656 research outputs found
Monte Carlo Study of Supernova Neutrino Spectra Formation
The neutrino flux and spectra formation in a supernova core is studied by
using a Monte Carlo code. The dominant opacity contribution for nu_mu and
nu_tau is elastic scattering on nucleons. In addition we switch on or off a
variety of processes which allow for the exchange of energy or the creation and
destruction of neutrino pairs, notably nucleon bremsstrahlung, the e^+ e^- pair
annihilation process and nu_e-bar nu_e -> nu_{mu,tau} nu_{mu,tau}-bar, recoil
and weak magnetism in elastic nucleon scattering, elastic scattering on
electrons and positrons and elastic scattering on electron neutrinos and
anti-neutrinos. The least important processes are neutrino-neutrino scattering
and e^+ e^- annihilation. The formation of the spectra and fluxes of nu_mu is
dominated by the nucleonic processes, i.e. bremsstrahlung and elastic
scattering with recoil, but also nu_e nu_e-bar annihilation and nu_mu e^\pm
scattering contribute significantly. When all processes are included, the
spectral shape of the emitted neutrino flux is always ``pinched,'' i.e. the
width of the spectrum is smaller than that of a thermal spectrum with the same
average energy. In all of our cases we find that the average nu_mu-bar energy
exceeds the average nu_e-bar energy by only a small amount, 10% being a typical
number. Weak magnetism effects cause the opacity of nu_mu to differ slightly
from that of nu_mu-bar, translating into differences of the luminosities and
average energies of a few percent. Depending on the density, temperature, and
composition profile, the flavor-dependent luminosities L_{nu_e}$, L_{nu_e-bar},
and L_{nu_mu} can mutually differ from each other by up to a factor of two in
either direction.Comment: 33 pages, 16 eps-figs, submitted to ApJ. Sections added: weak
magnetism, discussion of different analytic fits to the spectra and detailed
spectral shap
Prompt merger collapse and the maximum mass of neutron stars
We perform hydrodynamical simulations of neutron-star mergers for a large
sample of temperature-dependent, nuclear equations of state, and determine the
threshold mass above which the merger remnant promptly collapses to form a
black hole. We find that, depending on the equation of state, the threshold
mass is larger than the maximum mass of a non-rotating star in isolation by
between 30 and 70 per cent. Our simulations also show that the ratio between
the threshold mass and maximum mass is tightly correlated with the compactness
of the non-rotating maximum-mass configuration. We speculate on how this
relation can be used to derive constraints on neutron-star properties from
future observations.Comment: 6 pages, 3 figures, accepted for publication in Phys. Rev. Let
Ledoux-Convection in Protoneutron Stars --- a Clue to Supernova Nucleosynthesis?
Two-dimensional hydrodynamical simulations of the deleptonization of a newly
formed neutron star were performed. Driven by negative lepton fraction and
entropy gradients, convection starts near the neutrinosphere about 20-30 ms
after core bounce, but moves deeper into the protoneutron star, and after about
one second the whole protoneutron star is convective. The deleptonization of
the star proceeds much faster than in the corresponding spherically symmetrical
model because the lepton flux and the neutrino luminosities increase by up to a
factor of two. The convection below the neutrinosphere raises the
neutrinospheric temperatures and mean energies of the emitted neutrinos by
10-20%. This can have important implications for the supernova explosion
mechanism and changes the detectable neutrino signal from the Kelvin-Helmholtz
cooling of the protoneutron star. In particular, the enhanced electron neutrino
flux relative to the electron antineutrino flux during the early post-bounce
evolution might solve the overproduction problem of certain elements in the
neutrino-heated ejecta in models of type-II supernova explosions.Comment: 17 pages, LaTeX, 8 postscript figures, uses epsf.sty. To appear in
ApJ 473 (Letters), 1996 December 1
2D Multi-Angle, Multi-Group Neutrino Radiation-Hydrodynamic Simulations of Postbounce Supernova Cores
We perform axisymmetric (2D) multi-angle, multi-group neutrino
radiation-hydrodynamic calculations of the postbounce phase of core-collapse
supernovae using a genuinely 2D discrete-ordinate (S_n) method. We follow the
long-term postbounce evolution of the cores of one nonrotating and one
rapidly-rotating 20-solar-mass stellar model for ~400 milliseconds from 160 ms
to ~550 ms after bounce. We present a multi-D analysis of the multi-angle
neutrino radiation fields and compare in detail with counterpart simulations
carried out in the 2D multi-group flux-limited diffusion (MGFLD) approximation
to neutrino transport. We find that 2D multi-angle transport is superior in
capturing the global and local radiation-field variations associated with
rotation-induced and SASI-induced aspherical hydrodynamic configurations. In
the rotating model, multi-angle transport predicts much larger asymptotic
neutrino flux asymmetries with pole to equator ratios of up to ~2.5, while
MGFLD tends to sphericize the radiation fields already in the optically
semi-transparent postshock regions. Along the poles, the multi-angle
calculation predicts a dramatic enhancement of the neutrino heating by up to a
factor of 3, which alters the postbounce evolution and results in greater polar
shock radii and an earlier onset of the initially rotationally weakened SASI.
In the nonrotating model, differences between multi-angle and MGFLD
calculations remain small at early times when the postshock region does not
depart significantly from spherical symmetry. At later times, however, the
growing SASI leads to large-scale asymmetries and the multi-angle calculation
predicts up to 30% higher average integral neutrino energy deposition rates
than MGFLD.Comment: 20 pages, 21 figures. Minor revisions. Accepted for publication in
ApJ. A version with high-resolution figures may be obtained from
http://www.stellarcollapse.org/papers/Ott_et_al2008_multi_angle.pd
Electron Neutrino Pair Annihilation: A New Source for Muon and Tau Neutrinos in Supernovae
We show that in a supernova core the annihilation process nu_e nu_e-bar ->
nu_{mu,tau} nu_{mu,tau}-bar is always more important than the traditional
reaction e^+ e^- -> nu_{mu,tau} nu_{mu,tau}-bar as a source for muon and tau
neutrino pairs. We study the impact of the new process by means of a Monte
Carlo transport code with a static stellar background model and by means of a
self-consistent hydrodynamical simulation with Boltzmann neutrino transport.
Nucleon bremsstrahlung NN -> NN nu_{mu,tau} nu_{mu,tau}-bar is also included as
another important source term. Taking into account nu_e nu_e-bar -> nu_{mu,tau}
nu_{mu,tau}-bar increases the nu_mu and nu_tau luminosities by as much as 20%
while the spectra remain almost unaffected. In our hydrodynamical simulation
the shock was somewhat weakened. Elastic nu_{mu,tau} nu_e and nu_{mu,tau} nu_e
scattering is not negligible but less important than nu_{mu,tau} e^+ or e^-
scattering. Its influence on the nu_{mu,tau} fluxes and spectra is small after
all other processes have been included.Comment: 11 pages, 9 eps-figs, submitted to Ap
Neutrino Signal of Electron-Capture Supernovae from Core Collapse to Cooling
An 8.8 solar mass electron-capture supernova (SN) was simulated in spherical
symmetry consistently from collapse through explosion to nearly complete
deleptonization of the forming neutron star. The evolution time of about 9 s is
short because of nucleon-nucleon correlations in the neutrino opacities. After
a brief phase of accretion-enhanced luminosities (~200 ms), luminosity
equipartition among all species becomes almost perfect and the spectra of
electron antineutrinos and muon/tau antineutrinos very similar. We discuss
consequences for the neutrino-driven wind as a nucleosynthesis site and for
flavor oscillations of SN neutrinos.Comment: 4 pages, 4 eps figures; published as Physical Review Letters, vol.
104, Issue 25, id. 25110
Explosion Mechanisms of Core-Collapse Supernovae
Supernova theory, numerical and analytic, has made remarkable progress in the
past decade. This progress was made possible by more sophisticated simulation
tools, especially for neutrino transport, improved microphysics, and deeper
insights into the role of hydrodynamic instabilities. Violent, large-scale
nonradial mass motions are generic in supernova cores. The neutrino-heating
mechanism, aided by nonradial flows, drives explosions, albeit low-energy ones,
of ONeMg-core and some Fe-core progenitors. The characteristics of the neutrino
emission from new-born neutron stars were revised, new features of the
gravitational-wave signals were discovered, our notion of supernova
nucleosynthesis was shattered, and our understanding of pulsar kicks and
explosion asymmetries was significantly improved. But simulations also suggest
that neutrino-powered explosions might not explain the most energetic
supernovae and hypernovae, which seem to demand magnetorotational driving. Now
that modeling is being advanced from two to three dimensions, more realism, new
perspectives, and hopefully answers to long-standing questions are coming into
reach.Comment: 35 pages, 11 figures (29 eps files; high-quality versions can be
obtained upon request); accepted by Annual Review of Nuclear and Particle
Scienc
Neutrino-driven Explosions
The question why and how core-collapse supernovae (SNe) explode is one of the
central and most long-standing riddles of stellar astrophysics. A solution is
crucial for deciphering the SN phenomenon, for predicting observable signals
such as light curves and spectra, nucleosynthesis, neutrinos, and gravitational
waves, for defining the role of SNe in the evolution of galaxies, and for
explaining the birth conditions and properties of neutron stars (NSs) and
stellar-mass black holes. Since the formation of such compact remnants releases
over hundred times more energy in neutrinos than the SN in the explosion,
neutrinos can be the decisive agents for powering the SN outburst. According to
the standard paradigm of the neutrino-driven mechanism, the energy transfer by
the intense neutrino flux to the medium behind the stagnating core-bounce
shock, assisted by violent hydrodynamic mass motions (sometimes subsumed by the
term "turbulence"), revives the outward shock motion and thus initiates the SN
blast. Because of the weak coupling of neutrinos in the region of this energy
deposition, detailed, multidimensional hydrodynamic models including neutrino
transport and a wide variety of physics are needed to assess the viability of
the mechanism. Owing to advanced numerical codes and increasing supercomputer
power, considerable progress has been achieved in our understanding of the
physical processes that have to act in concert for the success of
neutrino-driven explosions. First studies begin to reveal observational
implications and avenues to test the theoretical picture by data from
individual SNe and SN remnants but also from population-integrated observables.
While models will be further refined, a real breakthrough is expected through
the next Galactic core-collapse SN, when neutrinos and gravitational waves can
be used to probe the conditions deep inside the dying star. (abridged)Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A.
Alsabti and P. Murdin, Springer. 54 pages, 13 figure
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