Improved table for cutting and welding

Welding table covered with parallel inverted steel angles improves metal torch cutting of various types and thicknesses

Perturbative evolution of the static configurations, quasinormal modes and quasi normal ringing in the Apostolatos - Thorne cylindrical shell model

We study the perturbative evolution of the static configurations, quasinormal modes and quasi normal ringing in the Apostolatos - Thorne cylindrical shell model. We consider first an expansion in harmonic modes and show that it provides a complete solution for the characteristic value problem for the finite perturbations of a static configuration. As a consequence of this completeness we obtain a proof of the stability of static solutions under this type of perturbations. The explicit expression for the mode expansion are then used to obtain numerical values for some of the quasi normal mode complex frequencies. Some examples involving the numerical evaluation of the integral mode expansions are described and analyzed, and the quasi normal ringing displayed by the solutions is found to be in agreement with quasi normal modes found previously. Going back to the full relativistic equations of motion we find their general linear form by expanding to first order about a static solution. We then show that the resulting set of coupled ordinary and partial differential equations for the dynamical variables of the system can be used to set an initial plus boundary values problem, and prove that there is an associated positive definite constant of the motion that puts absolute bounds on the dynamic variables of the system, establishing the stability of the motion of the shell under arbitrary, finite perturbations. We also show that the problem can be solved numerically, and provide some explicit examples that display the complete agreement between the purely numerical evolution and that obtained using the mode expansion, in particular regarding the quasi normal ringing that results in the evolution of the system. We also discuss the relation of the present work to some recent results on the same model that have appeared in the literature.Comment: 27 pages, 7 figure

The effect of the lateral interactions on the critical behavior of long straight rigid rods on two-dimensional lattices

Using Monte Carlo simulations and finite-size scaling analysis, the critical behavior of attractive rigid rods of length k (k-mers) on square lattices at intermediate density has been studied. A nematic phase, characterized by a big domain of parallel k-mers, was found. This ordered phase is separated from the isotropic state by a continuous transition occurring at a intermediate density \theta_c, which increases linearly with the magnitude of the lateral interactions.Comment: 7 pages, 6 figure

Accretion disks around black holes: dynamical evolution, meridional circulations and gamma ray bursts

We study the hydrodynamical evolution of massive accretion disks around black holes, formed when a neutron star is disrupted by a black hole in a binary system. Initial conditions are taken from 3D calculations of coalescing binaries. Assuming azimuthal symmetry, we follow the time dependence of the disk structure for 0.2 seconds. We use an ideal gas e.o.s., and assume that all the dissipated energy is radiated away. The disks evolve due to viscous stresses, modeled with an alpha law. We study the disk structure, and the strong meridional circulations that are established and persist throughout our calculations. These consist of strong outflows along the equatorial plane that reverse direction close to the surface of the disk and converge on the accretor. In the context of GRBs, we estimate the energy released from the system in neutrinos and through magnetic-dominated mechanisms, and find it can be as high as 10^52 erg and 10^51 erg respectively, during an estimated timescale of 0.1-0.2 seconds. neutrino-anti neutrino annihilation is likely to produce bursts from only an impulsive energy input (the annihilation luminosity scales as t^(-5/2)) and so would be unable to account for a large fraction of bursts with complicated light curves. However a gas mass ~0.1-0.25 Msun survives in the orbiting debris, enabling strong magnetic fields (~10^16 Gauss) to be anchored in the dense matter long enough to power short GRBs. We also investigate the continuous energy injection that arises as the black hole slowly swallows the rest of the disk and discuss its consequences on the GRB afterglow emission.Comment: Accepted for publication in ApJ, 30 pages, 7 figure

Comment on: "Effect of polydispersity on the ordering transition of adsorbed self-assembled rigid rods"

The critical behavior of self-assembled rigid rods on a square lattice was recently reinvestigated by Almarza et al. [Phys. Rev. E 82, 061117 (2010)]. Based on the Binder cumulants and the value of the critical exponent of the correlation length, the authors found that the isotropic-nematic phase transition occurring in the system is in the two-dimensional Ising universality class. This conclusion contrasts with that of a previous study [L\'opez et al., Phys. Rev. E 80, 040105 (R) (2009)] which indicates that the transition at intermediate density belongs to the q = 1 Potts universality class. Almarza et al. attributed the discrepancy to the use of the density as the control parameter by L\'opez et al. The present work shows that this suggestion is not sufficient, and that the discrepancy arises solely from the use of different statistical ensembles. Finally, the necessity of making corrections to the scaling functions in the canonical ensemble is discussed.Comment: 3 pages, 2 figures, accepted for publication in Physical Review

Search for the Higgs Boson H20H_2^0 at LHC in 3-3-1 Model

We present an analysis of production and signature of neutral Higgs boson ($H_{2}^{0}$) on the version of the 3-3-1 model containing heavy leptons at the Large Hadron Collider. We studied the possibility to identify it using the respective branching ratios. Cross section are given for the collider energy, $\sqrt{s} =$ 14 TeV. Event rates and significances are discussed for two possible values of integrated luminosity, 300 fb$^{-1}$ and 3000 fb$^{-1}$.Comment: 17 pages 7 figures. arXiv admin note: substantial text overlap with arXiv:1205.404

The properties of ten O-type stars in the low-metallicity galaxies IC 1613, WLM and NGC 3109

Massive stars likely played an important role in the reionization of the Universe, and the formation of the first black holes. Massive stars in low-metallicity environments in the local Universe are reminiscent of their high redshift counterparts. In a previous paper, we reported on indications that the stellar winds of low-metallicity O stars may be stronger than predicted, which would challenge the current paradigm of massive star evolution. In this paper, we aim to extend our initial sample of six O stars in low-metallicity environments by four. We aim to derive their stellar and wind parameters, and compare these to radiation-driven wind theory and stellar evolution models. We have obtained intermediate-resolution VLT/X-Shooter spectra of our sample of stars. We derive the stellar parameters by fitting synthetic fastwind line profiles to the VLT/X-Shooter spectra using a genetic fitting algoritm. We compare our parameters to evolutionary tracks and obtain evolutionary masses and ages. We also investigate the effective temperature versus spectral type calibration for SMC and lower metallicities. Finally, we reassess the wind momentum versus luminosity diagram. The derived parameters of our target stars indicate stellar masses that reach values of up to 50 $M_{\odot}$. The wind strengths of our stars are, on average, stronger than predicted from radiation-driven wind theory and reminiscent of stars with an LMC metallicity. We discuss indications that the iron content of the host galaxies is higher than originally thought and is instead SMC-like. We find that the discrepancy with theory is lessened, but remains significant for this higher metallicity. This may imply that our current understanding of the wind properties of massive stars, both in the local universe as well as at cosmic distances, remains incomplete.Comment: Accepted for publication in Astronomy and Astrophysics. 10 pages, 8 figure