1,944 research outputs found
Cold r-Process in Neutrino-Driven Winds
The r-process in a low temperature environment is explored, in which the
neutron emission by photodisintegration does not play a role (cold r-process).
A semi-analytic neutrino-driven wind model is utilized for this purpose. The
temperature in a supersonically expanding outflow can quickly drop to a few
10^8 K, where the (n, gamma)-(gamma, n) equilibrium is never achieved during
the heavy r-nuclei synthesis. In addition, the neutron capture competes with
the beta-decay owing to the low matter density. Despite such non-standard
physical conditions for the cold r-process, a solar-like r-process abundance
curve can be reproduced. The cold r-process predicts, however, the low lead
production compared to that expected in the traditional r-process conditions,
which can be a possible explanation for the low lead abundances found in a
couple of r-process-rich Galactic halo stars.Comment: 5 pages, 3 figures, accepted for publication in ApJ
Neutron-capture elements in the very metal-poor star HD88609: another st ar with excesses of light neutron-capture elements
We obtained a high resolution, high signal-to-noise UV-blue spectrum of the
extremely metal-poor red giant HD88609 to determine the abundances of heavy
elements. Nineteen neutron-capture elements are detected in the spectrum. Our
analysis revealed that this object has large excesses of light neutron-capture
elements while heavy neutron-capture elements are deficient. The abundance
pattern shows a continuously decreasing trend, as a function of atomic number,
from Sr to Yb, which is quite different from those in stars with excesses of
r-process elements. Such an abundance pattern is very similar to that of
HD122563 that was studied by our previous work. The results indicate that the
abundance pattern found in the two stars could represent the pattern produced
by the nucleosynthesis process that provided light neutron-capture elements in
the very early Galaxy.Comment: 18 pages, 6 figures, accepted for publication in Ap
The r-Process in the Proto-Neutron-Star Winds with Anisotropic Neutrino Emission
The astrophysical origin of the r-process nuclei is still unknown. Even the
most promising scenario, the neutrino-driven winds from a nascent neutron star,
encounters severe difficulties in obtaining requisite entropy and short dynamic
timescale for the r-process. In this study, the effect of anisotropy in
neutrino emission from a proto-neutron star surface is examined with
semi-analytic neutrino-driven wind models. The increase of neutrino number
density in the wind owing to the anisotropy is modeled schematically by
enhancing the effective neutrino luminosity. It is shown that the neutrino
heating rate from neutrino-antineutrino pair annihilation into
electron-positron pairs can significantly increase owing to the anisotropy and
play a dominant role for the heating of wind material. A factor of five
increase in the effective neutrino luminosity results in 50% higher entropy and
a factor of ten shorter dynamic timescale owing to this enhanced neutrino
heating. The nucleosynthesis calculations show that this change is enough for
the robust r-process, producing the third abundance peak A = 195 and beyond.
Future multi-dimensional studies with accurate neutrino transport will be
needed if such anisotropy relevant for the current scenario (more than a factor
of a few) is realized during the wind phase (~1-10 s).Comment: 8 pages, 3 figures, accepted for publication in ApJ Letter
The r-Process in Neutrino-Driven Winds from Nascent, "Compact" Neutron Stars of Core-Collapse Supernovae
We present calculations of r-process nucleosynthesis in neutrino-driven winds
from the nascent neutron stars of core-collapse supernovae. A full dynamical
reaction network for both the alpha-rich freezeout and the subsequent r-process
is employed. The physical properties of the neutrino-heated ejecta are deduced
from a general relativistic model in which spherical symmetry and steady flow
are assumed. Our results suggest that proto-neutron stars with a large
compaction ratio provide the most robust physical conditions for the r-process.
The third peak of the r-process is well reproduced in the winds from these
``compact'' proto-neutron stars even for a moderate entropy, \sim 100-200 N_A
k, and a neutrino luminosity as high as \sim 10^{52} ergs s^{-1}. This is due
to the short dynamical timescale of material in the wind. As a result, the
overproduction of nuclei with A \lesssim 120 is diminished (although some
overproduction of nuclei with A \approx 90 is still evident). The abundances of
the r-process elements per event is significantly higher than in previous
studies. The total-integrated nucleosynthesis yields are in good agreement with
the solar r-process abundance pattern. Our results have confirmed that the
neutrino-driven wind scenario is still a promising site in which to form the
solar r-process abundances. However, our best results seem to imply both a
rather soft neutron-star equation of state and a massive proto-neutron star
which is difficult to achieve with standard core-collapse models. We propose
that the most favorable conditions perhaps require that a massive supernova
progenitor forms a massive proto-neutron star by accretion after a failed
initial neutrino burst.Comment: 12 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Holographic Techni-dilaton
Techni-dilaton, a pseudo-Nambu-Goldstone boson of scale symmetry, was
predicted long ago in the Scale-invariant/Walking/Conformal Technicolor
(SWC-TC) as a remnant of the (approximate) scale symmetry associated with the
conformal fixed point, based on the conformal gauge dynamics of ladder
Schwinger-Dyson (SD) equation with non-running coupling. We study the
techni-dilaton as a flavor-singlet bound state of techni-fermions by including
the techni-gluon condensate (tGC) effect into the previous (bottom-up)
holographic approach to the SWC-TC, a deformation of the holographic QCD with
by large anomalous dimension . With
including a bulk scalar field corresponding to the gluon condensate, we first
improve the Operator Product Expansion of the current correlators so as to
reproduce gluonic term both in QCD and SWC-TC. We find in QCD about
(negative) contribution of gluon condensate to the meson mass. We
also calculate the oblique electroweak -parameter in the presence of the
effect of the tGC and find that for the fixed value of the tGC effects
dramatically reduce the flavor-singlet scalar (techni-dilaton) mass (in the unit of ), while the vector and axial-vector masses
and are rather insensitive to the tGC, where is the
decay constant of the techni-pion. If we use the range of values of tGC implied
by the ladder SD analysis of the non-perturbative scale anomaly in the large
QCD near the conformal window, the phenomenological constraint predicts the techni-dilaton mass GeV which is within
reach of LHC discovery.Comment: 28 pages, 11 eps files, typos corrected, references added, Fig.1
corrected, some discussions added, to be published in PR
Development of 3D CAD/FEM Analysis System for Natural Teeth and Jaw Bone Constructed from X-Ray CT Images
A three-dimensional finite element model of the lower first premolar, with the three layers of enamel, dentin, and pulp, and the mandible, with the two layers of cortical and cancellous bones, was directly constructed from noninvasively acquired CT images. This model was used to develop a system to analyze the stresses on the teeth and supporting bone structure during occlusion based on the finite element method and to examine the possibility of mechanical simulation
Three-dimensional finite element analysis of anterior two-unit cantilever resin-bonded fixed dental prostheses
The aim of this study was to evaluate the influence of different framework materials on biomechanical behaviour of anterior two-unit cantilever resin-bonded fixed dental prostheses (RBFDPs). A three-dimensional finite element model of a two-unit cantilever RBFDP replacing amaxillary lateral incisorwas created. Five frameworkmaterialswere evaluated: direct fibre-reinforced composite (FRC-Z250), indirect fibre-reinforced composite (FRC-ES), gold alloy (M), glass ceramic (GC), and zirconia (ZI). Finite element analysis was performed and stress distribution was evaluated. A similar stress pattern, with stress concentrations in the connector area, was observed in RBFDPs for all materials.Maximal principal stress showed a decreasing order: ZI >M>GC> FRC-ES > FRCZ250. The maximum displacement of RBFDPs was higher for FRC-Z250 and FRC-ES than for M, GC, and ZI. FE analysis depicted differences in location of the maximum stress at the luting cement interface between materials. For FRC-Z250 and FRC-ES, the maximum stress was located in the upper part of the proximal area of the retainer, whereas, for M, GC, and ZI, the maximum stress was located at the cervical outline of the retainer. The present study revealed differences in biomechanical behaviour between all RBFDPs.The general observation was that a RBFDP made of FRC provided a more favourable stress distribution
Enrichment of the r-process Element Europium in the Galactic Halo
We investigate the enrichment of europium, as a representative of r-process
elements, in the Galactic halo. In present chemical evolution models, stars are
assumed to be formed through shock processes by supernovae (SNe). The
enrichment of the interstellar medium is calculated by a one-zone approach. The
observed large dispersions in [Eu/Fe] for halo stars, converging with
increasing metallicity, can be explained with our models. In addition, the mass
range of SNe for the {\it r}-process site is constrained to be either stars of
or .Comment: 5 pages (including 4 figures), LaTeX, uses aas2pp4.sty, accepted to
ApJ
First-principles study on scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces
Scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces
are studied using first-principles calculations. Our results show that the
calculated filled-state images and local density of states are consistent with
recent experimental results, and the empty-state images appear significantly
different from the filled-state ones. To elucidate the origin of this
difference, we examined in detail the local density of states, which affects
the images, and found that the bonding and antibonding states of surface
silicon atoms largely affect the difference between the filled- and empty-state
images.Comment: 4 pages, and 4 figure
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