4,504 research outputs found
Hall drift in the crust of neutron stars - necessary for radio pulsar activity?
The radio pulsar models based on the existence of an inner accelerating gap
located above the polar cap rely on the existence of a small scale, strong
surface magnetic field . This field exceeds the dipolar field ,
responsible for the braking of the pulsar rotation, by at least one order of
magnitude. Neither magnetospheric currents nor small scale field components
generated during neutron star's birth can provide such field structures in old
pulsars. While the former are too weak to create G, the ohmic decay time of the latter is much shorter than
years. We suggest that a large amount of magnetic energy is stored in a
toroidal field component that is confined in deeper layers of the crust, where
the ohmic decay time exceeds years. This toroidal field may be created
by various processes acting early in a neutron star's life. The Hall drift is a
non-linear mechanism that, due to the coupling between different components and
scales, may be able to create the demanded strong, small scale, magnetic spots.
Taking into account both realistic crustal microphysics and a minimal cooling
scenario, we show that, in axial symmetry, these field structures are created
on a Hall time scale of - years. These magnetic spots can be
long-lived, thereby fulfilling the pre-conditions for the appearance of the
radio pulsar activity. Such magnetic structures created by the Hall drift are
not static, and dynamical variations on the Hall time scale are expected in the
polar cap region.Comment: 4 pages, 5 figures, contribution to the ERPM conferences, Zielona
Gora, April 201
Effect of lattice mismatch-induced strains on coupled diffusive and displacive phase transformations
Materials which can undergo slow diffusive transformations as well as fast
displacive transformations are studied using the phase-field method. The model
captures the essential features of the time-temperature-transformation (TTT)
diagrams, continuous cooling transformation (CCT) diagrams, and microstructure
formation of these alloys. In some materials systems there can exist an
intrinsic volume change associated with these transformations. We show that
these coherency strains can stabilize mixed microstructures (such as retained
austenite-martensite and pearlite-martensite mixtures) by an interplay between
diffusive and displacive mechanisms, which can alter TTT and CCT diagrams.
Depending on the conditions there can be competitive or cooperative nucleation
of the two kinds of phases. The model also shows that small differences in
volume changes can have noticeable effects on the early stages of martensite
formation and on the resulting microstructures.
-- Long version of cond-mat/0605577
-- Keywords: Ginzburg-Landau, martensite, pearlite, spinodal decomposition,
shape memory, microstructures, TTT diagram, CCT diagram, elastic compatibilityComment: 10 pages, 13 figures, long version of cond-mat/0605577. Physical
Review B, to appear in volume 75 (2007
A singular structure: Monopost made in composites
p. 3002-3012This work presents the design process, the analysis, and the performance of a cylindrical hollow monopost, made entirely in composites. It is about a translucent structure of height 40m, external diameter 1.60m and the average wall-thickness 11mm. The material is a polymer made up of vinylster resin and reinforced by glass fiber (GFRP). The manufactured processing used is filament winding. Moreover, due to geographical emplacement where it is situated, the structure has to support wind velocity value above 180Km/h and its elastic modulus of the material does not exceed 25GPa. Eventually, we was able to achieve an optimum solution and strentgh structure, considering and developing differents types of approaches and analysis, such as linear, non-linear and buckling.Rovira, JA.; Martin, P.; Pons, D.; Almerich Chulia, AI. (2009). A singular structure: Monopost made in composites. Editorial Universitat Politècnica de València. http://hdl.handle.net/10251/670
Generally covariant theories: the Noether obstruction for realizing certain space-time diffeomorphisms in phase space
Relying on known results of the Noether theory of symmetries extended to
constrained systems, it is shown that there exists an obstruction that prevents
certain tangent-space diffeomorphisms to be projectable to phase-space, for
generally covariant theories. This main result throws new light on the old fact
that the algebra of gauge generators in the phase space of General Relativity,
or other generally covariant theories, only closes as a soft algebra and not a
a Lie algebra.
The deep relationship between these two issues is clarified. In particular,
we see that the second one may be understood as a side effect of the procedure
to solve the first. It is explicitly shown how the adoption of specific
metric-dependent diffeomorphisms, as a way to achieve projectability, causes
the algebra of gauge generators (constraints) in phase space not to be a Lie
algebra --with structure constants-- but a soft algebra --with structure {\it
functions}.Comment: 22 pages, version to be published in Classical & Quantum Gravit
Population Synthesis of Isolated Neutron Stars with magneto-rotational evolution II: from radio-pulsars to magnetars
Population synthesis studies constitute a powerful method to reconstruct the
birth distribution of periods and magnetic fields of the pulsar population.
When this method is applied to populations in different wavelengths, it can
break the degeneracy in the inferred properties of initial distributions that
arises from single-band studies. In this context, we extend previous works to
include -ray thermal emitting pulsars within the same evolutionary model as
radio-pulsars. We find that the cumulative distribution of the number of X-ray
pulsars can be well reproduced by several models that, simultaneously,
reproduce the characteristics of the radio-pulsar distribution. However, even
considering the most favourable magneto-thermal evolution models with fast
field decay, log-normal distributions of the initial magnetic field
over-predict the number of visible sources with periods longer than 12 s. We
then show that the problem can be solved with different distributions of
magnetic field, such as a truncated log-normal distribution, or a binormal
distribution with two distinct populations. We use the observational lack of
isolated NSs with spin periods P>12 s to establish an upper limit to the
fraction of magnetars born with B > 10^{15} G (less than 1\%). As future
detections keep increasing the magnetar and high-B pulsar statistics, our
approach can be used to establish a severe constraint on the maximum magnetic
field at birth of NSs.Comment: 12 pages, 11 figures, 5 table
Reparameterization invariants for anisotropic Bianchi I cosmology with a massless scalar source
Intrinsic time-dependent invariants are constructed for classical, flat,
homogeneous, anisotropic cosmology with a massless scalar material source.
Invariance under the time reparameterization-induced canonical symmetry group
is displayed explicitly.Comment: 28 pages, to appear in General Relativity and Gravitation.
Substantial revisions: added foundational overview section 2, chose new
intrinsic time variable, worked with dimensionless variables, added appendix
with comparison and criticism of other approache
Gauge transformations in the Lagrangian and Hamiltonian formalisms of generally covariant theories
We study spacetime diffeomorphisms in Hamiltonian and Lagrangian formalisms
of generally covariant systems. We show that the gauge group for such a system
is characterized by having generators which are projectable under the Legendre
map. The gauge group is found to be much larger than the original group of
spacetime diffeomorphisms, since its generators must depend on the lapse
function and shift vector of the spacetime metric in a given coordinate patch.
Our results are generalizations of earlier results by Salisbury and
Sundermeyer. They arise in a natural way from using the requirement of
equivalence between Lagrangian and Hamiltonian formulations of the system, and
they are new in that the symmetries are realized on the full set of phase space
variables. The generators are displayed explicitly and are applied to the
relativistic string and to general relativity.Comment: 12 pages, no figures; REVTeX; uses multicol,fancyheadings,eqsecnum;
to appear in Phys. Rev.
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