33 research outputs found
Critical Surface for Explosions of Rotational Core-Collapse Supernovae
The effect of rotation on the explosion of core-collapse supernovae is
investigated systematically in three-dimensional simulations. In order to
obtain the critical conditions for explosion as a function of mass accretion
rate, neutrino luminosity, and specific angular momentum, rigidly rotating
matter was injected from the outer boundary with an angular momentum, which is
increased every 500 ms. It is found that there is a critical value of the
specific angular momentum, above which the standing shock wave revives, for a
given combination of mass accretion rate and neutrino luminosity, i.e. an
explosion can occur by rotation even if the neutrino luminosity is lower than
the critical value for a given mass accretion rate in non-rotational models.
The coupling of rotation and hydrodynamical instabilities plays an important
role to characterize the dynamics of shock revival for the range of specific
angular momentum that are supposed to be realistic. Contrary to expectations
from past studies, the most rapidly expanding direction of the shock wave is
not aligned with the rotation axis. Being perpendicular to the rotation axis on
average, it can be oriented in various directions. Its dispersion is small when
the spiral mode of the standing accretion shock instability (SASI) governs the
dynamics, while it is large when neutrino-driven convection is dominant. As a
result of the comparison between 2D and 3D rotational models, it is found that
m=!0 modes of neutrino-driven convection or SASI are important for shock
revival around the critical surface.Comment: First revised version, submitted to ApJ, 14 pages, 13 figures, 2
table
Three-Dimensional Simulations of Standing Accretion Shock Instability in Core-Collapse Supernovae
We have studied non-axisymmetric standing accretion shock instability, or
SASI, by 3D hydrodynamical simulations. This is an extention of our previous
study on axisymmetric SASI. We have prepared a spherically symmetric and steady
accretion flow through a standing shock wave onto a proto-neutron star, taking
into account a realistic equation of state and neutrino heating and cooling.
This unperturbed model is supposed to represent approximately the typical
post-bounce phase of core-collapse supernovae. We then have added a small
perturbation (~1%) to the radial velocity and computed the ensuing evolutions.
Not only axisymmetric but non-axisymmetric perturbations have been also
imposed. We have applied mode analysis to the non-spherical deformation of the
shock surface, using the spherical harmonics. We have found that (1) the growth
rates of SASI are degenerate with respect to the azimuthal index m of the
spherical harmonics Y_l^m, just as expected for a spherically symmetric
background, (2) nonlinear mode couplings produce only m=0 modes for the
axisymmetric perturbations, whereas m=!0 modes are also generated in the
non-axisymmetric cases according to the selection rule for the quadratic
couplings, (3) the nonlinear saturation level of each mode is lower in general
for 3D than for 2D because a larger number of modes are contributing to
turbulence in 3D, (4) low l modes are dominant in the nonlinear phase, (5) the
equi-partition is nearly established among different m modes in the nonlinear
phase, (6) the spectra with respect to l obey power laws with a slope slightly
steeper for 3D, and (7) although these features are common to the models with
and without a shock revival at the end of simulation, the dominance of low l
modes is more remarkable in the models with a shock revival.Comment: 37 pages, 16 figures, and 1 table, submitted to Ap
On the Neutrino Distributions in Phase Space for the Rotating Core-collapse Supernova Simulated with a Boltzmann-neutrino-radiation-hydrodynamics Code
With the Boltzmann-radiation-hydrodynamics code, which we have developed to
solve numerically the Boltzmann equations for neutrino transfer, the Newtonian
hydrodynamics equations, and the Newtonian self-gravity simultaneously and
consistently, we simulate the collapse of a rotating core of the progenitor
with a zero-age-main-sequence mass of and a shelluler rotation
of at the center. We pay particular attention in this
paper to the neutrino distribution in phase space, which is affected by the
rotation. By solving the Boltzmann equations directly, we can assess the
rotation-induced distortion of the angular distribution in momentum space,
which gives rise to the rotational component of the neutrino flux. We compare
the Eddington tensors calculated both from the raw data and from the M1-closure
approximation. We demonstrate that the Eddington tensor is determined by
complicated interplays of the fluid velocity and the neutrino interactions and
that the M1-closure, which assumes that the Eddington factor is determined by
the flux factor, fails to fully capture this aspect, especially in the vicinity
of the shock. We find that the error in the Eddington factor reaches in our simulation. This is due not to the resolution but to the different
dependence of the Eddington and flux factors on the angular profile of the
neutrino distribution function, and hence modification to the closure relation
is needed.Comment: 24 pages, 23 figures, 0 explosion, published in Ap
Effects of Rotation on Standing Accretion Shock Instability in Nonlinear Phase for Core-Collapse Supernovae
We studied the effects of rotation on standing accretion shock instability
(SASI) by performing three-dimensional hydrodynamics simulations. Taking into
account a realistic equation of state and neutrino heating/cooling, we prepared
a spherically symmmetric and steady accretion flow through a standing shock
wave onto a proto-neutron star (PNS). When the SASI entered the nonlinear
phase, we imposed uniform rotation on the flow advecting from the outer
boundary of the iron core, whose specific angular momentum was assumed to agree
with recent stellar evolution models. Using spherical harmonics in space and
Fourier decompositions in time, we performed mode analysis of the nonspherical
deformed shock wave to observe rotational effects on the SASI in the nonlinear
phase. We found that rotation imposed on the axisymmetric SASI did not make any
spiral modes and hardly affected sloshing modes, except for steady l=2, m=0
modes. In contrast, rotation imposed on the non-axisymmetric flow increased the
amplitude of spiral modes so that some spiral flows accreting on the PNS were
more clearly formed inside the shock wave than without rotation. The amplitudes
of spiral modes increased significantly with rotation in the progressive
direction.Comment: 27 pages, 11 figures, Submitted to Ap
Effects of Rotation on Stochasticity of Gravitational Waves in Nonlinear Phase of Core-Collapse Supernovae
By performing three-dimensional (3D) simulations that demonstrate the
neutrino-driven core-collapse supernovae aided by the standing accretion shock
instability (SASI), we study how the spiral modes of the SASI can have impacts
on the properties of the gravitational-wave (GW) emission. To see the effects
of rotation in the non-linear postbounce phase, we give a uniform rotation on
the flow advecting from the outer boundary of the iron core, whose specific
angular momentum is assumed to agree with recent stellar evolution models. We
compute fifteen 3D models in which the initial angular momentum as well as the
input neutrino luminosities from the protoneutron star are changed in a
systematic manner. By performing a ray-tracing analysis, we accurately estimate
the GW amplitudes generated by anisotropic neutrino emission. Our results show
that the gravitational waveforms from neutrinos in models that include rotation
exhibit a common feature otherwise they vary much more stochastically in the
absence of rotation. The breaking of the stochasticity stems from the excess of
the neutrino emission parallel to the spin axis. This is because the
compression of matter is more enhanced in the vicinity of the equatorial plane
due to the growth of the spiral SASI modes, leading to the formation of spiral
flows circulating around the spin axis with higher temperatures. We point out
that a recently proposed future space interferometers like Fabry-Perot type
DECIGO would permit detection of these signals for a Galactic supernova.Comment: 13 Figures, ApJ in pres
Stochastic Nature of Gravitational Waves from Supernova Explosions with Standing Accretion Shock Instability
We study properties of gravitational waves based on the three-dimensional
simulations, which demonstrate the neutrino-driven explosions aided by the
standing accretion shock instability (SASI). Pushed by evidence supporting slow
rotation prior to core-collapse, we focus on the asphericities in neutrino
emissions and matter motions outside the protoneutron star. By performing a
ray-tracing calculation in 3D, we estimate accurately the gravitational
waveforms from anisotropic neutrino emissions. In contrast to the previous work
assuming axisymmetry, we find that the gravitational waveforms vary much more
stochastically because the explosion anisotropies depend sensitively on the
growth of the SASI which develops chaotically in all directions. Our results
show that the gravitational-wave spectrum has its peak near Hz,
reflecting the SASI-induced matter overturns of ms. We point out
that the detection of such signals, possibly visible to the LIGO-class
detectors for a Galactic supernova, could be an important probe into the
long-veiled explosion mechanism.Comment: 4 pages, 3 figures, accepted by ApJ