323 research outputs found
Natal Kicks of Stellar-Mass Black Holes by Asymmetric Mass Ejection in Fallback Supernovae
Integrating trajectories of low-mass X-ray binaries containing black holes
within the Galactic potential, Repetto, Davies & Sigurdsson recently showed
that the large distances of some systems above the Galactic plane can only be
explained if black holes receive appreciable natal kicks. Surprisingly, they
found that the distribution of black hole kick velocities (rather than that of
the momenta) should be similar to that of neutron stars. Here I argue that this
result can be understood if neutron star and black hole kicks are a consequence
of large-scale asymmetries created in the supernova ejecta by the explosion
mechanism. The corresponding anisotropic gravitational attraction of the
asymmetrically expelled matter does not only accelerate new-born neutron stars
by the "gravitational tug-boat mechanism". It can also lead to delayed
black-hole formation by asymmetric fallback of the slowest parts of the initial
ejecta onto the transiently existing neutron star, in course of which the
momentum of the black hole can grow with the fallback mass. Black hole kick
velocities will therefore not be reduced by the ratio of neutron star to black
hole mass as would be expected for kicks caused by anisotropic neutrino
emission of the nascent neutron star.Comment: 7 pages, 1 figure (3 eps files); submitted to MNRA
Spatial distribution of radionuclides in 3D models of SN 1987A and Cas A
Fostered by the possibilities of multi-dimensional computational modeling, in
particular the advent of three-dimensional (3D) simulations, our understanding
of the neutrino-driven explosion mechanism of core-collapse supernovae (SNe)
has experienced remarkable progress over the past decade. First
self-consistent, first-principle models have shown successful explosions in 3D,
and even failed cases may be cured by moderate changes of the microphysics
inside the neutron star (NS), better grid resolution, or more detailed
progenitor conditions at the onset of core collapse, in particular large-scale
perturbations in the convective Si and O burning shells. 3D simulations have
also achieved to follow neutrino-driven explosions continuously from the
initiation of the blast wave, through the shock breakout from the progenitor
surface, into the radioactively powered evolution of the SN, and towards the
free expansion phase of the emerging remnant. Here we present results from such
simulations, which form the basis for direct comparisons with observations of
SNe and SN remnants in order to derive constraints on the still disputed
explosion mechanism. It is shown that predictions based on hydrodynamic
instabilities and mixing processes associated with neutrino-driven explosions
yield good agreement with measured NS kicks, light-curve properties of SN
1987A, and asymmetries of iron and 44Ti distributions observed in SN 1987A and
Cassiopeia A.Comment: 9 pages, 6 figures; submitted to: "SN 1987A, 30 years later",
Proceedings IAU Symposium No. 331, 2017; eds. M. Renaud et a
Three-Dimensional Simulations of Core-Collapse Supernovae: From Shock Revival to Shock Breakout
We present 3D simulations of core-collapse supernovae from blast-wave
initiation by the neutrino-driven mechanism to shock breakout from the stellar
surface, considering two 15 Msun red supergiants (RSG) and two blue supergiants
(BSG) of 15 Msun and 20 Msun. We demonstrate that the metal-rich ejecta in
homologous expansion still carry fingerprints of asymmetries at the beginning
of the explosion, but the final metal distribution is massively affected by the
detailed progenitor structure. The most extended and fastest metal fingers and
clumps are correlated with the biggest and fastest-rising plumes of
neutrino-heated matter, because these plumes most effectively seed the growth
of Rayleigh-Taylor (RT) instabilities at the C+O/He and He/H composition-shell
interfaces after the passage of the SN shock. The extent of radial mixing,
global asymmetry of the metal-rich ejecta, RT-induced fragmentation of initial
plumes to smaller-scale fingers, and maximal Ni and minimal H velocities do not
only depend on the initial asphericity and explosion energy (which determine
the shock and initial Ni velocities) but also on the density profiles and
widths of C+O core and He shell and on the density gradient at the He/H
transition, which lead to unsteady shock propagation and the formation of
reverse shocks. Both RSG explosions retain a great global metal asymmetry with
pronounced clumpiness and substructure, deep penetration of Ni fingers into the
H-envelope (with maximum velocities of 4000-5000 km/s for an explosion energy
around 1.5 bethe) and efficient inward H-mixing. While the 15 Msun BSG shares
these properties (maximum Ni speeds up to ~3500 km/s), the 20 Msun BSG develops
a much more roundish geometry without pronounced metal fingers (maximum Ni
velocities only ~2200 km/s) because of reverse-shock deceleration and
insufficient time for strong RT growth and fragmentation at the He/H interface.Comment: 21 pages, 15 figures; revised version with minor changes in Sect.1;
accepted by Astron. Astrophy
Resolution Study for Three-dimensional Supernova Simulations with the Prometheus-Vertex Code
We present a carefully designed, systematic study of the angular resolution
dependence of simulations with the Prometheus-Vertex neutrino-hydrodynamics
code. Employing a simplified neutrino heating-cooling scheme in the Prometheus
hydrodynamics module allows us to sample the angular resolution between 4
degrees and 0.5 degrees. With a newly-implemented static mesh refinement (SMR)
technique on the Yin-Yang grid, the angular coordinates can be refined in
concentric shells, compensating for the diverging structure of the spherical
grid. In contrast to previous studies with Prometheus and other codes, we find
that higher angular resolution and therefore lower numerical viscosity provides
more favorable explosion conditions and faster shock expansion. We discuss the
possible reasons for the discrepant results. The overall dynamics seem to
converge at a resolution of about 1 degree. Applying the SMR setup to
marginally exploding progenitors is disadvantageous for the shock expansion,
however, because kinetic energy of downflows is dissipated to internal energy
at resolution interfaces, leading to a loss of turbulent pressure support and a
steeper temperature gradient. We also present a way to estimate the numerical
viscosity on grounds of the measured turbulent kinetic-energy spectrum, leading
to smaller values that are better compatible with the flow behavior witnessed
in our simulations than results following calculations in previous literature.
Interestingly, the numerical Reynolds numbers in the turbulent, neutrino-heated
postshock layer (some 10 to several 100) are in the ballpark of expected
neutrino-drag effects on the relevant length scales in the turbulent postshock
layer. We provide a formal derivation and quantitative assessment of the
neutrino drag terms in an appendix.Comment: 37 pages, 14 figures, 4 tables; revised version with neutrino drag
discussion extended for numerical evaluation; accepted by Ap
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
A new multidimensional, energy-dependent two-moment transport code for neutrino-hydrodynamics
We present the new code ALCAR developed to model multidimensional, multi
energy-group neutrino transport in the context of supernovae and neutron-star
mergers. The algorithm solves the evolution equations of the 0th- and 1st-order
angular moments of the specific intensity, supplemented by an algebraic
relation for the 2nd-moment tensor to close the system. The scheme takes into
account frame-dependent effects of order O(v/c) as well as the most important
types of neutrino interactions. The transport scheme is significantly more
efficient than a multidimensional solver of the Boltzmann equation, while it is
more accurate and consistent than the flux-limited diffusion method. The
finite-volume discretization of the essentially hyperbolic system of moment
equations employs methods well-known from hydrodynamics. For the time
integration of the potentially stiff moment equations we employ a scheme in
which only the local source terms are treated implicitly, while the advection
terms are kept explicit, thereby allowing for an efficient computational
parallelization of the algorithm. We investigate various problem setups in one
and two dimensions to verify the implementation and to test the quality of the
algebraic closure scheme. In our most detailed test, we compare a fully
dynamic, one-dimensional core-collapse simulation with two published
calculations performed with well-known Boltzmann-type neutrino-hydrodynamics
codes and we find very satisfactory agreement.Comment: 30 pages, 12 figures. Revised version: several additional comments
and explanations, results remain unchanged. Accepted for publication in MNRA
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
Erratum: Progenitor-explosion connection and remnant birth masses for neutrino-driven supernovae of iron-core progenitors (2012, ApJ, 757, 69)
An erroneous interpretation of the hydrodynamical results led to an incorrect
determination of the fallback masses in Ugliano et al. (2012), which also (on a
smaller level) affects the neutron star masses provided in that paper. This
problem was already addressed and corrected in the follow-up works by Ertl et
al. (2015) and Sukhbold et al. (2015). Therefore, the reader is advised to use
the new data of the latter two publications. In the remaining text of this
Erratum we present the differences of the old and new fallback results in
detail and explain the origin of the mistake in the original analysis by
Ugliano et al. (2012).Comment: 3 pages, 2 figures; submitted to The Astrophysical Journa
Three-Dimensional Core-Collapse Supernova Simulations with Multi-Dimensional Neutrino Transport Compared to the Ray-by-Ray-plus Approximation
Self-consistent, time-dependent supernova (SN) simulations in three spatial
dimensions (3D) are conducted with the Aenus-Alcar code, comparing, for the
first time, calculations with fully multi-dimensional (FMD) neutrino transport
and the ray-by-ray-plus (RbR+) approximation, both based on a two-moment solver
with algebraic M1 closure. We find good agreement between 3D results with FMD
and RbR+ transport for both tested grid resolutions in the cases of a 20
solar-mass progenitor, which does not explode with the employed simplified set
of neutrino opacities, and of an exploding 9 solar-mass model. This is in stark
contrast to corresponding axisymmetric (2D) simulations, which confirm previous
claims that the RbR+ approximation can foster explosions in 2D in particular in
models with powerful axial sloshing of the stalled shock due to the standing
accretion shock instability (SASI). However, while local and instantaneous
variations of neutrino fluxes and heating rates can still be considerably
higher with RbR+ transport in 3D, the time-averaged quantities are very similar
to FMD results because of the absence of a fixed, artificial symmetry axis that
channels the flow. Therefore, except for stochastic fluctuations, the neutrino
signals and the post-bounce evolution of 3D simulations with FMD and RbR+
transport are also very similar, in particular for our calculations with the
better grid resolution. Higher spatial resolution has clearly a more important
impact than the differences by the two transport treatments. Our results back
up the use of the RbR+ approximation for neutrino transport in 3D SN modeling.Comment: 25 pages, 16 figures; referee comments included, new appendix added;
accepted by Ap
Parallelized Solution Method of the Three-dimensional Gravitational Potential on the Yin-Yang Grid
We present a new method for solving the three-dimensional gravitational
potential of a density field on the Yin-Yang grid. Our algorithm is based on a
multipole decomposition and completely symmetric with respect to the two
Yin-Yang grid patches. It is particularly efficient on distributed-memory
machines with a large number of compute tasks, because the amount of data being
explicitly communicated is minimized. All operations are performed on the
original grid without the need for interpolating data onto an auxiliary
spherical mesh.Comment: 8 pages, 4 figures; two minor additions after refereeing; accepted by
Ap
- âŠ