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&

L2 WRITING ANXIETY AND TEACHER’S COMMUNICATION BEHAVIOR AS PREDICTORS OF RESEARCH SELF-EFFICACY OF STUDENTS

The purpose of this study was to determine whether second language writing anxiety and teacher’ communication behavior significantly predict students’ research self-efficacy. The working group for the present study, in which quantitative descriptive-predictive design was employed, included 187 students enrolled in Research 1 and 2 subjects at UM Digos College. The participants answered adapted standard questionnaires to quantify which dimension/s of Second Language Writing Anxiety and Teacher’s Communication Behavior significantly predict/s research self-efficacy. The gathered data were interpreted using the mean in determining the central tendency and Pearson-r. Regression analysis was used to predict the value of research self-efficacy based on the value of writing anxiety and teachers’ communication behavior. The results revealed that the level of L2 writing anxiety of students is moderate. Likewise, the level of teacher’s communication behavior was revealed as high and the level of research self-efficacy is also high. The result also shows that there is no significant relationship between L2 writing anxiety and research self-efficacy but indicators of L2 writing anxiety posed a significant relationship with research self-efficacy while teachers’ communication behavior resulted in no significant relationship to research self-efficacy, but two indicators of TCB posed a significant relationship to RSE. The findings corroborated the four domains for SLWA and TCB, which are cognitive, somatic, challenging, and encouragement and praise; significantly predict research self-efficacy. The findings obtained in this study suggest that students look for ways to enhance their confidence in performing tasks related to research papers.  Article visualizations

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

Protoneutron star dynamos and pulsar magnetism

We have investigated the turbulent mean-field dynamo action in protoneutron stars that are subject to convective and neutron finger instabilities during the early evolutionary phase. While the first one develops mostly in the inner regions of the star, the second one is favored in the outer regions, where the Rossby number is much smaller and a mean-field dynamo action is more efficient. By solving the mean-field induction equation we have computed the critical spin period below which no dynamo action is possible and found it to be $\sim 1$ s for a wide range of stellar models and for both axisymmetric and non-axisymmetric magnetic fields. Because this critical period is substantially longer than the characteristic spin period of very young pulsars, we expect that a mean-field dynamo will be effective for most protoneutron stars. The saturation dipole field estimated by making use of the model of ``global'' quenching fits well the pulsar magnetic fields inferred from the spin-down data. Apart from the large scale magnetic field, our model predicts also a generation of small scale fields which are typically stronger than the poloidal field and can survive during the lifetime of pulsars. Extremely rapidly rotating protoneutron stars ($P \sim 1$ ms) may have the dipole field $\sim (3-6) \times 10^{14}$ G.Comment: 7 pages, 6 figures, to appear on A&

Hydromagnetic instabilities in protoneutron stars

The stability properties of newly born neutron stars, or proto--neutron stars, are considered. We take into account dissipative processes, such as neutrino transport and viscosity, in the presence of a magnetic field. In order to find the regions of the star subject to different sorts of instability, we derive the general instability criteria and apply it to evolutionary models of PNSs. The influence of the magnetic field on instabilities is analyzed and the critical magnetic field stabilizing the star is obtained. In the light of our results, we estimate of the maximum poloidal magnetic field that might be present in young pulsars or magnetars.Comment: 18 pages, 4 figures, to appear in Astrophysical Journa

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

Protoneutron star dynamos: pulsars, magnetars, and radio-silent X-ray emitting neutron stars

We discuss the mean-field dynamo action in protoneutron stars that are subject to instabilities during the early evolutionary phase. The mean field is generated in the neutron-finger unstable region where the Rossby number is $\sim 1$ and mean-field dynamo is efficient. Depending on the rotation rate, the mean-field dynamo can lead to the formation of three different types of pulsars. If the initial period of the protoneutron star is short, then the generated large-scale field is very strong ($> 3 \times 10^{13}$G) and exceeds the small-scale field at the neutron star surface. If rotation is moderate, then the pulsars are formed with more or less standard dipole fields ($< 3 \times 10^{13}$G) but with surface small-scale magnetic fields stronger than the dipole field. If rotation is very slow, then the mean-field dynamo does not operate, and the neutron star has no global field. Nevertheless, strong small-scale fields are generated in such pulsars, and they can manifest themselves as objects with very low spin-down rate but with a strong magnetic field inferred from the spectral features.Comment: 4 pages, 2 figures, to appear on A&