21 research outputs found
A Generalized Solution Method for Parallelized Computation of the Three-dimensional Gravitational Potential on a Multi-patch grid in Spherical Geometry
We present a generalized algorithm based on a spherical harmonics expansion
method for efficient computation of the three-dimensional gravitational
potential on a multi-patch grid in spherical geometry. Instead of solving for
the gravitational potential by superposition of separate contributions from the
mass density distribution on individual grid patch our new algorithm computes
directly the gravitational potential due to contributions from all grid patches
in one computation step, thereby reducing the computational cost of the gravity
solver. This is possible by considering a set of angular weights which are
derived from rotations of spherical harmonics functions defined in a global
coordinate system that is common for all grid patches. Additionally, our
algorithm minimizes data communication between parallel compute tasks by
eliminating its proportionality to the number of subdomains in the grid
configuration, making it suitable for parallelized computation on a multi-patch
grid configuration with any number of subdomains. Test calculations of the
gravitational potential of a tri-axial ellipsoidal body with constant mass
density on the Yin-Yang two-patch overset grid demonstrate that our method
delivers the same level of accuracy as a previous method developed for the
Yin-Yang grid, while offering improved computation efficiency and parallel
scaling behaviour.Comment: 12 pages, 5 figures; accepted for publication in Ap
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
The infancy of core-collapse supernova remnants
We present 3D hydrodynamic simulations of neutrino-driven supernovae (SNe)
with the PROMETHEUS-HOTB code, evolving the asymmetrically expanding ejecta
from shock breakout until they reach the homologous expansion phase after
roughly one year. Our calculations continue the simulations for two red
supergiant (RSG) and two blue supergiant (BSG) progenitors by Wongwathanarat et
al., who investigated the growth of explosion asymmetries produced by
hydrodynamic instabilities during the first second of the explosion and their
later fragmentation by Rayleigh-Taylor instabilities. We focus on the late time
acceleration and inflation of the ejecta caused by the heating due to the
radioactive decay of Ni to Fe and by a new outward-moving shock,
which forms when the reverse shock from the He/H-shell interface compresses the
central part of the ejecta. The mean velocities of the iron-rich ejecta
increase between 100 km/s and 350 km/s (8-30\%), and the fastest one
percent of the iron accelerates by up to 1000 km/s (20-25\%). This
'Ni-bubble effect', known from 1D models, accelerates the bulk of the nickel in
our 3D models and causes an inflation of the initially overdense Ni-rich
clumps, which leads to underdense, extended fingers, enveloped by overdense
skins of compressed surrounding matter. We also provide volume and surface
filling factors as well as a spherical harmonics analysis to characterize the
spectrum of Ni-clump sizes quantitatively. Three of our four models give volume
filling factors larger than 0.3, consistent with what is suggested for SN 1987A
by observations.Comment: 30 pages, 22 figures, 6 tables, extended discussion of correlation of
late and initial explosion asymmetries, accepted by MNRA
Supernova 1987A: neutrino-driven explosions in three dimensions and light curves
The well-studied type IIP SN 1987A, produced by the explosion of a blue
supergiant (BSG) star, is a touchstone for massive-star evolution, simulations
of neutrino-driven explosions, and modeling of light curves and spectra. In the
framework of the neutrino-driven mechanism, we study the dependence of
explosion properties on the structure of four different BSGs and compare the
corresponding light curves with observations of SN 1987A. We perform 3D
simulations with the PROMETHEUS code until about one day and map the results to
the 1D code CRAB for the light curve calculations. All of our 3D models with
explosion energies compatible with SN 1987A produce 56Ni in rough agreement
with the amount deduced from fitting the radioactively powered light-curve
tail. One of the progenitors yields maximum velocities of ~3000 km/s for the
bulk of ejected 56Ni, consistent with observations. In all of our models inward
mixing of hydrogen during the 3D evolution leads to minimum H-velocities below
100 km/s, in good agreement with spectral observations. The considered BSG
models, 3D explosion simulations, and light-curve calculations can thus explain
basic observational features of SN 1987A. However, all progenitors have too
large pre-SN radii to reproduce the narrow initial luminosity peak, and the
structure of their outer layers is not suitable to match the observed light
curve during the first 30-40 days. Only one stellar model has a structure of
the He core and the He/H composition interface that enables sufficient outward
mixing of 56Ni and inward mixing of hydrogen to produce a good match of the
dome-like shape of the observed light-curve maximum. But this model falls short
of the He-core mass of 6 Msun inferred from the absolute luminosity of the
pre-SN star. The lack of an adequate pre-SN model for SN 1987A is a pressing
challenge for the theory of massive-star evolution. (Abridged)Comment: 18 pages, 11 figures, 4 tables; revised version, accepted by Astron.
Astrophy
Modelling supernova nebular lines in 3D with
We present (EXplosive TRAnsient Spectral Simulator), a
newly developed code aimed at generating 3D spectra for supernovae in the
nebular phase by using modern multi-dimensional explosion models as input. It
is well established that supernovae are asymmetric by nature, and that the
morphology is encoded in the line profiles during the nebular phase, months
after the explosion. In this work, we use to study one such
simulation of a He-core explosion
(, erg)
modelled with the code and evolved to the homologous
phase. Our code calculates the energy deposition from the radioactive decay of
Ni Co Fe and uses this to
determine the Non-Local-Thermodynamic-Equilibrium temperature, excitation and
ionization structure across the nebula. From the physical condition solutions
we generate the emissivities to construct spectra depending on viewing angles.
Our results show large variations in the line profiles with viewing angles, as
diagnosed by the first three moments of the line profiles; shifts, widths, and
skewness. We compare line profiles from different elements, and study the
morphology of line-of-sight slices that determine the flux at each part of a
line profile. We find that excitation conditions can sometimes make the
momentum vector of the ejecta emitting in the excited states significantly
different from that of the bulk of the ejecta of the respective element, thus
giving blueshifted lines for bulk receding material, and vice versa. We compare
the 3.3 He-core model to observations of the Type Ib supernova SN
2007Y.Comment: 20 pages, 15 Figures 2 Tables. Accepted for publication in MNRA
Core-Collapse Supernovae: Explosion Dynamics, Neutrinos and Gravitational Waves
The quest for the supernova explosion mechanism has been one of the
outstanding challenges in computational astrophysics for several decades.
Simulations have now progressed to a stage at which the solution appears close
and neutrino and gravitational wave signals from self-consistent explosion
models are becoming available. Here we focus one of the recent advances in
supernova modeling, the inclusion of general relativity in multi-dimensional
neutrino hydrodynamics simulations, and present the latest simulation results
for an 11.2 and a 15 solar mass progenitor. We also mention 3D effects as
another aspect in supernova physics awaiting further, more thorough
investigation.Comment: Contribution to the Proceedings of HANSE 2011 workshop, 8 pages, 4
figure
Three-dimensional mixing and light curves: constraints on the progenitor of supernova 1987A
With the same method as used previously, we investigate neutrino-driven
explosions of a larger sample of blue supergiant models. The larger sample
includes three new presupernova stars. The results are compared with
light-curve observations of the peculiar type IIP SN 1987A. The explosions were
modeled in 3D with the neutrino-hydrodynamics code PROMETHEUS-HOTB, and
light-curve calculations were performed in spherical symmetry with the
radiation-hydrodynamics code CRAB. Our results confirm the basic findings of
the previous work: 3D neutrino-driven explosions with SN 1987A-like energies
synthesize an amount of Ni-56 that is consistent with the radioactive tail of
the light curve. Moreover, the models mix hydrogen inward to minimum velocities
below 400 km/s as required by spectral observations. Hydrodynamic simulations
with the new progenitor models, which possess smaller radii than the older
ones, show much better agreement between calculated and observed light curves
in the initial luminosity peak and during the first 20 days. A set of
explosions with similar energies demonstrated that a high growth factor of
Rayleigh-Taylor instabilities at the (C+O)/He composition interface combined
with a weak interaction of fast Rayleigh-Taylor plumes, where the reverse shock
occurs below the He/H interface, provides a sufficient condition for efficient
outward mixing of Ni-56 into the hydrogen envelope. This condition is realized
to the required extent only in one of the older stellar models, which yielded a
maximum velocity of around 3000 km/s for the bulk of ejected Ni-56, but failed
to reproduce the helium-core mass of 6 Msun inferred from the absolute
luminosity of the presupernova star. We conclude that none of the single-star
progenitor models proposed for SN 1987A to date satisfies all constraints set
by observations. (Abridged)Comment: 16 pages, 9 figures, 3 tables; accepted for publication in Astron.
Astrophy
X-ray Absorption in Young Core-Collapse Supernova Remnants
The material expelled by core-collapse supernova (SN) explosions absorbs
X-rays from the central regions. We use SN models based on three-dimensional
neutrino-driven explosions to estimate optical depths to the center of the
explosion, compare different progenitor models, and investigate the effects of
explosion asymmetries. The optical depths below 2 keV for progenitors with a
remaining hydrogen envelope are expected to be high during the first century
after the explosion due to photoabsorption. A typical optical depth is , where is the time since the explosion in units of 10
000 days (27 years) and the energy in units of keV. Compton
scattering dominates above 50 keV, but the scattering depth is lower and
reaches unity already at 1000 days at 1 MeV. The optical depths are
approximately an order of magnitude lower for hydrogen-stripped progenitors.
The metallicity of the SN ejecta is much higher than in the interstellar
medium, which enhances photoabsorption and makes absorption edges stronger.
These results are applicable to young SN remnants in general, but we explore
the effects on observations of SN 1987A and the compact object in Cas A in
detail. For SN 1987A, the absorption is high and the X-ray upper limits of
100 Lsun on a compact object are approximately an order of magnitude
less constraining than previous estimates using other absorption models. The
details are presented in an accompanying paper. For the central compact object
in Cas A, we find no significant effects of our more detailed absorption model
on the inferred surface temperature.Comment: 21 pages, 7 figures. Accepted for publication in ApJ. Updated to
match accepted version; added Section 2.5 on asymmetries and discussion on
homologous expansion in preamble of Section