2,740 research outputs found
Rapid neutron capture in supernova explosions
Rapid neutron capture in supernova explosion
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
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
Oscillations of the Eddington Capture Sphere
We present a toy model of mildly super-Eddington, optically thin accretion
onto a compact star in the Schwarzschild metric, which predicts periodic
variations of luminosity when matter is supplied to the system at a constant
accretion rate. These are related to the periodic appearance and disappearance
of the Eddington Capture Sphere. In the model the frequency is found to vary
inversely with the luminosity. If the input accretion rate varies (strictly)
periodically, the luminosity variation is quasi-periodic, and the quality
factor is inversely proportional to the relative amplitude of mass accretion
fluctuations, with its largest value approximately Q= 1/(10 |delta Mdot/Mdot|)
attained in oscillations at about 1 to 2 kHz frequencies for a 2 solar mass
star
Modeling core collapse supernovae in 2 and 3 dimensions with spectral neutrino transport
The overwhelming evidence that the core collapse supernova mechanism is
inherently multidimensional, the complexity of the physical processes involved,
and the increasing evidence from simulations that the explosion is marginal
presents great computational challenges for the realistic modeling of this
event, particularly in 3 spatial dimensions. We have developed a code which is
scalable to computations in 3 dimensions which couples PPM Lagrangian with
remap hydrodynamics [1], multigroup, flux-limited diffusion neutrino transport
[2], with many improvements), and a nuclear network [3]. The neutrino transport
is performed in a ray-by-ray plus approximation wherein all the lateral effects
of neutrinos are included (e.g., pressure, velocity corrections, advection)
except the transport. A moving radial grid option permits the evolution to be
carried out from initial core collapse with only modest demands on the number
of radial zones. The inner part of the core is evolved after collapse along
with the rest of the core and mantle by subcycling the lateral evolution near
the center as demanded by the small Courant times. We present results of 2-D
simulations of a symmetric and an asymmetric collapse of both a 15 and an 11 M
progenitor. In each of these simulations we have discovered that once the
oxygen rich material reaches the shock there is a synergistic interplay between
the reduced ram pressure, the energy released by the burning of the shock
heated oxygen rich material, and the neutrino energy deposition which leads to
a revival of the shock and an explosion.Comment: 10 pages, 3 figure
The natural resources of Bolinas Lagoon: their status and future
This publication is an integral part of the Department's high-priority inventory and assessment of coastal marshland and tideflat resources. It is intended as a guide for citizens, planners, administrators, and all others interested in the use and development of coastal lands and waters.
Although the resources and problems of Bolinas Lagoon have probably been the subject of more biological and physical investigations than any small estuarine area of the California coast, many of the pertinent reports and information are not readily available to the public.
Consequently, it is one purpose of this report to summarize the lagoon's history, ecological attractions, educational values and the problems facing its continued existence. At the same time, it should provide concerned citizens with a knowledge of the sources of additional and more specific information.
Publication of this report is consistent with the obligation of the Department of Fish and Game to do everything in its power to protect and maintain the State's fish and wildlife resources. Therefore, its purpose transcends local issues on pollution and development, and the Department is, in fact, submitting a report to the people on the status and future of part of its inheritance and the dowry of coming generations.
The report is the third of a scheduled series. It follows similar releases on Upper Newport Bay (Orange County) and Goleta Slough (Santa Barbara county) in March and June of 1970. Documentation of the resources of other critical areas is in progress. There will be future reports of this nature on Elkhorn Slough, Morro Bay, Tomales Bay, Humboldt Bay, and highly threatened marshlands in southern California. (137 pp.
Massive Stars in the Range : Evolution and Nucleosynthesis. II. the Solar Metallicity Models
We present the evolutionary properties of a set of massive stellar models
(namely 13, 15, 20 and 25 ) from the main sequence phase up to the
onset of the iron core collapse. All these models have initial solar chemical
composition, i.e. Y=0.285 and Z=0.02. A 179 isotope network, extending from
neutron up to and fully coupled to the evolutionary code has been
adopted from the Carbon burning onward. Our results are compared, whenever
possible, to similar computations available in literature.Comment: 42 pages, 18 figures, 26 tables, accepted for publicatin in ApJ
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
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