1,721,127 research outputs found
Revised conditions for MRI due to isorotation theorem
No full textWe re-analyze the physical conditions for Magneto-rotational Instability (MRI) within a steady axisymmetric stratified disk of plasma, in order to account for the so-called isorotation theory (the spatial profile of differential angular velocity depends on the magnetic flux surface). We develop the study of linear stability around an astrophysical background configuration, following the original derivation in [15], but implementing the isorotation condition as the orthogonality between the background magnetic field and the angular velocity gradient. We demonstrate that a dependence on the background magnetic field direction is restored in the dispersion relation and, hence, the emergence of MRI is also influenced by field orientation. © 2017 The Author(s
Semiclassical and quantum behavior of the Mixmaster model in the polymer approach for the isotropic Misner variable
No full textWe analyze the semi-classical and quantum behavior of the Bianchi IX Universe in the Polymer Quantum Mechanics framework, applied to the isotropic Misner variable, linked to the space volume of the model. The study is performed both in the Hamiltonian and field equations approaches, leading to the remarkable result of a still singular and chaotic cosmology, whose Poincaré return map asymptotically overlaps the standard Belinskii–Khalatnikov–Lifshitz one. In the quantum sector, we reproduce the original analysis due to Misner, within the revised Polymer approach and we arrive to demonstrate that the quantum numbers of the point-Universe still remain constants of motion. This issue confirms the possibility to have quasi-classical states up to the initial singularity. The present study clearly demonstrates that the asymptotic behavior of the Bianchi IX Universe towards the singularity is not significantly affected by the Polymer reformulation of the spatial volume dynamics both on a pure quantum and a semiclassical level. © 2018, The Author(s)
Bianchi I model as a prototype for a cyclical Universe
No full textWe analyze the dynamics of the Bianchi I model in the presence of stiff matter, an ultrarelativistic component and a small negative cosmological constant. We quantize this model in the framework of the polymer quantum mechanics, in order to introduce cut-off features in the minisuperspace dynamics. We then apply to the polymer Wheeler–DeWitt equation, emerging from the Dirac constraint, an adiabatic approximation a la Vilenkin, which treats the Universe volume as a quasi-classical variable, becoming de facto the dynamical clock for the pure quantum degrees of freedom, here identified in the Universe anisotropies. The main issue of the present analysis consists of determining a cyclical evolution for the Bianchi I model, oscillating between the Big-Bounce induced by the cut-off physics and the turning point due to the small cosmological constant. Furthermore, the mean value of the Universe anisotropy variables remains finite during the whole evolution, including the phase across the Big-Bounce. Such a feature, according to a suitable choice of the initial conditions makes the present cosmological paradigm, a viable scenario for the description of a possible primordial and late phases of the actual Universe. © 2017 The Author(s
Influence of toroidal magnetic field in multiaccreting tori
No full textWe analysed the effects of a toroidal magnetic field in the formation of several magnetized accretion tori, dubbed as ringed accretion discs (RADs), orbiting around one central Kerr supermassive black hole (SMBH) in active galactic nuclei (AGNs), where both corotating and counterotating discs are considered. Constraints on tori formation and emergence of RADs instabilities, accretion on to the central attractor and tori collision emergence, are investigated. The results of this analysis show that the role of the central BH spin-mass ratio, the magnetic field and the relative fluid rotation and tori rotation with respect the central BH, are crucial elements in determining the accretion tori features, providing ultimately evidence of a strict correlation between SMBH spin, fluid rotation, and magnetic fields in RADs formation and evolution. More specifically,we proved that magnetic field and discs rotation are in fact strongly constrained, as tori formation and evolution in RADs depend on the toroidal magnetic fields parameters. Eventually, this analysis identifies specific classes of tori, for restrict ranges of magnetic field parameter, that can be observed around some specific SMBHs identified by their dimensionless spin. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
Bianchi I cosmology in the presence of a causally regularized viscous fluid
No full textWe analyze the dynamics of a Bianchi I cosmology in the presence of a viscous fluid, causally regularized according to the Lichnerowicz approach. We show how the effect induced by shear viscosity is still able to produce a matter creation phenomenon, meaning that also in the regularized theory we address, the Universe is emerging from a singularity with a vanishing energy density value. We discuss the structure of the singularity in the isotropic limit, when bulk viscosity is the only retained contribution. We see that, as far as viscosity is not a dominant effect, the dynamics of the isotropic Universe possesses the usual non-viscous power-law behaviour but in correspondence to an effective equation of state, depending on the bulk viscosity coefficient. Finally., we show that, in the limit of a strong non-thermodynamical equilibrium of the Universe mimicked by a dominant contribution of the effective viscous pressure, a power-law inflation behaviour of the Universe appears, the cosmological horizons are removed and a significant amount of entropy is produced. © 2017, The Author(s)
Squeezing of toroidal accretion disks
No full textAccretion disks around very compact objects such as very massive black holes can grow according to thick toroidal models. We face the problem of defining how the thickness of a toroidal accretion disk spinning around a Schwarzschild black hole changes under the influence of a toroidal magnetic field and by varying the fluid angular momentum. We consider both a hydrodynamic and a magnetohydrodynamic disk based on the Polish doughnut thick model. We show that the torus thickness remains basically unaffected but tends to increase or decrease slightly depending on the balance of the magnetic, gravitational and centrifugal effects which the disk is subjected to. © Copyright EPLA, 2013
Thermomagnetic instability of a rotating magnetized plasma disc
No full textWe analyse the stability of a thin plasma disc which is rotating around a compact astrophysical object and is embedded in the strong magnetic field of such a source. The aim of this study is the determination of a newtype of unstable modes, able to replace the magnetorotational instability profile for low-ß values and for sufficiently small scales of the perturbations, where it fails. In particular, we consider the magnetohydrodynamical scheme including a non-zero Nernst coefficient, corresponding to first-order collisional effects. As a result, modes with imaginary frequency lead to an instability regime when the magnetic tension vanishes. Finally, we show that, even in the presence of resistive effects, it remains a good candidate to ensure the onset of a turbulent behaviour in the absence of the magnetorotational instability. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Counterexample of the magnetorotational instability in two-dimensional axial symmetry
No full textWe analyze a linear perturbation scheme for a two-dimensional background plasma, which is rotating at a differential frequency and is embedded in a poloidal magnetic field. The main two assumptions of the present study, which in turn are related, are (i) that the plasma profile is axially symmetric, both in the background and in the perturbation approximation, where the azimuthal magnetic field is requested to vanish identically, and (ii) that the angular frequency depends on the magnetic surface function only, still holds in the nonstationary regime, which, in the steady background equilibrium, is ensured by the validity of the corotation theorem. Indeed, such a restriction of the model is rather natural and it implies that the azimuthal component of the linear plasma shift is reabsorbed in the expression for the nonstationary electric field (in principle, at any order of approximation) and can no longer provide a nonzero azimuthal component of the magnetic tension field. As a result, the magnetorotational instability is suppressed and the magnetic field has the effect to stabilize the plasma configuration with respect to the pure hydrodynamical case. © 2013 American Physical Society
Plasma phenomenology in astrophysical systems: Radio-sources and jets
No full textWe review the plasma phenomenology in the astrophysical sources which show appreciable radio emissions, namely Radio-Jets from Pulsars, Microquasars, Quasars, and Radio-Active Galaxies. A description of their basic features is presented, then we discuss in some details the links between their morphology and the mechanisms that lead to the different radio-emissions, investigating especially the role played by the plasma configurations surrounding compact objects (Neutron Stars, Black Holes). For the sake of completeness, we briefly mention observational techniques and detectors, whose structure set them apart from other astrophysical instruments. The fundamental ideas concerning angular momentum transport across plasma accretion disks - together with the disk-source-jet coupling problem - are discussed, by stressing their successes and their shortcomings. An alternative scenario is then inferred, based on a parallelism between astrophysical and laboratory plasma configurations, where small-scale structures can be found. We will focus our attention on the morphology of the radio-jets, on their coupling with the accretion disks and on the possible triggering phenomena, viewed as profiles of plasma instabilities. © 2014 AIP Publishing LLC
The Jeans Mechanism and bulk-viscosity effects
No full textIn this paper we study the gravitational instability in presence of viscosity. In particular, the standard Jeans Mechanism is analyzed taking into account bulk-viscosity effects in the first-order Newtonian dynamics. We show how the perturbation evolution is dumped by dissipative processes and the top-down fragmentation mechanism is suppressed for strong viscous effects. The critical value of the Jeans Mass remains unchanged also in presence of bulk viscosity
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