48,002 research outputs found
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
Predicting the potential geographical distribution of the harlequin ladybird, Harmonia axyridis, using the CLIMEX model - BioControl
Harmonia axyridis (Pallas, 1773) (Coleoptera: Coccinellidae) is a ladybird beetle native to temperate and subtropical parts of Asia. Since 1916 populations of this species have been introduced throughout the world, either deliberately, or by accident through international transport. Harmonia axyridis was originally released as a classical biological control agent of aphid and coccid pests in orchards and forests, but since 1994 it is also available as a commercial product for augmentative control in field and greenhouse crops. It is a very voracious and effective natural enemy of aphids, psyllids and coccids in various agricultural and horticultural habitats and forests. During the past 20 years, however, it has successfully invaded non-target habitats in North America (since 1988), Europe (1999) and South America (2001) respectively in a short period of time, attacking a wide range of non-pest species in different insect orders. Becoming part of the agricultural commercial pathway, it is prone to being introduced into large areas across the world by accident. We use the CLIMEX programme (v2) to predict the potential geographical distribution of H. axyridis by means of matching the climate of its region of origin with other regions in the world and taking in account biological characteristics of the species. Establishment and spread seem likely in many regions across the world, including those areas which H. axyridis has already invaded (temperate Europe, North America). Based on the CLIMEX prediction a large part of Mediterranean Europe, South America, Africa, Australia and New Zealand seem highly suitable for long-term survival of H. axyridis as well. In addition we evaluate CLIMEX as a strategic tool for estimating establishment potential as part of an environmental risk assessment procedure for biological control agents we discuss biological and ecological aspects necessary to fine-tune its establishment and spread in areas after it has been introduce
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