253 research outputs found

    The intimate relation between the low T/W instability and the co-rotation point

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    We study the low T/W instability associated with the f-mode of differentially rotating stars. Our stellar models are described by a polytropic equation of state and the rotation profile is given by the standard j-constant law. The properties of the relevant oscillation modes, including the instability growth time, are determined from time evolutions of the linearised dynamical equations in Newtonian gravity. In order to analyse the instability we monitor also the canonical energy and angular momentum. Our results demonstrate that the l=m=2 f-mode becomes unstable as soon as a co-rotation point develops inside the star (i.e. whenever there is a point where the mode's pattern speed matches the bulk angular velocity). Considering various degrees of differential rotation, we show that the instability grows faster deep inside the co-rotation region and deduce an empirical relation that correlates the mode frequency and the star's parameters, which captures the main features of the l=m=2 f-mode growth time. This function is proportional to the product of the kinetic to gravitational energy ratio and the gradient of the star's spin, strengthening further the relationship between the co-rotation point and the low T/W instability. We briefly consider also the l=m=2 r-mode and demonstrate that it never moves far inside the co-rotation region even for significant differential rotation.Comment: 12 pages, 8 figures, 2 tables. Submitted to MNRA

    Non-linear Oscillations of Compact Stars and Gravitational Waves

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    This thesis investigates in the time domain a particular class of second order perturbations of a perfect fluid non-rotating compact star: those arising from the coupling between first order radial and non-radial perturbations. This problem has been treated by developing a gauge invariant formalism based on the 2-parameter perturbation theory (Sopuerta, Bruni and Gualtieri, 2004) where the radial and non-radial perturbations have been separately parameterized. The non-linear perturbations obey inhomogeneous partial differential equations, where the structure of the differential operator is given by the previous perturbative orders and the source terms are quadratic in the first order perturbations. In the exterior spacetime the sources vanish, thus the gravitational wave properties are completely described by the second order Zerilli and Regge-Wheeler functions. As main initial configuration we have considered a first order differentially rotating and radially pulsating star. Although at first perturbative order this configuration does not exhibit any gravitational radiation, we have found a new interesting gravitational signal at non-linear order, in which the radial normal modes are precisely mirrored. In addition, a resonance effect is present when the frequencies of the radial pulsations are close to the first axial w-mode. Finally, we have roughly estimated the damping times of the radial pulsations due to the non-linear gravitational emission. The coupling near the resonance results to be a very effective mechanism for extracting energy from the radial oscillations.Comment: 172 pages, 32 figures, PhD thesis, University of Portsmout

    On the magnetic field evolution timescale in superconducting neutron star cores

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    We revisit the various approximations employed to study the long-term evolution of the magnetic field in neutron star cores and discuss their limitations and possible improvements. A recent controversy on the correct form of the induction equation and the relevant evolution timescale in superconducting neutron star cores is addressed and clarified. We show that this ambiguity in the estimation of timescales arises as a consequence of nominally large terms that appear in the induction equation, but which are, in fact, mostly irrotational. This subtlety leads to a discrepancy by many orders of magnitude when velocity fields are absent or ignored. Even when internal velocity fields are accounted for, only the solenoidal part of the electric field contributes to the induction equation, which can be substantially smaller than the irrotational part. We also argue that stationary velocity fields must be incorporated in the slow evolution of the magnetic field as the next level of approximation.Comment: 6 pages, version accepted by MNRA

    The relevance of ambipolar diffusion for neutron star evolution

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    We study ambipolar diffusion in strongly magnetised neutron stars, with special focus on the effects of neutrino reaction rates and the impact of a superfluid/superconducting transition in the neutron star core. For axisymmetric magnetic field configurations, we determine the deviation from β−\beta-equilibrium induced by the magnetic force and calculate the velocity of the slow, quasi-stationary, ambipolar drift. We study the temperature dependence of the velocity pattern and clearly identify the transition to a predominantly solenoidal flow. For stars without superconducting/superfluid constituents and with a mixed poloidal-toroidal magnetic field of typical magnetar strength, we find that ambipolar diffusion proceeds fast enough to have a significant impact on the magnetic field evolution only at low core temperatures, T≲1−2×108T \lesssim 1-2\times10^8 K. The ambipolar diffusion timescale becomes appreciably shorter when fast neutrino reactions are present, because the possibility to balance part of the magnetic force with pressure gradients is reduced. We also find short ambipolar diffusion timescales in the case of superconducting cores for T≲109T \lesssim 10^9 K, due to the reduced interaction between protons and neutrons. In the most favourable scenario, with fast neutrino reactions and superconducting cores, ambipolar diffusion results in advection velocities of several km/kyr. This velocity can substantially reorganize magnetic fields in magnetar cores, in a way that can only be confirmed by dynamical simulations.Comment: 14 pages, 11 figures, version accepted for publication in MNRA

    Quasi-periodic oscillations in superfluid, relativistic magnetars with nuclear pasta phases

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    We study the torsional magneto-elastic oscillations of relativistic superfluid magnetars and explore the effects of a phase transition in the crust–core interface (nuclear pasta) which results in a weaker elastic response. Exploring various models with different extension of nuclear pasta phases, we find that the differences in the oscillation spectrum present in purely elastic modes (weak magnetic field) are smeared out with increasing strength of the magnetic field. For magnetar conditions, the main characteristic and features of models without nuclear pasta are preserved. We find, in general, two classes of magneto-elastic oscillations which exhibit a different oscillation pattern. For Bp 5 × 1014 G. We do not find any evidence of fundamental pure crustal modes in the low-frequency range (below 200 Hz) for Bp ≥ 1014 G.AP acknowledges support from the European Union under the Marie Sklodowska Curie Actions Individual Fellowship, grant agreement no 656370. This work is supported in part by the Spanish MINECO grant AYA2015-66899-C2-2-P, the programme PROMETEOII-2014-069 (Generalitat Valenciana), and by the NewCompstarCOST action MP1304

    Towards asteroseismology of core-collapse supernovae with gravitational-wave observations - I. Cowling approximation

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    Gravitational waves from core-collapse supernovae are produced by the excitation of different oscillation modes in the proto-neutron star (PNS) and its surroundings, including the shock. In this work we study the relationship between the post-bounce oscillation spectrum of the PNS-shock system and the characteristic frequencies observed in gravitational-wave signals from core-collapse simulations. This is a fundamental first step in order to develop a procedure to infer astrophysical parameters of the PNS formed in core-collapse supernovae. Our method combines information from the oscillation spectrum of the PNS, obtained through linear-perturbation analysis in general relativity of a background physical system, with information from the gravitational-wave spectrum of the corresponding non-linear, core-collapse simulation. Using results from the simulation of the collapse of a 35 M⊙M_{\odot} presupernova progenitor we show that both types of spectra are indeed related and we are able to identify the modes of oscillation of the PNS, namely g-modes, p-modes, hybrid modes, and standing-accretion-shock-instability (SASI) modes, obtaining a remarkably close correspondence with the time-frequency distribution of the gravitational-wave modes. The analysis presented in this paper provides a proof-of-concept that asteroseismology is indeed possible in the core-collapse scenario, and it may serve as a basis for future work on PNS parameter inference based on gravitational-wave observations

    The IGS-ETS in Bacillus (Insecta Phasmida): molecular characterization and the relevance of sex in ribosomal DNA evolution

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    <p>Abstract</p> <p>Background</p> <p>DNA encoding for ribosomal RNA (rDNA) is arranged in tandemly-repeated subunits, each containing ribosomal genes and non-coding spacers. Because tandemly-repeated, rDNA evolves under a balanced influence of selection and "concerted evolution", which homogenizes rDNA variants over the genome (through genomic turnover mechanisms) and the population (through sexuality).</p> <p>Results</p> <p>In this paper we analyzed the IGS-ETS of the automictic parthenogen <it>Bacillus atticus </it>and the bisexual <it>B. grandii</it>, two closely related stick-insect species. Both species share the same IGS-ETS structure and sequence, including a peculiar head-to-tail array of putative transcription enhancers, here named <it>Bag530</it>. Sequence variability of both IGS-ETS and <it>Bag530 </it>evidenced a neat geographic and subspecific clustering in <it>B. grandii</it>, while <it>B. atticus </it>shows a little but evident geographic structure. This was an unexpected result, since the parthenogen <it>B. atticus </it>should lack sequence fixation through sexuality. In <it>B. atticus </it>a new variant might spread in a given geographic area through colonization by an all-female clone, but we cannot discard the hypothesis that <it>B. atticus </it>was actually a bisexual taxon in that area at the time the new variant appeared. Moreover, a gene conversion event between two <it>Bag530 </it>variants of <it>B. grandii benazzii </it>and <it>B. grandii maretimi </it>suggested that rRNA might evolve according to the so-called "library hypothesis" model, through differential amplification of rDNA variants in different taxa.</p> <p>Conclusion</p> <p>On the whole, <it>Bacillus </it>rDNA evolution appears to be under a complex array of interacting mechanisms: homogenization may be achieved through genomic turnover that stabilizes DNA-binding protein interactions but, simultaneously, new sequence variants can be adopted, either by direct appearance of newly mutated repeats, or by competition among repeats, so that both DNA-binding proteins and repeat variants drive each other's evolution. All this, coupled with chromosome reshuffling due to sexuality (when present), might drive a quick fixation of new rDNA variants in the populations.</p

    Heritability of human "directed" functional connectome

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    IntroductionThe functional connectivity patterns in the brain are highly heritable; however, it is unclear how genetic factors influence the directionality of such "information flows." Studying the "directionality" of the brain functional connectivity and assessing how heritability modulates it can improve our understanding of the human connectome. MethodsHere, we investigated the heritability of "directed" functional connections using a state-space formulation of Granger causality (GC), in conjunction with blind deconvolution methods accounting for local variability in the hemodynamic response function. Such GC implementation is ideal to explore the directionality of functional interactions across a large number of networks. Resting-state functional magnetic resonance imaging data were drawn from the Human Connectome Project (total n = 898 participants). To add robustness to our findings, the dataset was randomly split into a "discovery" and a "replication" sample (each with n = 449 participants). The two cohorts were carefully matched in terms of demographic variables and other confounding factors (e.g., education). The effect of shared environment was also modeled. ResultsThe parieto- and prefronto-cerebellar, parieto-prefrontal, and posterior-cingulate to hippocampus connections showed the highest and most replicable heritability effects with little influence by shared environment. In contrast, shared environmental factors significantly affected the visuo-parietal and sensory-motor directed connectivity. ConclusionWe suggest a robust role of heritability in influencing the directed connectivity of some cortico-subcortical circuits implicated in cognition. Further studies, for example using task-based fMRI and GC, are warranted to confirm the asymmetric effects of genetic factors on the functional connectivity within cognitive networks and their role in supporting executive functions and learning
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