253 research outputs found
Neutrino Emission from Supernovae
Supernovae are the most powerful cosmic sources of MeV neutrinos. These
elementary particles play a crucial role when the evolution of a massive star
is terminated by the collapse of its core to a neutron star or a black hole and
the star explodes as supernova. The release of electron neutrinos, which are
abundantly produced by electron captures, accelerates the catastrophic infall
and causes a gradual neutronization of the stellar plasma by converting protons
to neutrons as dominant constituents of neutron star matter. The emission of
neutrinos and antineutrinos of all flavors carries away the gravitational
binding energy of the compact remnant and drives its evolution from the hot
initial to the cold final state. The absorption of electron neutrinos and
antineutrinos in the surroundings of the newly formed neutron star can power
the supernova explosion and determines the conditions in the innermost
supernova ejecta, making them an interesting site for the nucleosynthesis of
iron-group elements and trans-iron nuclei. In this Chapter the basic neutrino
physics in supernova cores and nascent neutron stars will be discussed. This
includes the most relevant neutrino production, absorption, and scattering
processes, elementary aspects of neutrino transport in dense environments, the
characteristic neutrino emission phases with their typical signal features, and
the perspectives connected to a measurement of the neutrino signal from a
future galactic supernova.Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A.
Alsabti and P. Murdin, Springer. 30 pages, 9 figure
Delayed neutrino-driven supernova explosions aided by the standing accretion-shock instability
We present results of 2D hydrodynamic simulations of stellar core collapse,
which confirm that the neutrino-heating mechanism remains viable for the
explosion of a wider mass range of supernova progenitors with iron cores. We
used an energy-dependent treatment of the neutrino transport based on the
"ray-by-ray plus" approximation, in which the number, energy, and momentum
equations are closed with a variable Eddington factor obtained by iteratively
solving a model Boltzmann equation. We focus on the evolution of a 15 Msun
progenitor and show that shock revival and the explosion are initiated at about
600 ms post bounce, powered by neutrino energy deposition. Similar to previous
findings for an 11.2 Msun star, but significantly later, the onset of the
explosion is fostered by the standing accretion shock instability (SASI). This
instability exhibits highest growth rates for the dipole and quadrupole modes,
which lead to large-amplitude bipolar shock oscillations and push the shock to
larger radii, thus increasing the time accreted matter is exposed to neutrino
heating in the gain layer. Therefore also convective overturn behind the shock
is strengthened. A "soft" nuclear equation of state that causes a rapid
contraction and a smaller radius of the forming neutron star and thus a fast
release of gravitational binding energy, seems to be more favorable for an
explosion. Rotation has the opposite effect because it leads to a more extended
and cooler neutron star and thus lower neutrino luminosities and mean energies
and overall less neutrino heating. Neutron star g-mode oscillations and the
acoustic mechanism play no important role in our simulations. (abridged)Comment: 46 pages, 20 figures, 59 eps files; submitted to ApJ; significantly
extended and revised version to account for referee comments; high-resolution
images can be obtained upon reques
Non-Radial Instabilities and Progenitor Asphericities in Core-Collapse Supernovae
Since core-collapse supernova simulations still struggle to produce robust
neutrino-driven explosions in 3D, it has been proposed that asphericities
caused by convection in the progenitor might facilitate shock revival by
boosting the activity of non-radial hydrodynamic instabilities in the
post-shock region. We investigate this scenario in depth using 42 relativistic
2D simulations with multi-group neutrino transport to examine the effects of
velocity and density perturbations in the progenitor for different perturbation
geometries that obey fundamental physical constraints (like the anelastic
condition). As a framework for analysing our results, we introduce
semi-empirical scaling laws relating neutrino heating, average turbulent
velocities in the gain region, and the shock deformation in the saturation
limit of non-radial instabilities. The squared turbulent Mach number, ,
reflects the violence of aspherical motions in the gain layer, and explosive
runaway occurs for ~0.3, corresponding to a reduction of the critical
neutrino luminosity by ~25% compared to 1D. In the light of this theory,
progenitor asphericities aid shock revival mainly by creating anisotropic mass
flux onto the shock: Differential infall efficiently converts velocity
perturbations in the progenitor into density perturbations (Delta rho/rho) at
the shock of the order of the initial convective Mach number Ma. The
anisotropic mass flux and ram pressure deform the shock and thereby amplify
post-shock turbulence. Large-scale (l=2,l=1) modes prove most conducive to
shock revival, whereas small-scale perturbations require unrealistically high
convective Mach numbers. Initial density perturbations in the progenitor are
only of order Ma^2 and therefore play a subdominant role.Comment: revised version, 34 pages, 24 figure
Global Anisotropy Versus Small-Scale Fluctuations in Neutrino Flux in Core-Collapse Supernova Explosions
Effects of small-scale fluctuations in the neutrino radiation on
core-collapse supernova explosions are examined. Through a parameter study with
a fixed radiation field of neutrinos, we find substantial differences between
the results of globally anisotropic neutrino radiation and those with
fluctuations. As the number of modes of fluctuations increases, the shock
positions, entropy distributions, and explosion energies approach those of
spherical explosion. We conclude that global anisotropy of the neutrino
radiation is the most effective mechanism of increasing the explosion energy
when the total neutrino luminosity is given. This supports the previous
statement on the explosion mechanism by Shimizu and coworkers.Comment: 14 pages, including 12 figures. To be published in the Astrophysical
Journa
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