Magneto-elastic oscillations of neutron stars with dipolar magnetic fields

By means of two dimensional, general-relativistic, magneto-hydrodynamical simulations we investigate the oscillations of magnetized neutron star models (magnetars) including the description of an extended solid crust. The aim of this study is to understand the origin of the QPOs observed in the giant flares of SGRs. We confirm the existence of three different regimes: (a) a weak magnetic field regime B<5 x 10^13 G, where crustal shear modes dominate the evolution; (b) a regime of intermediate magnetic fields 5 x 10^13 G<B< 10^15 G, where Alfv\'en QPOs are mainly confined to the core of the neutron star and the crustal shear modes are damped very efficiently; and (c) a strong field regime B>10^15 G, where magneto-elastic oscillations reach the surface and approach the behavior of purely Alfv\'en QPOs. When the Alfv\'en QPOs are confined to the core of the neutron star, we find qualitatively similar QPOs as in the absence of a crust. The lower QPOs associated with the closed field lines of the dipolar magnetic field configuration are reproduced as in our previous simulations without crust, while the upper QPOs connected to the open field lines are displaced from the polar axis. Additionally, we observe a family of edge QPOs. Our results do not leave much room for a crustal-mode interpretation of observed QPOs in SGR giant flares, but can accommodate an interpretation of these observations as originating from Alfv\'en-like, global, turning-point QPOs in models with dipolar magnetic field strengths in the narrow range of 5 x 10^15 G < B < 1.4 x 10^16 G. This range is somewhat larger than estimates for magnetic field strengths in known magnetars. The discrepancy may be resolved in models including a more complicated magnetic field structure or with models taking superfluidity of the neutrons and superconductivity of the protons in the core into account.Comment: 25 pages, 17 figures, 7 tables, minor corrections to match published version in MNRA

The effect of an organic pentasulfide EP additive in turning and milling operations

Coopération avec Ecole Centrale LyonBecause a cutting fluid could be equally used for different cutting operations, this study proposed to investigate the behavior of a well-known extreme-pressure additive (pentasulfide) in both turning and milling operations of a steel workpiece. The experimental approach is based on the coupling of mechanical tests (turning, milling, and tribological tests) with physico-chemical characterizations (Auger Electron Spectroscopy and X-Ray Photoelectron Spectroscopy) of the friction surfaces (chip and tool). In the case of milling, it was shown that the presence of a pentasulfide additive has a beneficial effect on the specific cutting energy (kc) and flank wear (Vb). These results are correlated with the presence of iron sulfides (FeS and FeS2) on the flank face of the cutter mill and on the chip face in contact with the mill. No such additive effects are found in case of turning. A lubrication model is proposed for the case of milling based on an indirect lubrication of the tool/workpiece and tool/chip contacts due to the transfer of iron and its reaction with sulfur compounds to produce iron sulfides. Because milling is a discontinuous cutting process, this lubrication mechanism is much more efficient than that observed in turning. Indeed, the tool faces are re-fed iron sulfides each time they leave the workpiece

Magneto-elastic oscillations of neutron stars: exploring different magnetic field configurations

We study magneto-elastic oscillations of highly magnetized neutron stars (magnetars) which have been proposed as an explanation for the quasi-periodic oscillations (QPOs) appearing in the decaying tail of the giant flares of soft gamma-ray repeaters (SGRs). We extend previous studies by investigating various magnetic field configurations, computing the Alfv\'en spectrum in each case and performing magneto-elastic simulations for a selected number of models. By identifying the observed frequencies of 28 Hz (SGR 1900+14) and 30 Hz (SGR 1806-20) with the fundamental Alfv\'en QPOs, we estimate the required surface magnetic field strength. For the magnetic field configurations investigated (dipole-like poloidal, mixed toroidal-poloidal with a dipole-like poloidal component and a toroidal field confined to the region of field lines closing inside the star, and for poloidal fields with an additional quadrupole-like component) the estimated dipole spin-down magnetic fields are between 8x10^14 G and 4x10^15 G, in broad agreement with spin-down estimates for the SGR sources producing giant flares. A number of these models exhibit a rich Alfv\'en continuum revealing new turning points which can produce QPOs. This allows one to explain most of the observed QPO frequencies as associated with magneto-elastic QPOs. In particular, we construct a possible configuration with two turning points in the spectrum which can explain all observed QPOs of SGR 1900+14. Finally, we find that magnetic field configurations which are entirely confined in the crust (if the core is assumed to be a type I superconductor) are not favoured, due to difficulties in explaining the lowest observed QPO frequencies (f<30 Hz).Comment: 21 pages, 16 figures, 6 tables, matched to version accepted by MNRAS with extended comparison/discussion to previous wor

Multi-phase-field analysis of short-range forces between diffuse interfaces

We characterize both analytically and numerically short-range forces between spatially diffuse interfaces in multi-phase-field models of polycrystalline materials. During late-stage solidification, crystal-melt interfaces may attract or repel each other depending on the degree of misorientation between impinging grains, temperature, composition, and stress. To characterize this interaction, we map the multi-phase-field equations for stationary interfaces to a multi-dimensional classical mechanical scattering problem. From the solution of this problem, we derive asymptotic forms for short-range forces between interfaces for distances larger than the interface thickness. The results show that forces are always attractive for traditional models where each phase-field represents the phase fraction of a given grain. Those predictions are validated by numerical computations of forces for all distances. Based on insights from the scattering problem, we propose a new multi-phase-field formulation that can describe both attractive and repulsive forces in real systems. This model is then used to investigate the influence of solute addition and a uniaxial stress perpendicular to the interface. Solute addition leads to bistability of different interfacial equilibrium states, with the temperature range of bistability increasing with strength of partitioning. Stress in turn, is shown to be equivalent to a temperature change through a standard Clausius-Clapeyron relation. The implications of those results for understanding grain boundary premelting are discussed.Comment: 24 pages, 28 figure

Simulated acoustic emissions from coupled strings

We consider traveling transverse waves on two identical uniform taut strings that are elastically coupled through springs that gradually decrease their stiffness over a region of finite length. The wave system can be decomposed into two modes: an in-phase mode ( + ) that is transparent to the coupling springs, and an out-of-phase mode ( − ) that engages the coupling springs and can resonate at a particular location depending on the excitation frequency. The system exhibits linear mode conversion whereby an incoming ( + ) wave is reflected back from the resonance location both as a propagating ( + ) wave and an evanescent ( − ) wave, while both types emerge as propagating forward through the resonance location. We match a local transition layer expansion to the WKB expansion to obtain estimates of the reflection and transmission coefficients. The reflected waves may be an analog for stimulated emissions from the ear

Dynamic evolution of interface roughness during friction and wear processes

Dynamic evolution of surface roughness and influence of initial roughness (Sa=0.282 to 6.73 µm) during friction and wear processes has been analyzed experimentally. The mirror polished and rough surfaces (28 samples in total) have been prepared by surface polishing on Ti-6Al-4V and AISI 1045 samples. Friction and wear have been tested in classical sphere/plane configuration using linear reciprocating tribometer with very small displacement from 130 to 200 microns. After an initial period of rapid degradation, dynamic evolution of surface roughness converges to certain level specific to a given tribosystem. However, roughness at such dynamic interface is still increasing and analysis of initial roughness influence revealed that to certain extent, a rheology effect of interface can be observed and dynamic evolution of roughness will depend on initial condition and history of interface roughness evolution. Multiscale analysis shows that morphology created in wear process is composed from nano, micro and macro scale roughness. Therefore, mechanical parts working under very severe contact conditions, like rotor/blade contact, screws, clutch etc. with poor initial surface finishing are susceptible to have much shorter lifetime than a quality finished parts

Break up of returning plasma after the 7 June 2011 filament eruption by Rayleigh-Taylor instabilities

A prominence eruption on 7 June 2011 produced spectacular curtains of plasma falling through the lower corona. At the solar surface they created an incredible display of extreme ultraviolet brightenings. The aim is to identify and analyze some of the local instabilities which produce structure in the falling plasma. The structures were investigated using SDO/AIA 171A and 193A images in which the falling plasma appeared dark against the bright coronal emission. Several instances of the Rayleigh-Taylor instability were investigated. In two cases the Alfven velocity associated with the dense plasma could be estimated from the separation of the Rayleigh-Taylor fingers. A second type of feature, which has the appearance of self-similar branching horns, is also discussed.Comment: 6 pages, 7 figures, submitted A&A. Movies are at http://www.mps.mpg.de/data/outgoing/innes/arcs_movie.avi and http://www.mps.mpg.de/data/outgoing/innes/horns_movie.av