γ) with 1.5 ≥ γ ≥ 4/3. We present the observational implication of our approach while studying the steady/persistent jet activities based on the accretion states of black holes. We discuss that our formalism seems to have the potential to explain the observed jet kinetic power for several Galactic black hole sources and active galactic nuclei.

  12. The central parsecs of M87: jet emission and an elusive accretion disc

    NASA Astrophysics Data System (ADS)

    Prieto, M. A.; Fernández-Ontiveros, J. A.; Markoff, S.; Espada, D.; González-Martín, O.

    2016-04-01

    We present the first simultaneous spectral energy distribution (SED) of M87 core at a scale of 0.4 arcsec ( ˜ 32 pc) across the electromagnetic spectrum. Two separate, quiescent, and active states are sampled that are characterized by a similar featureless SED of power-law form, and that are thus remarkably different from that of a canonical active galactic nuclei or a radiatively inefficient accretion source. We show that the emission from a jet gives an excellent representation of the core of M87 core covering ten orders of magnitude in frequency for both the active and the quiescent phases. The inferred total jet power is, however, one to two orders of magnitude lower than the jet mechanical power reported in the literature. The maximum luminosity of a thin accretion disc allowed by the data yields an accretion rate of < 6 × 10- 5 M⊙ yr- 1, assuming 10 per cent efficiency. This power suffices to explain M87 radiative luminosity at the jet frame, it is however two to three order of magnitude below that required to account for the jet's kinetic power. The simplest explanation is variability, which requires the core power of M87 to have been two to three orders of magnitude higher in the last 200 yr. Alternatively, an extra source of power may derive from black hole spin. Based on the strict upper limit on the accretion rate, such spin power extraction requires an efficiency an order of magnitude higher than predicted from magnetohydrodynamic simulations, currently in the few hundred per cent range.

  13. Implicit integrations for SPH in semi-Lagrangian approach: Application to the accretion disc modeling in a microquasar

    NASA Astrophysics Data System (ADS)

    Lanzafame, G.

    2013-03-01

    Current explicit integration techniques in fluid dynamics are deeply limited by the Courant-Friedrichs-Lewy condition of the time step progression, based on the adopted spatial resolution coupled with the maximum value between the kinetic velocity or the signal transmission speed in the computational domain. Eulerian implicit integration techniques, even though more time consuming, can allow us to perform stable computational fluid dynamics paying the price of a relatively larger inaccuracy in the calculations, without suffering such a strict temporal limitation. In this paper, we present a simple and effective scheme to perform free Lagrangian Smooth Particle Hydrodynamics (SPH) implicit integrations in the semi-Lagrangian approach without any Jacobian matrix inversion operations for viscous Navier-Stokes flows. Applications to SPH accretion disc simulation around a massive black hole (MBH) in a binary stellar system are shown, together with the comparison to the same results obtained according to the traditional explicit integration techniques. Some 1D and 2D critical tests are also discussed to check the validity of the technique.

  14. Transient jet formation and state transitions from large-scale magnetic reconnection in black hole accretion discs

    NASA Astrophysics Data System (ADS)

    Dexter, Jason; McKinney, Jonathan C.; Markoff, Sera; Tchekhovskoy, Alexander

    2014-05-01

    Magnetically arrested accretion discs (MADs), where the magnetic pressure in the inner disc is dynamically important, provide an alternative mechanism for regulating accretion to what is commonly assumed in black hole systems. We show that a global magnetic field inversion in the MAD state can destroy the jet, significantly increase the accretion rate, and move the effective inner disc edge in to the marginally stable orbit. Reconnection of the MAD field in the inner radii launches a new type of transient outflow containing hot plasma generated by magnetic dissipation. This transient outflow can be as powerful as the steady magnetically dominated Blandford-Znajek jet in the MAD state. The field inversion qualitatively describes many of the observational features associated with the high-luminosity hard-to-soft state transition in black hole X-ray binaries: the jet line, the transient ballistic jet, and the drop in rms variability. These results demonstrate that the magnetic field configuration can influence the accretion state directly, and hence the magnetic field structure is an important second parameter in explaining observations of accreting black holes across the mass and luminosity scales.

  15. Photon-conserving Comptonization in simulations of accretion discs around black holes

    NASA Astrophysics Data System (ADS)

    Sądowski, Aleksander; Narayan, Ramesh

    2015-12-01

    We introduce a new method for treating Comptonization in computational fluid dynamics. By construction, this method conserves the number of photons. Whereas the traditional `blackbody Comptonization' approach assumes that the radiation is locally a perfect blackbody and therefore uses a single parameter, the radiation temperature, to describe the radiation, the new `photon-conserving Comptonization' approach treats the photon gas as a Bose-Einstein fluid and keeps track of both the radiation temperature and the photon number density. We have implemented photon-conserving Comptonization in the general relativistic radiation magnetohydrodynamical code KORAL and we describe its impact on simulations of mildly supercritical black hole accretion discs. We find that blackbody Comptonization underestimates the gas and radiation temperature by up to a factor of 2 compared to photon-conserving Comptonization. This discrepancy could be serious when computing spectra. The photon-conserving simulation indicates that the spectral colour correction factor of the escaping radiation in the funnel region of the disc could be as large as 5.

  16. Looking into the inner black hole accretion disc with relativistic models of iron line

    NASA Astrophysics Data System (ADS)

    Svoboda, Jiri

    2010-07-01

    We discuss black hole spin measurements employing the relativistic iron line profiles in the X-ray domain. We investigate the iron line band for two representative sources -- MCG -6-30-15 (active galaxy) and GX 339-4 (X-ray binary). We compare two models of the broad iron line, LAOR and KYRLINE. We realise that the spin is currently determined entirely from the position of the marginally stable orbit while the effect of the spin on the overall line shape would be resolvable with higher resolution X-ray missions. We show that the precision of the spin measurements depends on an unknown angular distribution of the disc emission. We study how sensitive the spin determination is to the assumptions about the intrinsic angular distribution of the emitted photons. We find that the uncertainty of the directional emission distribution translates to 20% uncertainty in the determination of the radius of marginally stable orbit. We perform radiation transfer computations of an X-ray irradiated disc atmosphere (NOAR code) to determine the directionality of outgoing X-rays in the 2-10 keV energy band. Based on these computations, we find that from the simple formulae for the directionality, the isotropic case reproduces the simulated data with the best accuracy. The most frequently used limb-darkening law favours higher values of spin and, in addition, a steeper radial emissivity profile. Furthermore, we present a spectral analysis of an XMM-Newton observation of a Seyfert 1.5 galaxy IRAS 05078+1626 being the first X-ray spectroscopic study of this source. The lack of the significant relativistic blurring of the reflection model component suggests the accretion disc to be truncated at a farther radius.

  17. Fundamental stellar and accretion disc parameters of the eclipsing binary DQ Velorum

    NASA Astrophysics Data System (ADS)

    Barría, D.; Mennickent, R. E.; Schmidtobreick, L.; Djurašević, G.; Kołaczkowski, Z.; Michalska, G.; Vučković, M.; Niemczura, E.

    2013-04-01

    Context. To add to the growing collection of well-studied double periodic variables (DPVs) we have carried out the first spectroscopic and photometric analysis of the eclipsing binary DQ Velorum to obtain its main physical stellar and orbital parameters. Aims: Combining spectroscopic and photometric observations that cover several orbital cycles allows us to estimate the stellar properties of the binary components and the orbital parameters. We also searched for circumstellar material around the more massive star. Methods: We separated DQ Velorum composite spectra and measured radial velocities with an iterative method for double spectroscopic binaries. We obtained the radial velocity curves and calculated the spectroscopic mass ratio. We compared our single-lined spectra with a grid of synthetic spectra and estimated the temperature of the stars. We modeled the V-band light curve with a fitting method based on the simplex algorithm, which includes an accretion disc. To constrain the main stellar parameters we fixed the mass ratio and donor temperature to the values obtained by our spectroscopic analysis. Results: We obtain a spectroscopic mass ratio q = 0.31 ± 0.03 together with donor and gainer masses Md = 2.2 ± 0.2 M⊙, Mg = 7.3 ± 0.3 M⊙, the radii Rd = 8.4 ± 0.2 R⊙, Rg = 3.6 ± 0.2 R⊙ and temperatures Td = 9400 ± 100 K, Tg = 18 500 ± 500 K for the stellar components. We find that DQ Vel is a semi-detached system consisting of a B3V gainer and an A1III donor star plus an extended accretion disc around the gainer. The disc is filling 89% of the gainer Roche lobe with a temperature of 6580 ± 300 K at the outer radius. It has a concave shape that is thicker at its edge (de = 0.6 ± 0.1 R⊙) than at its centre (dc = 0.3 ± 0.1 R⊙). We find a significant sub-orbital frequency of 0.19 d-1 in the residuals of the V-band light curve, which we interpret as a pulsation of an slowly pulsating B-type (SPB) of a gainer star. We also estimate the distance to

  18. Long-term quasi-periodicity of 4U 1636-536 resulting from accretion disc instability

    NASA Astrophysics Data System (ADS)

    Wisniewicz, Mateusz; Zdziarski, Andrzej; Janiuk, Agnieszka; Rosinska, Dorota; Slowikowska, Agnieszka

    2016-07-01

    We present the results of a study of the low-mass X-ray binary 4U 1636-536. We have performed temporal analysis of all available RXTE/ASM, RXTE/PCA, Swift/BAT and MAXI data. We have confirmed the previously discovered quasi-periodicity of ˜45 d present during ˜2004, however we found it continued to 2006. At other epochs, the quasi-periodicity is only transient, and the quasi-period, if present, drifts. We have then applied a time-dependent accretion disc model to the interval with the significant X-ray quasi-periodicity. For our best model, the period and the amplitude of the theoretical light curve agree well with that observed. The modelled quasi-periodicity is due to the hydrogen thermal-ionization instability occurring in outer regions of the accretion disc. The model parameters are the average mass accretion rate (estimated from the light curves), and the accretion disc viscosity parameters, α_{cold} and α_{hot}, for the hot and cold phases, respectively. Our best model gives relatively low values of α_{cold} and α_{hot}.

  19. The mass donor star and the accretion disc of the dwarf nova V2051 Ophiuchi in the infrared

    NASA Astrophysics Data System (ADS)

    Wojcikiewicz, Eduardo; Baptista, Raymundo; Ribeiro, Tiago

    2016-07-01

    We report the analysis of infrared JHK_s high speed photometry of the dwarf nova V2051 Oph in quiescence. We model the ellipsoidal variations in the light curve to measure the fluxes of the mass donor star. Its colors are consistent with an M8 ± 1 spectral type with an equivalent blackbody temperature of T_{bb}= (2700± 300) K, in agreement with spectroscopic measurements and with theoretical expectation for donor stars at the same orbital period. We use the mass donor star fluxes and the Barnes & Evans relation to find a photometric parallax distance of (102 ± 16) pc to the binary. At this distance the outbursts of V2051 Oph occur at disc temperatures everywhere lower than the minimum/critical temperature predicted by the disc instability model, underscoring previous suggestions that they are powered by mass transfer bursts. We subtract the contribution of the mass donor star and apply eclipse mapping techniques to the remaining light curve in order to investigate the structure and emission of its accretion disc. The infrared accretion disc is bright and 'blue' in the inner regions and becomes progressively fainter and redder with increasing radii, indicating that the disc temperature decreases with radius. Bulges in the eclipse shape, more prominent in the H and K_s bands, lead to asymmetric arcs in the eclipse maps reminiscent of the spiral arms found in disc maps of outbursting dwarf novae. The arcs show an azimuthal extent of ˜90^o, extend from the intermediate to the outer disc regions (0.3-0.4 R_{L1}, where R_{L1} is the distance from disc center to the inner lagrangian point), and account for ≃ 30 per cent of the total flux in the H and K_s bands.

  20. Warping of an accretion disc and launching of a jet by a spinning black hole in NGC 4258

    NASA Astrophysics Data System (ADS)

    Wu, Qingwen; Yan, Hao; Yi, Zhu

    2013-12-01

    We fit the most up-to-date broad-band spectral energy distribution from radio to X-rays for NGC 4258 with a coupled accretion-jet model that surrounds a Kerr black hole (BH). Here, both the jet and the warped H2O maser disc are assumed to be triggered by a spinning BH through the Blandford-Znajek mechanism and the Bardeen-Petterson effect, respectively. The accretion flow consists of an inner radiatively inefficient accretion flow and an outer truncated standard thin disc, where the transition radius Rtr ≃ 3 × 103Rg for NGC 4258, based on the width and variability of its narrow Fe Kα line. The hybrid jet formation model, as a variant of the Blandford-Znajek model, is used to model the jet power. Therefore, we can estimate the accretion rate and BH spin through the two observed quantities (i.e. X-ray emission and jet power), where the observed jet power is estimated from the low-frequency radio emission. Using this method, we find that the BH of NGC 4258 should be mildly spinning with dimensionless spin parameter a* ≃ 0.7 ± 0.2. The outer thin disc mainly radiates at the near-infrared waveband and the jet contributes predominantly at the radio waveband. Using the above-estimated BH spin and the inferred accretion rate at the region of the maser disc based on the physical existence of the H2O maser, we find that the warp radius is ˜8.6 × 104Rg if it is driven by the Bardeen-Petterson effect. This is very consistent with the observational result.

  1. Global 3D simulations of disc accretion on to the classical T Tauri star V2129 Oph

    NASA Astrophysics Data System (ADS)

    Romanova, M. M.; Long, M.; Lamb, F. K.; Kulkarni, A. K.; Donati, J.-F.

    2011-02-01

    The magnetic field of the classical T Tauri star V2129 Oph can be modelled approximately by superposing slightly tilted dipole and octupole moments, with polar magnetic field strengths of 0.35 and 1.2 kG, respectively, as observed by Donati et al. Here we construct a numerical model of V2129 Oph incorporating this result and simulate accretion on to the star using a three-dimensional magnetohydrodynamic code. Simulations show that the disc is truncated by the dipole component and matter flows towards the star in two funnel streams. Closer to the star, the flow is redirected by the octupolar component, with some of the matter flowing towards the high-latitude poles, and the rest into the octupolar belts. The shape and position of the spots differ from those in a pure dipole case, where crescent-shaped spots are observed at the intermediate latitudes. Simulations show that if the disc is truncated at the distance of r≈ 6.2R★ which is comparable with the corotation radius, rcor≈ 6.8 R★, then the high-latitude polar spot dominates, but the accretion rate obtained from the simulations (and from the accompanying theoretical calculations) is about an order of magnitude lower than the observed one. The accretion rate matches the observed one if the disc is disrupted much closer to the star, at 3.4R★. However, in that case the octupolar belt spots strongly dominate. In the intermediate case of r≈ 4.3R★, the polar spots are sufficiently bright, and the accretion rate is within the error bar of the observed accretion rate, and this model can explain the observations. However, an even better match has been obtained in experiments with a dipole field twice as strong compared with one suggested by Donati et al. The torque on the star from the disc-magnetosphere interaction is small, and the time-scale of spin evolution, 2 × 107-6 × 108 yr is longer than the 2 × 106 yr age of V2129 Oph. This means that V2129 Oph probably lost most of its angular momentum in the

  2. Comparison of Ejection Events in the Jet and Accretion Disc Outflows in 3C 111

    NASA Technical Reports Server (NTRS)

    Tombesi, F.; Sambruna, R. M.; Marscher, A. P.; Jorstad, S. G.; Reynolds, C. S.; Markowtiz, A.

    2012-01-01

    We present a comparison of the parameters of accretion disc outflows and the jet of the broad-line radio galaxy 3C 111 on sub-pc scales. We make use of published X-ray observations of ultra-fast outflows (UFOs) and new 43 GHz VLBA images to track the jet knots ejection. We find that the superluminal jet coexists with the mildly relativistic outflows on sub-pc scales, possibly indicating a transverse stratification of a global flow. The two are roughly in pressure equilibrium, with the UFOs potentially providing additional support for the initial jet collimation. The UFOs are much more massive than the jet, but their kinetic power is probably about an order of magnitude lower, at least for the observations considered here. However, their momentum flux is equivalent and both of them are powerful enough to exert a concurrent feedback impact on the surrounding environment. A link between these components is naturally predicted in the context of MHD models for jet/outflow formation. However, given the high radiation throughput of AGNs, radiation pressure should also be taken into account. From the comparison with the long-term 2-10 keV RXTE light curve we find that the UFOs are preferentially detected during periods of increasing flux. We also find the possibility to place the UFOs within the known X-ray dips-jet ejection cycles, which has been shown to be a strong proof of the disc-jet connection, in analogue with stellar-mass black holes. However, given the limited number of observations presently available, these relations are only tentative and additional spectral monitoring is needed to test them conclusively.

  3. Comparison of ejection events in the jet and accretion disc outflows in 3C 111

    NASA Astrophysics Data System (ADS)

    Tombesi, F.; Sambruna, R. M.; Marscher, A. P.; Jorstad, S. G.; Reynolds, C. S.; Markowitz, A.

    2012-07-01

    We present a comparison of the parameters of accretion disc outflows and the jet of the broad-line radio galaxy 3C 111 on subparsec (sub-pc) scales. We make use of published X-ray observations of ultra-fast outflows (UFOs) and new 43-GHz Very Long Baseline Array images to track the jet knot ejection. We find that the superluminal jet coexists with the mildly relativistic outflows on sub-pc scales, possibly indicating a transverse stratification of a global flow. The two are roughly in pressure equilibrium, with the UFOs potentially providing additional support for the initial jet collimation. The UFOs are much more massive than the jet, but their kinetic power is probably about an order of magnitude lower, at least for the observations considered here. However, their momentum flux is equivalent and both of them are powerful enough to exert a concurrent feedback impact on the surrounding environment. A link between these components is naturally predicted in the context of magnetohydrodynamic models for jet/outflow formation. However, given the high radiation throughput of active galactic nuclei, radiation pressure should also be taken into account. From the comparison with the long-term 2-10 keV Rossi X-ray Timing Explorer light curve, we find that the UFOs are preferentially detected during periods of increasing flux. We also find the possibility to place the UFOs within the known X-ray dips-jet ejection cycles, which has been shown to be a strong proof of the disc-jet connection, in analogue with stellar mass black holes. However, given the limited number of observations presently available, these relations are only tentative and additional spectral monitoring is needed to test them conclusively.

  4. Active galactic nuclei at z ˜ 1.5 - III. Accretion discs and black hole spin

    NASA Astrophysics Data System (ADS)

    Capellupo, D. M.; Netzer, H.; Lira, P.; Trakhtenbrot, B.; Mejía-Restrepo, J.

    2016-07-01

    This is the third paper in a series describing the spectroscopic properties of a sample of 39 AGN at z ˜ 1.5, selected to cover a large range in black hole mass (MBH) and Eddington ratio (L/LEdd). In this paper, we continue the analysis of the VLT/X-shooter observations of our sample with the addition of nine new sources. We use an improved Bayesian procedure, which takes into account intrinsic reddening, and improved MBH estimates, to fit thin accretion disc (AD) models to the observed spectra and constrain the spin parameter (a*) of the central black holes. We can fit 37 out of 39 AGN with the thin AD model, and for those with satisfactory fits, we obtain constraints on the spin parameter of the BHs, with the constraints becoming generally less well defined with decreasing BH mass. Our spin parameter estimates range from ˜-0.6 to maximum spin for our sample, and our results are consistent with the `spin-up' scenario of BH spin evolution. We also discuss how the results of our analysis vary with the inclusion of non-simultaneous GALEX photometry in our thin AD fitting. Simultaneous spectra covering the rest-frame optical through far-UV are necessary to definitively test the thin AD theory and obtain the best constraints on the spin parameter.

  5. Active galactic nuclei at z ˜ 1.5: III. Accretion discs and Black Hole Spin

    NASA Astrophysics Data System (ADS)

    Capellupo, D. M.; Netzer, H.; Lira, P.; Trakhtenbrot, B.; Mejía-Restrepo, J.

    2016-04-01

    This is the third paper in a series describing the spectroscopic properties of a sample of 39 AGN at z ˜ 1.5, selected to cover a large range in black hole mass (MBH) and Eddington ratio (L/LEdd). In this paper, we continue the analysis of the VLT/X-shooter observations of our sample with the addition of 9 new sources. We use an improved Bayesian procedure, which takes into account intrinsic reddening, and improved MBH estimates, to fit thin accretion disc (AD) models to the observed spectra and constrain the spin parameter (a★) of the central black holes. We can fit 37 out of 39 AGN with the thin AD model, and for those with satisfactory fits, we obtain constraints on the spin parameter of the BHs, with the constraints becoming generally less well defined with decreasing BH mass. Our spin parameter estimates range from ˜-0.6 to maximum spin for our sample, and our results are consistent with the "spin-up" scenario of BH spin evolution. We also discuss how the results of our analysis vary with the inclusion of non-simultaneous GALEX photometry in our thin AD fitting. Simultaneous spectra covering the rest-frame optical through far-UV are necessary to definitively test the thin AD theory and obtain the best constraints on the spin parameter.

  6. Probing the Inner Relativistic Accretion Disc in 4u 1705-44 with RXTE and Xmm-Newton

    NASA Astrophysics Data System (ADS)

    di Salvo, Tiziana

    We propose to observe the X-ray burster Low Mass X-ray Binary 4U 1705- 44 with RXTE simultaneously with XMM-Newton for 200 ks. This observation will give us the possibility to confirm the disc reflection origin of the relativistic iron line observed by XMM in this source extending the spectral energy response to 200 keV and giving us the possibility to fit the broad band spectrum with self-consistent reflection models. The RXTE observation will also give us the possibility to detect kHz QPOs and to test the paradigm that their frequencies, as envisaged by most of the models, correspond to the Keplerian frequency at the inner accretion disc. This can be done by comparing the inner disc radius derived by the kHz QPO frequencies with that derived by fitting the relativistic iron line profile.

  7. Accretion disc dynamo activity in local simulations spanning weak-to-strong net vertical magnetic flux regimes

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg; Simon, Jacob B.; Armitage, Philip J.; Begelman, Mitchell C.

    2016-03-01

    Strongly magnetized accretion discs around black holes have attractive features that may explain enigmatic aspects of X-ray binary behaviour. The structure and evolution of these discs are governed by a dynamo-like mechanism, which channels part of the accretion power liberated by the magnetorotational instability (MRI) into an ordered toroidal magnetic field. To study dynamo activity, we performed three-dimensional, stratified, isothermal, ideal magnetohydrodynamic shearing box simulations. The strength of the self-sustained toroidal magnetic field depends on the net vertical magnetic flux, which we vary across almost the entire range over which the MRI is linearly unstable. We quantify disc structure and dynamo properties as a function of the initial ratio of mid-plane gas pressure to vertical magnetic field pressure, β _0^mid = p_gas / p_B. For 10^5 ≥ β _0^mid ≥ 10 the effective α-viscosity parameter scales as a power law. Dynamo activity persists up to and including β _0^mid = 10^2, at which point the entire vertical column of the disc is magnetic pressure dominated. Still stronger fields result in a highly inhomogeneous disc structure, with large density fluctuations. We show that the turbulent steady state βmid in our simulations is well matched by the analytic model of Begelman et al. describing the creation and buoyant escape of toroidal field, while the vertical structure of the disc can be broadly reproduced using this model. Finally, we discuss the implications of our results for observed properties of X-ray binaries.

  8. Super- and sub-Eddington accreting massive black holes: a comparison of slim and thin accretion discs through study of the spectral energy distribution

    NASA Astrophysics Data System (ADS)

    Castelló-Mor, N.; Netzer, H.; Kaspi, S.

    2016-05-01

    We employ optical and ultraviolet (UV) observations to present spectral energy distributions (SEDs) for two reverberation-mapped samples of super-Eddington and sub-Eddington active galactic nuclei (AGN) with similar luminosity distributions. The samples are fitted with accretion disc (AD) models in order to look for SED differences that depend on the Eddington ratio. The fitting takes into account measured black hole (BH) mass and accretion rates, BH spin and intrinsic reddening of the sources. All objects in both groups can be fitted by thin AD models over the range 0.2-1 μm with reddening as a free parameter. The intrinsic reddening required to fit the data are relatively small, E(B - V) ≤ 0.2 mag, except for one source. Super-Eddington AGN seems to require more reddening. The distribution of E(B - V) is similar to what is observed in larger AGN samples. The best-fitting disc models recover very well the BH mass and accretion for the two groups. However, the SEDs are very different, with super-Eddington sources requiring much more luminous far-UV continuum. The exact amount depends on the possible saturation of the UV radiation in slim discs. In particular, we derive for the super-Eddington sources a typical bolometric correction at 5100 Å of 60-150 compared with a median of ˜20 for the sub-Eddington AGN. The measured torus luminosity relative to λLλ(5100 Å) are similar in both groups. The αOX distribution is similar too. However, we find extremely small torus covering factors for super-Eddington sources, an order of magnitude smaller than those of sub-Eddington AGN. The small differences between the groups regarding the spectral range 0.2-22 μm, and the significant differences related to the part of the SED that we cannot observe may be consistent with some slim disc models. An alternative explanation is that present day slim-disc models overestimate the far-UV luminosity of such objects by a large amount.

  9. 2D segmentation of intervertebral discs and its degree of degeneration from T2-weighted magnetic resonance images

    NASA Astrophysics Data System (ADS)

    Castro-Mateos, Isaac; Pozo, José Maria; Lazary, Aron; Frangi, Alejandro F.

    2014-03-01

    Low back pain (LBP) is a disorder suffered by a large population around the world. A key factor causing this illness is Intervertebral Disc (IVD) degeneration, whose early diagnosis could help in preventing this widespread condition. Clinicians base their diagnosis on visual inspection of 2D slices of Magnetic Resonance (MR) images, which is subject to large interobserver variability. In this work, an automatic classification method is presented, which provides the Pfirrmann degree of degeneration from a mid-sagittal MR slice. The proposed method utilizes Active Contour Models, with a new geometrical energy, to achieve an initial segmentation, which is further improved using fuzzy C-means. Then, IVDs are classified according to their degree of degeneration. This classification is attained by employing Adaboost on five specific features: the mean and the variance of the probability map of the nucleus using two different approaches and the eccentricity of the fitting ellipse to the contour of the IVD. The classification method was evaluated using a cohort of 150 intervertebral discs assessed by three experts, resulting in a mean specificity (93%) and sensitivity (83%) similar to the one provided by every expert with respect to the most voted value. The segmentation accuracy was evaluated using the Dice Similarity Index (DSI) and Root Mean Square Error (RMSE) of the point-to-contour distance. The mean DSI ± 2 standard deviation was 91:7% ±5:6%, the mean RMSE was 0:82mm and the 95 percentile was 1:36mm. These results were found accurate when compared to the state-of-the-art.

  10. Multidimensional modelling of X-ray spectra for AGN accretion disc outflows - III. Application to a hydrodynamical simulation

    NASA Astrophysics Data System (ADS)

    Sim, S. A.; Proga, D.; Miller, L.; Long, K. S.; Turner, T. J.

    2010-11-01

    We perform multidimensional radiative transfer simulations to compute spectra for a hydrodynamical simulation of a line-driven accretion disc wind from an active galactic nucleus. The synthetic spectra confirm expectations from parametrized models that a disc wind can imprint a wide variety of spectroscopic signatures including narrow absorption lines, broad emission lines and a Compton hump. The formation of these features is complex with contributions originating from many of the different structures present in the hydrodynamical simulation. In particular, spectral features are shaped both by gas in a successfully launched outflow and in complex flows where material is lifted out of the disc plane but ultimately falls back. We also confirm that the strong Fe Kα line can develop a weak, red-skewed line wing as a result of Compton scattering in the outflow. In addition, we demonstrate that X-ray radiation scattered and reprocessed in the flow has a pivotal part in both the spectrum formation and determining the ionization conditions in the wind. We find that scattered radiation is rather effective in ionizing gas which is shielded from direct irradiation from the central source. This effect likely makes the successful launching of a massive disc wind somewhat more challenging and should be considered in future wind simulations.

  11. Accretion disc/corona emission from a radio-loud narrow-line Seyfert 1 galaxy PKS 0558-504

    NASA Astrophysics Data System (ADS)

    Ghosh, R.; Dewangan, G. C.; Raychaudhuri, B.

    2016-02-01

    Approximately 10-20 per cent of active galactic nuclei (AGN) are known to eject powerful jets from the innermost regions. There is very little observational evidence if the jets are powered by spinning black holes and if the accretion discs extend to the innermost regions in radio-loud AGN. Here, we study the soft X-ray excess, the hard X-ray spectrum and the optical/UV emission from the radio-loud narrow-line Seyfert 1 galaxy PKS 0558-504 using Suzaku and Swift observations. The broad-band X-ray continuum of PKS 0558-504 consists of a soft X-ray excess emission below 2 keV that is well described by a blackbody (kT ˜ 0.13 keV) and high-energy emission that is well described by a thermal Comptonization (compps) model with kTe ˜ 250 keV, optical depth τ ˜ 0.05 (spherical corona) or kTe ˜ 90 keV, τ ˜ 0.5 (slab corona). The Comptonizing corona in PKS 0558-504 is likely hotter than in radio-quiet Seyferts such as IC 4329A and Swift J2127.4+5654. The observed soft X-ray excess can be modelled as blurred reflection from an ionized accretion disc or optically thick thermal Comptonization in a low-temperature plasma. Both the soft X-ray excess emission when interpreted as the blurred reflection and the optical/UV to soft X-ray emission interpreted as intrinsic disc Comptonized emission implies spinning (a > 0.6) black hole. These results suggest that disc truncation at large radii and retrograde black hole spin both are unlikely to be the necessary conditions for launching the jets.

  12. Tilted black hole accretion disc models of Sagittarius A*: time-variable millimetre to near-infrared emission

    NASA Astrophysics Data System (ADS)

    Dexter, Jason; Fragile, P. Chris

    2013-07-01

    High-resolution, multiwavelength and time-domain observations of the Galactic Centre black hole candidate, Sgr A*, allow for a direct test of contemporary accretion theory. Most models assume alignment between the accretion disc and black hole angular momentum axes, but this is not necessarily the case for geometrically thick accretion flows like that on to Sgr A*. Instead, we calculate images and spectra from a set of numerical simulations of accretion flows misaligned (`tilted') by 15° from the black hole spin axis and compare them with millimetre (mm) to near-infrared (NIR) observations. Non-axisymmetric standing shocks from eccentric fluid orbits dominate the emission, leading to a wide range of possible image morphologies. The strong effects of disc tilt lead to poorly constrained model parameters. These results suggest that previous parameter estimates from fitting aligned models, including estimates of the dimensionless black hole spin, likely only apply for small values of spin or tilt (upper limits of a < 0.3 or β < 15°). At 1.3 mm, the black hole images have crescent morphologies as in the aligned case, and the black hole shadow may still be accessible to future very long baseline interferometry (mm-VLBI) observations. Shock heating leads to multiple populations of electrons, some at high energies (Te > 1012 K). These electrons can naturally produce the observed NIR flux, spectral index and rapid variability (`flaring'). This NIR emission is uncorrelated with that in the mm, which also agrees with observations. These are the first numerical models to explain the time-variable mm to NIR emission of Sgr A*. Predictions of the model include significant structural changes observable with mm-VLBI on both the dynamical (hour) and Lense-Thirring precession (day-year) time-scales, and ≃ 30-50 μas changes in centroid position from extreme gravitational lensing events during NIR flares, detectable with the future VLT instrument GRAVITY. We further predict

  13. Convergence of smoothed particle hydrodynamics simulations of self-gravitating accretion discs: sensitivity to the implementation of radiative cooling

    NASA Astrophysics Data System (ADS)

    Rice, W. K. M.; Forgan, D. H.; Armitage, P. J.

    2012-02-01

    Recent simulations of self-gravitating accretion discs, carried out using a three-dimensional smoothed particle hydrodynamics (SPH) code by Meru & Bate, have been interpreted as implying that three-dimensional global discs fragment much more easily than would be expected from a two-dimensional local model. Subsequently, global and local two-dimensional models have been shown to display similar fragmentation properties, leaving it unclear whether the three-dimensional results reflect a physical effect or a numerical problem associated with the treatment of cooling or artificial viscosity in SPH. Here, we study how fragmentation of self-gravitating disc flows in SPH depends upon the implementation of cooling. We run disc simulations that compare a simple cooling scheme, in which each particle loses energy based upon its internal energy per unit mass, with a method in which the cooling is derived from a smoothed internal energy density field. For the simple per particle cooling scheme, we find a significant increase in the minimum cooling time-scale for fragmentation with increasing resolution, matching previous results. Switching to smoothed cooling, however, results in lower critical cooling time-scales, and tentative evidence for convergence at the highest spatial resolution tested. We conclude that precision studies of fragmentation using SPH require careful consideration of how cooling (and, probably, artificial viscosity) is implemented, and that the apparent non-convergence of the fragmentation boundary seen in prior simulations is likely a numerical effect. In real discs, where cooling is physically smoothed by radiative transfer effects, the fragmentation boundary is probably displaced from the two-dimensional value by a factor that is only of the order of unity.

  14. Long-term nonlinear behaviour of the magnetorotational instability in a localized model of an accretion disc

    NASA Astrophysics Data System (ADS)

    Silvers, L. J.

    2008-04-01

    For more than a decade, the so-called shearing-box model has been used to study the fundamental local dynamics of accretion discs. This approach has proved to be very useful because it allows high-resolution and long-term studies to be carried out, studies that would not be possible for a global disc. Localized disc studies have largely focused on examining the rate of enhanced transport of angular momentum, essentially a sum of the Reynolds and Maxwell stresses. The dominant radial-azimuthal component of this stress tensor is, in the classic Shakura-Sunyaev model, expressed as a constant α times the pressure. Previous studies have estimated α based on a modest number of orbital times. Here we use much longer baselines, and perform a cumulative average for α. Great care must be exercised when trying to extract numerical α values from simulations: dissipation scales, computational box aspect ratio, and even numerical algorithms can all affect the result. This study suggests that estimating α becomes more, not less, difficult as computational power increases.

  15. ICEG2D: An Integrated Software Package for Automated Prediction of Flow Fields for Single-Element Airfoils with Ice Accretion

    NASA Technical Reports Server (NTRS)

    Thompson, David S.; Soni, Bharat K.

    2000-01-01

    An integrated software package, ICEG2D, was developed to automate computational fluid dynamics (CFD) simulations for single-element airfoils with ice accretion. ICEG2D is designed to automatically perform three primary functions: (1) generating a grid-ready, surface definition based on the geometrical characteristics of the iced airfoil surface, (2) generating a high-quality grid using the generated surface point distribution, and (3) generating the input and restart files needed to run the general purpose CFD solver NPARC. ICEG2D can be executed in batch mode using a script file or in an interactive mode by entering directives from a command line. This report summarizes activities completed in the first year of a three-year research and development program to address issues related to CFD simulations for aircraft components with ice accretion. Specifically, this document describes the technology employed in the software, the installation procedure, and a description of the operation of the software package. Validation of the geometry and grid generation modules of ICEG2D is also discussed.

  16. HST Spatially Resolved Spectra of the Accretion Disc and Gas Stream of the Nova-Like Variable UX Ursae Majoris

    NASA Technical Reports Server (NTRS)

    Baptista, Raymundo; Horne, Keith; Wade, Richard A.; Hubeny, Ivan; Long, Knox S.; Rutten, Rene G. M.

    1998-01-01

    Time-resolved eclipse spectroscopy of the nova-like variable UX UMa obtained with the Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) on 1994 August and November is analysed with eclipse mapping techniques to produce spatially resolved spectra of its accretion disk and gas stream as a function of distance from the disk centre. The inner accretion disk is characterized by a blue continuum filled with absorption bands and lines, which cross over to emission with increasing disk radius, similar to that reported at optical wavelengths. The comparison of spatially resolved spectra at different azimuths reveals a significant asymmetry in the disk emission at ultraviolet (UV) wavelengths, with the disk side closest to the secondary star showing pronounced absorption by an 'iron curtain' and a Balmer jump in absorption. These results suggest the existence of an absorbing ring of cold gas whose density and/or vertical scale increase with disk radius. The spectrum of the infalling gas stream is noticeably different from the disc spectrum at the same radius suggesting that gas overflows through the impact point at the disk rim and continues along the stream trajectory, producing distinct emission down to 0.1 R(sub LI). The spectrum of the uneclipsed light shows prominent emission lines of Lyalpha, N v lambda1241, SiIV Lambda 1400, C IV Lambda 1550, HeII Lambda 1640, and MgII Lambda 2800, and a UV continuum rising towards longer wavelengths. The Balmer jump appears clearly in emission indicating that the uneclipsed light has an important contribution from optically thin gas. The lines and optically thin continuum emission are most probably emitted in a vertically extended disk chromosphere + wind. The radial temperature profiles of the continuum maps are well described by a steady-state disc model in the inner and intermediate disk regions (R greater than or equal to 0.3R(sub LI) ). There is evidence of an increase in the mass accretion rate from August to November

  17. The structure of accretion discs around low-mass young stars

    NASA Astrophysics Data System (ADS)

    Lachaume, R.

    2003-07-01

    It is nowadays widely accepted that low mass star formation initiated from a molecular cloud undergoes a phase where the central object is surrounded by a disc, in which planets may form later. The study of such a disc mainly aims at the understanding of the evolutionary sequence of star formation and of planet formation. Radiative transfer in this disc is of high importance, for it is heated by viscous dissipation or stellar irradiation for instance. The production and transportation of thermal energy in this disc therefore conditions its chemical and physical properties inside. It has countless consequences in terms of structure. I shall present a study of the discs based on a deep analytical description of the radiative transfer and a delay of the numerical implementation. This method allows a better grasp on processes and physical conditions in these objects. I shall constrain model parameters and establish a new diagnosis combining the spectral energy distribution and the visibilities obtained with optical long-baseline interferometers. The former observable is a well known technique, and the latter new and promising: it provides information at the scale of one astronomical unit for the closest star forming regions. I shall start a generalisation of the analytical studies of the radiative transfer in stellar atmospheres, with two striking discrepancies: viscous heating occurs everywhere in the disc and the surface is illuminated by the star. I apply this formalism in a numerical simulation of a disc heated by viscosity alone. I then develop a simplified version of this transfer in a two-layer disc: the surface is heated by the interior and by the star, and the interior by viscosity and by the surface. This simplification allow to derive a simplified analytical description of the physical conditions in a disc heated by the two processes mentioned above. I shall tackle the interpretation of observations in the end. I shall of the possibilities brought by

  18. ICEG2D (v2.0) - An Integrated Software Package for Automated Prediction of Flow Fields for Single-Element Airfoils With Ice Accretion

    NASA Technical Reports Server (NTRS)

    Thompson David S.; Soni, Bharat K.

    2001-01-01

    An integrated geometry/grid/simulation software package, ICEG2D, is being developed to automate computational fluid dynamics (CFD) simulations for single- and multi-element airfoils with ice accretions. The current version, ICEG213 (v2.0), was designed to automatically perform four primary functions: (1) generate a grid-ready surface definition based on the geometrical characteristics of the iced airfoil surface, (2) generate high-quality structured and generalized grids starting from a defined surface definition, (3) generate the input and restart files needed to run the structured grid CFD solver NPARC or the generalized grid CFD solver HYBFL2D, and (4) using the flow solutions, generate solution-adaptive grids. ICEG2D (v2.0) can be operated in either a batch mode using a script file or in an interactive mode by entering directives from a command line within a Unix shell. This report summarizes activities completed in the first two years of a three-year research and development program to address automation issues related to CFD simulations for airfoils with ice accretions. As well as describing the technology employed in the software, this document serves as a users manual providing installation and operating instructions. An evaluation of the software is also presented.

  19. On different types of instabilities in black hole accretion discs: implications for X-ray binaries and active galactic nuclei

    NASA Astrophysics Data System (ADS)

    Janiuk, Agnieszka; Czerny, Bożena

    2011-07-01

    We discuss two important instability mechanisms that may lead to the limit-cycle oscillations of the luminosity of the accretion discs around compact objects: ionization instability and radiation pressure instability. Ionization instability is well established as a mechanism of X-ray novae eruptions in black hole binary systems, but its applicability to active galactic nuclei (AGN) is still problematic. Radiation pressure theory has still a very weak observational background in any of these sources. In this paper, we attempt to confront the parameter space of these instabilities with the observational data. At the basis of this simple survey of sources properties, we argue that the radiation pressure instability is likely to be present in several Galactic sources with the Eddington ratios being above 0.15 and in AGN with the Eddington ratio above 0.025. Our results favour the parametrization of the viscosity through the geometrical mean of the radiation and gas pressure in both Galactic sources and AGN. More examples of the quasi-regular outbursts in the time-scales of 100 s in Galactic sources and hundreds of years in AGN are needed to formulate firm conclusions. We also show that the disc sizes in the X-ray novae are consistent with the ionization instability. This instability may also considerably influence the lifetime cycle and overall complexity in the supermassive black hole environment.

  20. Transient accretion disc-like envelope in the symbiotic binary BF Cygni during its 2006-2015 optical outburst

    NASA Astrophysics Data System (ADS)

    Tomov, N. A.; Tomova, M. T.; Bisikalo, D. V.

    2015-09-01

    The optical light of the symbiotic binary BF Cyg during its last eruption after 2006 shows orbital variations because of an eclipse of the outbursting compact object. The first orbital minimum is deeper than the following ones. Moreover, the Balmer profiles of this system acquired additional satellite components indicating a bipolar collimated outflow at one time between the first and second orbital minima. This behaviour is interpreted in the framework of the model of a collimated stellar wind from the outbursting object. It is supposed that one extended disc-like envelope covering the accretion disc of the compact object and collimating its stellar wind forms in the period between the first and second minima. The uneclipsed part of this envelope is responsible for the decrease of the depth of the orbital minimum. The calculated U BVR_{ C}I_{ C} fluxes of this uneclipsed part are in agreement with the observed residual of the depths of the first and second orbital minima. The parameters of the envelope require that it is the main emitting region of the line Hα but the Hα profile is less determined from its rotation and mostly from other mechanisms. It is concluded that the envelope is a transient nebular region and its destruction determines the increase of the depth of the orbital minimum with fading of the optical light.

  1. Time-dependent models of accretion discs with nuclear burning following the tidal disruption of a white dwarf by a neutron star

    NASA Astrophysics Data System (ADS)

    Margalit, Ben; Metzger, Brian D.

    2016-09-01

    We construct time-dependent one-dimensional (vertically averaged) models of accretion discs produced by the tidal disruption of a white dwarf (WD) by a binary neutron star (NS) companion. Nuclear reactions in the disc mid-plane burn the WD matter to increasingly heavier elements at sequentially smaller radii, releasing substantial energy which can impact the disc dynamics. A model for disc outflows is employed, by which cooling from the outflow balances other sources of heating (viscous, nuclear) in regulating the Bernoulli parameter of the mid-plane to a fixed value ≲0. We perform a comprehensive parameter study of the compositional yields and velocity distributions of the disc outflows for WDs of different initial compositions. For C/O WDs, the radial composition profile of the disc evolves self-similarly in a quasi-steady-state manner, and is remarkably robust to model parameters. The nucleosynthesis in helium WD discs does not exhibit this behaviour, which instead depends sensitively on factors controlling the disc mid-plane density (e.g. the strength of the viscosity, α). By the end of the simulation, a substantial fraction of the WD mass is unbound in outflows at characteristic velocities of ˜109 cm s-1. The outflows from WD-NS merger discs contain 10-4-3 × 10-3 M⊙ of radioactive 56Ni, resulting in fast (˜ week long) dim (˜1040 erg s-1) optical transients; shock heating of the ejecta by late-time outflows may increase the peak luminosity to ˜1043 erg s-1. The accreted mass on to the NS is probably not sufficient to induce gravitational collapse, but may be capable of spinning up the NS to periods of ˜10 ms, illustrating the feasibility of this channel in forming isolated millisecond pulsars.

  2. Accretion disc-corona and jet emission from the radio-loud Narrow Line Seyfert 1 galaxy RXJ1633.3+4719

    NASA Astrophysics Data System (ADS)

    Mallick, Labani; Dewangan, G. C.; Gandhi, P.; Misra, R.; Kembhavi, A. K.

    2016-05-01

    We perform X-ray/UV spectral and X-ray variability studies of the radio-loud Narrow Line Seyfert 1 (NLS1) galaxy RXJ1633.3+4719 using XMM-Newton and Suzaku observations from 2011 and 2012. The 0.3-10 keV spectra consist of an ultra-soft component described by an accretion disc blackbody (kT_in = 39.6^{+11.2}_{-5.5} eV) and a power-law due to the thermal Comptonization (Γ =1.96^{+0.24}_{-0.31}) of the disc emission. The disc temperature inferred from the soft excess is at least a factor of two lower than that found for the canonical soft excess emission from radio-quiet NLS1s. The UV spectrum is described by a power-law with photon index 3.05^{+0.56}_{-0.33}. The observed UV emission is too strong to arise from the accretion disc or the host galaxy, but can be attributed to a jet. The X-ray emission from RXJ1633.3+4719 is variable with fractional variability amplitude Fvar=13.5±1.0%. In contrast to radio-quiet AGN, X-ray emission from the source becomes harder with increasing flux. The fractional RMS variability increases with energy and the RMS spectrum is well described by a constant disc component and a variable power-law continuum with the normalization and photon index being anti-correlated. Such spectral variability cannot be caused by variations in the absorption and must be intrinsic to the hot corona. Our finding of possible evidence for emission from the inner accretion disc, jet and hot corona from RXJ1633.3+4719 in the optical to X-ray bands makes this object an ideal target to probe the disc-jet connection in AGN.

  3. Accretion disc-corona and jet emission from the radio-loud narrow-line Seyfert 1 galaxy RX J1633.3+4719

    NASA Astrophysics Data System (ADS)

    Mallick, Labani; Dewangan, G. C.; Gandhi, P.; Misra, R.; Kembhavi, A. K.

    2016-08-01

    We perform X-ray/ultraviolet (UV) spectral and X-ray variability studies of the radio-loud narrow-line Seyfert 1 (NLS1) galaxy RX J1633.3+4719 using XMM-Newton and Suzaku observations from 2011 and 2012. The 0.3-10 keV spectra consist of an ultrasoft component described by an accretion disc blackbody (kT_in = 39.6^{+11.2}_{-5.5} eV) and a power law due to the thermal Comptonization (Γ = 1.96^{+0.24}_{-0.31}) of the disc emission. The disc temperature inferred from the soft excess is at least a factor of 2 lower than that found for the canonical soft excess emission from radio-quiet NLS1s. The UV spectrum is described by a power law with photon index 3.05^{+0.56}_{-0.33}. The observed UV emission is too strong to arise from the accretion disc or the host galaxy, but can be attributed to a jet. The X-ray emission from RX J1633.3+4719 is variable with fractional variability amplitude Fvar = 13.5 ± 1.0 per cent. In contrast to radio-quiet active galactic nuclei (AGN), X-ray emission from the source becomes harder with increasing flux. The fractional rms variability increases with energy and the rms spectrum is well described by a constant disc component and a variable power-law continuum with the normalization and photon index being anticorrelated. Such spectral variability cannot be caused by variations in the absorption and must be intrinsic to the hot corona. Our finding of possible evidence for emission from the inner accretion disc, jet and hot corona from RX J1633.3+4719 in the optical to X-ray bands makes this object an ideal target to probe the disc-jet connection in AGN.

  4. Testing black hole neutrino-dominated accretion discs for long-duration gamma-ray bursts

    NASA Astrophysics Data System (ADS)

    Song, Cui-Ying; Liu, Tong; Gu, Wei-Min; Tian, Jian-Xiang

    2016-05-01

    Long-duration gamma-ray bursts (LGRBs) are generally considered to originate from the massive collapsars. It is believed that the central engine of gamma-ray bursts (GRBs) is a neutrino-dominated accretion flow (NDAF) around a rotating stellar-mass black hole (BH). The neutrino annihilation above the NDAF is a feasible mechanism to power GRB. In this work, we analyse the distributions of the isotropic gamma-ray-radiated energy and jet kinetic energy of 48 LGRBs. According to the NDAF and fireball models, we estimate the mean accreted masses of LGRBs in our sample to investigate whether the NDAFs can power LGRBs with the reasonable BH parameters and conversion efficiency of neutrino annihilation. The results indicate that most of the values of the accreted masses are less than 5 M⊙ for the extreme Kerr BHs and high conversion efficiency. It suggests that the NDAFs may be suitable for most of LGRBs except for some extremely high energy sources.

  5. Accretion disc geometry evolution of GRS 1915+105 during its plataeu states

    NASA Astrophysics Data System (ADS)

    Sarathi Pal, Partha; Gopal Dutta, Broja; Chakrabarti, Sandip Kumar

    2016-07-01

    The evolution of Time-lags are correlated with the accretion geometry during Plateau states of GRS 1915+105. We find that the lag spectrum for the χ_3 class is different from that of χ_1, χ_2 and χ_4 classes. Hard lags occur only when Comptonizing efficiency (CE) is about 0.9% for different plateau states and its evolution follows the sequence of class transitions suggested on the basis of CE parameter. We conclude that the variation of time lags could be due to change of size of the CENBOL which is often triggered by Compton cooling process.

  6. A relativistically smeared spectrum in the neutron star X-ray binary 4U 1705-44: looking at the inner accretion disc with X-ray spectroscopy

    NASA Astrophysics Data System (ADS)

    di Salvo, T.; D'Aí, A.; Iaria, R.; Burderi, L.; Dovčiak, M.; Karas, V.; Matt, G.; Papitto, A.; Piraino, S.; Riggio, A.; Robba, N. R.; Santangelo, A.

    2009-10-01

    Iron emission lines at 6.4-6.97 keV, identified with fluorescent Kα transitions, are among the strongest discrete features in the X-ray band. These are therefore one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper, we present a recent XMM-Newton observation of the X-ray burster 4U 1705-44, where we clearly detect a relativistically smeared iron line at about 6.7 keV, testifying with high statistical significance that the line profile is distorted by high-velocity motion in the accretion disc. As expected from disc reflection models, we also find a significant absorption edge at about 8.3 keV; this feature appears to be smeared, and is compatible with being produced in the same region where the iron line is produced. From the line profile, we derive the physical parameters of the inner accretion disc with large precision. The line is identified with the Kα transition of highly ionized iron, FeXXV, the inner disc radius is Rin = 14 +/- 2 Rg (where Rg is the Gravitational radius, GM/c2), the emissivity dependence from the disc radius is r-2.27+/-0.08, the inclination angle with respect to the line of sight is i = 39° +/- 1°. Finally, the XMM-Newton spectrum shows evidences of other low-energy emission lines, which again appear broad and their profiles are compatible with being produced in the same region where the iron line is produced.

  7. Hydromagnetic flows from accretion discs and the production of radio jets

    NASA Astrophysics Data System (ADS)

    Blandford, R. D.; Payne, D. G.

    1982-06-01

    The possibility is examined that angular momentum is removed magnetically from an accretion disk by field lines that leave the disk surface, and is eventually carried off in a jet moving perpendicular to the disk. The mechanism is illustrated by a self-similar MHD solution, with the gas being regarded as cold and starting from rest at the equatorial plane, with the disk itself in Keplerian orbit about a black hole. It is shown that a centrifugally driven outflow of matter from the disk is possible if the poloidal component of the magnetic field makes an angle of less than 60 deg with disk surface. At large distances the outflow forms a pair of collimated, antiparallel jets, while close to the disk it is probably driven by gas pressure in a hot, magnetically dominated corona.

  8. Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes

    NASA Technical Reports Server (NTRS)

    T.Dauser; Garcia, J.; Wilms, J.; Boeck, M.; Brenneman, L. W.; Falanga, M.; Fukumura, Keigo; Reynolds, C. S.

    2013-01-01

    X-ray irradiation of the accretion disc leads to strong reflection features, which are then broadened and distorted by relativistic effects. We present a detailed, general relativistic approach to model this irradiation for different geometries of the primary X-ray source. These geometries include the standard point source on the rotational axis as well as more jet-like sources, which are radially elongated and accelerating. Incorporating this code in the RELLINE model for relativistic line emission, the line shape for any configuration can be predicted. We study how different irradiation geometries affect the determination of the spin of the black hole. Broad emission lines are produced only for compact irradiating sources situated close to the black hole. This is the only case where the black hole spin can be unambiguously determined. In all other cases the line shape is narrower, which could either be explained by a low spin or an elongated source. We conclude that for those cases and independent of the quality of the data, no unique solution for the spin exists and therefore only a lower limit of the spin value can be given

  9. On the mechanism of self gravitating Rossby interfacial waves in proto-stellar accretion discs

    NASA Astrophysics Data System (ADS)

    Yellin-Bergovoy, Ron; Heifetz, Eyal; Umurhan, Orkan M.

    2016-05-01

    The dynamical response of edge waves under the influence of self-gravity is examined in an idealized two-dimensional model of a proto-stellar disc, characterized in steady state as a rotating vertically infinite cylinder of fluid with constant density except for a single density interface at some radius r0. The fluid in basic state is prescribed to rotate with a Keplerian profile $\\Omega_k(r)\\sim r^{-3/2}$ modified by some additional azimuthal sheared flow. A linear analysis shows that there are two azimuthally propagating edge waves, kin to the familiar Rossby waves and surface gravity waves in terrestrial studies, which move opposite to one another with respect to the local basic state rotation rate at the interface. Instability only occurs if the radial pressure gradient is opposite to that of the density jump (unstably stratified) where self-gravity acts as a wave stabilizer irrespective of the stratification of the system. The propagation properties of the waves are discussed in detail in the language of vorticity edge waves. The roles of both Boussinesq and non-Boussinesq effects upon the stability and propagation of these waves with and without the inclusion of self-gravity are then quantified. The dynamics involved with self-gravity non- Boussinesq effect is shown to be a source of vorticity production where there is a jump in the basic state density, in addition, self-gravity also alters the dynamics via the radial main pressure gradient, which is a Boussinesq effect . Further applications of these mechanical insights are presented in the conclusion including the ways in which multiple density jumps or gaps may or may not be stable.

  10. X-ray diagnostics of chemical composition of the accretion disc and donor star in ultra-compact X-ray binaries

    NASA Astrophysics Data System (ADS)

    Koliopanos, Filippos; Gilfanov, Marat; Bildsten, Lars

    2013-06-01

    Non-solar composition of the donor star in ultra-compact X-ray binaries (UCXBs) may have a pronounced effect on the fluorescent lines appearing in their spectra due to reprocessing of primary radiation by the accretion disc and the white dwarf surface. We show that the most dramatic and easily observable consequence of the anomalous C/O abundance is the significant, by more than an order of magnitude, attenuation of the Kα line of iron. It is caused by screening of the presence of iron by oxygen - in the C/O-dominated material the main interaction process for an E ≈ 7 keV photon is absorption by oxygen rather than by iron, contrary to the solar composition case. Ionization of oxygen at high mass accretion rates adds a luminosity dependence to this behaviour - the iron line is significantly suppressed only at low luminosity, log (LX) ≲ 37-37.5, and should recover its nominal strength at higher luminosity. The increase of the equivalent width of the Kα lines of carbon and oxygen, on the other hand, saturates at rather moderate values. Screening by He is less important, due to its low ionization threshold and because in the accretion disc it is mostly ionized. Consequently, in the case of the He-rich donor, the iron line strength remains close to its nominal value, determined by the iron abundance in the accretion disc. This opens the possibility of constraining the nature of donor stars in UCXBs by means of X-ray spectroscopy with moderate energy resolution.

  11. XMM-Newton observations of UW CrB: detection of X-ray bursts and evidence for accretion disc evolution

    NASA Astrophysics Data System (ADS)

    Hakala, Pasi; Ramsay, Gavin; Muhli, Panu; Charles, Phil; Hannikainen, Diana; Mukai, Koji; Vilhu, Osmi

    2005-01-01

    UW CrB (MS 1603+2600) is a peculiar short-period X-ray binary that exhibits extraordinary optical behaviour. The shape of the optical light curve of the system changes drastically from night to night, without any changes in overall brightness. Here we report X-ray observations of UW CrB obtained with XMM-Newton. We find evidence for several X-ray bursts, confirming a neutron star primary. This considerably strengthens the case that UW CrB is an accretion disc corona system located at a distance of at least 5-7 kpc (3-5 kpc above the Galactic plane). The X-ray and Optical Monitor (ultraviolet-optical) light curves show remarkable shape variation from one observing run to another, which we suggest are due to large-scale variations in the accretion disc shape resulting from a warp that periodically obscures the optical and soft X-ray emission. This is also supported by the changes in phase-resolved X-ray spectra.

  12. Particle acceleration from an inner accretion disc into compact corona and further out: case of an organised magnetic field near a supermassive black hole

    NASA Astrophysics Data System (ADS)

    Karas, Vladimir; Kopacek, Ondrej; Kunneriath, Devaky; Kovar, Jiri; Slany, Petr

    2016-04-01

    Upcoming observational techniques in X-rays and millimeter spectral bands will allow to probe the inner corona of accretion discs near supermassive black holes. Size of this region only a few gravitational radii has been inferred from various circumstantial evidence. To populate ithe region with particles, pair-creation in ergosphere and transport of particles via accretion have been invoked.Electromagnetic fields are a likely agent of acceleration in strong gravity of a rotating black hole. We put forward a scenario with an organised component of the magnetic field near a supermassive black hole. An emergent flow of particles may be induced in a preferentially bi-polar direction. Our mechanism does not seem to be capable of producing ultra-high energy cosmic rays but it does expel particles along unbound trajectories.The mentioned concept is relevant also from a purely theoretical viewpoint of dynamical properties of particle motion in General Relativity, namely, the onset of chaos near a black hole. We conclude that the role of black-hole spin in setting the chaos is more complicated than initially thought (based on http://arxiv.org/abs/1408.2452).

  13. X-ray Dips Followed by Superluminal Ejections as Evidence for An Accretion Disc Feeding the Jet in A Radio Galaxy

    NASA Technical Reports Server (NTRS)

    Marscher, Alan P.; Jorstad, Svetlana G.; Gomez, Jose-Luis; Aller, Margo F.; Terasranta, Harri; Lister, Matthew L.; Stirling, Alastair, M.

    2002-01-01

    Accretion onto black holes is thought to power the relativistic jets and other high-energy phenomena in both active galactic nuclei (AGNs) and the "microquasar" binary systems located in our Galaxy. However, until now there has been insufficient multifrequency monitoring to establish a direct observational link between the black hole and the jet in an AGE. This contrasts with the case of microquasars, in which superluminal features appear and propagate down the radio jet shortly after sudden decreases in the X-ray flux. Such an X-ray dip is most likely caused by the disappearance of a section of the inner accretion disc, part of which falls past the event horizon and the remainder of which is injected into the jet. This infusion of energy generates a disturbance that propagates down the jet, creating the appearance of a superluminal bright spot. Here we report the results of three years of intensive monitoring of the X-ray and radio emission of the Seyfert-like radio galaxy 3C 120. As in the case of microquasars, dips in the X-ray emission are followed by ejections of bright superluminal knots in the radio jet. Comparison of the characteristic length and time scales allows us to infer that the rotational states of the black holes in these two objects are different.

  14. Binary accretion rates: dependence on temperature and mass ratio

    NASA Astrophysics Data System (ADS)

    Young, M. D.; Clarke, C. J.

    2015-09-01

    We perform a series of 2D smoothed particle hydrodynamics simulations of gas accretion on to binaries via a circumbinary disc, for a range of gas temperatures and binary mass ratios (q). We show that increasing the gas temperature increases the accretion rate on to the primary for all values of the binary mass ratio: for example, for q = 0.1 and a fixed binary separation, an increase of normalized sound speed by a factor of 5 (from our `cold' to `hot' simulations) changes the fraction of the accreted gas that flows on to the primary from 10 to ˜40 per cent. We present a simple parametrization for the average accretion rate of each binary component accurate to within a few per cent and argue that this parametrization (rather than those in the literature based on warmer simulations) is relevant to supermassive black hole accretion and all but the widest stellar binaries. We present trajectories for the growth of q during circumbinary disc accretion and argue that the period distribution of stellar `twin' binaries is strong evidence for the importance of circumbinary accretion. We also show that our parametrization of binary accretion increases the minimum mass ratio needed for spin alignment of supermassive black holes to q ˜ 0.4, with potentially important implications for the magnitude of velocity kicks acquired during black hole mergers.

  15. Analytical computation of two integrals, appearing in the theory of elliptical accretion discs. III. Solving of the full set of auxiliary integrals, containing logarithmic functions into their integrands

    NASA Astrophysics Data System (ADS)

    Dimitrov, Dimitar

    2014-09-01

    The present investigation encloses the started in the earlier papers [3] and [4] analytical evaluations of some kinds definite integrals. These solutions are necessary steps towards the revealing the mathematical structure of the dynamical equation, governing the properties of the stationary elliptical accretion discs, which apse lines of all orbits are in line with each other[5]. Though the considered here task, at first glance, may seem as a purely mathematical one, there are some restrictions of physical nature on the variables, entering as arguments into the integrals. In this paper we resolve analytically the following two definite integrals, including into their nominators (as a factor) the logarithmic function ln(1 + ecosφ). Concretely, we find in an explicit form the solutions of the integrals Li(e,ė) ≡ 2 π ≡ ∫[ln(1 + ecosφ)](1 + ecosφ) – 1[1 + (e – ė)cosφ] – i dφ, (i = 0,…, 3), 0 2 π and Kj(e,ė) ≡ ∫[ln(1 + ecosφ)] [1 +(e – ė)cosφ] – j dφ, (j = 1, …, 5). 0 Here we have used the following notations: φ is the azimutal angle. The integration over φ from 0 to 2π means an averaging over the whole trajectory for each disc particle. Each such particle spirals inward to the center of the disc, moving on (quasi-) elliptical orbits with focal parameters p. These parameters p are allowed to vary for different elliptical orbits.In the our approach of computations, we treat e(u) and ė(u) as independent variables. The physically imposed restrictions (which, to some extend, lead to simplifications of the problems) are |e(u)| < 1,|ė(u)| < 1 and |e(u) – ė(u)| < 1 for all admitted values of u. That is to say, between the innermost and outermost orbits of the disc. Consequently, the established in this paper analytical solutions for the integrals Li(e,ė), (i = 0,…, 3) and Kj(e,ė), (j = 1, …, 5), are, probably, not the most general ones, even in the domain of the real analysis. But nevertheless, they are sufficient for our

  16. The structure of protoplanetary discs around evolving young stars

    NASA Astrophysics Data System (ADS)

    Bitsch, Bertram; Johansen, Anders; Lambrechts, Michiel; Morbidelli, Alessandro

    2015-03-01

    The formation of planets with gaseous envelopes takes place in protoplanetary accretion discs on time scales of several million years. Small dust particles stick to each other to form pebbles, pebbles concentrate in the turbulent flow to form planetesimals and planetary embryos and grow to planets, which undergo substantial radial migration. All these processes are influenced by the underlying structure of the protoplanetary disc, specifically the profiles of temperature, gas scale height, and density. The commonly used disc structure of the minimum mass solar nebula (MMSN) is a simple power law in all these quantities. However, protoplanetary disc models with both viscous and stellar heating show several bumps and dips in temperature, scale height, and density caused by transitions in opacity, which are missing in the MMSN model. These play an important role in the formation of planets, since they can act as sweet spots for forming planetesimals via the streaming instability and affect the direction and magnitude of type-I migration. We present 2D simulations of accretion discs that feature radiative cooling and viscous and stellar heating, and they are linked to the observed evolutionary stages of protoplanetary discs and their host stars. These models allow us to identify preferred planetesimal and planet formation regions in the protoplanetary disc as a function of the disc's metallicity, accretion rate, and lifetime. We derive simple fitting formulae that feature all structural characteristics of protoplanetary discs during the evolution of several Myr. These fits are straightforward for applying to modelling any growth stage of planets where detailed knowledge of the underlying disc structure is required. Appendix A is available in electronic form at http://www.aanda.org

  17. Long-Term Properties of Accretion Discs in X-ray Binaries. 1; The Variable Third Period in SMC X-1

    NASA Technical Reports Server (NTRS)

    Charles, P. A.; Clarkson, W. I.; Coe, M. J.; Laycock, S.; Tout, M.; Wilson, C.; Six, N. Frank (Technical Monitor)

    2002-01-01

    Long term X-ray monitoring data from the RXTE All Sky Monitor (ASM) reveal that the third (superorbital) period in SMC X-1 is not constant but varies between 40-60 days. A dynamic power spectrum analysis indicates that the third period has been present continuously throughout the five years of ASM observations. This period changed smoothly from 60 days to 45 days and then returned to its former value, on a timescale of approximately 1600 days. During the nearly 4 years of overlap between the CGRO & RXTE missions, the simultaneous BATSE hard X-ray data confirm this variation in SMC X-1. Sources of systematic error and possible artefacts are investigated and found to be incapable of reproducing the results reported here. Our disco cry of such an instability in the superorbital period of SMC X-1 is interpreted in the context of recent theoretical studies of warped, precessing accretion discs. We find that the behaviour of SMC X-1 is consistent with a radiation - driven warping model.

  18. The long-term evolution of photoevaporating transition discs with giant planets

    NASA Astrophysics Data System (ADS)

    Rosotti, Giovanni P.; Ercolano, Barbara; Owen, James E.

    2015-12-01

    Photoevaporation and planet formation have both been proposed as mechanisms responsible for the creation of a transition disc. We have studied their combined effect through a suite of 2D simulations of protoplanetary discs undergoing X-ray photoevaporation with an embedded giant planet. In a previous work, we explored how the formation of a giant planet triggers the dispersal of the inner disc by photoevaporation at earlier times than what would have happened otherwise. This is particularly relevant for the observed transition discs with large holes and high mass accretion rates that cannot be explained by photoevaporation alone. In this work, we significantly expand the parameter space investigated by previous simulations. In addition, the updated model includes thermal sweeping, needed for studying the complete dispersal of the disc. After the removal of the inner disc, the disc is a non-accreting transition disc, an object that is rarely seen in observations. We assess the relative length of this phase, to understand if it is long lived enough to be found observationally. Depending on the parameters, especially on the X-ray luminosity of the star, we find that the fraction of time spent as a non-accretor greatly varies. We build a population synthesis model to compare with observations and find that in general thermal sweeping is not effective enough to destroy the outer disc, leaving many transition discs in a relatively long lived phase with a gas-free hole, at odds with observations. We discuss the implications for transition disc evolution. In particular, we highlight the current lack of explanation for the missing non-accreting transition discs with large holes, which is a serious issue in the planet hypothesis.

  19. The broad emission-line region: the confluence of the outer accretion disc with the inner edge of the dusty torus

    NASA Astrophysics Data System (ADS)

    Goad, M. R.; Korista, K. T.; Ruff, A. J.

    2012-11-01

    We have investigated the observational characteristics of a class of broad emission line region (BLR) geometries that connect the outer accretion disc with the inner edge of the dusty toroidal obscuring region (TOR). We suggest that the BLR consists of photoionized gas of densities which allow for efficient cooling by ultraviolet (UV)/optical emission lines and of incident continuum fluxes which discourage the formation of grains, and that such gas occupies the range of distance and scale height between the continuum-emitting accretion disc and the dusty TOR. As a first approximation, we assume a population of clouds illuminated by ionizing photons from the central source, with the scale height of the illuminated clouds growing with increasing radial distance, forming an effective surface of a 'bowl'. Observer lines of sight which peer into the bowl lead to a Type 1 active galactic nuclei (AGN) spectrum. We assume that the gas dynamics are dominated by gravity, and we include in this model the effects of transverse Doppler shift (TDS), gravitational redshift (GR) and scale-height-dependent macroturbulence. Our simple model reproduces many of the commonly observed phenomena associated with the central regions of AGN, including (i) the shorter than expected continuum-dust delays (geometry), (ii) the absence of response in the core of the optical recombination lines on short time-scales (geometry/photoionization), (iii) an enhanced redwing response on short time-scales (GR and TDS), (iv) the observed differences between the delays for high- and low-ionization lines (photoionization), (v) identifying one of the possible primary contributors to the observed line widths for near face-on systems even for purely transverse motion (GR and TDS), (vi) a mechanism responsible for producing Lorentzian profiles (especially in the Balmer and Mg II emission lines) in low-inclination systems (turbulence), (vii) the absence of significant continuum-emission-line delays between the

  20. AGN flickering and chaotic accretion

    NASA Astrophysics Data System (ADS)

    King, Andrew; Nixon, Chris

    2015-10-01

    Observational arguments suggest that the growth phases of the supermassive black holes in active galactic nuclei have a characteristic time-scale ˜105 yr. We show that this is the time-scale expected in the chaotic accretion picture of black hole feeding, because of the effect of self-gravity in limiting the mass of any accretion-disc feeding event.

  1. Have protoplanetary discs formed planets?

    NASA Astrophysics Data System (ADS)

    Greaves, J. S.; Rice, W. K. M.

    2010-09-01

    It has recently been noted that many discs around T Tauri stars appear to comprise only a few Jupiter masses of gas and dust. Using millimetre surveys of discs within six local star formation regions, we confirm this result, and find that only a few per cent of young stars have enough circumstellar material to build gas giant planets, in standard core accretion models. Since the frequency of observed exoplanets is greater than this, there is a `missing-mass' problem. As alternatives to simply adjusting the conversion of dust flux to disc mass, we investigate three other classes of solution. Migration of planets could hypothetically sweep up the disc mass reservoir more efficiently, but trends in multiplanet systems do not support such a model, and theoretical models suggest that the gas accretion time-scale is too short for migration to sweep the disc. Enhanced inner-disc mass reservoirs are possible, agreeing with predictions of disc evolution through self-gravity, but not adding to millimetre dust flux as the inner disc is optically thick. Finally, the incidence of massive discs is shown to be higher at the protostellar stages, Classes 0 and I, where discs substantial enough to form planets via core accretion are abundant enough to match the frequency of exoplanets. Gravitational instability may also operate in the Class 0 epoch, where half the objects have potentially unstable discs of >~30 per cent of the stellar mass. However, recent calculations indicate that forming gas giants inside 50 au by instability is unlikely, even in such massive discs. Overall, the results presented suggest that the canonically `protoplanetary' discs of Class II T Tauri stars have globally low masses in dust observable at millimetre wavelengths, and conversion to larger bodies (anywhere from small rocks up to planetary cores) must already have occurred.

  2. The star formation history and accretion-disc fraction among the K-type members of the Scorpius-Centaurus OB association

    NASA Astrophysics Data System (ADS)

    Pecaut, Mark J.; Mamajek, Eric E.

    2016-09-01

    We present results of a spectroscopic survey for new K- and M-type members of Scorpius-Centaurus (Sco-Cen), the nearest OB Association (˜100-200 pc). Using an X-ray, proper motion and colour-magnitude selected sample, we obtained spectra for 361 stars, for which we report spectral classifications and Li and Hα equivalent widths. We identified 156 new members of Sco-Cen, and recovered 51 previously published members. We have combined these with previously known members to form a sample of 493 solar-mass (˜0.7-1.3 M⊙) members of Sco-Cen. We investigated the star formation history of this sample, and re-assessed the ages of the massive main-sequence turn-off and the G-type members in all three subgroups. We performed a census for circumstellar discs in our sample using WISE infrared data and find a protoplanetary disc fraction for K-type stars of 4.4^{+1.6}_{-0.9} per cent for Upper Centaurus-Lupus and Lower Centaurus-Crux at ˜16 Myr and 9.0^{+4.0}_{-2.2} per cent for Upper Scorpius at ˜10 Myr. These data are consistent with a protoplanetary disc e-folding time-scale of ˜4-5 Myr for ˜1 M⊙ stars, twice that previously quoted, but consistent with the Bell et al. revised age scale of young clusters. Finally, we construct an age map of Scorpius-Centaurus which clearly reveals substructure consisting of concentrations of younger and older stars. We find evidence for strong age gradients within all three subgroups. None of the subgroups are consistent with being simple, coeval populations which formed in single bursts, but likely represents a multitude of smaller star formation episodes of hundreds to tens of stars each.

  3. Accretion and Subduction of Oceanic Lithosphere: 2D and 3D Seismic Studies of Off-Axis Magma Lenses at East Pacific Rise 9°37-40'N Area and Downgoing Juan de Fuca Plate at Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Han, Shuoshuo

    Two thirds of the Earth's lithosphere is covered by the ocean. The oceanic lithosphere is formed at mid-ocean ridges, evolves and interacts with the overlying ocean for millions of years, and is eventually consumed at subduction zones. In this thesis, I use 2D and 3D multichannel seismic (MCS) data to investigate the accretionary and hydrothermal process on the ridge flank of the fast-spreading East Pacific Rise (EPR) at 9°37-40'N and the structure of the downgoing Juan de Fuca plate at the Cascadia subduction zone offshore Oregon and Washington. Using 3D multichannel seismic (MCS) data, I image a series of off-axis magma lenses (OAML) in the middle or lower crust, 2-10 km from the ridge axis at EPR 9°37-40'N. The large OAMLs are associated with Moho travel time anomalies and local volcanic edifices above them, indicating off-axis magmatism contributes to crustal accretion though both intrusion and eruption (Chapter 1). To assess the effect of OAMLs on the upper crustal structure, I conduct 2-D travel time tomography on downward continued MCS data along two across-axis lines above a prominent OAML in our study area. I find higher upper crustal velocity in a region ~ 2 km wide above this OAML compared with the surrounding crust. I attribute these local anomalies to enhanced precipitation of alteration minerals in the pore space of upper crust associated with high-temperature off-axis hydrothermal circulation driven by the OAML (Chapter 2). At Cascadia, a young and hot end-member of the global subduction system, the state of hydration of the downgoing Juan de Fuca (JdF) plate is important to a number of subduction processes, yet is poorly known. As local zones of higher porosity and permeability, faults constitute primary conduits for seawater to enter the crust and potentially uppermost mantle. From pre-stack time migrated MCS images, I observe pervasive faulting in the sediment section up to 200 km from the deformation front. Yet faults with large throw and

  4. Chondrule transport in protoplanetary discs

    NASA Astrophysics Data System (ADS)

    Goldberg, Aaron Z.; Owen, James E.; Jacquet, Emmanuel

    2015-10-01

    Chondrule formation remains one of the most elusive early Solar system events. Here, we take the novel approach of employing numerical simulations to investigate chondrule origin beyond purely cosmochemical methods. We model the transport of generically produced chondrules and dust in a 1D viscous protoplanetary disc model in order to constrain the chondrule formation events. For a single formation event we are able to match analytical predictions of the memory they retain of each other (complementarity), finding that a large mass accretion rate (≳10-7 M⊙ yr-1) allows for delays on the order of the disc's viscous time-scale between chondrule formation and chondrite accretion. Further, we find older discs to be severely diminished of chondrules, with accretion rates ≲10-9 M⊙ yr-1 for nominal parameters. We then characterize the distribution of chondrule origins in both space and time, as functions of disc parameters and chondrule formation rates, in runs with continuous chondrule formation and both static and evolving discs. Our data suggest that these can account for the observed diversity between distinct chondrite classes, if some diversity in accretion time is allowed for.

  5. Planetesimal formation in self-gravitating discs

    NASA Astrophysics Data System (ADS)

    Gibbons, P. G.; Rice, W. K. M.; Mamatsashvili, G. R.

    2012-10-01

    We study particle dynamics in local two-dimensional simulations of self-gravitating accretion discs with a simple cooling law. It is well known that the structure which arises in the gaseous component of the disc due to a gravitational instability can have a significant effect on the evolution of dust particles. Previous results using global simulations indicate that spiral density waves are highly efficient at collecting dust particles, creating significant local overdensities which may be able to undergo gravitational collapse. We expand on these findings using a range of cooling times to mimic the conditions at a large range of radii within the disc. Here we use the PENCIL code to solve the 2D local shearing sheet equations for gas on a fixed grid together with the equations of motion for solids coupled to the gas solely through aerodynamic drag force. We find that spiral density waves can create significant enhancements in the surface density of solids, equivalent to 1-10 cm sized particles in a disc following the profiles of Clarke around an ˜1 M⊙ star, causing it to reach concentrations several orders of magnitude larger than the particles mean surface density. We also study the velocity dispersion of the particles, finding that the spiral structure can result in the particle velocities becoming highly ordered, having a narrow velocity dispersion. This implies low relative velocities between particles, which in turn suggest that collisions are typically low energy, lessening the likelihood of grain destruction. Both these findings suggest that the density waves that arise due to gravitational instabilities in the early stages of star formation provide excellent sites for the formation of large, planetesimal-sized objects.

  6. MASS ACCRETION RATE OF ROTATING VISCOUS ACCRETION FLOW

    SciTech Connect

    Park, Myeong-Gu

    2009-11-20

    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 flow profile from the spherical Bondi flow, and hence its mass accretion rate, but most work on disc accretion has taken the mass flux to be given with the relation between that parameter and external conditions left uncertain. Within the framework of a slim alpha disk, we have constructed global solutions of the rotating, viscous, hot accretion flow in the Paczynski-Wiita potential and determined its mass accretion rate as a function of density, temperature, and angular momentum of gas at the outer boundary. We find that the low angular momentum flow resembles the spherical Bondi flow and its mass accretion rate approaches the Bondi accretion rate for the same density and temperature at the outer boundary. The high angular momentum flow on the other hand is the conventional hot accretion disk with advection, but its mass accretion rate can be significantly smaller than the Bondi accretion rate with the same boundary conditions. We also find that solutions exist only within a limited range of dimensionless mass accretion rate m-dotident toM-dot/M-dot{sub B}, where M-dot is the mass accretion rate and M-dot{sub B} is the Bondi accretion rate: when the temperature at the outer boundary is equal to the virial temperature, solutions exist only for 0.05approxaccretion rate is roughly independent of the radius of the outer boundary but inversely proportional to the angular momentum at the outer boundary and proportional to the viscosity parameter, m-dotapprox =9.0 alphalambda{sup -1} when 0.1 approx

  7. Galactic Fountains and Gas Accretion

    NASA Astrophysics Data System (ADS)

    Marinacci, F.; Binney, J.; Fraternali, F.; Nipoti, C.; Ciotti, L.; Londrillo, P.

    2010-06-01

    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 subsequent accretion are caused by the passage of cold galactic-fountain clouds through the hot corona. The Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas causes coronal gas to condense in the cloud's wake. For likely parameters of the Galactic corona and of typical fountain clouds we obtain a global accretion rate of the order of that required to feed the star formation.

  8. Hydrodynamics of embedded planets' first atmospheres - I. A centrifugal growth barrier for 2D flows

    NASA Astrophysics Data System (ADS)

    Ormel, Chris W.; Kuiper, Rolf; Shi, Ji-Ming

    2015-01-01

    In the core accretion paradigm of planet formation, gas giants only form a massive atmosphere after their progenitors exceeded a threshold mass: the critical core mass. Most (exo)planets, being smaller and rock/ice-dominated, never crossed this line. Nevertheless, they were massive enough to attract substantial amounts of gas from the disc, while their atmospheres remained in pressure-equilibrium with the disc. Our goal is to characterize the hydrodynamical properties of the atmospheres of such embedded planets and the implications for their (long-term) evolution. In this paper - the first in series - we start to investigate the properties of an isothermal and inviscid flow past a small, embedded planet by conducting local, 2D hydrodynamical simulations. Using the PLUTO code, we confirm that the flow is steady and bound. This steady outcome is most apparent for the log-polar grid (with the grid spacing proportional to the distance from the planet). For low-mass planets, Cartesian grids are somewhat less efficient as they have difficulty to follow the circular, large speeds in the deep atmosphere. Relating the amount of rotation to the gas fraction of the atmosphere, we find that more massive atmospheres rotate faster - a finding consistent with Kelvin's circulation theorem. Rotation therefore limits the amount of gas that planets can acquire from the nebula. Dependent on the Toomre-Q parameter of the circumstellar disc, the planet's atmosphere will reach Keplerian rotation before self-gravity starts to become important.

  9. Proto-planetary disc evolution and dispersal

    NASA Astrophysics Data System (ADS)

    Rosotti, Giovanni Pietro

    2015-05-01

    Planets form from gas and dust discs in orbit around young stars. The timescale for planet formation is constrained by the lifetime of these discs. The properties of the formed planetary systems depend thus on the evolution and final dispersal of the discs, which is the main topic of this thesis. Observations reveal the existence of a class of discs called "transitional", which lack dust in their inner regions. They are thought to be the last stage before the complete disc dispersal, and hence they may provide the key to understanding the mechanisms behind disc evolution. X-ray photoevaporation and planet formation have been studied as possible physical mechanisms responsible for the final dispersal of discs. However up to now, these two phenomena have been studied separately, neglecting any possible feedback or interaction. In this thesis we have investigated what is the interplay between these two processes. We show that the presence of a giant planet in a photo-evaporating disc can significantly shorten its lifetime, by cutting the inner regions from the mass reservoir in the exterior of the disc. This mechanism produces transition discs that for a given mass accretion rate have larger holes than in models considering only X-ray photo-evaporation, constituting a possible route to the formation of accreting transition discs with large holes. These discs are found in observations and still constitute a puzzle for the theory. Inclusion of the phenomenon called "thermal sweeping", a violent instability that can destroy a whole disc in as little as 10 4 years, shows that the outer disc left can be very short-lived (depending on the X-ray luminosity of the star), possibly explaining why very few non accreting transition discs are observed. However the mechanism does not seem to be efficient enough to reconcile with observations. In this thesis we also show that X-ray photo-evaporation naturally explains the observed correlation between stellar masses and accretion

  10. On the evolution of the protolunar disc.

    PubMed

    Ward, William R

    2014-09-13

    The structure and viscous evolution of a post-impact, protolunar disc is examined. The equations for a silicate disc in two-phase (vapour-liquid) equilibrium are employed to derive an analytical solution to vertical structure. Both a vertically mixed phase disc and a stratified disc, where a magma layer exists in the mid-plane surrounded by a vapour reservoir, are considered. The former largely reproduces the low gas mass fraction, x≪1, profiles of the disc described in earlier literature that proposed that the disc would hover on the brink of gravitational instability. In the latter, the vapour layer has x∼1 and is generally gravitationally stable, while the magma layer is vigorously unstable. The viscous evolution of the stratified model is then explored. Initially, the disc quickly settles to a quasi-steady state with a vapour reservoir containing the majority of the disc mass. The magma layer viscously spreads on a time scale of approximately 3-4 years, during which vapour continuously condenses into droplets that settle to the mid-plane, maintaining the magma surface density in spite of disc spreading. Material flowing inwards is accreted by the Earth; material flowing outwards past the Roche boundary can become incorporated into accreting moonlets. This evolution persists until the vapour reservoir is depleted in approximately 50-100 years, depending on its initial mass. PMID:25114314

  11. Simulations of the Galactic Centre Stellar Discs In a Warped Disc Origin Scenario

    NASA Astrophysics Data System (ADS)

    Ulubay-Siddiki, A.; Bartko, H.

    2012-07-01

    The Galactic Center (GC) hosts a population of young stars some of which seem to form a system of mutually inclined warped discs. While the presence of young stars in the close vicinity of the massive black hole is already problematic, their orbital configuration makes the situation even more puzzling. We present a possible warped disc origin scenario for these stars, which assumes an initially flat accretion disc which develops a warp through Pringle instability, or Bardeen-Petterson Effect. By working out the critical radii and the time scales involved, we argue that disc warping is plausible for GC parameters. We construct time evolution models for such discs considering the discs' self-gravity, and the torques exerted by the surrounding old star cluster. Our simulations suggest that the best agreement for a purely self-gravitating model is obtained for a disc-to-black hole mass ratio of Md/Mbh ~ 0.001.

  12. Planetary growth by the accretion of pebbles

    NASA Astrophysics Data System (ADS)

    Lambrechts, Michiel; Johansen, Anders; Bitsch, Bertram; Morbidelli, Alessandro

    2015-11-01

    Pebbles, approximately cm-sized solids that drift through a protoplanetary disc, provide a reservoir of material that can be efficiently accreted by planetary embryos due to the dissipating effect of gas drag (Lambrechts & Johansen, 2012).Here, we will highlight the robust implications of pebble accretion on the formation of planets throughout the protoplanetary disc.In the outer disc, icy pebbles form by coagulation and consequently start drifting inwards. Nevertheless, we find that the pebble surface densities are sufficiently high to form giant planets on wide orbits, before the gas disc disperses after a few Myr (Lambrechts & Johansen, 2014). Growth is only halted when cores reach sizes of around 10 Earth masses, when their gravity creates pressure bumps trapping the inwards drifting pebbles.This accretion cutoff triggers the attraction of a massive gaseous envelope. Additionally, the fast growth of giant planets prevents the loss of the cores by type-I migration (Lambrechts et al 2014, Bitsch et al 2015).Closer to the star, interior to the ice line, pebble accretion takes on a different form. There, chondrule-sized particles lead to the formation of much smaller, Mars-sized embryos, before the pebble flux is terminated by the growth of the gas giants (Morbidelli et al, 2015). We will also discuss ongoing work on the conditions under which much larger Super-Earths can form.

  13. The effects of a magnetic field on planetary migration in laminar and turbulent discs

    NASA Astrophysics Data System (ADS)

    Comins, Megan L.; Romanova, Marina M.; Koldoba, Alexander V.; Ustyugova, Galina V.; Blinova, Alisa A.; Lovelace, Richard V. E.

    2016-07-01

    We investigate the migration of low-mass planets (1, 5 and 20 M⊕) in accretion discs threaded with a magnetic field using 2D magnetohydrodynamic code in polar coordinates. We observed that, in the case of a strong azimuthal magnetic field where the plasma parameter is β ˜ 2-4, density waves at the magnetic resonances exert a positive torque on the planet and may slow down or reverse its migration. However, when the magnetic field is weaker (i.e. the plasma parameter β is relatively large), then non-axisymmetric density waves excited by the planet lead to growth of the radial component of the field and, subsequently, to development of the magnetorotational instability, such that the disc becomes turbulent. Migration in a turbulent disc is stochastic, and the migration direction may change as such. To understand migration in a turbulent disc, both the interaction between a planet and individual turbulent cells, as well as the interaction between a planet and ordered density waves, have been investigated.

  14. The effects of a magnetic field on planetary migration in laminar and turbulent discs

    NASA Astrophysics Data System (ADS)

    Comins, Megan L.; Romanova, Marina M.; Koldoba, Alexander V.; Ustyugova, Galina V.; Blinova, Alisa A.; Lovelace, Richard V. E.

    2016-04-01

    We investigate the migration of low-mass planets (1M⊕, 5M⊕ and 20M⊕) in accretion discs threaded with a magnetic field using 2D MHD code in polar coordinates. We observed that, in the case of a strong azimuthal magnetic field where the plasma parameter is β ˜ 2 - 4, density waves at the magnetic resonances exert a positive torque on the planet and may slow down or reverse its migration. However, when the magnetic field is weaker (i.e., the plasma parameter β is relatively large), then non-axisymmetric density waves excited by the planet lead to growth of the radial component of the field and, subsequently, to development of the magneto-rotational instability, such that the disc becomes turbulent. Migration in a turbulent disc is stochastic, and the migration direction may change as such. To understand migration in a turbulent disc, both the interaction between a planet and individual turbulent cells, as well as the interaction between a planet and ordered density waves, have been investigated.

  15. From birth to death of protoplanetary discs: modelling their formation, evolution and dispersal

    NASA Astrophysics Data System (ADS)

    Kimura, Shigeo S.; Kunitomo, Masanobu; Takahashi, Sanemichi Z.

    2016-09-01

    The formation, evolution and dispersal processes of protoplanetary discs are investigated and the disc lifetime is estimated. The gravitational collapse of a pre-stellar core forms both a central star and a protoplanetary disc. The central star grows by accretion from the disc and irradiation by the central star heats up the disc and generates a thermal wind, which results in the disc's dispersal. Using the one-dimensional diffusion equation, we calculate the evolution of protoplanetary discs numerically. To calculate the disc evolution from formation to dispersal, we add source and sink terms that represent gas accretion from pre-stellar cores and photoevaporation, respectively. We find that the disc lifetimes of typical pre-stellar cores are around 2-4 million years (Myr). A pre-stellar core with high angular momentum forms a larger disc with a long lifetime, while a disc around an X-ray-luminous star has a short lifetime. Integrating disc lifetimes under various masses and angular velocities of pre-stellar cores and X-ray luminosities of young stellar objects, we obtain the disc fraction at a given stellar age and mean lifetime of the disc. Our model indicates that the mean lifetime of a protoplanetary disc is 3.7 Myr, which is consistent with the observational estimate from young stellar clusters. We also find that the dispersion of X-ray luminosity is needed to reproduce the observed disc fraction.

  16. Aniso2D

    Energy Science and Technology Software Center (ESTSC)

    2005-07-01

    Aniso2d is a two-dimensional seismic forward modeling code. The earth is parameterized by an X-Z plane in which the seismic properties Can have monoclinic with x-z plane symmetry. The program uses a user define time-domain wavelet to produce synthetic seismograms anrwhere within the two-dimensional media.

  17. Jets from magnetized accretion disks

    NASA Astrophysics Data System (ADS)

    Matsumoto, Ryoji

    When an accretion disk is threaded by large scale poloidal magnetic fields, the injection of magnetic helicity from the accretion disk drives bipolar outflows. We present the results of global magnetohydrodynamic (MHD) simulations of jet formation from a torus initially threaded by vertical magnetic fields. After the torsional Alfvén waves generated by the injected magnetic twists propagate along the large-scale magnetic field lines, magnetically driven jets emanate from the surface of the torus. Due to the magnetic pinch effect, the jets are collimated along the rotation axis. Since the jet formation process extracts angular momentum from the disk, it enhances the accretion rate of the disk material. Through three-dimensional (3D) global MHD simulations, we confirmed previous 2D results that the magnetically braked surface of the disk accretes like an avalanche. Owing to the growth of non-axisymmetric perturbations, the avalanche flow breaks up into spiral channels. Helical structure also appears inside the jet. When magnetic helicity is injected into closed magnetic loops connecting the central object and the accretion disk, it drives recurrent magnetic reconnection and outflows.

  18. Reconstructing the star formation history of the Milky Way disc(s) from chemical abundances

    NASA Astrophysics Data System (ADS)

    Snaith, O.; Haywood, M.; Di Matteo, P.; Lehnert, M. D.; Combes, F.; Katz, D.; Gómez, A.

    2015-06-01

    We develop a chemical evolution model to study the star formation history of the Milky Way. Our model assumes that the Milky Way has formed from a closed-box-like system in the inner regions, while the outer parts of the disc have experienced some accretion. Unlike the usual procedure, we do not fix the star formation prescription (e.g. Kennicutt law) to reproduce the chemical abundance trends. Instead, we fit the abundance trends with age to recover the star formation history of the Galaxy. Our method enables us to recover the star formation history of the Milky Way in the first Gyrs with unprecedented accuracy in the inner (R < 7-8 kpc) and outer (R > 9-10 kpc) discs, as sampled in the solar vicinity. We show that half the stellar mass formed during the thick-disc phase in the inner galaxy during the first 4-5 Gyr. This phase was followed by a significant dip in star formation activity (at 8-9 Gyr) and a period of roughly constant lower-level star formation for the remaining 8 Gyr. The thick-disc phase has produced as many metals in 4 Gyr as the thin-disc phase in the remaining 8 Gyr. Our results suggest that a closed-box model is able to fit all the available constraints in the inner disc. A closed-box system is qualitatively equivalent to a regime where the accretion rate maintains a high gas fraction in the inner disc at high redshift. In these conditions the SFR is mainly governed by the high turbulence of the interstellar medium. By z ~ 1 it is possible that most of the accretion takes place in the outer disc, while the star formation activity in the inner disc is mostly sustained by the gas that is not consumed during the thick-disc phase and the continuous ejecta from earlier generations of stars. The outer disc follows a star formation history very similar to that of the inner disc, although initiated at z ~ 2, about 2 Gyr before the onset of the thin-disc formation in the inner disc.

  19. Mesh2d

    SciTech Connect

    Greg Flach, Frank Smith

    2011-12-31

    Mesh2d is a Fortran90 program designed to generate two-dimensional structured grids of the form [x(i),y(i,j)] where [x,y] are grid coordinates identified by indices (i,j). The x(i) coordinates alone can be used to specify a one-dimensional grid. Because the x-coordinates vary only with the i index, a two-dimensional grid is composed in part of straight vertical lines. However, the nominally horizontal y(i,j0) coordinates along index i are permitted to undulate or otherwise vary. Mesh2d also assigns an integer material type to each grid cell, mtyp(i,j), in a user-specified manner. The complete grid is specified through three separate input files defining the x(i), y(i,j), and mtyp(i,j) variations.

  20. Mesh2d

    Energy Science and Technology Software Center (ESTSC)

    2011-12-31

    Mesh2d is a Fortran90 program designed to generate two-dimensional structured grids of the form [x(i),y(i,j)] where [x,y] are grid coordinates identified by indices (i,j). The x(i) coordinates alone can be used to specify a one-dimensional grid. Because the x-coordinates vary only with the i index, a two-dimensional grid is composed in part of straight vertical lines. However, the nominally horizontal y(i,j0) coordinates along index i are permitted to undulate or otherwise vary. Mesh2d also assignsmore » an integer material type to each grid cell, mtyp(i,j), in a user-specified manner. The complete grid is specified through three separate input files defining the x(i), y(i,j), and mtyp(i,j) variations.« less

  1. The inner cavity of the circumnuclear disc

    NASA Astrophysics Data System (ADS)

    Blank, M.; Morris, M. R.; Frank, A.; Carroll-Nellenback, J. J.; Duschl, W. J.

    2016-06-01

    The circumnuclear disc (CND) orbiting the Galaxy's central black hole is a reservoir of material that can ultimately provide energy through accretion, or form stars in the presence of the black hole, as evidenced by the stellar cluster that is presently located at the CND's centre. In this paper, we report the results of a computational study of the dynamics of the CND. The results lead us to question two paradigms that are prevalent in previous research on the Galactic Centre. The first is that the disc's inner cavity is maintained by the interaction of the central stellar cluster's strong winds with the disc's inner rim, and secondly, that the presence of unstable clumps in the disc implies that the CND is a transient feature. Our simulations show that, in the absence of a magnetic field, the interaction of the wind with the inner disc rim actually leads to a filling of the inner cavity within a few orbital time-scales, contrary to previous expectations. However, including the effects of magnetic fields stabilizes the inner disc rim against rapid inward migration. Furthermore, this interaction causes instabilities that continuously create clumps that are individually unstable against tidal shearing. Thus the occurrence of such unstable clumps does not necessarily mean that the disc is itself a transient phenomenon. The next steps in this investigation are to explore the effect of the magnetorotational instability on the disc evolution and to test whether the results presented here persist for longer time-scales than those considered here.

  2. Vertical 2D Heterostructures

    NASA Astrophysics Data System (ADS)

    Lotsch, Bettina V.

    2015-07-01

    Graphene's legacy has become an integral part of today's condensed matter science and has equipped a whole generation of scientists with an armory of concepts and techniques that open up new perspectives for the postgraphene area. In particular, the judicious combination of 2D building blocks into vertical heterostructures has recently been identified as a promising route to rationally engineer complex multilayer systems and artificial solids with intriguing properties. The present review highlights recent developments in the rapidly emerging field of 2D nanoarchitectonics from a materials chemistry perspective, with a focus on the types of heterostructures available, their assembly strategies, and their emerging properties. This overview is intended to bridge the gap between two major—yet largely disjunct—developments in 2D heterostructures, which are firmly rooted in solid-state chemistry or physics. Although the underlying types of heterostructures differ with respect to their dimensions, layer alignment, and interfacial quality, there is common ground, and future synergies between the various assembly strategies are to be expected.

  3. The Origin and Evolution of Transition Discs: Successes, Problems, and Open Questions

    NASA Astrophysics Data System (ADS)

    Owen, James E.

    2016-02-01

    Transition discs are protoplanetary discs that show evidence for large holes or wide gaps (with widths comparable to their radii) in their dust component. These discs could be giving us clues about the disc destruction mechanism or hints about the location and time-scales for the formation of planets. However, at the moment there remain key gaps in our theoretical understanding. The vast majority of transition discs are accreting onto their central stars, indicating that-at least close to the star-dust has been depleted from the gas by a very large amount. In this review, we discuss evidence for two distinct populations of transition discs: mm-faint-those with low mm-fluxes, small holes (≲ 10 AU), and low accretion rates (~ 10-10 - 10-9 M⊙ yr-1) and mm-bright-discs with large mm-fluxes, large holes (≳ 20 AU), and high accretion rates ~ 10-8 M⊙ yr-1. MM-faint transition discs are consistent with what would naively be expected from a disc undergoing dispersal; however, mm-bright discs are not, and are likely to be rare and long-lived objects. We discuss the two commonly proposed mechanisms for creating transition discs: photoevaporation and planet-disc interactions, with a particular emphasis on how they would evolve in these models, comparing these predictions to the observed population. More theoretical work on explaining the lack of optically thick, non-accreting transition discs is required in both the photoevaporation and planetary hypothesis, before we can start to use transition discs to constrain models of planet formation. Finally, we suggest that the few discs with primordial looking spectral energy distribution, but serendipitously imaged showing large cavities in the mm (e.g. MWC758 and WSB 60) may represent a hidden population of associated objects. Characterising and understanding how these objects fit into the overall paradigm may allow us to unravel the mystery of transition discs.

  4. Intermediate mass black holes in AGN discs - I. Production and growth

    NASA Astrophysics Data System (ADS)

    McKernan, B.; Ford, K. E. S.; Lyra, W.; Perets, H. B.

    2012-09-01

    Here we propose a mechanism for efficiently growing intermediate mass black holes (IMBH) in discs around supermassive black holes. Stellar mass objects can efficiently agglomerate when facilitated by the gas disc. Stars, compact objects and binaries can migrate, accrete and merge within discs around supermassive black holes. While dynamical heating by cusp stars excites the velocity dispersion of nuclear cluster objects (NCOs) in the disc, gas in the disc damps NCO orbits. If gas damping dominates, NCOs remain in the disc with circularized orbits and large collision cross-sections. IMBH seeds can grow extremely rapidly by collisions with disc NCOs at low relative velocities, allowing for super-Eddington growth rates. Once an IMBH seed has cleared out its feeding zone of disc NCOs, growth of IMBH seeds can become dominated by gas accretion from the active galactic nucleus (AGN) disc. However, the IMBH can migrate in the disc and expand its feeding zone, permitting a super-Eddington accretion rate to continue. Growth of IMBH seeds via NCO collisions is enhanced by a pile-up of migrators. We highlight the remarkable parallel between the growth of IMBH in AGN discs with models of giant planet growth in protoplanetary discs. If an IMBH becomes massive enough it can open a gap in the AGN disc. IMBH migration in AGN discs may stall, allowing them to survive the end of the AGN phase and remain in galactic nuclei. Our proposed mechanisms should be more efficient at growing IMBH in AGN discs than the standard model of IMBH growth in stellar clusters. Dynamical heating of disc NCOs by cusp stars is transferred to the gas in an AGN disc helping to maintain the outer disc against gravitational instability. Model predictions, observational constraints and implications are discussed in a companion paper (Paper II).

  5. Janus discs.

    PubMed

    Walther, Andreas; André, Xavier; Drechsler, Markus; Abetz, Volker; Müller, Axel H E

    2007-05-16

    We describe the synthesis and the solution properties of sheet- and disclike Janus particles, containing an inner crosslinked polybutadiene (PB) layer and two different outer sides of polystyrene (PS) and poly(tert-butyl methacrylate) (PtBMA). The structures formed upon adsorption of the flat Janus particles onto solid substrates as well as in THF solution are investigated. The Janus discs are obtained in a template-assisted synthetic pathway followed by sonication. Selectively crosslinking the lamellar PB domains in a well-ordered lamellar microphase-separated bulk morphology of PS-block-PB-block-PtBMA (SBT) block terpolymers leads to the conservation of the compartmentalization of the two outer blocks. Sonication of the crosslinked block terpolymer templates renders soluble sheet- and disclike Janus particles, the size of which can be tuned from the micrometer range down to the nanometer scale. Small-angle X-ray scattering, transmission electron microscopy, dynamic light scattering, scanning force microscopy, and scanning electron microscopy are used to characterize the template-assisted synthetic process and the solution properties. Cryogenic transmission electron microscopy in THF and TEM of particles, embedded into a photo-crosslinkable silicon oil, indicate a supramolecular aggregation behavior of the Janus discs in concentrated solutions. Pendant drop tensiometry demonstrates that Janus sheets and discs can be used to stabilize liquid-liquid interfaces, rendering these materials interesting for future applications. PMID:17441717

  6. Modelling and observations of molecules in discs around young stars

    NASA Astrophysics Data System (ADS)

    Ilee, John David

    2013-04-01

    This thesis contains a study of molecules within circumstellar discs around young stars. Firstly, the chemistry of a disc around a young, Class 0 protostar is modelled. Such discs are thought to be massive, and thus experience gravitational instabilities, which produce spiral density waves. These affect the chemistry in three ways; by desorbing molecules from dust grains, by providing extra energy for new reactions to take place, and by mixing the internal structure of the disc to provide a rich chemistry near the midplane. Secondly, high resolution near-infrared spectra of 20 massive young stellar objects are presented. The objects display CO first overtone bandhead emission, which is excited in the conditions expected within circumstellar discs. The emission is modelled using a simple analytic model of a Keplerian disc, and good fits are found to all spectra. On average, the discs correspond to being geometrically thin, spread across a wide range of inclinations. The discs are located within the dust sublimation radius, providing strong evidence that the CO emission originates in small gaseous discs, supporting the scenario in which massive stars form via disc accretion. Finally, medium resolution near-infrared spectra of 5 Herbig Ae/Be stars are presented. The spectra cover both CO bandhead and Br gamma emission. Accretion rates are derived from the measuring the Br gamma emission and through modelling the CO emission, however these accretion rates are found to be inconsistent. High resolution archival data of one of the targets is presented, and it is shown that this CO disc model is unable to fit the high resolution data. Therefore, it is concluded that to properly fit CO spectra, high resolution data are needed, and that previously published information determined from low resolution spectra should be treated with caution.

  7. Gravitoturbulence in magnetized protostellar discs

    NASA Astrophysics Data System (ADS)

    Riols, A.; Latter, H.

    2016-08-01

    Gravitational instability (GI) features in several aspects of protostellar disc evolution, most notably in angular momentum transport, fragmentation, and the outbursts exemplified by FU Ori and EX Lupi systems. The outer regions of protostellar discs may also be coupled to magnetic fields, which could then modify the development of GI. To understand the basic elements of their interaction, we perform local 2D ideal and resistive magnetohydrodynamics simulations with an imposed toroidal field. In the regime of moderate plasma beta, we find that the system supports a hot gravitoturbulent state, characterized by considerable magnetic energy and stress and a surprisingly large Toomre parameter Q ≳ 10. This result has potential implications for disc structure, vertical thickness, ionization, etc. Our simulations also reveal the existence of long-lived and dense `magnetic islands' or plasmoids. Lastly, we find that the presence of a magnetic field has little impact on the fragmentation criterion of the disc. Though our focus is on protostellar discs, some of our results may be relevant for the outer radii of AGN.

  8. Artificial Disc Replacement

    MedlinePlus

    ... treat this condition, alternatives to disc replacement include fusion, nonoperative care or no treatment. Typically, surgery is ... operative treatment for disc pain has been spinal fusion. This is a surgical procedure in which disc ...

  9. Growth of Gas-giant Cores in Protoplanetary Discs

    NASA Astrophysics Data System (ADS)

    Lambrechts, Michiel

    2011-09-01

    The core accretion scenario is the most successful theoretical model for gas-giant formation. However, the initial growth of the core depends on arbitrary assumptions on planetesimal sizes. Growing the solid core before gas dissipation is problematic due to the long time-scale for run-away accretion, especially in the outer distant regions of a protoplanetary disc. We have studied the dynamics of gas-coupled cm-sized pebbles, gravitationally interacting with larger than km-sized cores. The Pencil Code is used to correctly model the gas drag hydrodynamics. Interestingly, the presence of pebbles in the gaseous disc influences both the dynamics (through dynamical friction) and growth rate of the gas-giant core. Under favourable conditions, i.e. unity mid-plane dust-to-gas ratio and particle growth to mm and cm sizes, pebble accretion turns out to be significantly faster than run-away accretion of planetesimals.

  10. Redundant disc

    NASA Technical Reports Server (NTRS)

    Barack, W. N.; Domas, P. A.; Beekman, S. W. (Inventor)

    1978-01-01

    A rotatable disc is described that consists of parallel plates tightly joined together for rotation about a hub. Each plate is provided with several angularly projecting spaced lands. The lands of each plate are interposed in alternating relationship between the lands of the next adjacent plate. In this manner, circumferential displacement of adjacent sectors in any one plate is prevented in the event that a crack develops. Each plate is redundantly sized so that, in event of structural failure of one plate, the remaining plates support a proportionate share of the load of the failed plate. The plates are prevented from separating laterally through the inclusion of generally radially extending splines which are inserted to interlock cooperating, circumferentially adjacent lands.

  11. Intervertebral disc disease.

    PubMed

    Simpson, S T

    1992-07-01

    This article describes the functional anatomy of intervertebral discs and their relationship to the vertebrae and spinal cord. The pathologic events and clinical complications of intervertebral disc disease are described. A discussion of proper staging of disc disease and appropriate conservative management of degenerative disc disease is included. PMID:1641922

  12. Partial accretion in the propeller stage of accreting millisecond X-ray pulsars

    NASA Astrophysics Data System (ADS)

    Gungor, Can; Gogus, Ersin; Eksi, Kazim Yavuz; Guver, Tolga

    2016-07-01

    Accreting millisecond X-ray pulsars (AMXPs) are very important objects for studying the stages of disk - magnetosphere interaction as these objects may show different stages in an observable duration. A typical X-ray light curve of an outburst of AMXP has a fast rise and an exponential decay phases. Most of the outbursts have a knee where the flux goes from the slow decay stage to the rapid decay stage. This knee may be linked to the transition from accretion to propeller stage. Since, after the knee, the X-ray luminosity of the source is still higher than its quiescent level, the accretion from inner disc must be continuing in the propeller stage with a lower fraction than in the accretion stage. The X-ray does not only come from accretion onto the poles but the inner parts of the disk may also contribute to the total X-ray luminosity. To infer what fraction (f) of the inflowing matter accretes onto the star the light curve in the propeller stage, one should first separate the emission originating from the disk and obtain a light curve of X-ray emission only from the magnetic poles. We provide a new method to infer from the observational data the fraction of accreting matter onto the neutron star pole to the mass transferring from outer layers of the disc to the inner disc (f), as a function of the fastness parameter (ω_{*}), assuming the knee is due to the transition from accretion to the propeller stage. We transform X-ray luminosities to the mass fraction, f, and the time scale of outburst to fastness parameter, ω_*. It allows us to compare different types of outbursts of an AMXP in f - ω_* space which is universal for a unique system. We analysed the Rossi X-ray Timing Explorer/Proportional Counter Array (RXTE/PCA) observations of the 2000 and the 2011 outbursts and the Swift Gamma-Ray Burst Mission/X-ray Telescope (SWIFT/XRT) data of the 2013 outburst of the most known AMXP, Aql X-1 using a combination of blackbody representing hot spot, disk blackbody

  13. MHD of accretion-disk flows

    NASA Astrophysics Data System (ADS)

    Yankova, Krasimira

    2015-01-01

    Accretion is one of the most important problems of astrophysics concerning the transfer of matter and the transformation of energy into space. Process represents a falling of the substance on a cosmic object from the surrounding area and is a powerful gravitational mechanism for the production of radiation. Accretion disc effectively converts the mass of the substance by viscous friction and released potential energy transformed into radiation by particle collisions. Accretion onto compact object shows high energy efficiency and temporal variability in a broad class of observational data in all ranges. In the disks of these objects are developed a series instabilities and structures that govern the distribution of the energy. They are expressed in many variety non-stationary phenomena that we observe. That is why we propose generalized model of magnetized accretion disk with advection, which preserves the nonlinearity of the problem. We study interaction of the plasmas flow with the magnetic field, and how this affects the self-organizing disk. The aim of the work is to describe the accretion flow in detail, in his quality of the open astrophysical system, to investigate the evolution and to reveal the mechanisms of the structuring the disk-corona system for to interpret correctly the high energy behavior of such sources.

  14. Disc-planet interactions in subkeplerian discs

    NASA Astrophysics Data System (ADS)

    Paardekooper, S.-J.

    2009-11-01

    Context: One class of protoplanetary disc models, the X-wind model, predicts strongly subkeplerian orbital gas velocities, a configuration that can be sustained by magnetic tension. Aims: We investigate disc-planet interactions in these subkeplerian discs, focusing on orbital migration for low-mass planets and gap formation for high-mass planets. Methods: We use linear calculations and nonlinear hydrodynamical simulations to measure the torque and look at gap formation. In both cases, the subkeplerian nature of the disc is treated as a fixed external constraint. Results: We show that, depending on the degree to which the disc is subkeplerian, the torque on low-mass planets varies between the usual type I torque and the one-sided outer Lindblad torque, which is also negative but an order of magnitude stronger. In strongly subkeplerian discs, corotation effects can be ignored, making migration fast and inward. Gap formation near the planet's orbit is more difficult in such discs, since there are no resonances close to the planet accommodating angular momentum transport. The location of the gap is shifted inwards with respect to the planet, leaving the planet on the outside of a surface density depression. Conclusions: Depending on the degree to which a protoplanetary disc is subkeplerian, disc-planet interactions can be very different from the usual Keplerian picture, making these discs in general more hazardous for young planets.

  15. Appearance of Keplerian discs orbiting Kerr superspinars

    NASA Astrophysics Data System (ADS)

    Stuchlík, Zdeněk; Schee, Jan

    2010-11-01

    We study optical phenomena related to the appearance of Keplerian accretion discs orbiting Kerr superspinars predicted by string theory. The superspinar exterior is described by standard Kerr naked singularity geometry breaking the black hole limit on the internal angular momentum (spin). We construct local photon escape cones for a variety of orbiting sources that enable us to determine the superspinars silhouette in the case of distant observers. We show that the superspinar silhouette depends strongly on the assumed edge where the external Kerr spacetime is joined to the internal spacetime governed by string theory and significantly differs from the black hole silhouette. The appearance of the accretion disc is strongly dependent on the value of the superspinar spin in both their shape and frequency shift profile. Apparent extension of the disc grows significantly with the growing spin, while the frequency shift grows with the descending spin. This behaviour differs substantially from the appearance of discs orbiting black holes enabling thus, at least in principle, to distinguish clearly the Kerr superspinars and black holes. In vicinity of a Kerr superspinar the non-escaped photons have to be separated to those captured by the superspinar and those being trapped in its strong gravitational field leading to self-illumination of the disc that could even influence its structure and cause self-reflection effect of radiation of the disc. The amount of trapped photons grows with descending superspinar spin. We thus can expect significant self-illumination effects in the field of Kerr superspinars with near-extreme spin a ~ 1.

  16. High divergent 2D grating

    NASA Astrophysics Data System (ADS)

    Wang, Jin; Ma, Jianyong; Zhou, Changhe

    2014-11-01

    A 3×3 high divergent 2D-grating with period of 3.842μm at wavelength of 850nm under normal incidence is designed and fabricated in this paper. This high divergent 2D-grating is designed by the vector theory. The Rigorous Coupled Wave Analysis (RCWA) in association with the simulated annealing (SA) is adopted to calculate and optimize this 2D-grating.The properties of this grating are also investigated by the RCWA. The diffraction angles are more than 10 degrees in the whole wavelength band, which are bigger than the traditional 2D-grating. In addition, the small period of grating increases the difficulties of fabrication. So we fabricate the 2D-gratings by direct laser writing (DLW) instead of traditional manufacturing method. Then the method of ICP etching is used to obtain the high divergent 2D-grating.

  17. Swept wing ice accretion modeling

    NASA Technical Reports Server (NTRS)

    Potapczuk, Mark G.; Bidwell, Colin S.

    1990-01-01

    An effort to develop a three-dimensional modeling method was initiated. This first step towards creation of a complete aircraft icing simulation code builds on previously developed methods for calculating three-dimensional flow fields and particle trajectories combined with a two-dimensional ice accretion calculation along coordinate locations corresponding to streamlines. This work is a demonstration of the types of calculations necessary to predict a three-dimensional ice accretion. Results of calculations using the 3-D method for a MS-317 swept wing geometry are projected onto a 2-D plane normal to the wing leading edge and compared to 2-D results for the same geometry. It is anticipated that many modifications will be made to this approach, however, this effort will lay the groundwork for future modeling efforts. Results indicate that the flow field over the surface and the particle trajectories differed for the two calculations. This led to lower collection efficiencies, convective heat transfer coefficients, freezing fractions, and ultimately ice accumulation for the 3-D calculation.

  18. Self-Organised Criticality in Astrophysical Accretion Systems

    NASA Astrophysics Data System (ADS)

    Dendy, R. O.; Helander, P.; Tagger, M.

    Self-organised criticality (SOC) has been proposed as a potentially powerful unifying paradigm for interpreting non-diffusive avalanche-type transport in laboratory, space and astrophysical plasmas. After reviewing the most promising astrophysical sites where SOC might be observable, we consider the theoretical arguments for supposing that SOC can occur in accretion discs. Perhaps the most rigorous evidence is provided by numerical modelling of energy dissipation due to magnetohydrodynamic turbulence in accretion discs by G. Geertsema & A. Achterberg (Astron. Astrophys. 255, 427 (1992)); we investigate how “sandpile”-type dynamics arise in this model. It is concluded that the potential sites for SOC in accretion systems are numerous and observationally accessible, and that theoretical support for the possible occurrence of SOC can be derived from first principles.

  19. Properties of optically thick coronae around accreting black holes

    NASA Astrophysics Data System (ADS)

    Belmont, R.; Różańska, A.; Malzac, J.; Czerny, B.; Petrucci, P.-O.

    2015-12-01

    Accreting black holes are complex sources exhibiting several spectral components (disc, jet, hot corona etc). The exact nature and the interplay between these components is still uncertain, and constraining the accretion flow in the vicinity of the compact object has become a key problem to understand the general physics of accretion and ejection. In the past years, the X-ray spectra of several X-ray binaries and AGN have suggested the existence of a new type of coronae in the inner part of their accretion disk. These coronae are warm (about 1 keV) and have Thomson optical depths of about τ ≈ 10, much larger than the standard comptonizing medium inferred in black hole systems. However, simple radiative models based on the diffusion approximation are unable to sustain a large temperature over such high optical depths, therefore questioning existence of these thick coronae. Here we investigate the radiative and hydrostatic properties of slabs, thick coronae covering a standard accretion disc. A precise modelling of the radiation transfer shows that the observed temperature inversion can be reproduced, provided that most of the accretion power is dissipated in this upper layer and that the medium is strongly magnetised.

  20. Two-dimensional vortices and accretion disks

    NASA Astrophysics Data System (ADS)

    Nauta, Michiel Doede

    2000-01-01

    Observations show that there are disks around certain stars that slowly rain down on the central (compact) object: accretion disks. The rate of depletion of the disk might be slow but is still larger than was expected on theoretical grounds. That is why it has been suggested that the disks are turbulent. Because the disk is thin and rotating this turbulence might be related to two-dimensional (2D) turbulence which is characterized by energy transfers towards small wave numbers and the formation of 2D-vortices. This hypothesis is investigated in this thesis by numerical simulations. After an introduction, the numerical algorithm that was inplemented is discussed together with its relation to an accretion disk. It performs well under the absence of discontinuities. The code is used to study 2D-turbulence under the influence of background rotation with compressibility and a shearing background flow. The first is found to be of little consequence but the shear flow alters 2D-turbulence siginificantly. Only prograde vortices of enough strength are able to withstand the shear flow. The size of the vortices in the cross stream direction is also found to be smaller than the equivalent of the thickness of an accretion disk. These circulstances imply that the assumption of two-dimensionality is questionable so that 2D-vortices might not abound in accretion disks. However, the existence of such vortices is not ruled out and one such a cortex is studied in detail in chapter 4. The internal structure of the vortex is well described by a balance between Coriolis, centrifugal and pressure forces. The vortex is also accompanied by two spiral compressible waves. These are not responsible for the azimuthal drift of the vortex, which results from secondary vortices, but they might be related to the small radial drift that is observed. Radial drift leads to accretion but it is not very efficient. Multiple vortex interactions are the topic of tha last chapter and though interesting the

  1. ACCRETION ONTO INTERMEDIATE-MASS BLACK HOLES REGULATED BY RADIATIVE FEEDBACK. I. PARAMETRIC STUDY FOR SPHERICALLY SYMMETRIC ACCRETION

    SciTech Connect

    Park, Kwang Ho; Ricotti, Massimo E-mail: ricotti@astro.umd.edu

    2011-09-20

    We study the effect of radiative feedback on accretion onto intermediate-mass black holes (IMBHs) using the hydrodynamical code ZEUS-MP with a radiative transfer algorithm. In this paper, the first of a series, we assume accretion from a uniformly dense gas with zero angular momentum and extremely low metallicity. Our one-dimensional (1D) and 2D simulations explore how X-ray and UV radiation emitted near the black hole regulates the gas supply from large scales. Both 1D and 2D simulations show similar accretion rates and periods between peaks in accretion, meaning that the hydro-instabilities that develop in 2D simulations do not affect the mean flow properties. We present a suite of simulations exploring accretion across a large parameter space, including different radiative efficiencies and radiation spectra, black hole masses, density, and temperature, T{sub {infinity}}, of the neighboring gas. In agreement with previous studies, we find regular oscillatory behavior of the accretion rate, with duty cycle {approx}6%, mean accretion rate 3% (T{sub {infinity}}/10{sup 4} K){sup 2.5} of the Bondi rate and peak accretion {approx}10 times the mean for T{sub {infinity}} ranging between 3000 K and 15, 000 K. We derive parametric formulae for the period between bursts, the mean accretion rate, and the peak luminosity of the bursts and thus provide a formulation of how feedback-regulated accretion operates. The temperature profile of the hot ionized gas is crucial in determining the accretion rate, while the period of the bursts is proportional to the mean size of the Stroemgren sphere, and we find qualitatively different modes of accretion in the high versus low density regimes. We also find that a softer radiation spectrum produces a higher mean accretion rate. However, it is still unclear what the effect of a significant time delay is between the accretion rate at our inner boundary and the output luminosity. Such a delay is expected in realistic cases with non

  2. Video Discs in Education.

    ERIC Educational Resources Information Center

    Barker, Philip

    1986-01-01

    This discussion of the use of images in learning processes focuses on recent developments in optical storage disc technology, particularly compact disc read-only (CD-ROM) and optical video discs. Interactive video systems and user interfaces are described, and applications in education and industry in the United Kingdom are reviewed. (Author/LRW)

  3. Turbine disc sealing assembly

    DOEpatents

    Diakunchak, Ihor S.

    2013-03-05

    A disc seal assembly for use in a turbine engine. The disc seal assembly includes a plurality of outwardly extending sealing flange members that define a plurality of fluid pockets. The sealing flange members define a labyrinth flow path therebetween to limit leakage between a hot gas path and a disc cavity in the turbine engine.

  4. Observations of accreting pulsars

    NASA Technical Reports Server (NTRS)

    Prince, Thomas A.; Bildsten, Lars; Chakrabarty, Deepto; Wilson, Robert B.; Finger, Mark H.

    1994-01-01

    We discuss recent observations of accreting binary pulsars with the all-sky BATSE instrument on the Compton Gamma Ray Observatory. BATSE has detected and studied nearly half of the known accreting pulsar systems. Continuous timing studies over a two-year period have yielded accurate orbital parameters for 9 of these systems, as well as new insights into long-term accretion torque histories.

  5. Relativistic simulation of flip-flop instabilities of Bondi-Hoyle accretion and quasi-periodic oscillations

    NASA Astrophysics Data System (ADS)

    Dönmez, O.

    2012-10-01

    It is known from recent numerical calculations that Bondi-Hoyle accretion creates a shock cone behind compact objects. This type of accretion leads to instabilities, which can explain certain astrophysical phenomena. In this paper, our main goal is to find the flip-flop behaviour of the shock cone in the relativistic region. In order to do so we have modelled the dynamics of a shock cone around non-rotating and rotating black holes at the equatorial plane in 2D. The effects of the various parameters on the shock cones and instabilities, such as the asymptotic velocity, sound speed, Mach number and adiabatic index, are studied. We have determined the mass accretion rate, shock opening angle, shock cone oscillation, quasi-periodic oscillations (QPOs), and growth rate of instabilities to reveal the disc properties and its radiation. We have discovered, for the first time, flip-flop instabilities around a black hole in the relativistic region by solving the general relativistic hydrodynamical equations. The flip-flop instabilities are found for sound speeds Cs, ∞ < 0.2 with moderate Mach numbers (˜M=3 and M=4 for Cs, ∞ = 0.1 or M=7 and M=8 for Cs, ∞ = 0.05). Our calculation clearly confirms that the shock cone should be detached from the black hole in the Bondi-Hoyle accretion flow with Γ ≥ 2 for non-rotating and rotating black holes. Results reveal that the flip-flopping shock cone not only creates a torque effect on the black hole but also produces continuous X-ray flares with a certain frequency. Furthermore, QPOs originate inside the shock cone and are stronger in regions that have a radius of a few gravitational radii away from the centre owing to the flip-flop oscillation. Finally, our results are compared with the results of numerical and theoretical calculations in Newtonian hydrodynamics, and it is found that they are in good agreement.

  6. AnisWave 2D

    Energy Science and Technology Software Center (ESTSC)

    2004-08-01

    AnisWave2D is a 2D finite-difference code for a simulating seismic wave propagation in fully anisotropic materials. The code is implemented to run in parallel over multiple processors and is fully portable. A mesh refinement algorithm has been utilized to allow the grid-spacing to be tailored to the velocity model, avoiding the over-sampling of high-velocity materials that usually occurs in fixed-grid schemes.

  7. Stellar and gaseous disc structures in cosmological galaxy equilibrium models

    NASA Astrophysics Data System (ADS)

    Rathaus, Ben; Sternberg, Amiel

    2016-05-01

    We present `radially resolved equilibrium models' for the growth of stellar and gaseous discs in cosmologically accreting massive haloes. Our focus is on objects that evolve to redshifts z ˜ 2. We solve the time-dependent equations that govern the radially dependent star formation rates, inflows and outflows from and to the inter- and circumgalactic medium, and inward radial gas flows within the discs. The stellar and gaseous discs reach equilibrium configurations on dynamical time-scales much shorter than variations in the cosmological dark matter halo growth and baryonic accretions rates. We show analytically that mass and global angular momentum conservation naturally give rise to exponential gas and stellar discs over many radial length-scales. As expected, the gaseous discs are more extended as set by the condition Toomre Q < 1 for star formation. The discs rapidly become baryon dominated. For massive, 5 × 1012 M⊙ haloes at redshift z = 2, we reproduced the typical observed star formation rates of ˜100 M⊙ yr-1, stellar masses ˜9 × 1010 M⊙, gas contents ˜1011 M⊙, half-mass sizes of 4.5 and 5.8 kpc for the stars and gas, and characteristic surface densities of 500 and 400 M⊙ pc-2 for the stars and gas.

  8. On the evolution of the Snow Line in Protoplanetary Discs

    NASA Astrophysics Data System (ADS)

    Martin, Rebecca G.; Livio, Mario

    2014-01-01

    We examine the evolution of the snow line in a protoplanetary disc. If the magneto-rotational instability (MRI) drives turbulence throughout the disc, there is a unique snow line outside of which the disc is icy. The snow line moves closer to the star as the infall accretion rate drops. Because the snow line moves inside the radius of the Earth's orbit, the formation of our water-devoid planet is difficult with this model. However, protoplanetary discs are not likely to be sufficiently ionised to be fully turbulent. A dead zone at the mid-plane slows the flow of material through the disc and a global steady state cannot be achieved. We model the evolution of the snow line also in a disc with a dead zone. As the mass is accumulating, the outer parts of the dead zone become self gravitating, heat the massive disc and thus the outer snow line does not come inside the radius of the Earth's orbit. With this model there is sufficient time and mass in the disc for the Earth to form from water-devoid planetesimals at a radius of 1AU. Furthermore, the additional inner icy region within the dead zone predicted by this model may allow for the formation of giant planets close to their host star without the need for much migration.

  9. Accretion phase of star formation in clouds with different metallicities

    NASA Astrophysics Data System (ADS)

    Machida, Masahiro N.; Nakamura, Teppei

    2015-04-01

    The main accretion phase of star formation is investigated in clouds with different metallicities in the range 0 ≤ Z ≤ Z⊙, resolving the protostellar radius. Starting from a near-equilibrium prestellar cloud, we calculate the cloud evolution up to ˜100 yr after the first protostar forms. Star formation differs considerably between clouds with lower (Z ≤ 10-4 Z⊙) and higher (Z > 10-4 Z⊙) metallicities. Fragmentation frequently occurs and many protostars appear without a stable circumstellar disc in lower-metallicity clouds. In these clouds, although protostars mutually interact and some are ejected from the cloud centre, many remain as a small stellar cluster. In contrast, higher-metallicity clouds produce a single protostar surrounded by a nearly stable rotation-supported disc. In these clouds, although fragmentation occasionally occurs in the disc, the fragments migrate inwards and finally fall on to the central protostar. The difference in cloud evolution is due to different thermal evolutions and mass accretion rates. The thermal evolution of the cloud determines the emergence and lifetime of the first core. The first core develops prior to the formation of a protostar in higher-metallicity clouds, whereas no (obvious) first core appears in lower-metallicity clouds. The first core evolves into a circumstellar disc with a spiral pattern, which effectively transfers the angular momentum outwards and suppresses frequent fragmentation. In lower-metallicity clouds, the higher mass accretion rate increases the disc surface density within a very short time, rendering the disc unstable to self-gravity and inducing vigorous fragmentation.

  10. Hoyle-Lyttleton Accretion from a Planar Wind

    NASA Astrophysics Data System (ADS)

    Raymer, Eric

    2014-01-01

    Two-dimensional hydrodynamic simulations of Hoyle-Lyttleton accretion have informed predictions about the evolution of wind-driven accretion systems for over two decades. These simulations frequently exhibit dramatic nonlinear behavior such as the flip-flop instability and the formation of transient accretion disks. During disk accretion, the mass accretion rate is suppressed and angular momentum accretion occurs at quasi-Keplerian rates. These results have been used to interpret neutron star accretion from the equatorially enhanced wind of a Be star in Be/X-ray Binaries. We employ large-scale hydrodynamic simulations to investigate whether the flip-flop instability is possible in three dimensions or is simply a consequence of the restrictions on a 2D flow. We do not observe the flip-flop instability in 3D for any values of the wind scale height or density. Moreover, the angular momentum vector of the accreting gas is typically found to be in the plane of the disk wind rather than perpendicular to it as one might expect based on the results of 2D planar simulations. We measure large-scale asymmetries about the plane of the disk wind that arise due to rotational flow near the accretor. Gas is driven above and below the plane, where it interacts with the bow shock and results in a time-varying shock structure. Winds with scale heights of 0.25 Ra enter locked rotation modes that remain stable for the duration of our computational runs. During this phase, the mass accretion rate is suppressed by up to two orders of magnitude below the analytical prediction and angular momentum accretion occurs at sub-Keplerian values.

  11. Accretion flow diagnostics with X-ray spectral timing: the hard state of SWIFT J1753.5-0127

    NASA Astrophysics Data System (ADS)

    Cassatella, P.; Uttley, P.; Maccarone, T. J.

    2012-12-01

    Recent XMM-Newton studies of X-ray variability in the hard states of black hole X-ray binaries (BHXRBs) indicate that the variability is generated in the 'standard' optically thick accretion disc that is responsible for the multi-colour blackbody emission. The variability originates in the disc as mass-accretion fluctuations and propagates through the disc to 'light up' inner disc regions, eventually modulating the power-law emission that is produced relatively centrally. Both the covariance spectra and time-lags that cover the soft bands strongly support this scenario. Here, we present a comparative spectral-timing study of XMM-Newton data from the BHXRB SWIFT J1753.5-0127 in a bright 2009 hard state with that from the significantly fainter 2006 hard state to show for the first time the change in disc spectral-timing properties associated with a global increase in both the accretion rate and the relative contribution of the disc emission to the bolometric luminosity. We show that, although there is strong evidence for intrinsic disc variability in the more luminous hard state, the disc variability amplitude is suppressed relative to that of the power-law emission, which contrasts with the behaviour at lower luminosities where the disc variability is slightly enhanced when compared with the power-law variations. Furthermore, in the higher luminosity data the disc variability below 0.6 keV becomes incoherent with the power-law and higher energy disc emission at frequencies below 0.5 Hz, in contrast with the coherent variations seen in the 2006 data. We explain these differences and the associated complex lags in the 2009 data in terms of the fluctuating disc model, where the increase in accretion rate seen in 2009 leads to more pronounced and extended disc emission. If the variable signals are generated at small radii in the disc, the variability of disc emission can be naturally suppressed by the fraction of unmodulated disc emission arising from larger radii

  12. Density, Velocity and Ionization Structure in Accretion-Disc Winds

    NASA Technical Reports Server (NTRS)

    Sonneborn, George (Technical Monitor); Long, Knox

    2004-01-01

    This was a project to exploit the unique capabilities of FUSE to monitor variations in the wind- formed spectral lines of the luminous, low-inclination, cataclysmic variables(CV) -- RW Sex. (The original proposal contained two additional objects but these were not approved.) These observations were intended to allow us to determine the relative roles of density and ionization state changes in the outflow and to search for spectroscopic signatures of stochastic small-scale structure and shocked gas. By monitoring the temporal behavior of blue-ward extended absorption lines with a wide range of ionization potentials and excitation energies, we proposed to track the changing physical conditions in the outflow. We planned to use a new Monte Carlo code to calculate the ionization structure of and radiative transfer through the CV wind. The analysis therefore was intended to establish the wind geometry, kinematics and ionization state, both in a time-averaged sense and as a function of time.

  13. 2D kinematic signatures of boxy/peanut bulges

    NASA Astrophysics Data System (ADS)

    Iannuzzi, Francesca; Athanassoula, E.

    2015-07-01

    We study the imprints of boxy/peanut structures on the 2D line-of-sight kinematics of simulated disc galaxies. The models under study belong to a family with varying initial gas fraction and halo triaxiality, plus few other control runs with different structural parameters; the kinematic information was extracted using the Voronoi-binning technique and parametrized up to the fourth order of a Gauss-Hermite series. Building on a previous work for the long-slit case, we investigate the 2D kinematic behaviour in the edge-on projection as a function of the boxy/peanut strength and position angle; we find that for the strongest structures the highest moments show characteristic features away from the mid-plane in a range of position angles. We also discuss the masking effect of a classical bulge and the ambiguity in discriminating kinematically this spherically symmetric component from a boxy/peanut bulge seen end-on. Regarding the face-on case, we extend existing results to encompass the effect of a second buckling and find that this phenomenon spurs an additional set of even deeper minima in the fourth moment. Finally, we show how the results evolve when inclining the disc away from perfectly edge-on and face-on. The behaviour of stars born during the course of the simulations is discussed and confronted to that of the pre-existing disc. The general aim of our study is providing a handle to identify boxy/peanut structures and their properties in latest generation Integral Field Unit observations of nearby disc galaxies.

  14. Why stellar feedback promotes disc formation in simulated galaxies

    NASA Astrophysics Data System (ADS)

    Übler, Hannah; Naab, Thorsten; Oser, Ludwig; Aumer, Michael; Sales, Laura V.; White, Simon D. M.

    2014-09-01

    We study how feedback influences baryon infall on to galaxies using cosmological, zoom-in simulations of haloes with present mass Mvir = 6.9 × 1011 to 1.7 × 1012 M⊙. Starting at z = 4 from identical initial conditions, implementations of weak and strong stellar feedback produce bulge- and disc-dominated galaxies, respectively. Strong feedback favours disc formation: (1) because conversion of gas into stars is suppressed at early times, as required by abundance matching arguments, resulting in flat star formation histories and higher gas fractions; (2) because 50 per cent of the stars form in situ from recycled disc gas with angular momentum only weakly related to that of the z = 0 dark halo; (3) because late-time gas accretion is typically an order of magnitude stronger and has higher specific angular momentum, with recycled gas dominating over primordial infall; (4) because 25-30 per cent of the total accreted gas is ejected entirely before z ˜ 1, removing primarily low angular momentum material which enriches the nearby intergalactic medium. Most recycled gas roughly conserves its angular momentum, but material ejected for long times and to large radii can gain significant angular momentum before re-accretion. These processes lower galaxy formation efficiency in addition to promoting disc formation.

  15. Theory of wind accretion

    NASA Astrophysics Data System (ADS)

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.

    2014-01-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

  16. On the possibility of a warped disc origin of the inclined stellar discs at the Galactic Centre

    NASA Astrophysics Data System (ADS)

    Ulubay-Siddiki, A.; Bartko, H.; Gerhard, O.

    2013-01-01

    The central parsec of our Galaxy hosts a population of young stars. At distances of r ˜ 0.03-0.5 pc, most of these stars seem to form a system of mutually inclined discs of clockwise and counterclockwise rotating stars. We present a possible warped disc origin scenario for these stars assuming that an initially flat accretion disc becomes warped due to a central radiation source via the Pringle instability or due to a spinning black hole via the Bardeen-Petterson effect before it cools, fragments and forms stars. From simple arguments, we show that this is plausible if the star formation efficiency is high, ɛSF ≲ 1, and the viscosity parameter α ˜ 0.1. After fragmentation, we model the disc as a collection of concentric, circular rings tilted with respect to each other, and construct time evolution models of warped discs for mass ratios and other parameters relevant to the Galactic Centre environment, but also for more massive discs. We take into account the disc's self-gravity in the non-linear regime and the torques exerted by a slightly flattened surrounding star cluster. Our simulations show that a self-gravitating low-mass disc (Md/Mbh ˜ 0.001) precesses with its integrity maintained in the lifetime of the stars, but precesses essentially freely when the torques from a non-spherical cluster are included. An intermediate-mass disc (Md/Mbh ˜ 0.01) breaks into pieces, which precess as independent discs in the self-gravity-only case, and become disrupted in the presence of the star cluster torques. Finally, for a high-mass disc (Md/Mbh ˜ 0.1), the evolution is dominated by self-gravity and the disc is broken but not dissolved. The time-scale after which the disc breaks into pieces scales almost linearly with Md/Mbh for self-gravitating models. Typical values are longer than the age of the stars for Md/Mbh ˜ 0.001, and are in the range ˜8 × 104-105 yr for Md/Mbh ˜ 0.1-0.01, respectively. None of these discs explains the two Galactic Centre discs with

  17. Inner disc obscuration in GRS 1915+105 based on relativistic slim disc model

    NASA Astrophysics Data System (ADS)

    Vierdayanti, K.; Sadowski, A.; Mineshige, S.; Bursa, M.

    2013-11-01

    We study the observational signatures of the relativistic slim disc of 10 M⊙ black hole, in a wide range of mass accretion rate, dot{m}, dimensionless spin parameter, a*, and viewing angle, i. In general, the innermost temperature, Tin, increases with the increase of i for a fixed value of dot{m} and a*, due to the Doppler effect. However, for i > 50° and dot{m}>dot{m}_turn, Tin starts to decrease with the increase of dot{m}. This is a result of self-obscuration - the radiation from the innermost hot part of the disc is blocked by the surrounding cooler part. The value of dot{m}_turn and the corresponding luminosities depend on a* and i. Such obscuration effects cause an interesting behaviour on the disc luminosity (Ldisc)-Tin plane for high inclinations. In addition to the standard disc branch which appears below dot{m}_turn and which obeys L_disc ∝ T_in4 relation, another branch above dot{m}_turn, which is nearly horizontal, may be observed at luminosities close to the Eddington luminosity. We show that these features are likely observed in a Galactic X-ray source, GRS 1915+105. We support a high spin parameter (a* > 0.9) for GRS 1915+105 since otherwise the high value of Tin and small size of the emitting region (rin < 1rS) cannot be explained.

  18. Stacking up 2D materials

    NASA Astrophysics Data System (ADS)

    Mayor, Louise

    2016-05-01

    Graphene might be the most famous example, but there are other 2D materials and compounds too. Louise Mayor explains how these atomically thin sheets can be layered together to create flexible “van der Waals heterostructures”, which could lead to a range of novel applications.

  19. Structure formation in gas-rich galactic discs with finite thickness: from discs to rings

    NASA Astrophysics Data System (ADS)

    Behrendt, M.; Burkert, A.; Schartmann, M.

    2015-03-01

    Gravitational instabilities play an important role in structure formation of gas-rich high-redshift disc galaxies. In this paper, we revisit the axisymmetric perturbation theory and the resulting growth of structure by taking the realistic thickness of the disc into account. In the unstable regime, which corresponds for thick discs to a Toomre parameter below the critical value Q0, crit = 0.696, we find a fastest growing perturbation wavelength that is always a factor 1.93 times larger than in the classical razor-thin disc approximation. This result is independent of the adopted disc scaleheight and by this independent of temperature and surface density. In order to test the analytical theory, we compare it with a high-resolution hydrodynamical simulation of an isothermal gravitationally unstable gas disc with the typical vertical sech2 density profile and study its break up into rings that subsequently fragment into dense clumps. In the first phase, rings form, that organize themselves discretely, with distances corresponding to the local fastest growing perturbation wavelength. We find that the disc scaleheight has to be resolved initially with five or more grid cells in order to guarantee proper growth of the ring structures, which follow the analytical prediction. These rings later on contract to a thin and dense line, while at the same time accreting more gas from the inter-ring region. It is these dense, circular filaments, that subsequently fragment into a large number of clumps. Contrary to what is typically assumed, the clump sizes are therefore not directly determined by the fastest growing wavelength.

  20. MRI-driven accretion on to magnetized stars: global 3D MHD simulations of magnetospheric and boundary layer regimes

    NASA Astrophysics Data System (ADS)

    Romanova, M. M.; Ustyugova, G. V.; Koldoba, A. V.; Lovelace, R. V. E.

    2012-03-01

    We discuss results of global three-dimensional magnetohydrodynamic simulations of accretion on to a rotating magnetized star with a tilted dipole magnetic field, where the accretion is driven by the magnetorotational instability (MRI). The simulations show that MRI-driven turbulence develops in the disc, and angular momentum is transported outwards primarily due to the magnetic stress. The turbulent flow is strongly inhomogeneous and the densest matter is in azimuthally stretched turbulent cells. We investigate two regimes of accretion: a magnetospheric regime and a boundary layer (BL) regime. In the magnetospheric regime, the magnetic field of the star is dynamically important: the accretion disc is truncated by the star's magnetic field within a few stellar radii from the star's surface, and matter flows to the star in funnel streams. The funnel streams flow towards the south and north magnetic poles but are not equal due to the inhomogeneity of the flow. The hotspots on the stellar surface are not symmetric as well. In the BL regime, the magnetic field of the star is dynamically unimportant, and matter accretes on to the surface of the star through the BL. The magnetic field in the inner disc is strongly amplified by the shear of the accretion flow, and the matter and magnetic stresses become comparable. Accreting matter forms a belt-shaped hot region on the surface of the star. The belt has inhomogeneous density distribution which varies in time due to variable accretion rate. The peaks in the variability curve are associated with accretion of individual turbulent cells. They show 20-50 per cent density amplifications at periods of ˜5-10 dynamical time-scales at the surface of the star. Spiral waves in the disc are excited in both magnetospheric and BL regimes of accretion. Results of simulations can be applied to classical T Tauri stars, accreting brown dwarfs, millisecond pulsars, dwarf novae cataclysmic variables and other stars with magnetospheres smaller

  1. Black hole mergers: do gas discs lead to spin alignment?

    NASA Astrophysics Data System (ADS)

    Lodato, Giuseppe; Gerosa, Davide

    2013-02-01

    In this Letter, we revisit arguments suggesting that the Bardeen-Petterson effect can coalign the spins of a central supermassive black hole binary accreting from a circumbinary (or circumnuclear) gas disc. We improve on previous estimates by adding the dependence on system parameters and noting that the non-linear nature of warp propagation in a thin viscous disc affects alignment. This reduces the disc's ability to communicate the warp, and can severely reduce the effectiveness of disc-assisted spin alignment. We test our predictions with a Monte Carlo realization of random misalignments and accretion rates, and we find that the outcome depends strongly on the spin magnitude. We estimate a generous upper limit to the probability of alignment by making assumptions which favour it throughout. Even with these assumptions, about 40 per cent of black holes with a ≳ 0.5 do not have time to align with the disc. If the residual misalignment is not small and it is maintained down to the final coalescence phase, this can give a powerful recoil velocity to the merged hole. Highly spinning black holes are thus more likely being subject to strong recoils, the occurrence of which is currently debated.

  2. On the role of self-organised criticality in accretion systems

    NASA Astrophysics Data System (ADS)

    Dendy, R. O.; Helander, P.; Tagger, M.

    1998-09-01

    Self-organised criticality (SOC) has been suggested as a potentially powerful unifying paradigm for interpreting the structure of, and signals from, accretion systems. After reviewing the most promising sites where SOC might be observable, we consider the theoretical arguments for supposing that SOC can occur in accretion discs. Perhaps the most rigorous evidence is provided by numerical modelling of energy dissipation due to magnetohydrodynamic turbulence in accretion discs by G Geertsema & A Achterberg (A&A 255, 427 (1992)); we investigate how ``sandpile"-type dynamics arise in this model. It is concluded that the potential sites for SOC in accretion systems are numerous and observationally accessible, and that theoretical support for the possible occurrence of SOC can be derived from first principles.

  3. On the formation of a quasi-stationary twisted disc after a tidal disruption event

    NASA Astrophysics Data System (ADS)

    Xiang-Gruess, M.; Ivanov, P. B.; Papaloizou, J. C. B.

    2016-08-01

    We investigate misaligned accretion discs formed after tidal disruption events that occur when a star encounters a supermassive black hole. We employ the linear theory of warped accretion discs to find the shape of a disc for which the stream arising from the disrupted star provides a source of angular momentum that is misaligned with that of the black hole. For quasi-steady configurations we find that when the warp diffusion or propagation time is large compared to the local mass accretion time and/or the natural disc alignment radius is small, misalignment is favoured. These results have been verified using SPH simulations. We also simulated 1D model discs including gas and radiation pressure. As accretion rates initially exceed the Eddington limit the disc is initially advection dominated. Assuming the α model for the disc, where it can be thermally unstable it subsequently undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from ˜1 to ˜10-3 which is reflected in changes in the degree of disc misalignment at the stream impact location. For maximal black hole rotation and sufficiently large values of viscosity parameter α > ˜0.01 - 0.1 the ratio of the disc inclination to that of the initial stellar orbit is estimated to be 0.1 - 0.2 in the advection dominated state, while reaching of order unity in the low state. Misalignment descreases with decrease of α, but increases as the black hole rotation parameter decreases. Thus, it is always significant when the latter is small.

  4. Chaotic Accretion and Merging Supermassive Black Holes

    NASA Astrophysics Data System (ADS)

    Nixon, Christopher James

    2012-09-01

    The main driver of the work in this thesis is the idea of chaotic accretion in galaxy centres. Most research in this area focuses on orderly or coherent accretion where supermassive black holes or supermassive black hole binaries are fed with gas always possessing the same sense of angular momentum. If instead gas flows in galaxies are chaotic, feeding occurs through randomly oriented depositions of gas. Previous works show that this chaotic mode of feeding can explain some astrophysical phenomena, such as the lack of correlation between host galaxy structure and the direction of jets. It has also been shown that by keeping the black hole spin low this feeding mechanism can grow supermassive black holes from stellar mass seeds. In this thesis I show that it also alleviates the "final parsec problem" by facilitating the merger of two supermassive black holes, and the growth of supermassive black holes through rapid accretion. I also develop the intriguing possibility of breaking a warped disc into two or more distinct planes.

  5. Rotating Bondi Accretion Flow

    NASA Astrophysics Data System (ADS)

    Park, Myeong-Gu; Han, Du-Hwan

    2016-06-01

    The characteristics of accretion flow onto a black hole are determined by the physical condition of gas at large radius. When the gas has no angular momentum and is polytropic, the accretion flow becomes the classic Bondi flow. The mass accretion rate in such case is an eigenvalue and uniquely determined by the density and the temperature of the surrounding gas for a given black hole mass. When the gas has angular momentum above some critical value, the angular momentum of the gas should be removed by viscosity to reach the black hole horizon. We study, within the slim disk approximation, rotating polytropic accretion flow with alpha viscosity as an an extension of the Bondi flow. The characteristics of the accretion flow are now determined by the temperature, density, and angular momentum of the gas at the outer boundary. We explore the effects of the viscosity parameter and the outer boundary radius on the physical characteristic of the flow, especially on the mass accretion rate, and compare the result with previous works of Park (2009) and Narayan & Fabian (2011).

  6. MOSS2D V1

    Energy Science and Technology Software Center (ESTSC)

    2001-01-31

    This software reduces the data from two-dimensional kSA MOS program, k-Space Associates, Ann Arbor, MI. Initial MOS data is recorded without headers in 38 columns, with one row of data per acquisition per lase beam tracked. The final MOSS 2d data file is reduced, graphed, and saved in a tab-delimited column format with headers that can be plotted in any graphing software.

  7. Geometry of X-ray sources in accreting black-hole binaries

    NASA Astrophysics Data System (ADS)

    Zdziarski, Andrzej

    2016-07-01

    The structure of the X-ray sources in the hard spectral state of accreting black-hole binaries has been a subject of intense debate. The paradigm dominant for many years postulated that the accretion disc in the hard state is truncated at some radius >> the innermost stable orbit (ISCO) whereas the disc reaches the ISCO in the soft state. This paradigm explains a large body of observed phenomena, including the spectral and variability differences between the states and outbursts of transient sources, proceeding from quiescence (where no disc is present) through the hard state to the peak flux in the soft state. On the other hand, there have been numerous claims in recent years that the disc extends to the ISCO in the hard state. Also, the primary X-ray source has been postulated to consist of a compact source on-axis of the rotating black hole (a lamppost). Those claims are based on observations of broad Fe K lines and of soft X-ray components interpreted as blackbody-emitting accretion discs. I will discuss arguments for and against the disc truncation and the lamppost geometry based on current spectral and timing results.

  8. Estimating the fossil disc mass during supermassive black hole mergers: the importance of torque implementation

    NASA Astrophysics Data System (ADS)

    Tazzari, M.; Lodato, G.

    2015-05-01

    In this paper, we revisit the issue of estimating the `fossil' disc mass in the circumprimary disc, during the merger of a supermassive black hole binary. As the binary orbital decay speeds up due to the emission of gravitational waves, the gas in the circumprimary disc might be forced to accrete rapidly and could in principle provide a significant electromagnetic counterpart to the gravitational wave emission. Since the luminosity of such flare is proportional to the gaseous mass in the circumprimary disc, estimating such mass accurately is important. Previous investigations of this issue have produced contradictory results, with some authors estimating super-Eddington flares and large disc mass, while others suggesting that the `fossil' disc mass is very low, even less than a Jupiter mass. Here, we perform simple 1D calculations to show that such very low estimates of the disc mass are an artefact of the specific implementation of the tidal torque in 1D models. In particular, for moderate mass ratios of the binary, the usual formula for the torque used in 1D models significantly overestimates the width of the gap induced by the secondary and this artificially leads to a very small leftover circumprimary disc. Using a modified torque, calibrated to reproduce the correct gap width as estimated by 3D models, leads to fossil disc masses of the order of one solar mass. The rapid accretion of the whole circumprimary disc would produce peak luminosities of the order of 1-20 times the Eddington luminosity. Even if a significant fraction of the gas escapes accretion by flowing out the secondary orbit during the merger (an effect not included in our calculations), we would still predict close to Eddington luminosities that might be easily detected.

  9. Jet and accretion power in the most powerful Fermi blazars

    NASA Astrophysics Data System (ADS)

    Ghisellini, G.; Tavecchio, F.; Ghirlanda, G.

    2009-11-01

    Among the blazars detected by the Fermi satellite, we have selected the 23 blazars that in the 3months of survey had an average γ-ray luminosity above 1048ergs-1. For 17 out of the 23 sources we found and analysed X-ray and optical-ultraviolet data taken by the Swift satellite. With these data, implemented by archival and not simultaneous data, we construct the spectral energy distributions, and interpreted them with a simple one-zone, leptonic, synchrotron and inverse Compton model. When possible, we also compare different high-energy states of single sources, like 0528+134 and 3C454.3, for which multiple good sets of multiwavelength data are available. In our powerful blazars the high energy emission always dominates the electromagnetic output, and the relatively low level of the synchrotron radiation often does not hide the accretion disc emission. We can then constrain the black hole mass and the disc luminosity. Both are large (i.e. masses equal or greater than 109M solar and disc luminosities above 10 per cent of Eddington). By modelling the non-thermal continuum we derive the power that the jet carries in the form of bulk motion of particles and fields. On average, the jet power is found to be slightly larger than the disc luminosity, and proportional to the mass accretion rate.

  10. Nanoimprint lithography: 2D or not 2D? A review

    NASA Astrophysics Data System (ADS)

    Schift, Helmut

    2015-11-01

    Nanoimprint lithography (NIL) is more than a planar high-end technology for the patterning of wafer-like substrates. It is essentially a 3D process, because it replicates various stamp topographies by 3D displacement of material and takes advantage of the bending of stamps while the mold cavities are filled. But at the same time, it keeps all assets of a 2D technique being able to pattern thin masking layers like in photon- and electron-based traditional lithography. This review reports about 20 years of development of replication techniques at Paul Scherrer Institut, with a focus on 3D aspects of molding, which enable NIL to stay 2D, but at the same time enable 3D applications which are "more than Moore." As an example, the manufacturing of a demonstrator for backlighting applications based on thermally activated selective topography equilibration will be presented. This technique allows generating almost arbitrary sloped, convex and concave profiles in the same polymer film with dimensions in micro- and nanometer scale.

  11. Star Formation and Gas Accretion in Nearby Galaxies

    NASA Astrophysics Data System (ADS)

    Yim, Kijeong; van der Hulst, J. M.

    2016-08-01

    In order to quantify the relationship between gas accretion and star formation, we analyse a sample of 29 nearby galaxies from the WHISP survey which contains galaxies with and without evidence for recent gas accretion. We compare combined radial profiles of FUV (GALEX) and IR 24 μm (Spitzer) characterizing distributions of recent star formation with radial profiles of CO (IRAM, BIMA, or CARMA) and H I (WSRT) tracing molecular and atomic gas contents to examine star formation efficiencies in symmetric (quiescent), asymmetric (accreting), and interacting (tidally disturbed) galaxies. In addition, we investigate the relationship between star formation rate and H I in the outer discs for the three groups of galaxies. We confirm the general relationship between gas surface density and star formation surface density, but do not find a significant difference between the three groups of galaxies.

  12. Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion.

    PubMed

    Johansen, Anders; Low, Mordecai-Mark Mac; Lacerda, Pedro; Bizzarro, Martin

    2015-04-01

    Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas drag-assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo runaway accretion of chondrules within ~3 My, forming planetary embryos up to Mars's size along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size sorting of chondrules consistent with chondrites. Accretion of millimeter-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disc lifetime outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles in the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts. PMID:26601169

  13. Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion

    PubMed Central

    Johansen, Anders; Low, Mordecai-Mark Mac; Lacerda, Pedro; Bizzarro, Martin

    2015-01-01

    Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas drag–assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo runaway accretion of chondrules within ~3 My, forming planetary embryos up to Mars’s size along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size sorting of chondrules consistent with chondrites. Accretion of millimeter-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disc lifetime outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles in the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts. PMID:26601169

  14. Constraining black-hole spin using disc tomography

    NASA Astrophysics Data System (ADS)

    Middleton, Matthew

    2013-10-01

    The emission from the inner accretion disc of low mass, high mass accretion rate AGN extends into the soft X-ray bandpass. Where orbiting material covers or reveals each side of the disc in turn, we can study the region of strong gravity and determine the spin and inclination directly. RX J1301.9+2747, shows flares as a long-lived feature of its lightcurve which are most likely due to gaps in an obscuring shroud. By comparing to the predictions from ray-tracing codes, the data imply the spin and inclination to be low. We propose a 210 ks observation (including overheads and background flaring) to more robustly test our new technique and tightly constrain the spin and inclination, or otherwise provide a unique view of the mechanism responsible for the soft excess in this source.

  15. Formation of primordial supermassive stars by burst accretion

    NASA Astrophysics Data System (ADS)

    Sakurai, Y.; Hosokawa, T.; Yoshida, N.; Yorke, H. W.

    2015-09-01

    Recent observations show that supermassive black holes (BHs) with ˜109 M⊙ exist at redshift z ≳ 6. A promising formation channel is the so-called direct collapse model, which posits that a massive seed BH forms through gravitational collapse of a ˜105 M⊙ supermassive star (SMS). We study the evolution of such an SMS growing by rapid mass accretion. In particular, we examine the impact of time-dependent mass accretion of repeating burst and quiescent phases expected to occur with a self-gravitating circumstellar disc. We show that protostars growing via episodic accretion can substantially contract during the quiescent phases, in contrast to the case of constant mass accretion, whereby the star expands roughly monotonically. The stellar effective temperature and ionizing photon emissivity increase accordingly, which can cause strong ionizing feedback and halt the mass accretion. With a fixed duration of the quiescent phase Δtq, this contraction occurs in early evolutionary phases, i.e. for M* ≲ 103 M⊙ with Δtq ≃ 103 yr. For later epochs and larger masses but the same Δtq, contraction is negligible even during quiescent phases. With larger Δtq, however, the star continues to contract during quiescent phases even for the higher stellar masses. We show that this behaviour is well understood by comparing the interval time and the thermal relaxation time for a bloated surface layer. We conclude that the feedback becomes effective, if Δtq ≳ 103 yr, which is possible in an accretion disc forming in the direct collapse model.

  16. Iron K-Alpha Lines from Ionized Discs in Z-Type X-Ray

    NASA Astrophysics Data System (ADS)

    Brandt, W. M.; Matt, G.

    1994-06-01

    The properties of the iron Kα line emitted by a photoionized accretion disc in a Z-type X-ray binary system are determined for different values of the mass accretion rate and disc inclination angle. We consider a geometry where the accretion disc is illuminated by X-rays emitted from a spherical, optically thick Comptonized cloud, representing the magnetosphere on the horizontal branch, and the magnetosphere plus puffed accretion disc on the normal and flaring branches. We find that line profiles can be complex due to Doppler, relativistic, shadowing, line-blending, resonant trapping and other effects. The iron Kα line equivalent width, intrinsic width and centroid energy vary in a tortuous manner as a function of the accretion rate, cloud radius and inclination angle. We examine the reasons for this behaviour and discuss how it will affect inferences about the physical states of Z-sources based on their iron line properties. We compare our results with available data, in particular on Cygnus X-2, and indicate how tests of Z-source models could be made using high-energy-resolution measurements like those expected from AS CA.

  17. Precision diagnostic disc injections.

    PubMed

    Fortin, J D

    2000-07-01

    Spinal pain is an important public health problem affecting the population indiscriminately. The structures responsible for pain in the spine include the vertebrae, intervertebral discs, spinal cord, nerve roots, facet joints, ligaments, muscles, atlanto-occipital joints, atlanto-axial joints, and sacroiliac joints. Even though disc herniation, facet joints, strained muscles, and torn ligaments have been attributed to be the cause of most spinal pain, either in the neck and upper extremities, upper and mid back, or low back and lower extremities, disorders of the disc other than disc herniation have been implicated more frequently than any other disorders. Once stifled by misinformation, discography now has applications in a number of clinical settings. While cervical and lumbar discography is well studied and well known, thoracic discography is in its nascent stages of clinical application. The value of discography lies in its ability to produce pain and thereby identify a "pain generator." This allows treatment to be based on the specific cause of pain. The three primary components of diagnostic disc injection are: provocation/analgesia, discometry, and nucleography. Despite the recent exponential growth of noninvasive spinal technology, diagnostic disc injection remains the sole direct method for definitively determining whether a disc is a physiological pain generator. It is clear that discography is a safe and powerful complement to the overall clinical context. PMID:16906185

  18. Percutaneous laser disc decompression.

    PubMed

    Choy, D S

    1995-06-01

    Herniated disc disease has an incidence of 1.7% in the U.S. Heretofore, open operative procedures were the rule for this condition when conservative measures were ineffective. Choy and Ascher introduced this new technique in February 1986 using a Nd:YAG laser introduced into the disc through an optical fiber in a needle. Percutaneous laser disc decompression is based on the principle that in an enclosed hydraulic space, such as an intact disc, a small reduction in volume is associated with a disproportionate fall in pressure. In the disc, this partial vacuum causes the herniated portion to move away from the nerve root back toward the center of the disc. This technique has been taught worldwide and is being performed in most of Europe, Japan, the United States, and Korea. In this special issue devoted to percutaneous laser disc decompression (PLDD), we will set forth the basic science of PLDD, patient selection, use of the holmium:YAG, and the Nd:YAG lasers, operative technique, and results. PMID:10150634

  19. Structure of the accretion disk in the dwarf nova IP Pegasi

    NASA Astrophysics Data System (ADS)

    Goranskij, V. P.; Lyutyj, V. M.; Shugarov, S. Yu.

    1985-10-01

    The brightness distribution over the edge-on accretion disc is investigated by means of photoelectric UBV observations of eclipses in the close binary system IP Pegasi. The radiation of a hot spot is predominant. The brightness and location of the spot in the disc change depending on the phase of activity. In a quiet state the disc is surrounded by an extended obscure ring, a small sector of which is shining when the star brightness falls down after the outburst. The observations support the model of a strengthening gas flow from the secondary star to explain the cause of outbursts.

  20. Giant planet formation in radially structured protoplanetary discs

    NASA Astrophysics Data System (ADS)

    Coleman, Gavin A. L.; Nelson, Richard P.

    2016-08-01

    Our recent N-body simulations of planetary system formation, incorporating models for the main physical processes thought to be important during the building of planets (i.e. gas disc evolution, migration, planetesimal/boulder accretion, gas accretion onto cores, etc.), have been successful in reproducing some of the broad features of the observed exoplanet population (e.g. compact systems of low mass planets, hot Jupiters), but fail completely to form any surviving cold Jupiters. The primary reason for this failure is rapid inward migration of growing protoplanets during the gas accretion phase, resulting in the delivery of these bodies onto orbits close to the star. Here, we present the results of simulations that examine the formation of gas giant planets in protoplanetary discs that are radially structured due to spatial and temporal variations in the effective viscous stresses, and show that such a model results in the formation of a population of cold gas giants. Furthermore, when combined with models for disc photoevaporation and a central magnetospheric cavity, the simulations reproduce the well-known hot-Jupiter/cold-Jupiter dichotomy in the observed period distribution of giant exoplanets, with a period valley between 10-100 days.

  1. Giant planet formation in radially structured protoplanetary discs

    NASA Astrophysics Data System (ADS)

    Coleman, Gavin A. L.; Nelson, Richard P.

    2016-08-01

    Our recent N-body simulations of planetary system formation, incorporating models for the main physical processes thought to be important during the building of planets (i.e. gas disc evolution, migration, planetesimal/boulder accretion, gas accretion on to cores, etc.), have been successful in reproducing some of the broad features of the observed exoplanet population (e.g. compact systems of low-mass planets, hot Jupiters), but fail completely to form any surviving cold Jupiters. The primary reason for this failure is rapid inward migration of growing protoplanets during the gas accretion phase, resulting in the delivery of these bodies on to orbits close to the star. Here, we present the results of simulations that examine the formation of gas giant planets in protoplanetary discs that are radially structured due to spatial and temporal variations in the effective viscous stresses, and show that such a model results in the formation of a population of cold gas giants. Furthermore, when combined with models for disc photoevaporation and a central magnetospheric cavity, the simulations reproduce the well-known hot-Jupiter/cold-Jupiter dichotomy in the observed period distribution of giant exoplanets, with a period valley between 10 and 100 d.

  2. From Disc Wind Models to Observations of TTauri Microjets

    NASA Astrophysics Data System (ADS)

    Ferreira, Jonathan; Casse, Fabien; Garcia, Paulo; Darren, O'brien; Sylvie, Cabrit; Catherine, Dougados; Pesenti, Nicolas; Luc, Binette

    Two decades after their discovery jets from accreting young stars still represent a major challenge for theorists. Several theoretical scenarii have been proposed but only models involving large scale magnetic fields have proved capable of producing self-collimated jets. However the launching region remains unknown: is it the star the surrounding accretion disc or their interaction zone? Progresses in high angular resolution offer now the opportunity to test the various proposed models. I will first review the results on magnetized disc winds based on the only MHD model describing self-consistently these accretion-ejection structures. Then I will show how the thermal and ionization states of the outflowing matter can be consistently computed once the dominant heating source has been chosen (ambipolar diffusion alfven wave damping or some local mechanical heating). A set of observational predictions (emission maps line fluxes/ratios and line profiles) for selected optical forbidden lines can then be calculated. As an illustration I will compare these predictions with new sub-arcsecond spectroimaging observations of the DG Tau and RW Aur jets and discuss the constraints they set on disc winds in TTauri stars.

  3. Ringed Accretion Disks: Instabilities

    NASA Astrophysics Data System (ADS)

    Pugliese, D.; Stuchlík, Z.

    2016-04-01

    We analyze the possibility that several instability points may be formed, due to the Paczyński mechanism of violation of mechanical equilibrium, in the orbiting matter around a supermassive Kerr black hole. We consider a recently proposed model of a ringed accretion disk, made up by several tori (rings) that can be corotating or counter-rotating relative to the Kerr attractor due to the history of the accretion process. Each torus is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. We prove that the number of the instability points is generally limited and depends on the dimensionless spin of the rotating attractor.

  4. The Formation and Fragmentation of Primordial Protostellar Discs

    NASA Astrophysics Data System (ADS)

    Clark, Paul C.; Glover, Simon C. O.; Smith, Rowan J.; Greif, Thomas H.; Klessen, Ralf S.; Bromm, Volker

    2010-11-01

    We study the formation and evolution of the protostellar discs that form around the first stars in the Universe. Using sink particles, we replace the gravitationally bound gas at densities higher than 1015 cm-3 and radii greater than 3 AU from the central protostellar core, with an accreting point mass that is able to gravitationally interact with the surrounding gas. We find the disc is gravitationally (or `Toomre') unstable, and is dominated by a strong m = 2 spiral mode. Although the angular momentum transport is dominated by a combination of gravitational torques and Reynolds stresses, which are extremely efficient mechanisms, the disc is unable to process the infalling material and grows increasingly gravitationally unstable. During the build-up of the disc, the temperature in the gas is regulated by a combination of H2 line cooling, collision-induced emission and H2 dissociation, which together help to offset heating from the gravitational collapse and feedback from the protostar. Once the disc starts to fragment, H2 dissociation keeps the gas almost isothermal as the collapse of the fragment progresses. The fragmentation occurs when the protostar/disc system is only 230 yr old and at a distance of ~20 AU from its sibling, by which point the central protostar has a mass of ~1 Msolar. Given the angular momentum of the new protostellar system, it is likely that the protostars will grow to become a massive binary system.

  5. Circumplanetary disc or circumplanetary envelope?

    NASA Astrophysics Data System (ADS)

    Szulágyi, J.; Masset, F.; Lega, E.; Crida, A.; Morbidelli, A.; Guillot, T.

    2016-08-01

    We present three-dimensional simulations with nested meshes of the dynamics of the gas around a Jupiter mass planet with the JUPITER and FARGOCA codes. We implemented a radiative transfer module into the JUPITER code to account for realistic heating and cooling of the gas. We focus on the circumplanetary gas flow, determining its characteristics at very high resolution (80 per cent of Jupiter's diameter). In our nominal simulation where the temperature evolves freely by the radiative module and reaches 13000 K at the planet, a circumplanetary envelope was formed filling the entire Roche lobe. Because of our equation of state is simplified and probably overestimates the temperature, we also performed simulations with limited maximal temperatures in the planet region (1000, 1500, and 2000 K). In these fixed temperature cases circumplanetary discs (CPDs) were formed. This suggests that the capability to form a CPD is not simply linked to the mass of the planet and its ability to open a gap. Instead, the gas temperature at the planet's location, which depends on its accretion history, plays also fundamental role. The CPDs in the simulations are hot and cooling very slowly, they have very steep temperature and density profiles, and are strongly sub-Keplerian. Moreover, the CPDs are fed by a strong vertical influx, which shocks on the CPD surfaces creating a hot and luminous shock-front. In contrast, the pressure supported circumplanetary envelope is characterized by internal convection and almost stalled rotation.

  6. Herniated Lumbar Disc

    MedlinePlus

    ... 50. A herniated lumbar disc may also cause back pain, although back pain alone (without leg pain) can have many causes ... 90% success); surgery is less effective in relieving back pain. Nonsurgical treatment Your doctor may prescribe nonsurgical treatments ...

  7. Effect of Be disc evolution on global one-armed oscillations

    NASA Astrophysics Data System (ADS)

    Oktariani, F.; Okazaki, A. T.; Kunjaya, C.; Aprilia

    2016-07-01

    We study the effect of density distribution evolution on the global one-armed oscillation modes in low-viscosity discs around isolated and binary Be stars. Observations show that some Be stars exhibit evidence of formation and dissipation of the equatorial disc. In this paper, we first calculate the density evolution in discs around isolated Be stars. To model the formation stage of the disc, we inject mass at a radius just outside the star at a constant rate for 30-50 yr. As the disc develops, the density distribution approaches the form of the steady disc solution. Then, we turn-off the mass injection to model the disc dissipation stage. The innermost part of the disc starts accretion, and a gap forms between the star and the disc. Next, we calculate the one-armed modes at several epochs. We neglect the effect of viscosity because the time-scale of oscillations is much shorter than the disc evolution time-scale for low viscosity. In the disc formation stage, the eigenfrequency increases with time towards the value for the steady state disc. On the other hand, one-armed eigenmodes in dissipating Be discs have significantly higher eigenfrequencies and narrower propagation regions. Observationally, such a change of mode characteristics can be taken as an evidence for gap opening around the star. In binary Be stars, the characteristics of the disc evolution and the eigenmodes are qualitatively the same as in isolated Be stars, but quantitatively, they have shorter evolution time-scales and higher eigenfrequencies, which is in agreement with the observed trend.

  8. Line-driven ablation of circumstellar discs – I. Optically thin decretion discs of classical Oe/Be stars

    PubMed Central

    Kee, Nathaniel Dylan; Owocki, Stanley; Sundqvist, J. O.

    2016-01-01

    discussion of future extensions to study line-driven ablation of denser, optically thick, accretion discs of pre-main-sequence massive stars. PMID:27346978

  9. Line-driven ablation of circumstellar discs - I. Optically thin decretion discs of classical Oe/Be stars

    NASA Astrophysics Data System (ADS)

    Kee, Nathaniel Dylan; Owocki, Stanley; Sundqvist, J. O.

    2016-05-01

    discussion of future extensions to study line-driven ablation of denser, optically thick, accretion discs of pre-main-sequence massive stars.

  10. Disc Motor: Conventional and Superconductor Simulated Results Analysis

    NASA Astrophysics Data System (ADS)

    Inácio, David; Martins, João; Neves, Mário Ventim; Álvarez, Alfredo; Rodrigues, Amadeu Leão

    Taking into consideration the development and integration of electrical machines with lower dimensions and higher performance, this paper presents the design and development of a three-phase axial flux disc motor, with 50 Hz frequency supply. It is made with two conventional semi-stators and a rotor, which can be implemented with a conventional aluminum disc or a high temperature-superconducting disc. The analysis of the motor characteristics is done with a 2D commercial finite elements package, being the modeling performed as a linear motor. The obtained results allow concluding that the superconductor motor provides a higher force than the conventional one. The conventional disc motor presents an asynchronous behavior, like a conventional induction motor, while the superconductor motor presents both synchronous and asynchronous behaviors.

  11. Herniated lumbar disc

    PubMed Central

    2011-01-01

    Introduction Herniated lumbar disc is a displacement of disc material (nucleus pulposus or annulus fibrosis) beyond the intervertebral disc space. The highest prevalence is among people aged 30 to 50 years, with a male to female ratio of 2:1. There is little evidence to suggest that drug treatments are effective in treating herniated disc. Methods and outcomes We conducted a systematic review and aimed to answer the following clinical questions: What are the effects of drug treatments, non-drug treatments, and surgery for herniated lumbar disc? We searched: Medline, Embase, The Cochrane Library, and other important databases up to June 2010 (Clinical Evidence reviews are updated periodically; please check our website for the most up-to-date version of this review). We included harms alerts from relevant organisations such as the US Food and Drug Administration (FDA) and the UK Medicines and Healthcare products Regulatory Agency (MHRA). Results We found 37 systematic reviews, RCTs, or observational studies that met our inclusion criteria. We performed a GRADE evaluation of the quality of evidence for interventions. Conclusions In this systematic review, we present information relating to the effectiveness and safety of the following interventions: acupuncture, advice to stay active, analgesics, antidepressants, bed rest, corticosteroids (epidural injections), cytokine inhibitors (infliximab), discectomy (automated percutaneous, laser, microdiscectomy, standard), exercise therapy, heat, ice, massage, muscle relaxants, non-steroidal anti-inflammatory drugs (NSAIDs), percutaneous disc decompression, spinal manipulation, and traction. PMID:21711958

  12. The Stability of Hoyle-Lyttleton Accretion in Three Dimensions

    NASA Astrophysics Data System (ADS)

    Blondin, John M.; Raymer, E.

    2011-05-01

    The gravitational accretion of gas onto a compact star moving supersonically through a uniform ambient medium is dynamically unstable in the restricted case of two-dimensional planar geometry (a cylindrical star). Numerical simulations in 3D (e.g., the series of papers by Ruffert) show some hint of instability, but not the dramatic flip-flop seen in 2D planar simulations. We extend the recent 2D numerical simulations of Blondin and Pope (2009) to 3D using the overset spherical grid approach developed by Kageyama and Sato (2004). Using this grid geometry on current supercomputers allows us to simulate the smallest accretors studied in previous 3D work, but with an order of magnitude higher spatial resolution. For an ideal gas with a ratio of specific heats of 5/3, we find relatively minor time variability in the subsonic flow between the head of the accretion bow shock and the accreting star. Overall the bow shock and mass accretion rate remain nearly constant in time, with negligible angular momentum accreted onto the compact star.

  13. The physics of 2D microfluidic droplet ensembles

    NASA Astrophysics Data System (ADS)

    Beatus, Tsevi; Bar-Ziv, Roy H.; Tlusty, Tsvi

    2012-07-01

    We review non-equilibrium many-body phenomena in ensembles of 2D microfluidic droplets. The system comprises of continuous two-phase flow with disc-shaped droplets driven in a channel, at low Reynolds number of 10-4-10-3. The basic physics is that of an effective potential flow, governed by the 2D Laplace equation, with multiple, static and dynamic, boundaries of the droplets and the walls. The motion of the droplets induces dipolar flow fields, which mediate 1/r2 hydrodynamic interaction between the droplets. Summation of these long-range 2D forces over droplet ensembles converges, in contrast to the divergence of the hydrodynamic forces in 3D. In analogy to electrostatics, the strong effect of boundaries on the equations of motion is calculated by means of image dipoles. We first consider the dynamics of droplets flowing in a 1D crystal, which exhibits unique phonon-like excitations, and a variety of nonlinear instabilities-all stemming from the hydrodynamic interactions. Narrowing the channel results in hydrodynamic screening of the dipolar interactions, which changes salient features of the phonon spectra. Shifting from a 1D ordered crystal to 2D disordered ensemble, the hydrodynamic interactions induce collective density waves and shocks, which are superposed on single-droplet randomized motion and dynamic clustering. These collective modes originate from density-velocity coupling, whose outcome is a 1D Burgers equation. The rich observational phenomenology and the tractable theory render 2D droplet ensembles a suitable table-top system for studying non-equilibrium many-body physics with long-range interactions.

  14. The formation and evolution of wind-capture discs in binary systems

    NASA Astrophysics Data System (ADS)

    Huarte-Espinosa, M.; Carroll-Nellenback, J.; Nordhaus, J.; Frank, A.; Blackman, E. G.

    2013-07-01

    We study the formation, evolution and physical properties of accretion discs formed via wind capture in binary systems. Using the adaptive mesh refinement (AMR) code AstroBEAR, we have carried out high-resolution 3D simulations that follow a stellar mass secondary in the corotating frame as it orbits a wind producing asymptotic giant branch (AGB) primary. We first derive a resolution criteria, based on considerations of Bondi-Hoyle flows, that must be met in order to properly resolve the formation of accretion discs around the secondary. We then compare simulations of binaries with three different orbital radii (Ro = 10, 15, 20 au). Discs are formed in all three cases, however, the size of the disc and, most importantly, its accretion rate decreases with orbital radii. In addition, the shape of the orbital motions of material within the disc becomes increasingly elliptical with increasing binary separation. The flow is mildly unsteady with `fluttering' around the bow shock observed. The discs are generally well aligned with the orbital plane after a few binary orbits. We do not observe the presence of any large-scale, violent instabilities (such as the flip-flop mode). For the first time, moreover, it is observed that the wind component that is accreted towards the secondary has a vortex tube-like structure, rather than a column-like one as it was previously thought. In the context of AGB binary systems that might be precursors to pre-planetary nebula (PPN) and planetary nebula (PN), we find that the wind accretion rates at the chosen orbital separations are generally too small to produce the most powerful outflows observed in these systems if the companions are main-sequence stars but marginally capable if the companions are white dwarfs. It is likely that many of the more powerful PPN and PN involve closer binaries than the ones considered here. The results also demonstrate principles of broad relevance to all wind-capture binary systems.

  15. Composition of early planetary atmospheres - I. Connecting disc astrochemistry to the formation of planetary atmospheres

    NASA Astrophysics Data System (ADS)

    Cridland, A. J.; Pudritz, R. E.; Alessi, M.

    2016-09-01

    We present a model of the early chemical composition and elemental abundances of planetary atmospheres based on the cumulative gaseous chemical species that are accreted on to planets forming by core accretion from evolving protoplanetary discs. The astrochemistry of the host disc is computed using an ionization-driven, non-equilibrium chemistry network within viscously evolving disc models. We accrete gas giant planets whose orbital evolution is controlled by planet traps using the standard core accretion model and track the chemical composition of the material that is accreted on to the protoplanet. We choose a fiducial disc model and evolve planets in three traps - water ice line, dead zone and heat transition. For a disc with a lifetime of 4.1 Myr, we produce two hot Jupiters (M = 1.43, 2.67 MJupiter, r = 0.15, 0.11 au) in the heat transition and ice line trap and one failed core (M = 0.003 MJupiter, r = 3.7 au) in the dead zone. These planets are found with mixing ratios for CO and H2O of 1.99 × 10-4 and 5.0 × 10-4, respectively, for both hot Jupiters. Additionally, for these planets we find CO2 and CH4, with mixing ratios of 1.8 × 10-6 → 9.8 × 10-10 and 1.1 × 10-8 → 2.3 × 10-10, respectively. These ranges correspond well with the mixing ratio ranges that have been inferred through the detection of emission spectra from hot Jupiters by multiple authors. We compute a carbon-to-oxygen ratio of 0.227 for the ice line planet and 0.279 for the heat transition planet. These planets accreted their gas inside the ice line, hence the sub-solar C/O.

  16. Patterns of disc-jet-wind coupling in black hole binaries

    NASA Astrophysics Data System (ADS)

    Fender, R.

    2015-07-01

    In this talk I will present the current state of the art in our understanding of the connection between accretion state and feedback in black hole X-ray binaries. In particular I will discuss how the X-ray accretion states, defined by their spectral and temporal properties, relate to phases of the production of relativistic (radio) jets and accretion disc winds. I will furthermore discuss how these patterns of behaviour contribute to the overall kinetic and radiative feedback during an outburst, and how comparable they may be to similar behaviour in neutron star X-ray binaries and supermassive black holes in active galactic nuclei.

  17. A carbon dwarf wearing a Necklace: first proof of accretion in a post-common-envelope binary central star of a planetary nebula with jets

    NASA Astrophysics Data System (ADS)

    Miszalski, Brent; Boffin, Henri M. J.; Corradi, Romano L. M.

    2013-01-01

    The formation of collimated outflows or jets in planetary nebulae (PNe) is not well understood. There is no evidence for active accretion discs in PNe, making it difficult to decide which of the several proposed jet formation scenarios may be correct. A handful of wide binary central stars of PNe are known to have accreted carbon and slow neutron capture (s-process) enhanced material, the immediate progenitors of barium stars; however, no close binary analogues are known to have passed through a common-envelope (CE) phase. Here we present spectroscopy of the Necklace taken near light-curve minimum that for the first time reveals a carbon-rich (C/O > 1) companion, a carbon dwarf, in a post-CE central star. As unevolved stars do not produce carbon, the chemical enhancement of the secondary can only be explained by accretion from the primary. Accretion most likely happened prior to the CE phase via wind accretion as not enough material can be accreted during the short CE phase. The pair of jets in the Necklace, which are observed to be older than the PN, are therefore likely to have been launched from an accretion disc around the companion during this early accretion phase. This discovery adds significant weight to the emerging scenario that jets in post-CE PNe are primarily launched by an accretion disc around a main-sequence companion before the CE phase.

  18. Terrane accretion: Insights from numerical modelling

    NASA Astrophysics Data System (ADS)

    Vogt, Katharina; Gerya, Taras

    2016-04-01

    The oceanic crust is not homogenous, but contains significantly thicker crust than norm, i.e. extinct arcs, spreading ridges, detached continental fragments, volcanic piles or oceanic swells. These (crustal) fragments may collide with continental crust and form accretionary complexes, contributing to its growth. We analyse this process using a thermo-mechanical computer model (i2vis) of an ocean-continent subduction zone. In this model the oceanic plate can bend spontaneously under the control of visco-plastic rheologies. It moreover incorporates effects such as mineralogical phase changes, fluid release and consumption, partial melting and melt extraction. Based on our 2-D experiments we suggest that the lithospheric buoyancy of the downgoing slab and the rheological strength of crustal material may result in a variety of accretionary processes. In addition to terrane subduction, we are able to identify three distinct modes of terrane accretion: frontal accretion, basal accretion and underplating plateaus. We show that crustal fragments may dock onto continental crust and cease subduction, be scrapped off the downgoing plate, or subduct to greater depth prior to slab break off and subsequent exhumation. Direct consequences of these processes include slab break off, subduction zone transference, structural reworking, formation of high-pressure terranes, partial melting and crustal growth.

  19. Neutron star accretion and the neutrino fireball

    SciTech Connect

    Colgate, S.A.; Herant, M.E.; Benz, W.

    1991-11-26

    The mixing necessary to explain the ``Fe`` line widths and possibly the observed red shifts of 1987A is explained in terms of large scale, entropy conserving, up and down flows (calculated with a smooth particle 2-D code) taking place between the neutron star and the explosion shock wave due to the gravity and neutrino deposition. Depending upon conditions of entropy and mass flux further accretion takes place in single events, similar to relaxation oscillator, fed by the downward flows of low entropy matter. The shock, in turn, is driven by the upflow of the buoyant high entropy bubbles. Some accretion events will reach a temperature high enough to create a neutrino ``fireball,`` a region hot enough, 11 Mev, so as to be partially opaque to its own (neutrino) radiation. The continuing neutrino deposition drives the explosion shock until the entropy of matter flowing downwards onto the neutron star is high enough to prevent further accretion. This process should result in a robust supernova explosion.

  20. A Database of Supercooled Large Droplet Ice Accretions

    NASA Technical Reports Server (NTRS)

    VanZante, Judith Foss

    2007-01-01

    A unique, publicly available database regarding supercooled large droplet ice accretions has been developed in NASA Glenn's Icing Research Tunnel. Identical cloud and flight conditions were generated for five different airfoil models. The models chosen represent a variety of aircraft types from the horizontal stabilizer of a large trans-port aircraft to the wings of regional, business, and general aviation aircraft. In addition to the standard documentation methods of 2D ice shape tracing and imagery, ice mass measurements were also taken. This database will also be used to validate and verify the extension of the ice accretion code, LEWICE, into the SLD realm.

  1. A Database of Supercooled Large Droplet Ice Accretions [Supplement

    NASA Technical Reports Server (NTRS)

    VanZante, Judith Foss

    2007-01-01

    A unique, publicly available database regarding supercooled large droplet (SLD) ice accretions has been developed in NASA Glenn's Icing Research Tunnel. Identical cloud and flight conditions were generated for five different airfoil models. The models chosen represent a variety of aircraft types from the horizontal stabilizer of a large transport aircraft to the wings of regional, business, and general aviation aircraft. In addition to the standard documentation methods of 2D ice shape tracing and imagery, ice mass measurements were also taken. This database will also be used to validate and verify the extension of the ice accretion code, LEWICE, into the SLD realm.

  2. Thanatology in protoplanetary discs. The combined influence of Ohmic, Hall, and ambipolar diffusion on dead zones

    NASA Astrophysics Data System (ADS)

    Lesur, Geoffroy; Kunz, Matthew W.; Fromang, Sébastien

    2014-06-01

    Protoplanetary discs are poorly ionised due to their low temperatures and high column densities and are therefore subject to three "non-ideal" magnetohydrodynamic (MHD) effects: Ohmic dissipation, ambipolar diffusion, and the Hall effect. The existence of magnetically driven turbulence in these discs has been a central question since the discovery of the magnetorotational instability (MRI). Early models considered Ohmic diffusion only and led to a scenario of layered accretion, in which a magnetically "dead" zone in the disc midplane is embedded within magnetically "active" surface layers at distances of about 1-10 au from the central protostellar object. Recent work has suggested that a combination of Ohmic dissipation and ambipolar diffusion can render both the midplane and surface layers of the disc inactive and that torques due to magnetically driven outflows are required to explain the observed accretion rates. We reassess this picture by performing three-dimensional numerical simulations that include all three non-ideal MHD effects for the first time. We find that the Hall effect can generically "revive" dead zones by producing a dominant azimuthal magnetic field and a large-scale Maxwell stress throughout the midplane, provided that the angular velocity and magnetic field satisfy Ω·B > 0. The attendant large magnetic pressure modifies the vertical density profile and substantially increases the disc scale height beyond its hydrostatic value. Outflows are produced but are not necessary to explain accretion rates ≲ 10-7 M⊙ yr-1. The flow in the disc midplane is essentially laminar, suggesting that dust sedimentation may be efficient. These results demonstrate that if the MRI is relevant for driving mass accretion in protoplanetary discs, one must include the Hall effect to obtain even qualitatively correct results. Appendices are available in electronic form at http://www.aanda.org

  3. Disc instability models for X-ray transients: evidence for evaporation and low α-viscosity?

    NASA Astrophysics Data System (ADS)

    Menou, Kristen; Hameury, Jean-Marie; Lasota, Jean-Pierre; Narayan, Ramesh

    2000-05-01

    We construct time-dependent models of accretion discs around black holes and neutron stars. We investigate the effect that evaporation of the inner disc regions during quiescence has upon the predictions of the disc instability model (DIM) for these systems. We do not include irradiation of the disc in the models. Removing the inner, most unstable, parts of the accretion disc increases the predicted recurrence times. However, DIMs with values of the viscosity parameter αhot~0.1 and αcold~0.02 (values typically used in applications of the DIM to standard dwarf nova outbursts) fail to reproduce the long recurrence times of soft X-ray transients (unless we resort to fine-tuning the parameters), independent of the evaporation strength. We show that models in which evaporation is included and a smaller value of αcold (~0.005) used do reproduce the long recurrence times and the accretion rates at the level of the Eddington rate observed in outburst. The large difference between the values of αhot and αcold, if confirmed once disc irradiation is included, suggests that several viscosity mechanisms operate in these accretion discs. For some parameter sets our models predict re-flares during the decline from outburst. The re-flares are a physical property of the model and result from the formation of a heating front in the wake of an initial cooling front, and subsequent multiple front reflections. The re-flares disappear in low-α models where front reflection cannot occur.

  4. Comments on ``Angular Momentum Transport in Quasi-Keplerian Accretion Disks''

    NASA Astrophysics Data System (ADS)

    Hayashi, E.; Isaka, H.; Matsuda, T.

    2005-06-01

    Subramanian, Pujari and Becker (2004) claim that the correct expression for the angular momentum transport in an accretion disc, which is proportional to d Ω/d R, can be derived on the basis of the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disc. We study their argument and show that their derivation contains several fundamental errors: 1) the biased choice of the desired formula from an infinite number of formulae; 2) the biased choice of parcel trajectories; and 3) confusion regarding the reference frames. Following 1) we could derive, for example, a (invalid) formula in which the angular momentum transport is proportional to d vφ/d R, and from 2) we could even prove that the angular momentum transport is either inward or null. We present the correct approach to the problem of angular momentum transport in an accretion disc in terms of mean free path theory.

  5. Implementation and Validation of 3-D Ice Accretion Measurement Methodology

    NASA Technical Reports Server (NTRS)

    Lee, Sam; Broeren, Andy P.; Kreeger, Richard E.; Potapczuk, Mark; Utt, Lloyd

    2014-01-01

    A research program has been implemented to develop and validate the use of a commercial 3-D laser scanning system to record ice accretion geometry in the NASA Icing Research Tunnel. A main component of the program was the geometric assessment of the 3- D laser scanning system on a 2-D (straight wing) and a 3-D (swept wing) airfoil geometries. This exercise consisted of comparison of scanned ice accretion to castings of the same ice accretion. The scan data were also used to create rapid prototype artificial ice shapes that were scanned and compared to the original ice accretion. The results from geometric comparisons on the straight wing showed that the ice shape models generated through the scan/rapid prototype process compared reasonably well with the cast shapes. Similar results were obtained with the geometric comparisons on the swept wing. It was difficult to precisely compare the scans of the cast shapes to the original ice accretion scans because the cast shapes appear to have shrunk during the mold/casting process by as much as 0.10-inch. However the comparison of the local ice-shape features were possible and produced better results. The rapid prototype manufacturing process was shown to reproduce the original ice accretion scan normally within 0.01-inch.

  6. Implementation and Validation of 3-D Ice Accretion Measurement Methodology

    NASA Technical Reports Server (NTRS)

    Lee, Sam; Broeren, Andy; Kreeger, Richard; Potapczuk, Mark; Utt, Lloyd

    2014-01-01

    A research program has been implemented to develop and validate the use of a commercial 3-D laser scanning system to record ice accretion geometry in the NASA Icing Research Tunnel. A main component of the program was the geometric assessment of the 3-D laser scanning system on a 2-D (straight wing) and a 3-D (swept wing) airfoil geometries. This exercise consisted of comparison of scanned ice accretion to castings of the same ice accretion. The scan data were also used to create rapid prototype artificial ice shapes that were scanned and compared to the original ice accretion.The results from geometric comparisons on the straight wing showed that the ice shape models generated through the scanrapid prototype process compared reasonably well with the cast shapes. Similar results were obtained with the geometric comparisons on the swept wing. It was difficult to precisely compare the scans of the cast shapes to the original ice accretion scans because the cast shapes appear to have shrunk during the moldcasting process by as much as 0.10-inch. However the comparison of the local ice-shape features were possible and produced better results. The rapid prototype manufacturing process was shown to reproduce the original ice accretion scan normally within 0.01-inch.

  7. Magnetospheric accretion in EX Lupi

    NASA Astrophysics Data System (ADS)

    Abraham, Peter; Kospal, Agnes; Bouvier, Jerome

    2016-08-01

    We propose to observe EX Lup, the prototype of the EXor class of young eruptive stars, in order to understand how the accretion process works in the quiescent system. Here, we request 2.6 hours of telescope time on Spitzer, to carry out a mid-infrared photometric monitoring, which we will supplement with simultaneous ground-based optical and near-infrared data. The multi-wavelength light curves will allow us to reliably separate the effects of fluctuating accretion rate from the rotation of the star. By analyzing the variations of the accretion rate we will determine whether EX Lup accretes through a few stable accretion columns or several short-lived random accretion streams. With this campaign, EX Lup will become one of the T Tauri systems where the accretion process is best understood.

  8. Accretion of southern Alaska

    USGS Publications Warehouse

    Hillhouse, J.W.

    1987-01-01

    Paleomagnetic data from southern Alaska indicate that the Wrangellia and Peninsular terranes collided with central Alaska probably by 65 Ma ago and certainly no later than 55 Ma ago. The accretion of these terranes to the mainland was followed by the arrival of the Ghost Rocks volcanic assemblage at the southern margin of Kodiak Island. Poleward movement of these terranes can be explained by rapid motion of the Kula oceanic plate, mainly from 85 to 43 Ma ago, according to recent reconstructions derived from the hot-spot reference frame. After accretion, much of southwestern Alaska underwent a counterclockwise rotation of about 50 ?? as indicated by paleomagnetic poles from volcanic rocks of Late Cretaceous and Early Tertiary age. Compression between North America and Asia during opening of the North Atlantic (68-44 Ma ago) may account for the rotation. ?? 1987.

  9. Going with the flow: using gas clouds to probe the accretion flow feeding Sgr A*

    NASA Astrophysics Data System (ADS)

    McCourt, Michael; Madigan, Ann-Marie

    2016-01-01

    The massive black hole in our Galactic centre, Sgr A*, accretes only a small fraction of the gas available at its Bondi radius. The physical processes determining this accretion rate remain unknown, partly due to a lack of observational constraints on the gas at distances between ˜10 and ˜105 Schwarzschild radii (Rs) from the black hole. Recent infrared observations identify low-mass gas clouds, G1 and G2, moving on highly eccentric, nearly co-planar orbits through the accretion flow around Sgr A*. Although it is not yet clear whether these objects contain embedded stars, their extended gaseous envelopes evolve independently as gas clouds. In this paper we attempt to use these gas clouds to constrain the properties of the accretion flow at ˜103 Rs. Assuming that G1 and G2 follow the same trajectory, we model the small differences in their orbital parameters as evolution resulting from interaction with the background flow. We find evolution consistent with the G-clouds originating in the clockwise disc. Our analysis enables the first unique determination of the rotation axis of the accretion flow: we localize the rotation axis to within 20°, finding an orientation consistent with the parsec-scale jet identified in X-ray observations and with the circumnuclear disc, a massive torus of molecular gas ˜1.5 pc from Sgr A*. This suggests that the gas in the accretion flow comes predominantly from the circumnuclear disc, rather than the winds of stars in the young clockwise disc. This result will be tested by the Event-Horizon Telescope within the next year. Our model also makes testable predictions for the orbital evolution of G1 and G2, falsifiable on a 5-10 year time-scale.

  10. Boundary between stable and unstable regimes of accretion. Ordered and chaotic unstable regimes

    NASA Astrophysics Data System (ADS)

    Blinova, A. A.; Romanova, M. M.; Lovelace, R. V. E.

    2016-07-01

    We present a new study of the Rayleigh-Taylor unstable regime of accretion on to rotating magnetized stars in a set of high grid resolution three-dimensional magnetohydrodynamic simulations performed in low-viscosity discs. We find that the boundary between the stable and unstable regimes is determined almost entirely by the fastness parameter ωs = Ω⋆/ΩK(rm), where Ω⋆ is the angular velocity of the star and ΩK(rm) is the angular velocity of the Keplerian disc at the disc-magnetosphere boundary r = rm. We found that accretion is unstable if ωs ≲ 0.6. Accretion through instabilities is present in stars with different magnetospheric sizes. However, only in stars with relatively small magnetospheres, rm/R⋆ ≲ 7, do the unstable tongues produce chaotic hotspots on the stellar surface and irregular light curves. At even smaller values of the fastness parameter, ωs ≲ 0.45, multiple irregular tongues merge, forming one or two ordered unstable tongues that rotate with the angular frequency of the inner disc. This transition occurs in stars with even smaller magnetospheres, rm/R⋆ ≲ 4.2. Most of our simulations were performed at a small tilt of the dipole magnetosphere, Θ = 5°, and a small viscosity parameter α = 0.02. Test simulations at higher α values show that many more cases become unstable, and the light curves become even more irregular. Test simulations at larger tilts of the dipole Θ show that instability is present, however, accretion in two funnel streams dominates if Θ ≳ 15°. The results of these simulations can be applied to accreting magnetized stars with relatively small magnetospheres: Classical T Tauri stars, accreting millisecond X-ray pulsars, and cataclysmic variables.