El Artista Enfermo
Ricardo de la Fuente Ballesteros2001, SIGLO DIECINUEVE (Literatura hispánica)
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Abstract
Neutron stars, and magnetars in particular, are known to host the strongest magnetic fields in the Universe. The origin of these strong fields is a matter of controversy. In this preliminary work, via numerical simulations, we study, for the first time in non-ideal general relativistic magnetohydrodynamic (GRMHD) regime, the growth of the magnetic field due to the action of the mean-field dynamo due to subscale, unresolved turbulence. The dynamo process, combined with the differential rotation of the (proto-)star, is able to produce an exponential growth of any initial magnetic seed field up to the values required to explain the observations. By varying the dynamo coefficient we obtain different growth rates. We find a quasi-linear dependence of the growth rates on the intensity of the dynamo. Furthermore, the time interval in which exponential growth occurs and the growth rates also seems to depend on the initial configuration of the magnetic field.
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The Small-Scale Dynamo and Non-Ideal Magnetohydrodynamics in Primordial Star Formation
The Astrophysical Journal, 2012
We study the amplification of magnetic fields during the formation of primordial halos. The turbulence generated by gravitational infall motions during the formation of the first stars and galaxies can amplify magnetic fields very efficiently and on short timescales up to dynamically significant values. Using the Kazantsev theory, which describes the so-called small-scale dynamo-a magnetohydrodynamical process converting kinetic energy from turbulence into magnetic energy-we can then calculate the growth rate of the small-scale magnetic field. Our calculations are based on a detailed chemical network and we include non-ideal magnetohydrodynamical effects such as ambipolar diffusion and Ohmic dissipation. We follow the evolution of the magnetic field up to larger scales until saturation occurs on the Jeans scale. Assuming a weak magnetic seed field generated by the Biermann battery process, both Burgers and Kolmogorov turbulence lead to saturation within a rather small density range. Such fields are likely to become relevant after the formation of a protostellar disk and, thus, could influence the formation of the first stars and galaxies in the universe.
Magnetic field amplification in proto-neutron stars
Astronomy and Astrophysics, 2008
Aims. During the first 40 s after their birth, proto-neutron stars are expected to be subject to at least two types of instability. The first one, the convective instability, is excited in the inner regions, where the entropy gradient produces a Rayleigh-type convection. The second one, the neutron-finger instability, is instead excited in the outer layers where the lepton gradients are large. Both instabilities involve convective motions and hence can trigger dynamo actions that may be responsible for the large magnetic fields in neutron stars and magnetars. However, because they have rather different mean turbulent velocities, they are also likely to give rise to different types of dynamo. Methods. We have solved the mean-field induction equation in a simplified one-dimensional model of both the convective and the neutron-finger instability zones. Although very idealized, the model includes the nonlinearities introduced by the feedback processes that tend to saturate the growth of the magnetic field (α-quenching) and suppress its turbulent diffusion (η-quenching). The possibility of a dynamo action is studied within a dynamical model of turbulent diffusivity where the boundary of the unstable zone is allowed to move. A large number of numerical simulations have been performed in which the relevant parameters, such as the spin-period, the strength of the differential rotation, the intensity of the initial magnetic field, and the extent of the neutron finger instability zone, have been suitably varied. Results. We show that the dynamo action can also be operative within a dynamical model of turbulent diffusivity and that the amplification of the magnetic field can still be very effective. Furthermore, we confirm the existence of a critical spin-period, below which the dynamo is always excited independently of the degree of differential rotation, and whose value is related to the size of the neutron-finger instability zone. We provide a relation for the intensity of the final field as a function of the spin of the star and of its differential rotation. Conclusions. Although they were obtained by using a toy model, we expect that our results are able to capture the qualitative and asymptotic behaviour of a mean-field dynamo action developing in the neutron-finger instability zone. Overall, we find that such a dynamo is very efficient in producing magnetic fields well above equipartition, and thus that it could represent a possible explanation for the large surface magnetic fields observed in neutron stars.
On generation of magnetic field in astrophysical bodies
2001
The generation of magnetic field in astrophysical bodies, e.g., galaxies, stars, planets, is one of the outstanding theoretical problems of physics and astrophysics. The initial magnetic fields of galaxies and stars are weak, and are amplified by the turbulent motion of the plasma. The generated field gets saturated due to nonlinear interactions. The above process is called "dynamo" action. Qualitatively, the magnetic field is amplified by the stretching of the field lines due to turbulent plasma motion. A fraction of kinetic energy of the plasma is spent in increasing the tension of the magnetic field lines, which effectively enhances the magnetic field strength. Current dynamo theories of are of two types, kinematic and dynamic. In the kinematic theories, one studies the evolution of magnetic field under a prescribed velocity field. In kinematic α-dynamo, the averaged nonlinear term u × b (u, b are velocity and magnetic field fluctuations respectively) is replaced by a constant α times mean magnetic field B 0 . This process, which is valid for small magnetic field fluctuations, yields linear equations that can be solved for a given boundary condition and external forcing fields 1−3 . In dynamic theories 4−6 , the modification of velocity field by the magnetic field (back reaction) is taken into account. Using a different approach, here we compute energy transfer rates from velocity field to magnetic field using field-theoretic method. The striking result of our field theoretic calculation is that there is a large energy transfer rate from the large-scale velocity field to the large-scale magnetic field. We claim that the growth of large-scale magnetic energy is primarily due to this transfer. We reached the above conclusion without any linear approximation like that in α-dynamo.
Relativistic magnetohydrodynamics winds from rotating neutron stars
Monthly Notices of the Royal Astronomical Society, 2006
We solve for the time-dependent dynamics of axisymmetric, general relativistic magnetohydrodynamic winds from rotating neutron stars. The mass-loss rate as a function of latitude is obtained self-consistently as a solution to the magnetohydrodynamics equations, subject to a finite thermal pressure at the stellar surface. We consider both monopole and dipole magnetic field geometries and we explore the parameter regime extending from low magnetization (low σ 0), almost thermally driven winds to high magnetization (high σ 0), relativistic Poyntingflux-dominated outflows (σ = B 2 /4πργc 2 β 2 ≈ σ 0 /γ ∞ ,β = v/c with σ 0 = ω 2 2 /Ṁ, where ω is the rotation rate, is the open magnetic flux, andṀ is the mass flux). We compute the angular momentum and rotational energy-loss rates as a function of σ 0 and compare with analytic expectations from the classical theory of pulsars and magnetized stellar winds. In the case of the monopole, our high-σ 0 calculations asymptotically approach the analytic force-free limit. If we define the spindown rate in terms of the open magnetic flux, we similarly reproduce the spindown rate from recent force-free calculations of the aligned dipole. However, even for σ 0 as high as ∼20, we find that the location of the Y-type point (r Y), which specifies the radius of the last closed field line in the equatorial plane, is not the radius of the Light Cylinder R L = c/ω (R = cylindrical radius), as has previously been assumed in most estimates and force-free calculations. Instead, although the Alfvén radius at intermediate latitudes quickly approaches R L as σ 0 exceeds unity, r Y remains significantly less than R L. In addition, r Y is a weak function of σ 0 , suggesting that high magnetizations may be required to quantitatively approach the force-free magnetospheric structure, with r Y = R L. Because r Y < R L , our calculated spindown rates thus exceed the classic 'vacuum dipole' rate: equivalently, for a given spindown rate, the corresponding dipole field is smaller than traditionally inferred. In addition, our results suggest a braking index generically less than 3. We discuss the implications of our results for models of rotation-powered pulsars and magnetars, both in their observed states and in their hypothesized rapidly rotating initial states.
Life Cycles of Magnetic Fields in Stellar Evolution
Arxiv preprint arXiv: …, 2009
Small-scale dynamo action during the formation of the first stars and galaxies
Astronomy & Astrophysics, 2010
We explore the amplification of magnetic seeds during the formation of the first stars and galaxies. During gravitational collapse, turbulence is created from accretion shocks, which may act to amplify weak magnetic fields in the protostellar cloud. Numerical simulations showed that such turbulence is sub-sonic in the first star-forming minihalos, and highly supersonic in the first galaxies with virial temperatures larger than 10 4 K. We investigate the magnetic field amplification during the collapse both for Kolmogorov and Burgers-type turbulence with a semi-analytic model that incorporates the effects of gravitational compression and small-scale dynamo amplification. We find that the magnetic field may be substantially amplified before the formation of a disk. On scales of 1/10 of the Jeans length, saturation occurs after ∼10 8 yr. Although the saturation behaviour of the small-scale dynamo is still somewhat uncertain, we expect a saturation field strength of the order ∼10 −7 n 0.5 G in the first star-forming halos, with n the number density in cgs units. In the first galaxies with higher turbulent velocities, the magnetic field strength may be increased by an order of magnitude, and saturation may occur after 10 6 −10 7 yr. In the Kolmogorov case, the magnetic field strength on the integral scale (i.e. the scale with most magnetic power) is higher due to the characteristic power-law indices, but the difference is less than a factor of 2 in the saturated phase. Our results thus indicate that the precise scaling of the turbulent velocity with length scale is of minor importance. They further imply that magnetic fields will be significantly enhanced before the formation of a protostellar disk, where they may change the fragmentation properties of the gas and the accretion rate.
Magnetic fields in the formation of the first stars – II. Results
Monthly Notices of the Royal Astronomical Society, 2022
Beginning with cosmological initial conditions at z = 100, we simulate the effects of magnetic fields on the formation of Population III stars and compare our results with the predictions of Paper I. We use gadget-2 to follow the evolution of the system while the field is weak. We introduce a new method for treating kinematic fields by tracking the evolution of the deformation tensor. The growth rate in this stage of the simulation is lower than expected for diffuse astrophysical plasmas, which have a very low resistivity (high magnetic Prandtl number); we attribute this to the large numerical resistivity in simulations, corresponding to a magnetic Prandtl number of order unity. When the magnetic field begins to be dynamically significant in the core of the minihalo at z = 27, we map it onto a uniform grid and follow the evolution in an adaptive mesh refinement, MHD simulation in orion2. The nonlinear evolution of the field in the orion2 simulation violates flux-freezing and is consistent with the theory proposed by Xu & Lazarian. The fields approach equipartition with kinetic energy at densities ∼ 10 10 -10 12 cm -3 . When the same calculation is carried out in orion2 with no magnetic fields, several protostars form, ranging in mass from ∼ 1 to 30 M ⊙ ; with magnetic fields, only a single ∼ 30 M ⊙ protostar forms by the end of the simulation. Magnetic fields thus suppress the formation of low-mass Pop III stars, yielding a top-heavy Pop III IMF and contributing to the absence of observed Pop III stars.
Magnetic fields in the first galaxies: Dynamo amplification and limits from reionization
Arxiv preprint arXiv: …, 2011
We discuss the amplification of magnetic fields by the small-scale dynamo, a process that could efficiently produce strong magnetic fields in the first galaxies. In addition, we derive constraints on the primordial field strength from the epoch of reionization. *
Galactic winds and the origin of large-scale magnetic fields
Astronomy & Astrophysics
Context. Observations of dwarf galaxies suggest the presence of large-scale magnetic fields. However the size and slow rotation of these galaxies appear insufficient to support a mean-field dynamo action to excite such fields. Aims. Here we suggest a new mechanism to explain large-scale magnetic fields in galaxies that are too small to support mean-field dynamo action. The key idea is that we do not identify large-scale and mean magnetic fields. In our scenario the magnetic structures originate from a small-scale dynamo which produces small-scale magnetic field in the galactic disc and a galactic wind that transports this field into the galactic halo where the large turbulent diffusion increases the scale and order of the field. As a result, the magnetic field becomes large-scale; however its mean value remains vanishing in a strict sense. Methods. We verify the idea by numerical modelling of two distinct simplified configurations, a thin disc model using the no-z approximation, and an axisymmetric model using cylindrical r, z coordinates. Results. Each of these allows reduction of the problem to two spatial dimensions. Taken together, the models support the proposition that the general trends will persist in a fully 3D model. We demonstrate that a pronounced large-scale pattern can develop in the galactic halo for a wide choice of the dynamo governing parameters. Conclusions. We believe that our mechanism can be relevant to explaining the presence of the fields observed in the halos of dwarf galaxies, and maybe elsewhere. We emphasize that detailed modelling of the proposed scenario needs 3D simulations, and adjustment to the specific dynamo governing parameters of dwarf galaxies.
Evolution of random initial magnetic fields in stably stratified and barotropic stars
Monthly Notices of the Royal Astronomical Society, 2022
Long-lived magnetic fields are known to exist in upper main-sequence stars, white dwarfs, and neutron stars. In order to explore possible equilibrium configurations of the magnetic field inside these stars, we have performed 3D magnetohydrodynamic simulations of the evolution of initially random magnetic fields in stably stratified and barotropic stars with an ideal-gas equation of state using the pencil code, a high-order finite-difference code for compressible hydrodynamic flows in the presence of magnetic fields. In barotropic (isentropic) stars, we confirm previous results in the sense that all initial magnetic fields we tried decay away, unable to reach a stable equilibrium. In the case of stably stratified stars (with radially increasing specific entropy), initially random magnetic fields appear to always evolve to a stable equilibrium. However, the nature of this equilibrium depends on the dissipation mechanisms considered. If magnetic diffusivity (or hyper-diffusivity) is in...
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A large-scale dynamo and magnetoturbulence in rapidly rotating core-collapse supernovae
Nature, 2015
Magnetohydrodynamic (MHD) turbulence is of key importance in many highenergy astrophysical systems, where MHD instabilities can amplify local magnetic field over very short time scales 1,2. Specifically, the magnetorotational instability (MRI) and dynamo action 3,4 have been suggested as a mechanism to grow magnetar-strength magnetic field (≥ 10 15 G) and magnetorotationally power the explosion 5-7 of a rotating massive star 8,9. Such stars are progenitor candidates for type Ic-bl hypernova explosions 10,11 and make up all supernovae connected to long ! 2 gamma-ray bursts (GRBs) 12,13. The MRI has been studied with local 14,15,16 highresolution shearing box simulations in 3D or with global 2D simulations 17 , but it is an open question whether MRI-driven turbulence can result in the creation of a large-scale ordered and dynamically relevant field. Here we report results from global 3D general-relativistic magnetohydrodynamic (GRMHD) turbulence simulations and show that MRI-driven MHD turbulence in rapidly rotating protoneutron stars produces an inverse cascade of energy. We find a large-scale ordered toroidal field that is consistent with the formation of bipolar magnetorotationally driven outflows. Our results demonstrate that rapidly rotating massive stars are plausible progenitors for both type Ic-bl supernovae 10,18,19 and long GRBs, present a viable formation scenario for magnetars 20,21 , and may account for potentially magnetar-powered superluminous ! 8
A new scenario for magnetar formation: Tayler-Spruit dynamo in a proto-neutron star spun up by fallback
Astronomy & Astrophysics
Magnetars are isolated young neutron stars characterised by the most intense magnetic fields known in the Universe, which power a wide variety of high-energy emissions from giant flares to fast radio bursts. The origin of their magnetic field is still a challenging question. In situ magnetic field amplification by dynamo action could potentially generate ultra-strong magnetic fields in fast-rotating progenitors. However, it is unclear whether the fraction of progenitors harbouring fast core rotation is sufficient to explain the entire magnetar population. To address this point, we propose a new scenario for magnetar formation involving a slowly rotating progenitor, in which a slow-rotating proto-neutron star is spun up by the supernova fallback. We argue that this can trigger the development of the Tayler-Spruit dynamo while other dynamo processes are disfavoured. Using the findings of previous studies of this dynamo and simulation results characterising the supernova fallback, we d...
Dynamos and cosmic magnetic fields
Physics Reports, 1997
This paper discusses the origin of the galactic magnetic field. The theory of the mean field dynamo in the interstellar medium is reviewed and shown to be flawed because it ignores the strong amplification of small-scale magnetic fields. An alternative origin is offered. It is proposed that the galactic fields are created in the protogalaxy by protogalactic turbulence. It is shown that they are first created from zero by the turbulence through the Biermann battery mechanism. The resulting weak seed fields are then amplified by the dynamo action of the protogalactic turbulence up to a field strength adequate for a primordial field origin of the galactic magnetic field. It is suggested that the amplification of the small-scale fields, that are a problem for the interstellar origin, are suppressed in the protogalaxy by collisionless processes that act on scales smaller than the mean free path. Since the relative size of the mean free path is quite large in the protogalaxy, the dynamo would generate only large-scale fields. After compression this field could become the galactic field. It is possible that no further amplification of it need occur in the interstellar medium.
Magnetic Fields In Stars And Disks
Journal of the Korean Astronomical Society
Magnetic fields are thought to play a role in a wide variety of important astrophysical processes, from angular momentum transport and jet formation in accretion disks to corona formation in stars. Unfortunately, the dynamics of magnetic fields in astrophysical plasmas are extremely complicated, and the success of current theoretical models and computer simulations seems to be inversely correlated with the amount of observational detail available to us. Here I will discuss some of the more striking conflicts between numerical simulations and observations, and present an explanation for them based on an important dynamical process which is not adequately modeled in current numerical simulations. These processes will lead to the formation of flux tubes in stars and accretion disks, in accordance with observations. I will discuss some of the implications of flux tube formation for stellar and accretion disk dynamos. Key Words : magnetic fields, turbulence, accretion I. INTRODUCTION Mag...
Magnetohydrodynamic experiments on cosmic magnetic fields
ZAMM, 2008
It is widely known that cosmic magnetic fields, i.e. the fields of planets, stars, and galaxies, are produced by the hydromagnetic dynamo effect in moving electrically conducting fluids. It is less well known that cosmic magnetic fields play also an active role in cosmic structure formation by enabling outward transport of angular momentum in accretion disks via the magnetorotational instability (MRI). Considerable theoretical and computational progress has been made in understanding both processes. In addition to this, the last ten years have seen tremendous efforts in studying both effects in liquid metal experiments. In 1999, magnetic field self-excitation was observed in the large scale liquid sodium facilities in Riga and Karlsruhe. Recently, self-excitation was also obtained in the French "von Kármán sodium" (VKS) experiment. An MRIlike mode was found on the background of a turbulent spherical Couette flow at the University of Maryland. Evidence for MRI as the first instability of an hydrodynamically stable flow was obtained in the "Potsdam Rossendorf Magnetic Instability Experiment" (PROMISE). In this review, the history of dynamo and MRI related experiments is delineated, and some directions of future work are discussed.
The basic role of magnetic fields in stellar evolution
Magnetic field is playing an important role at all stages of star evolution from star formation to the endpoints. The main effects are briefly reviewed. We also show that O-type stars have large convective envelopes, where convective dynamo could work. There, fields in magnetostatic balance have intensities of the order of 100 G.
Turbulence and Magnetic Fields in the Large-Scale Structure of the Universe
Science, 2008
Magnetic fields appear to be ubiquitous in astrophysical environments. Their existence in the intracluster medium is established through observations of synchrotron emission and Faraday rotation. On the other hand, the nature of magnetic fields outside of clusters, where observations are scarce and controversial, remains largely unknown. In this chapter, we review recent developments in our understanding of the nature and origin of intergalactic magnetic fields, and in particular, intercluster fields. A plausible scenario for the origin of galactic and intergalactic magnetic fields is for seed fields, created in the early universe, to be amplified by turbulent flows induced during the formation of the large scale structure. We present several mechanisms for the generation of seed fields both before and after recombination. We then discuss the evolution and role of magnetic fields during the formation of the first starts. We describe the turbulent amplification of seed fields during the formation of large scale structure and the nature of the magnetic fields that arise. Finally, we discuss implications of intergalactic magnetic fields.
Magnetic fields and massive star formation
Proceedings of the International Astronomical Union, 2005
Magnetic fields may be observed via the Zeeman effect, linear polarization of dust emission, and linear polarization of spectral-line emission. Useful parameters that can be inferred from observations are the mass-to-flux ratio M/Φ and the scaling of field strength with density. The former tells us whether magnetic fields exert sufficient pressure to provide support against gravitational contraction; the latter tells whether or not magnetic fields are sufficiently strong to determine the nature (spherical or disk geometry) of the contraction. Examples of massive star formation regions for which detailed observations have been made of magnetic field strengths and morphologies include DR21OH, OMC1, and S106; observational results for these regions and relevant results for the diffuse ISM and masers will be reviewed. Results are that the strength of interstellar magnetic fields remains invariant at B ∼ 6µG between 0.1 cm −3 < n(H) < 10 3 cm −3 , but increases as B ∝ ρ 0. 4−0. 5 for 10 3 cm −3 < n(H2) < 10 8 cm −3. Moreover, M/Φ is significantly subcritical (strong B with respect to gravity) in diffuse H I clouds that are not self-gravitating, but becomes approximately critical in high-density molecular cloud cores. This suggests that GMCs form primarily by accumulation of matter along magnetic field lines, a process that will increase density but not magnetic field strength. How clumps in GMCs evolve will then depend critically on the M/Φ ratio in each clump.
Magnetic field amplification by the small-scale dynamo in the early Universe
Physical Review D, 2014
Relativistic MHD modeling of magnetized neutron stars, pulsar winds, and their nebulae
Plasma Physics and Controlled Fusion, 2017
Neutron stars are among the most fascinating astrophysical sources, being characterized by strong gravity, densities about the nuclear one or even above, and huge magnetic fields. Their observational signatures can be extremely diverse across the electromagnetic spectrum, ranging from the periodic and lowfrequency signals of radio pulsars, up to the abrupt high-energy gamma-ray flares of magnetars, where energies of ∼ 10 46 erg are released in a few seconds. Fastrotating and highly magnetized neutron stars are expected to launch powerful relativistic winds, whose interaction with the supernova remnants gives rise to the non-thermal emission of pulsar wind nebulae, which are known cosmic accelerators of electrons and positrons up to PeV energies. In the extreme cases of protomagnetars (magnetic fields of ∼ 10 15 G and millisecond periods), a similar mechanism is likely to provide a viable engine for the still mysterious gammaray bursts. The key ingredient in all these spectacular manifestations of neutron stars is the presence of strong magnetic fields in their constituent plasma. Here we will present recent updates of a couple of state-of-the-art numerical investigations by the high-energy astrophysics group in Arcetri: a comprehensive modeling of the steady-state axisymmetric structure of rotating magnetized neutron stars in general relativity, and dynamical 3-D MHD simulations of relativistic pulsar winds and their associated nebulae.
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