27,924 research outputs found
Gravitational Collapse and Neutrino Emission of Population III Massive Stars
Pop III stars are the first stars in the universe. They do not contain metals
and their formation and evolution may be different from that of stars of later
generations. In fact, according to the theory of star formation, Pop III stars
might have very massive components (). In this paper,
we compute the spherically symmetric gravitational collapse of these Pop III
massive stars. We solve the general relativistic hydrodynamics and neutrino
transfer equations simultaneously, treating neutrino reactions in detail.
Unlike supermassive stars (), the stars of concern in
this paper become opaque to neutrinos. The collapse is simulated until after an
apparent horizon is formed. We confirm that the neutrino transfer plays a
crucial role in the dynamics of gravitational collapse, and find also that the
-equilibration leads to a somewhat unfamiliar evolution of electron
fraction. Contrary to the naive expectation, the neutrino spectrum does not
become harder for more massive stars. This is mainly because the neutrino
cooling is more efficient and the outer core is more massive as the stellar
mass increases. Here the outer core is the outer part of the iron core falling
supersonically. We also evaluate the flux of relic neutrino from Pop III
massive stars. As expected, the detection of these neutrinos is difficult for
the currently operating detectors. However, if ever observed, the spectrum will
enable us to obtain the information on the formation history of Pop III stars.
We investigate 18 models covering the mass range of ,
making this study the most detailed numerical exploration of spherical
gravitational collapse of Pop III massive stars. This will also serve as an
important foundation for multi-dimensional investigations.Comment: 32 pages, 11 figs, submitted to Ap
Electrochemical kinetics and dimensional considerations at the nanoscale
It is shown that the consideration of the density of states variation in
nanoscale electrochemical systems yields modulations in the rate constant and
concomitant electrical currents. The proposed models extend the utility of
Marcus-Hush-Chidsey (MHC) kinetics to a larger class of materials and could be
used as a test of dimensional character. The implications of the study are of
much significance to an understanding and modulation of charge transfer
nanostructured electrodes.Comment: 15 pages, 6 figure
Primordial Black Holes from Polynomial Potentials in Single Field Inflation
Within canonical single field inflation models, we provide a method to
reverse engineer and reconstruct the inflaton potential from a given power
spectrum. This is not only a useful tool to find a potential from observational
constraints, but also gives insight into how to generate a large amplitude
spike in density perturbations, especially those that may lead to primordial
black holes (PBHs). In accord with other works, we find that the usual
slow-roll conditions need to be violated in order to generate a significant
spike in the spectrum. We find that a way to achieve a very large amplitude
spike in single field models is for the classical roll of the inflaton to
over-shoot a local minimum during inflation. We provide an example of a quintic
polynomial potential that implements this idea and leads to the observed
spectral index, observed amplitude of fluctuations on large scales, significant
PBH formation on small scales, and is compatible with other observational
constraints. We quantify how much fine-tuning is required to achieve this in a
family of random polynomial potentials, which may be useful to estimate the
probability of PBH formation in the string landscape.Comment: 13 pages in double column format, 5 figures. V2: Added references and
small clarification
Vacuum Decay in Real Time and Imaginary Time Formalisms
We analyze vacuum tunneling in quantum field theory in a general formalism by
using the Wigner representation. In the standard instanton formalism, one
usually approximates the initial false vacuum state by an eigenstate of the
field operator, imposes Dirichlet boundary conditions on the initial field
value, and evolves in imaginary time. This approach does not have an obvious
physical interpretation. However, an alternative approach does have a physical
interpretation: in quantum field theory, tunneling can happen via classical
dynamics, seeded by initial quantum fluctuations in both the field and its
momentum conjugate, which was recently implemented in Ref. [1]. We show that
the Wigner representation is a useful framework to calculate and understand the
relationship between these two approaches. We find there are two, related,
saddle point approximations for the path integral of the tunneling process: one
corresponds to the instanton solution in imaginary time and the other one
corresponds to classical dynamics from initial quantum fluctuations in real
time. The classical approximation for the dynamics of the latter process is
justified only in a system with many degrees of freedom, as can appear in field
theory due to high occupancy of nucleated bubbles, while it is not justified in
single particle quantum mechanics, as we explain. We mention possible
applications of the real time formalism, including tunneling when the instanton
vanishes, or when the imaginary time contour deformation is not possible, which
may occur in cosmological settings.Comment: 10 pages in double column format, 2 figures. V2: Further
clarifications. Updated to resemble version published in PR
Monopole Excitation to Cluster States
We discuss strength of monopole excitation of the ground state to cluster
states in light nuclei. We clarify that the monopole excitation to cluster
states is in general strong as to be comparable with the single particle
strength and shares an appreciable portion of the sum rule value in spite of
large difference of the structure between the cluster state and the
shell-model-like ground state. We argue that the essential reasons of the large
strength are twofold. One is the fact that the clustering degree of freedom is
possessed even by simple shell model wave functions. The detailed feature of
this fact is described by the so-called Bayman-Bohr theorem which tells us that
SU(3) shell model wave function is equivalent to cluster model wave function.
The other is the ground state correlation induced by the activation of the
cluster degrees of freedom described by the Bayman-Bohr theorem. We
demonstrate, by deriving analytical expressions of monopole matrix elements,
that the order of magnitude of the monopole strength is governed by the first
reason, while the second reason plays a sufficient role in reproducing the data
up to the factor of magnitude of the monopole strength. Our explanation is made
by analysing three examples which are the monopole excitations to the
and states in O and the one to the state in C.
The present results imply that the measurement of strong monopole transitions
or excitations is in general very useful for the study of cluster states.Comment: 11 pages, 1 figure: revised versio
Reflection equation for the N=3 Cremmer-Gervais R-matrix
We consider the reflection equation of the N=3 Cremmer-Gervais R-matrix. The
reflection equation is shown to be equivalent to 38 equations which do not
depend on the parameter of the R-matrix, q. Solving those 38 equations. the
solution space is found to be the union of two types of spaces, each of which
is parametrized by the algebraic variety and .Comment: 28 pages, revised versio
Reflectivity measurements in uniaxial superconductors: a methodological discussion applied to the case of La(2-x)Sr(x)CuO(4)
Most of the novel superconductors are uniaxial crystals, with metallic planes
() orthogonal to an insulating axis (). Far-infrared measurements of the
reflectivity provide valuable information on their low-energy
electrodynamics, but involve delicate experimental issues. Two of them are a
possible contamination of from the c axis and the
extrapolation of the data to =0, both above and below .
Here we discuss quantitatively these issues with particular regard to
LaSrCuO, one of the most studied high- materials.Comment: 13 pages with 3 Fig
The Grounds For Time Dependent Market Potentials From Dealers' Dynamics
We apply the potential force estimation method to artificial time series of
market price produced by a deterministic dealer model. We find that dealers'
feedback of linear prediction of market price based on the latest mean price
changes plays the central role in the market's potential force. When markets
are dominated by dealers with positive feedback the resulting potential force
is repulsive, while the effect of negative feedback enhances the attractive
potential force.Comment: 9 pages, 3 figures, proceedings of APFA
Search for effective models of stripes in the cuprates
We argue that effective 1D models of stripes in the cuprate superconductors
can be constructed by studying ground states and elementary excitations of
domain walls in 2D model antiferromagnets. This method, applied to the t-J
model with Ising anisotropy, yields two such limiting cases: an ordinary 1D
electron gas and a 1D gas of holons strongly coupled to transversal
fluctuations of the stripe.Comment: LaTeX2e, 2 pages, 2 EPS figures, a style file include
Synthetic Observations of Carbon Lines of Turbulent Flows in Diffuse Multiphase Interstellar Medium
We examine observational characteristics of multi-phase turbulent flows in
the diffuse interstellar medium (ISM) using a synthetic radiation field of
atomic and molecular lines. We consider the multi-phase ISM which is formed by
thermal instability under the irradiation of UV photons with moderate visual
extinction . Radiation field maps of C, C, and CO line
emissions were generated by calculating the non-local thermodynamic equilibrium
(nonLTE) level populations from the results of high resolution hydrodynamic
simulations of diffuse ISM models. By analyzing synthetic radiation field of
carbon lines of [\ion{C}{2}] 158 m, [\ion{C}{1}] (809 GHz),
(492 GHz), and CO rotational transitions, we found a high ratio
between the lines of high- and low-excitation energies in the diffuse
multi-phase interstellar medium. This shows that simultaneous observations of
the lines of warm- and cold-gas tracers will be useful in examining the thermal
structure, and hence the origin of diffuse interstellar clouds.Comment: 16 pages, 10 figures : accepted for publication in ApJ. PDF version
with high resolution figures is available
(http://yso.mtk.nao.ac.jp/~ymasako/paper/ms_hires.pdf
- …