Non-Radial Instabilities and Progenitor Asphericities in Core-Collapse Supernovae

Since core-collapse supernova simulations still struggle to produce robust neutrino-driven explosions in 3D, it has been proposed that asphericities caused by convection in the progenitor might facilitate shock revival by boosting the activity of non-radial hydrodynamic instabilities in the post-shock region. We investigate this scenario in depth using 42 relativistic 2D simulations with multi-group neutrino transport to examine the effects of velocity and density perturbations in the progenitor for different perturbation geometries that obey fundamental physical constraints (like the anelastic condition). As a framework for analysing our results, we introduce semi-empirical scaling laws relating neutrino heating, average turbulent velocities in the gain region, and the shock deformation in the saturation limit of non-radial instabilities. The squared turbulent Mach number, , reflects the violence of aspherical motions in the gain layer, and explosive runaway occurs for ~0.3, corresponding to a reduction of the critical neutrino luminosity by ~25% compared to 1D. In the light of this theory, progenitor asphericities aid shock revival mainly by creating anisotropic mass flux onto the shock: Differential infall efficiently converts velocity perturbations in the progenitor into density perturbations (Delta rho/rho) at the shock of the order of the initial convective Mach number Ma. The anisotropic mass flux and ram pressure deform the shock and thereby amplify post-shock turbulence. Large-scale (l=2,l=1) modes prove most conducive to shock revival, whereas small-scale perturbations require unrealistically high convective Mach numbers. Initial density perturbations in the progenitor are only of order Ma^2 and therefore play a subdominant role.Comment: revised version, 34 pages, 24 figure

Variational approach to interfaces in random media: negative variances and replica symmetry breaking

A Gaussian variational approximation is often used to study interfaces in random media. By considering the 1+1 dimensional directed polymer in a random medium, it is shown here that the variational Ansatz typically leads to a negative variance of the free energy. The situation improves by taking into account more and more steps of replica symmetry breaking. For infinite order breaking the variance is zero (i.e. subextensive). This situation is reminiscent of the negative entropies in mean field spin glass models, which were also eliminated by considering infinite order replica symmetry breaking.Comment: 8 pages revtex. Submitted to Journal de Physique (France

Finite-Size Effects in Lattice QCD with Dynamical Wilson Fermions

As computing resources are limited, choosing the parameters for a full Lattice QCD simulation always amounts to a compromise between the competing objectives of a lattice spacing as small, quarks as light, and a volume as large as possible. Aiming to push unquenched simulations with the Wilson action towards the computationally expensive regime of small quark masses we address the question whether one can possibly save computing time by extrapolating results from small lattices to the infinite volume, prior to the usual chiral and continuum extrapolations. In the present work the systematic volume dependence of simulated pion and nucleon masses is investigated and compared with a long-standing analytic formula by Luescher and with results from Chiral Perturbation Theory. We analyze data from Hybrid Monte Carlo simulations with the standard (unimproved) two-flavor Wilson action at two different lattice spacings of a=0.08fm and 0.13fm. The quark masses considered correspond to approximately 85 and 50% (at the smaller a) and 36% (at the larger a) of the strange quark mass. At each quark mass we study at least three different lattices with L/a=10 to 24 sites in the spatial directions (L=0.85-2.08fm).Comment: 21 pages, 20 figures, REVTeX 4; v2: caption of Fig.7 corrected, one reference adde

New Two-Dimensional Models of Supernova Explosions by the Neutrino-Heating Mechanism: Evidence for Different Instability Regimes in Collapsing Stellar Cores

The neutrino-driven explosion mechanism for core-collapse supernovae in its modern flavor relies on the additional support of hydrodynamical instabilities in achieving shock revival. Two possible candidates, convection and the so-called standing accretion shock instability (SASI), have been proposed for this role. In this paper, we discuss new successful simulations of supernova explosions that shed light on the relative importance of these two instabilities. While convection has so far been observed to grow first in self-consistent hydrodynamical models with multi-group neutrino transport, we here present the first such simulation in which the SASI grows faster while the development of convection is initially inhibited. We illustrate the features of this SASI-dominated regime using an explosion model of a 27 solar mass progenitor, which is contrasted with a convectively-dominated model of an 8.1 solar mass progenitor with subsolar metallicity, whose early post-bounce behavior is more in line with previous 11.2 and 15 solar mass explosion models. We analyze the conditions discriminating between the two different regimes, showing that a high mass-accretion rate and a short advection time-scale are conducive for strong SASI activity. We also briefly discuss some important factors for capturing the SASI-driven regime, such as general relativity, the progenitor structure, a nuclear equation of state leading to a compact proto-neutron star, and the neutrino treatment. Finally, we evaluate possible implications of our findings for 2D and 3D supernova simulations. Our results show that a better understanding of the SASI and convection in the non-linear regime is required.Comment: 12 pages, 13 figures; revised version accepted for publication in Ap

THE NON-ENGLISH LECTURERS’ READING COMPETENCE IN READING ENGLISH TEXT AT HIGHER EDUCATION IN CENTRAL JAVA

Abstract Nowadays, the government offers a lot of scholarships to non-English lecturers to study abroad. However, only a few that could get the scholarship due to their inability to communicate in English or to comprehend English texts. In this paper, therefore, the writers want to find out whether or not the non-English lecturers could get some benefits to comprehend the English texts after being given the manual book developed by the researchers. Besides, the researchers also want to find out the elements of the text that are easy or difficult to be understood by them. The method used in this research was experimental research with the population of non English lecturers at Kopertis Wilayah VI. One hundred (100) people from several universities in Central Java were taken as the sample. Two kinds of instrument for collecting the data, namely reading test and interview were used. Reading test material was taken from the modification of the TOEFL test with the intention to know the ability of non English lecturers in reading English texts and which elements of the text are difficult or easy to be mastered by the lecturers. The interview was used to know the lecturers’ perception on the appropriateness of the manual book given and whether they got some benefits from it

Collective Atomic Motion in an Optical Lattice formed inside a High Finesse Cavity

We report on collective non-linear dynamics in an optical lattice formed inside a high finesse ring cavity in a so far unexplored regime, where the light shift per photon times the number of trapped atoms exceeds the cavity resonance linewidth. We observe bistability and self-induced squeezing oscillations resulting from the retro-action of the atoms upon the optical potential wells. We can well understand most of our observations within a simplified model assuming adiabaticity of the atomic motion. Non-adiabatic aspects of the atomic motion are reproduced by solving the complete system of coupled non-linear equations of motion for hundred atoms.Comment: 4 pages, 5 figure