158 research outputs found
Spin-flavor precession of Dirac neutrinos in dense matter and its potential in core-collapse supernovae
We calculate the spin-flavor precession (SFP) of Dirac neutrinos induced by
strong magnetic fields and finite neutrino magnetic moments in dense matter. As
found in the case of Majorana neutrinos, the SFP of Dirac neutrinos is enhanced
by the large magnetic field potential and suppressed by large matter potentials
composed of the baryon density and the electron fraction. The SFP is possible
irrespective of the large baryon density when the electron fraction is close to
1/3. The diagonal neutrino magnetic moments that are prohibited for Majorana
neutrinos enable the spin precession of Dirac neutrinos without any flavor
mixing. With supernova hydrodynamics simulation data, we discuss the
possibility of the SFP of both Dirac and Majorana neutrinos in core-collapse
supernovae. The SFP of Dirac neutrinos occurs at a radius where the electron
fraction is 1/3. The required magnetic field of the proto-neutron star for the
SFP is a few G at any explosion time. For the Majorana neutrinos, the
required magnetic field fluctuates from G to G. Such a
fluctuation of the magnetic field is more sensitive to the numerical scheme of
the neutrino transport in the supernova simulation.Comment: 14 pages, 10 figure
Numerical Study on GRB-Jet Formation in Collapsars
Two-dimensional magnetohydrodynamic simulations are performed using the
ZEUS-2D code to investigate the dynamics of a collapsar that generates a GRB
jet, taking account of realistic equation of state, neutrino cooling and
heating processes, magnetic fields, and gravitational force from the central
black hole and self gravity. It is found that neutrino heating processes are
not so efficient to launch a jet in this study. It is also found that a jet is
launched mainly by B_\phi fields that are amplified by the winding-up effect.
However, since the ratio of total energy relative to the rest mass energy in
the jet is not so high as several hundred, we conclude that the jets seen in
this study are not be a GRB jet. This result suggests that general relativistic
effects, which are not included in this study, will be important to generate a
GRB jet. Also, the accretion disk with magnetic fields may still play an
important role to launch a GRB jet, although a simulation for much longer
physical time (\sim 10-100 s) is required to confirm this effect. It is shown
that considerable amount of 56Ni is synthesized in the accretion disk. Thus
there will be a possibility for the accretion disk to supply sufficient amount
of 56Ni required to explain the luminosity of a hypernova. Also, it is shown
that neutron-rich matter due to electron captures with high entropy per baryon
is ejected along the polar axis. Moreover, it is found that the electron
fraction becomes larger than 0.5 around the polar axis near the black hole by
\nu_e capture at the region. Thus there will be a possibility that r-process
and r/p-process nucleosynthesis occur at these regions. Finally, much neutrons
will be ejected from the jet, which suggests that signals from the neutron
decays may be observed as the delayed bump of afterglow or gamma-rays.Comment: 54 pages with 19 postscript figures. Accepted for publication in ApJ.
High resolution version is available at
http://www2.yukawa.kyoto-u.ac.jp/~nagataki/collapsar.pd
Neutrino oscillations in magnetically driven supernova explosions
We investigate neutrino oscillations from core-collapse supernovae that
produce magnetohydrodynamic (MHD) explosions. By calculating numerically the
flavor conversion of neutrinos in the highly non-spherical envelope, we study
how the explosion anisotropy has impacts on the emergent neutrino spectra
through the Mikheyev-Smirnov-Wolfenstein effect. In the case of the inverted
mass hierarchy with a relatively large theta_(13), we show that survival
probabilities of electron type neutrinos and antineutrinos seen from the
rotational axis of the MHD supernovae (i.e., polar direction), can be
significantly different from those along the equatorial direction. The event
numbers of electron type antineutrinos observed from the polar direction are
predicted to show steepest decrease, reflecting the passage of the
magneto-driven shock to the so-called high-resonance regions. Furthermore we
point out that such a shock effect, depending on the original neutrino spectra,
appears also for the low-resonance regions, which leads to a noticeable
decrease in the electron type neutrino signals. This reflects a unique nature
of the magnetic explosion featuring a very early shock-arrival to the resonance
regions, which is in sharp contrast to the neutrino-driven delayed supernova
models. Our results suggest that the two features in the electron type
antineutrinos and neutrinos signals, if visible to the Super-Kamiokande for a
Galactic supernova, could mark an observational signature of the magnetically
driven explosions, presumably linked to the formation of magnetars and/or
long-duration gamma-ray bursts.Comment: 25 pages, 21 figures, JCAP in pres
Turbulent magnetic field amplification from spiral SASI modes in core-collapse supernovae
We describe the initial implementation of magnetohydrodynamics (MHD) in our
astrophysical simulation code \genasis. Then, we present MHD simulations
exploring the capacity of the stationary accretion shock instability (SASI) to
generate magnetic fields by adding a weak magnetic field to an initially
spherically symmetric fluid configuration that models a stalled shock in the
post-bounce supernova environment. Upon perturbation and nonlinear SASI
development, shear flows associated with the spiral SASI mode contributes to a
widespread and turbulent field amplification mechanism. While the SASI may
contribute to neutron star magnetization, these simulations do not show
qualitatively new features in the global evolution of the shock as a result of
SASI-induced magnetic field amplification.Comment: 15 pages, 7 figures, To appear in the Journal of Physics: Conference
Series. Proceedings of the IUPAP Conference on Computational Physics
(CCP2011
CASTRO: A New Compressible Astrophysical Solver. III. Multigroup Radiation Hydrodynamics
We present a formulation for multigroup radiation hydrodynamics that is
correct to order using the comoving-frame approach and the
flux-limited diffusion approximation. We describe a numerical algorithm for
solving the system, implemented in the compressible astrophysics code, CASTRO.
CASTRO uses an Eulerian grid with block-structured adaptive mesh refinement
based on a nested hierarchy of logically-rectangular variable-sized grids with
simultaneous refinement in both space and time. In our multigroup radiation
solver, the system is split into three parts, one part that couples the
radiation and fluid in a hyperbolic subsystem, another part that advects the
radiation in frequency space, and a parabolic part that evolves radiation
diffusion and source-sink terms. The hyperbolic subsystem and the frequency
space advection are solved explicitly with high-order Godunov schemes, whereas
the parabolic part is solved implicitly with a first-order backward Euler
method. Our multigroup radiation solver works for both neutrino and photon
radiation.Comment: accepted by ApJS, 27 pages, 20 figures, high-resolution version
available at https://ccse.lbl.gov/Publications/wqzhang/castro3.pd
Crucial Physical Dependencies of the Core-Collapse Supernova Mechanism
We explore with self-consistent 2D F{\sc{ornax}} simulations the dependence
of the outcome of collapse on many-body corrections to neutrino-nucleon cross
sections, the nucleon-nucleon bremsstrahlung rate, electron capture on heavy
nuclei, pre-collapse seed perturbations, and inelastic neutrino-electron and
neutrino-nucleon scattering. Importantly, proximity to criticality amplifies
the role of even small changes in the neutrino-matter couplings, and such
changes can together add to produce outsized effects. When close to the
critical condition the cumulative result of a few small effects (including
seeds) that individually have only modest consequence can convert an anemic
into a robust explosion, or even a dud into a blast. Such sensitivity is not
seen in one dimension and may explain the apparent heterogeneity in the
outcomes of detailed simulations performed internationally. A natural
conclusion is that the different groups collectively are closer to a realistic
understanding of the mechanism of core-collapse supernovae than might have
seemed apparent.Comment: 25 pages; 10 figure
On the spherical-axial transition in supernova remnants
A new law of motion for supernova remnant (SNR) which introduces the quantity
of swept matter in the thin layer approximation is introduced. This new law of
motion is tested on 10 years observations of SN1993J. The introduction of an
exponential gradient in the surrounding medium allows to model an aspherical
expansion. A weakly asymmetric SNR, SN1006, and a strongly asymmetric SNR,
SN1987a, are modeled. In the case of SN1987a the three observed rings are
simulated.Comment: 19 figures and 14 pages Accepted for publication in Astrophysics &
Space Science in the year 201
Effect of Collective Neutrino Oscillations on the Neutrino Mechanism of Core-Collapse Supernovae
In the seconds after collapse of a massive star, the newborn proto-neutron
star (PNS) radiates neutrinos of all flavors. The absorption of electron-type
neutrinos below the radius of the stalled shockwave may drive explosions (the
"neutrino mechanism"). Because the heating rate is proportional to the square
of neutrino energy, flavor conversion of mu and tau neutrinos to electron-type
neutrinos via collective neutrino oscillations (CnuO) may in principle increase
the heating rate and drive explosions. In order to assess the potential
importance of CnuO for the shock revival, we solve the steady-state boundary
value problem of spherically-symmetric accretion between the PNS surface (r_nu)
and the shock (r_S), including a scheme for flavor conversion via CnuO. For a
given r_nu, PNS mass (M), accretion rate (Mdot), and assumed values of the
neutrino energies from the PNS, we calculate the critical neutrino luminosity
above which accretion is impossible and explosion results. We show that CnuO
can decrease the critical luminosity by a factor of at most ~1.5, but only if
the flavor conversion is fully completed inside r_S and if there is no matter
suppression. The magnitude of the effect depends on the model parameters (M,
Mdot, and r_nu) through the shock radius and the physical scale for flavor
conversion. We quantify these dependencies and find that CnuO could lower the
critical luminosity only for small M and Mdot, and large r_nu. However, for
these parameter values CnuO are suppressed due to matter effects. By
quantifying the importance of CnuO and matter suppression at the critical
neutrino luminosity for explosion, we show in agreement with previous studies
that CnuO are unlikely to affect the neutrino mechanism of core-collapse
supernovae significantly.Comment: 8 pages, 3 figures, accepted to MNRA
Core collapse supernovae in the QCD phase diagram
We compare two classes of hybrid equations of state with a hadron-to-quark
matter phase transition in their application to core collapse supernova
simulations. The first one uses the quark bag model and describes the
transition to three-flavor quark matter at low critical densities. The second
one employs a Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) model with
parameters describing a phase transition to two-flavor quark matter at higher
critical densities. These models possess a distinctly different temperature
dependence of their transition densities which turns out to be crucial for the
possible appearance of quark matter in supernova cores. During the early post
bounce accretion phase quark matter is found only if the phase transition takes
place at sufficiently low densities as in the study based on the bag model. The
increase critical density with increasing temperature, as obtained for our PNJL
parametrization, prevents the formation of quark matter. The further evolution
of the core collapse supernova as obtained applying the quark bag model leads
to a structural reconfiguration of the central proto-neutron star where, in
addition to a massive pure quark matter core, a strong hydrodynamic shock wave
forms and a second neutrino burst is released during the shock propagation
across the neutrinospheres. We discuss the severe constraints in the freedom of
choice of quark matter models and their parametrization due to the recently
observed 2 solar mass pulsar and their implications for further studies of core
collapse supernovae in the QCD phase diagram.Comment: 19 pages, 4 figures, CPOD2010 conference proceedin
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