Relativistic r-modes and shear viscosity

We derive the relativistic equations for stellar perturbations, including in a consistent way shear viscosity in the stress-energy tensor, and we numerically integrate our equations in the case of large viscosity. We consider the slow rotation approximation, and we neglect the coupling between polar and axial perturbations. In our approach, the frequency and damping time of the emitted gravitational radiation are directly obtained. We find that, approaching the inviscid limit from the finite viscosity case, the continuous spectrum is regularized. Constant density stars, polytropic stars, and stars with realistic equations of state are considered. In the case of constant density stars and polytropic stars, our results for the viscous damping times agree, within a factor two, with the usual estimates obtained by using the eigenfunctions of the inviscid limit. For realistic neutron stars, our numerical results give viscous damping times with the same dependence on mass and radius as previously estimated, but systematically larger of about 60%.Comment: 8 pages, 7 figures, to appear in the Proceedings of the Albert Einstein Century International Conference, Paris, France, July 200

Anisotropic convection in rotating proto-neutron stars

We study the conditions for convective instability in rotating, non-magnetic proto--neutron stars. The criteria that determine stability of nascent neutron stars are analogous to the Solberg--Hoiland conditions but including the presence of lepton gradients. Our results show that, for standard angular velocity profiles, convectively unstable modes with wave-vectors parallel to the rotation axis are suppressed by a stable angular momentum profile, while unstable modes with wave-vectors perpendicular to the axis remain unaltered. Since the wave-vector is perpendicular to the velocity perturbation, the directional selection of the unstable modes may result in fluid motions along the direction of the rotation axis. This occurs in rigidly rotating stars as well as in the inner core of differentially rotating stars. Our results provide a natural source of asymmetry for proto--neutron stars with the only requirement that angular velocities be of the order of the convective characteristic frequency.Comment: 5 pages, 4 figures, final version to appear in A&

Energy versus information based estimations of dissipation using a pair of magnetic colloidal particles

Using the framework of stochastic thermodynamics, we present an experimental study of a doublet of magnetic colloidal particles which is manipulated by a time-dependent magnetic field. Due to hydrodynamic interactions, each bead experiences a state-dependent friction, which we characterize using a hydrodynamic model. In this work, we compare two estimates of the dissipation in this system: the first one is energy based since it relies on the measured interaction potential, while the second one is information based since it uses only the information content of the trajectories. While the latter only offers a lower bound of the former, we find it to be simple to implement and of general applicability to more complex systems.Comment: Main text: 5 pages, 4 figures. Supplementary material: 5 pages, 5 figure

Z_2-gradings of Clifford algebras and multivector structures

Let Cl(V,g) be the real Clifford algebra associated to the real vector space V, endowed with a nondegenerate metric g. In this paper, we study the class of Z_2-gradings of Cl(V,g) which are somehow compatible with the multivector structure of the Grassmann algebra over V. A complete characterization for such Z_2-gradings is obtained by classifying all the even subalgebras coming from them. An expression relating such subalgebras to the usual even part of Cl(V,g) is also obtained. Finally, we employ this framework to define spinor spaces, and to parametrize all the possible signature changes on Cl(V,g) by Z_2-gradings of this algebra.Comment: 10 pages, LaTeX; v2 accepted for publication in J. Phys.

Evidence for Heating of Neutron Stars by Magnetic Field Decay

We show the existence of a strong trend between neutron star surface temperature and the dipolar component of the magnetic field extending through three orders of field magnitude, a range that includes magnetars, radio-quiet isolated neutron stars, and many ordinary radio pulsars. We suggest that this trend can be explained by the decay of currents in the crust over a time scale of few Myr. We estimate the minimum temperature that a NS with a given magnetic field can reach in this interpretation.Comment: 4 pages, 1 figures, version accepted for publication in Phys. Rev. Let

Síndrome de Mazabraud: Presentación de un caso clínico y revisión de la literatura

El síndrome de Mazabraud se define como la asociación de una tumoración de partes blandas, el mixoma intramuscular, y un tumor óseo, la displasia fibrosa. Se realiza una revisión bibliográfica de los casos publicados en la literatura, desde que en 1957, Mazabraud acuñó esta asociación como síndrome. El caso clínico corresponde a un varón de 43 años de edad con antecedentes de nódulo frío tiroideo, pigmentación cutánea y lesiones óseas en pelvis y tercio proximal fémur derecho, que acudió a la consulta por presentar una tumoración de partes blandas a nivel del muslo derecho de unos meses de evolución. Tras realizar biopsia de la misma se diagnosticó de mixoma intramuscular. El paciente se intervino quirúrgicamente practicándose resección amplia de la lesión. Hasta la fecha el enfermo no ha presentado recidiva de la lesión de partes blandas, y la displasia ósea no ha sufrido transformación maligna.Mazabraud's síndrome is the association of a soft-tissue tumour, intramuscular myxonma and a bone tumour, fibrous dysplasia. We review the literature since Mazabraud, in 1957 described this syndrome. We report a 43 years male who consulted for a soft-time mass in the right thigh. He also had bone lesions in pelvis and right femur that were diagnosted of fibrous dysplasia. We made a biopsy of soft-tissue tumour and it was diagnosed of intramuscular myxom. The patient was operated and the mass was removed. Till now, there is no recurrence of the myxom and there is no malignant degeneration of the fibrous dysplasia

Convection in protoneutron stars and the structure of surface magnetic fields in pulsars

We consider generation and evolution of small-scale magnetic fields in neutron stars. These fields can be generated by small-scale turbulent dynamo action soon after the collapse when the proto-neutron star is subject to convective and neutron finger instabilities. After instabilities stop, small-scale fields should be frozen into the crust that forms initially at high density about 10^{14} g/cm^{3} and then spreads to the surface. Because of high crustal conductivity, magnetic fields with the lengthscale about 1-3 km can survive in the crust as long as 10-100 Myr and form a sunspot-like structure at the surface of radiopulsars

Turning Points in the Evolution of Isolated Neutron Stars' Magnetic Fields

During the life of isolated neutron stars (NSs) their magnetic field passes through a variety of evolutionary phases. Depending on its strength and structure and on the physical state of the NS (e.g. cooling, rotation), the field looks qualitatively and quantitatively different after each of these phases. Three of them, the phase of MHD instabilities immediately after NS's birth, the phase of fallback which may take place hours to months after NS's birth, and the phase when strong temperature gradients may drive thermoelectric instabilities, are concentrated in a period lasting from the end of the proto--NS phase until 100, perhaps 1000 years, when the NS has become almost isothermal. The further evolution of the magnetic field proceeds in general inconspicuous since the star is in isolation. However, as soon as the product of Larmor frequency and electron relaxation time, the so-called magnetization parameter, locally and/or temporally considerably exceeds unity, phases, also unstable ones, of dramatic changes of the field structure and magnitude can appear. An overview is given about that field evolution phases, the outcome of which makes a qualitative decision regarding the further evolution of the magnetic field and its host NS.Comment: References updated, typos correcte