5,520 research outputs found
Magnetic fields in axisymmetric neutron stars
No full textWe derive general equations for axisymmetric Newtonian magnetohydrodynamics and use these as the basis of a code for calculating equilibrium configurations of rotating magnetized neutron stars in a stationary state. We investigate the field configurations that result from our formalism, which include purely poloidal, purely toroidal and mixed fields. For the mixed-field formalism, the toroidal component appears to be bounded at less than 7 per cent. We calculate distortions induced both by magnetic fields and by rotation. From our non-linear work, we are able to look at the realm of validity of perturbative work: we find for our results that perturbative-regime formulae for magnetic distortions agree to within 10 per cent of the non-linear results if the ellipticity is less than 0.15 or the average field strength is less than 10^17 G. We also consider how magnetized equilibrium structures vary for different polytropic indices
Non-rigid precession of magnetic stars
No full textStars are, generically, rotating and magnetized objects with a misalignment between their magnetic and rotation axes. Since a magnetic field induces a permanent distortion to its host, it provides effective rigidity even to a fluid star, leading to bulk stellar motion that resembles free precession. This bulk motion is, however, accompanied by induced interior velocity and magnetic field perturbations, which are oscillatory on the precession time-scale. Extending previous work, we show that these quantities are described by a set of second-order perturbation equations featuring cross-terms scaling with the product of the magnetic and centrifugal distortions to the star. For the case of a background toroidal field, we reduce these to a set of differential equations in radial functions, and find a method for their solution. The resulting magnetic field and velocity perturbations show complex multipolar structure and are strongest towards the centre of the star
Oscillations and instabilities in neutron stars with poloidal magnetic fields
No full textWe study the time evolution of non-axisymmetric linear perturbations of a rotating magnetized neutron star, whose magnetic field is purely poloidal. The background stellar configurations are generated self-consistently, with multipolar field configurations and allowing for distortions to the density distribution from rotational and magnetic forces. The perturbations split into two symmetry classes, with perturbations in one class being dominated by an instability generic to poloidal fields, which is localized around the ‘neutral line’ where the background field vanishes. Rotation acts to reduce the effect of this instability. Perturbations in the other symmetry class do not suffer this instability and in this case we are able to resolve Alfvén oscillations, whose restoring force is the magnetic field; this is the first study of non-axisymmetric Alfvén modes of a star with a poloidal field. We find no evidence that these modes form a continuum. In a rotating magnetized star, we find that there are no pure Alfvén modes or pure inertial modes, but hybrids of these. We discuss the nature of magnetic instabilities and oscillations in magnetars and pulsars, finding the dominant Alfvén mode from our simulations has a frequency comparable with observed magnetar quasi-periodic oscillations (QPOs).<br/
Instabilities in neutron stars with toroidal magnetic fields
No full textWe study m= 1 oscillations and instabilities of magnetized neutron stars, by numerical time evolution of linear perturbations of the system. The background stars are stationary equilibrium configurations with purely toroidal magnetic fields. We find that an m= 1 instability of toroidal magnetic fields, already known from local analyses, may also be found in our relatively low-resolution global study. We present quantitative results for the instability growth rate and its suppression by rotation. The instability is discussed as a possible trigger mechanism for soft gamma repeater flares. Although our primary focus is evolutions of magnetized stars, we also consider perturbations about unmagnetized background stars in order to study m= 1 inertial modes. We track these modes up to break-up frequency ?K, extending known slow-rotation results.<br/
Algebraic K-theory of groups wreath product with finite groups
No full textAbstractThe Farrell–Jones Fibered Isomorphism Conjecture for the stable topological pseudoisotopy theory has been proved for several classes of groups. For example, for discrete subgroups of Lie groups [F.T. Farrell, L.E. Jones, Isomorphism conjectures in algebraic K-theory, J. Amer. Math. Soc. 6 (1993) 249–297], virtually poly-infinite cyclic groups [F.T. Farrell, L.E. Jones, Isomorphism conjectures in algebraic K-theory, J. Amer. Math. Soc. 6 (1993) 249–297], Artin braid groups [F.T. Farrell, S.K. Roushon, The Whitehead groups of braid groups vanish, Internat. Math. Res. Notices 10 (2000) 515–526], a class of virtually poly-surface groups [S.K. Roushon, The isomorphism conjecture for 3-manifold groups and K-theory of virtually poly-surface groups, math.KT/0408243, K-Theory, in press] and virtually solvable linear group [F.T. Farrell, P.A. Linnell, K-Theory of solvable groups, Proc. London Math. Soc. (3) 87 (2003) 309–336]. We extend these results in the sense that if G is a group from the above classes then we prove the conjecture for the wreath product G≀H for H a finite group. The need for this kind of extension is already evident in [F.T. Farrell, S.K. Roushon, The Whitehead groups of braid groups vanish, Internat. Math. Res. Notices 10 (2000) 515–526; S.K. Roushon, The Farrell–Jones isomorphism conjecture for 3-manifold groups, math.KT/0405211, K-Theory, in press; S.K. Roushon, The isomorphism conjecture for 3-manifold groups and K-theory of virtually poly-surface groups, math.KT/0408243, K-Theory, in press]. We also prove the conjecture for some other classes of groups
Tussen Onderdanen, Rijksgenoten en Nederlanders : Nederlandse politici over burgers uit Oost en West en Nederland, 1945-2005
No full textSpijkerboer, T.P. [Promotor]Walsum, S.K. van [Copromotor
Oscillations of rotating magnetized neutron stars with purely toroidal magnetic fields
No full textWe investigate the oscillation spectrum of rotating Newtonian neutron stars endowed with purely toroidal magnetic fields, using a time-evolution code to evolve linear perturbations in the Cowling approximation. The background star is generated by numerically solving the magnetohydrodynamics equilibrium equations and may be non-spherical by virtue of both rotation and magnetic effects; hence, our perturbations and background are fully consistent. Whilst the background field is purely toroidal, the perturbed field is mixed poloidal–toroidal. From Fourier analysis of the perturbations, we are able to identify a number of magnetically restored Alfvén (or a) modes. We show that in a rotating star pure inertial and a-modes are replaced by hybrid magneto-inertial modes, which reduce to a-modes in the non-rotating limit and inertial modes in the non-magnetic limit. We show that the r-mode instability is suppressed by magnetic fields in sufficiently slowly rotating stars. In addition, we determine magnetic frequency shifts in the f-mode. We discuss the astrophysical relevance of our results, in particular for magnetar oscillations
Short comments on technical note—The EOQ and EPQ models with shortages derived without derivatives
No full textWe study the paper of Ronald, Yang and Chu that was recently published in the International Journal of Production Economics to find the optimal solution for the EOQ and EPQ models without derivatives. We will offer a simple algebraic method to replace their sophisticated algebraic skill, and then we point out a possible direction for further research
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