3,678 research outputs found
Nuclear liquid-gas phase transition and supernovae evolution
It is shown that the large density fluctuations appearing at the onset of the
first order nuclear liquid-gas phase transition can play an important role in
the supernovae evolution. Due to these fluctuations, the neutrino gas may be
trapped inside a thin layer of matter near the proto-neutron star surface. The
resulting increase of pressure may induce strong particle ejection a few
hundred milliseconds after the bounce of the collapse, contributing to the
revival of the shock wave. The Hartree-Fock+RPA scheme, with a finite-range
nucleon-nucleon effective interaction, is employed to estimate the effects of
the neutrino trapping due to the strong density fluctuations, and to discuss
qualitatively the consequences of the suggested new scenario.Comment: version2 - precise that nuclear liquid-gas phase transition is 1st
order and the unique instable mode is isoscala
Contestable adulthood: variability and disparity in markers for negotiating the transition to adulthood
Recent research has identified a discreet set of subjective markers that are seen as characterizing the transition to adulthood. The current study challenges this coherence by examining the disparity and variability in young peopleâs selection of such criteria. Four sentence-completion cues corresponding to four differentcontexts in which adult status might be contested were given to 156 British 16- to 17-year-olds. Their qualitative responses were analyzed to
explore patterns whilst capturing some of their richness and diversity. An astonishing amount of variability emerged, both within and between cued contexts.The implications of this variability for how the transition to adulthood is experienced are explored. The argument is made that markers of the transition to adulthood are not merely reflective of the bioâpsychoâsocial development of
young people. Rather, adulthood here is seen as an essentially contested concept,located within the discursive interactional environment in which young people participate
Expression, Purification, Crystallization and Preliminary X-Ray Analysis of \u3cem\u3ePseudomonas aeuginosa\u3c/em\u3e AlgX
AlgX is a periplasmic protein required for the production of the exopolysaccharide alginate in Pseudomonas sp. and Azotobacter vinelandii. AlgX has been overexpressed and purified and diffraction-quality crystals have been grown using iterative seeding and the hanging-drop vapor-diffusion method. The crystals grew as flat plates with unit-cell parameters a=46.4, b=120.6, c=86.9Ă
, ÎČ=95.7°. The cyrstals exhibited the symmetry of space group P21 and diffracted to a minimimum d-spacing of 2.1Ă
. On the basis of the Matthews coefficient (VM=2.25Ă
3 Da-1), two molecules were estimated to be present in the asymmetric unit
Two-Dimensional Hydrodynamics of Pre-Core Collapse: Oxygen Shell Burning
By direct hydrodynamic simulation, using the Piecewise Parabolic Method (PPM)
code PROMETHEUS, we study the properties of a convective oxygen burning shell
in a SN 1987A progenitor star prior to collapse. The convection is too
heterogeneous and dynamic to be well approximated by one-dimensional
diffusion-like algorithms which have previously been used for this epoch.
Qualitatively new phenomena are seen.
The simulations are two-dimensional, with good resolution in radius and
angle, and use a large (90-degree) slice centered at the equator. The
microphysics and the initial model were carefully treated. Many of the
qualitative features of previous multi-dimensional simulations of convection
are seen, including large kinetic and acoustic energy fluxes, which are not
accounted for by mixing length theory. Small but significant amounts of
carbon-12 are mixed non-uniformly into the oxygen burning convection zone,
resulting in hot spots of nuclear energy production which are more than an
order of magnitude more energetic than the oxygen flame itself. Density
perturbations (up to 8%) occur at the `edges' of the convective zone and are
the result of gravity waves generated by interaction of penetrating flows into
the stable region. Perturbations of temperature and electron fraction at the
base of the convective zone are of sufficient magnitude to create angular
inhomogeneities in explosive nucleosynthesis products, and need to be included
in quantitative estimates of yields. Combined with the plume-like velocity
structure arising from convection, the perturbations will contribute to the
mixing of nickel-56 throughout supernovae envelopes. Runs of different
resolution, and angular extent, were performed to test the robustness of theseComment: For mpeg movies of these simulations, see
http://www.astrophysics.arizona.edu/movies.html Submitted to the
Astrophysical Journa
Analytical representations of unified equations of state of neutron-star matter
Analytical representations are derived for two equations of state (EOSs) of
neutron-star matter: FPS and SLy. Each of these EOSs is unified, that is, it
describes the crust and the core of a neutron star using the same physical
model. Two versions of the EOS parametrization are considered. In the first
one, pressure and mass density are given as functions of the baryon density. In
the second version, pressure, mass density, and baryon density are given as
functions of the pseudo-enthalpy, which makes this representation particularly
useful for 2-D calculations of stationary rotating configurations of neutron
stars.Comment: 7 pages, 5 figures, 3 tables, accepted by A&A. In v.2, auxiliary fits
(15) and (16) are correcte
Inclusive Masculinity and Facebook Photographs Among Early Emerging Adults at a British University
Central to debates about the construction of masculinity in sociology is the influence of culture and what constitutes acceptable displays of masculinity. This article adopts a novel approach in examining this question. It adopts a summative content analysis, combined with a semiotic analysis, of 1,100 Facebook photographs, in order to explore the underlying meanings within the photos and the performances of masculinity. Facebook photographs from 44, straight, White, male, early emerging adults attending the same university are used as a representation of an individualâs ideal self. These are then analyzed in order to determine the behaviors endorsed by peer culture. It was found that the sample overwhelmingly adopted inclusive behaviors (including homosocial tactility, dancing, and kissing each other), and inclusive masculinity theory was utilized to contextualize participantsâ constructions of masculinity. Thus, this research shows that emerging adult males at this university construct their masculine identities away from previous orthodox archetypes. It is argued that the reducing importance of gendered behavior patterns may represent an adoption of what are perceived as wider cultural norms and act as a symbol of adulthood to these early emerging adults
Screened thermonuclear reactions and predictive stellar evolution of detached double-lined eclipsing binaries
The low energy fusion cross sections of charged-particle nuclear reactions
(and the respective reaction rates) in stellar plasmas are enhanced due to
plasma screening effects. We study the impact of those effects on predictive
stellar evolution simulations for detached double-lined eclipsing binaries. We
follow the evolution of binary systems (pre-main sequence or main sequence
stars) with precisely determined radii and masses from 1.1Mo to 23Mo (from
their birth until their present state). The results indicate that all the
discrepancies between the screened and unscreened models (in terms of
luminosity, stellar radius, and effective temperature) are within the
observational uncertainties. Moreover, no nucleosynthetic or compositional
variation was found due to screening corrections. Therefore all thermonuclear
screening effects on the charged-particle nuclear reactions that occur in the
binary stars considered in this work (from their birth until their present
state) can be totally disregarded. In other words, all relevant
charged-particle nuclear reactions can be safely assumed to take place in a
vacuum, thus simplifying and accelerating the simulation processes.Comment: 5 RevTex pages,no figures. Accepted for publication in Phys.Rev.
Observational Tests and Predictive Stellar Evolution II: Non-standard Models
We examine contributions of second order physical processes to results of
stellar evolution calculations amenable to direct observational testing. In the
first paper in the series (Young et al. 2001) we established baseline results
using only physics which are common to modern stellar evolution codes. In the
current paper we establish how much of the discrepancy between observations and
baseline models is due to particular elements of new physics. We then consider
the impact of the observational uncertainties on the maximum predictive
accuracy achievable by a stellar evolution code. The sun is an optimal case
because of the precise and abundant observations and the relative simplicity of
the underlying stellar physics. The Standard Model is capable of matching the
structure of the sun as determined by helioseismology and gross surface
observables to better than a percent. Given an initial mass and surface
composition within the observational errors, and no additional constraints for
which the models can be optimized, it is not possible to predict the sun's
current state to better than ~7%. Convectively induced mixing in radiative
regions, seen in multidimensional hydrodynamic simulations, dramatically
improves the predictions for radii, luminosity, and apsidal motions of
eclipsing binaries while simultaneously maintaining consistency with observed
light element depletion and turnoff ages in young clusters (Young et al. 2003).
Systematic errors in core size for models of massive binaries disappear with
more complete mixing physics, and acceptable fits are achieved for all of the
binaries without calibration of free parameters. The lack of accurate abundance
determinations for binaries is now the main obstacle to improving stellar
models using this type of test.Comment: 33 pages, 8 figures, accepted for publication in the Astrophysical
Journa
A Model For the Formation of High Density Clumps in Proto-Planetary Nebulae
The detection of NaCl at large radii in the Egg Nebula, CRL 2688, requires
densities of 10^7 - 10^8 cm^-3 in a thick shell of r ~ a few X 10^17 cm. To
explain these results, a mechanism is needed for producing high densities at a
considerable distance from the central star. In two dimensional simulations of
the interaction of the fast wind with an ambient medium, the material becomes
thermally unstable. The resulting clumps can achieve the requisite conditions
for NaCl excitation. We present 2D models with simple physics as
proof-of-principle calculations to show that the clumping behavior is robust.
Clumping is a natural outcome of cooling in the colliding wind model and
comparable to that inferred from observations.Comment: 12 pages, 3 figures. Accepted for publication in the Astrophysical
Journal Letter
Bondi Accretion and the Problem of the Missing Isolated Neutron Stars
A large number of neutron stars (NSs), ~10^9, populate the Galaxy, but only a
tiny fraction of them is observable during the short radio pulsar lifetime. The
majority of these isolated NSs, too cold to be detectable by their own thermal
emission, should be visible in X-rays as a result of accretion from the
interstellar medium. The ROSAT all sky survey has however shown that such
accreting isolated NSs are very elusive: only a few tentative candidates have
been identified, contrary to theoretical predictions that up to several
thousands should be seen. We suggest that the fundamental reason for this
discrepancy lies in the use of the standard Bondi formula to estimate the
accretion rates. We compute the expected source counts using updated estimates
of the pulsar velocity distribution, realistic hydrogen atmosphere spectra, and
a modified expression for the Bondi accretion rate as suggested by recent MHD
simulations, and supported by direct observations in the case of accretion
around supermassive black holes in nearby galaxies and in our own. We find
that, whereas the inclusion of atmospheric spectra partly compensates for the
reduction in the counts due to the higher mean velocities of the new
distribution, the modified Bondi formula dramatically suppresses the source
counts. The new predictions are consistent with a null detection at the ROSAT
sensitivity.Comment: accepted to ApJ; 19 pages, 4 figure
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