3,587 research outputs found
The Three Dimensional Evolution to Core Collapse of a Massive Star
We present the first three dimensional (3D) simulation of the final minutes
of iron core growth in a massive star, up to and including the point of core
gravitational instability and collapse. We self-consistently capture the
development of strong convection driven by violent Si burning in the shell
surrounding the iron core. This convective burning builds the iron core to its
critical (Chandrasekhar) mass and collapse ensues, driven by electron capture
and photodisintegration. The non-spherical structure and motion (turbulent
fluctuations) generated by 3D convection is substantial at the point of
collapse. We examine the impact of such physically-realistic 3D initial
conditions on the core-collapse supernova mechanism using 3D simulations
including multispecies neutrino leakage. We conclude that non-spherical
progenitor structure should not be ignored, and has a significant and favorable
impact on the likelihood for neutrino-driven explosions.Comment: 7 pages, 5 figures, accepted for publication in ApJ Letters. Movies
may be viewed at http://flash.uchicago.edu/~smc/progen3
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
Ultrasonic Microdissection of Rat Cerebellum for Scanning Electron Microscopy
The cerebelli of rats were initially fixed with aldehydes (modified Karnovsky\u27s fixative; 503 mOsM/L) by cardiac perfusion. Blocks of tissue were razor-cut, usually longitudinal to folia, and immersed in the same fluid for 2-4 hours. Three separate methods of treatment followed: (1) immersion in 1% aqueous boric acid, or (2) in 2% phosphate buffered OsO4 followed by boric acid or (3) in an 8/2 mixture of boric acid and OsO4. After 18-48 hours immersion the blocks were dehydrated in ascending grades of acetone. They were then exposed to ultrasound in 100% acetone at frequencies of 80 kHz or 40 kHz for 10 to 20 minutes.
Microdissection of cut surfaces (erosion) occurs after all three treatments. It is least extensive after boric acid, moderate after OsO4 and greatest after the combined mixture. All cerebellar cell types are recognizable as are numerous fibers according to morphology and position. Variable erosion accommodates analysis of different levels of neural organization. In general, structural situations not involving great depth of field are best revealed by H3BO3 or OsO4. Blood vascular relationships to other structures are best demonstrated in deeply eroded specimens
Theoretical Studies of Accretion of Matter onto White Dwarfs and the Single Degenerate Scenario for Supernovae of Type Ia
We present a brief summary of the Single Degenerate Scenario for the
progenitors of Type Ia Supernovae in which it is assumed that a low mass
carbon-oxygen white dwarf is growing in mass as a result of accretion from a
secondary star in a close binary system. Recent hydrodynamic simulations of
accretion of solar material onto white dwarfs without mixing always produce a
thermonuclear runaway and steady burning does not occur. For a broad range in
WD mass (0.4 Solar masses to 1.35 Solar Masses), the maximum ejected material
occurs for the 1.25 Solar Mass sequences and then decreases as the white dwarf
mass decreases. Therefore, the white dwarfs are growing in mass as a
consequence of the accretion of solar material and as long as there is no
mixing of accreted material with core material. In contrast, a thermonuclear
runaway in the accreted hydrogen-rich layers on the low luminosity WDs in close
binary systems where mixing of core matter with accreted material has occurred
is the outburst mechanism for Classical, Recurrent, and Symbiotic novae. The
differences in characteristics of these systems is likely the WD mass and mass
accretion rate. The high levels of enrichment of CN ejecta in elements ranging
from carbon to sulfur confirm that there is dredge-up of matter from the core
of the WD and enable them to contribute to the chemical enrichment of the
interstellar medium. Therefore, studies of CNe can lead to an improved
understanding of Galactic nucleosynthesis, some sources of pre-solar grains,
and the Extragalactic distance scale. The characteristics of the outburst
depend on the white dwarf mass, luminosity, mass accretion rate, and the
chemical composition of both the accreting material and WD material. The
properties of the outburst also depends on when, how, and if the accreted
layers are mixed with the WD core and the mixing mechanism is still unknown.Comment: 25 Pages, Bulletin of the Astronomical Society of India (BASI) in
pres
Securing Personal Information Assets: Testing Antecedents of Behavioral Intentions
Due to the increased global reliance on information technology, and the prominence of information resources value, identity theft is a problem domain effecting millions of computer users annually. The realities of identity theft are highly visible in the global media, although empirical investigations on the topic are limited. The purpose of this study is to identify and analyze perceptions of personal information (e.g., identity) as it relates to perceived threats, mitigation, perceived risks, and intended safe information practice intentions. We propose a risk analysis model based on theoretical variables that have been researched and extensively used in both government and private sector organizations. The model is empirically tested using LISREL to perform structural equation modeling. Findings indicate support for a relationship between risk and both 1) behavioral intentions to perform safe information practices and 2) personal information asset value
A Finite Difference Representation of Neutrino Radiation Hydrodynamics in Spherically Symmetric General Relativistic Space-Time
We present an implicit finite difference representation for general
relativistic radiation hydrodynamics in spherical symmetry. Our code,
Agile-Boltztran, solves the Boltzmann transport equation for the angular and
spectral neutrino distribution functions in self-consistent simulations of
stellar core collapse and postbounce evolution. It implements a dynamically
adaptive grid in comoving coordinates. Most macroscopically interesting
physical quantities are defined by expectation values of the distribution
function. We optimize the finite differencing of the microscopic transport
equation for a consistent evolution of important expectation values. We test
our code in simulations launched from progenitor stars with 13 solar masses and
40 solar masses. ~0.5 s after core collapse and bounce, the protoneutron star
in the latter case reaches its maximum mass and collapses further to form a
black hole. When the hydrostatic gravitational contraction sets in, we find a
transient increase in electron flavor neutrino luminosities due to a change in
the accretion rate. The muon- and tauon-neutrino luminosities and rms energies,
however, continue to rise because previously shock-heated material with a
non-degenerate electron gas starts to replace the cool degenerate material at
their production site. We demonstrate this by supplementing the concept of
neutrinospheres with a more detailed statistical description of the origin of
escaping neutrinos. We compare the evolution of the 13 solar mass progenitor
star to simulations with the MGFLD approximation, based on a recently developed
flux limiter. We find similar results in the postbounce phase and validate this
MGFLD approach for the spherically symmetric case with standard input physics.Comment: reformatted to 63 pages, 24 figures, to be published in ApJ
Collapsars - Gamma-Ray Bursts and Explosions in "Failed Supernovae"
Using a two-dimensional hydrodynamics code (PROMETHEUS), we study the
continued evolution of rotating massive helium stars whose iron core collapse
does not produce a successful outgoing shock, but instead forms a black hole.
We study the formation of a disk, the associated flow patterns, and the
accretion rate for disk viscosity parameter, alpha ~ 0.001 and 0.1. For the
standard 14 solar mass model the average accretion rate for 15 s is 0.07 solar
masses per second and the total energy deposited along the rotational axes by
neutrino annihilation is (1 - 14) x 10**51 erg, depending upon the evolution of
the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of
this energy in the polar regions results in strong relativistic outflow - jets
beamed to about 1.5% of the sky. The jets remain highly focused, and are
capable of penetrating the star in 5 - 10 s. After the jet breaks through the
surface of the star, highly relativistic flow can commence. Because of the
sensitivity of the mass ejection and jets to accretion rate, angular momentum,
and disk viscosity, and the variation of observational consequences with
viewing angle, a large range of outcomes is possible ranging from bright GRBs
like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are
also possible as the jet first breaks out of the star. While only a small
fraction of supernovae make GRBs, we predict that all GRBs longer than a few
seconds will make supernovae similar to SN 1998bw. However, hard, energetic
GRBs shorter than a few seconds will be difficult to make in this model.Comment: Latex, 66 pages including 27 figures (9 color), Submitted to The
Astrophysical Journal, latex uses aaspp4.sty. Figures also available at
http://www.ucolick.org/~andre
The case against the progenitor's carbon-to-oxygen ratio as a source of peak luminosity variations in Type Ia supernovae
One of the major challenges for theoretical modeling of Type Ia supernova
explosions is to explain the diversity of these events and the empirically
established correlation between their peak luminosity and light curve shape. In
the framework of the so-called Chandrasekhar mass models, the progenitor's
carbon-to-oxygen ratio has been suggested to be a principal source of peak
luminosity variations due to a variation in the production of radioactive
Ni during the explosion. The underlying idea is that an enhanced carbon
mass fraction should result in a more vigorous explosion since here the energy
release from nuclear reactions is increased. It was suspected that this would
produce a higher amount of Ni in the ejecta. In this letter we describe
a mechanism resulting from an interplay between nucleosynthesis and turbulent
flame evolution which counteracts such an effect. Based on three-dimensional
simulations we argue that it is nearly balanced and only minor differences in
the amount of synthesized Ni with varying carbon mass fraction in the
progenitor can be expected. Therefore this progenitor parameter is unlikely to
account for the observed variations in Type Ia supernova luminosity. We discuss
possible effects on the calibration of cosmological measurements.Comment: 5 pages, 4 figures, resolution of Figs. 1 and 2 is reduced, submitted
to A&A Letter
Differential Sensitivity Between a Virtual Reality Balance Module and Clinically Used Concussion Balance Modalities
Balance assessments are part of the recommended clinical concussion evaluation, along with computerized neuropsychological testing and self-reported symptoms checklists. New technology has allowed for the creation of virtual reality (VR) balance assessments to be used in concussion care, but there is little information on the sensitivity and specificity of these evaluations. The purpose of this study is to establish the sensitivity and specificity of a VR balance module for detecting lingering balance deficits clinical concussion care
The effect of 12C + 12C rate uncertainties on the evolution and nucleosynthesis of massive stars
[Shortened] The 12C + 12C fusion reaction has been the subject of
considerable experimental efforts to constrain uncertainties at temperatures
relevant for stellar nucleosynthesis. In order to investigate the effect of an
enhanced carbon burning rate on massive star structure and nucleosynthesis, new
stellar evolution models and their yields are presented exploring the impact of
three different 12C + 12C reaction rates. Non-rotating stellar models were
generated using the Geneva Stellar Evolution Code and were later post-processed
with the NuGrid Multi-zone Post-Processing Network tool. The enhanced rate
causes core carbon burning to be ignited more promptly and at lower
temperature. This reduces the neutrino losses, which increases the core carbon
burning lifetime. An increased carbon burning rate also increases the upper
initial mass limit for which a star exhibits a convective carbon core. Carbon
shell burning is also affected, with fewer convective-shell episodes and
convection zones that tend to be larger in mass. Consequently, the chance of an
overlap between the ashes of carbon core burning and the following carbon shell
convection zones is increased, which can cause a portion of the ashes of carbon
core burning to be included in the carbon shell. Therefore, during the
supernova explosion, the ejecta will be enriched by s-process nuclides
synthesized from the carbon core s process. The yields were used to estimate
the weak s-process component in order to compare with the solar system
abundance distribution. The enhanced rate models were found to produce a
significant proportion of Kr, Sr, Y, Zr, Mo, Ru, Pd and Cd in the weak
component, which is primarily the signature of the carbon-core s process.
Consequently, it is shown that the production of isotopes in the Kr-Sr region
can be used to constrain the 12C + 12C rate using the current branching ratio
for a- and p-exit channels.Comment: The paper contains 17 figures and 7 tables. Table 7 will be published
in full online onl
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