2,465 research outputs found
Stellar neutrino energy loss rates due to Mg suitable for O+Ne+Mg core simulations
Neutrino losses from proto-neutron stars play a pivotal role to decide if
these stars would be crushed into black holes or explode as supernovae. Recent
observations of subluminous Type II-P supernovae (e.g., 2005cs, 2003gd, 1999br,
1997D) were able to rejuvenate the interest in 8-10 M stars which
develop O+Ne+Mg cores. Simulation results of O+Ne+Mg cores show varying results
in converting the collapse into an explosion. The neutrino energy loss rates
are important input parameters in core collapse simulations. Proton-neutron
quasi-particle random phase approximation (pn-QRPA) theory has been used for
calculation of neutrino energy loss rates due to Mg in stellar matter.
The rates are presented on a detailed density-temperature grid suitable for
simulation purposes. The calculated neutrino energy loss rates are enhanced up
to more than one order of magnitude compared to the shell model calculations
and favor a lower entropy for the core of these massive stars.Comment: 20 pages, 4 figures, 2 table
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
Rapid neutron capture in supernova explosions
Rapid neutron capture in supernova explosion
Recommended from our members
E-Process - Its Components And Their Neutron Excesses
NSF GP-18335, GP-32051NASA NGR-44-006-159Astronom
E-Process - Its Components And Their Neutron Excesses
NSF GP-18335, GP-32051NASA NGR-44-006-159Astronom
Chemical equilibrium and stable stratification of a multi-component fluid: thermodynamics and application to neutron stars
A general thermodynamic argument shows that multi-component matter in full
chemical equilibrium, with uniform entropy per baryon, is generally stably
stratified. This is particularly relevant for neutron stars, in which the
effects of entropy are negligible compared to those of the equilibrium
composition gradient established by weak interactions. It can therefore be
asserted that, regardless of the uncertainties in the equation of state of
dense matter, neutron stars are stably stratified. This has important,
previously discussed consequences for their oscillation modes, magnetic field
evolution, and internal angular momentum transport.Comment: AASTeX, 8 pages, including 1 PS figure. Accepted for publication in
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
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
Spectral and Polarization Sensitivity of the Dipteran Visual System
Spectral and polarization sensitivity measurements were made at several levels (retina, first and third optic ganglion, cervical connective, behavior) of the dipteran visual nervous system. At all levels, it was possible to reveal contributions from the retinular cell subsystem cells 1 to 6 or the retinular cell subsystem cells 7 and 8 or both. Only retinular cells 1 to 6 were directly studied, and all possessed the same spectral sensitivity characterized by two approximately equal sensitivity peaks at 350 and 480 nm. All units of both the sustaining and on-off variety in the first optic ganglion exhibited the same spectral sensitivity as that of retinular cells 1 to 6. It was possible to demonstrate for motion detection and optomotor responses two different spectral sensitivities depending upon the spatial wavelength of the stimulus. For long spatial wavelengths, the spectral sensitivity agreed with retinular cells 1 to 6; however, the spectral sensitivity at short spatial wavelengths was characterized by a single peak at 465 nm reflecting contributions from the (7, 8) subsystem. Although the two subsystems exhibited different spectral sensitivities, the difference was small and no indication of color discrimination mechanisms was observed. Although all retinular cells 1 to 6 exhibited a preferred polarization plane, sustaining and on-off units did not. Likewise, motion detection and optomotor responses were insensitive to the polarization plane for long spatial wavelength stimuli; however, sensitivity to select polarization planes was observed for short spatial wavelengths
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