699 research outputs found
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
Neutron-star radius constraints from GW170817 and future detections
We introduce a new, powerful method to constrain properties of neutron stars
(NSs). We show that the total mass of GW170817 provides a reliable constraint
on the stellar radius if the merger did not result in a prompt collapse as
suggested by the interpretation of associated electromagnetic emission. The
radius R_1.6 of nonrotating NSs with a mass of 1.6 M_sun can be constrained to
be larger than 10.68_{-0.04}^{+0.15} km, and the radius R_max of the
nonrotating maximum mass configuration must be larger than 9.60_{-0.03}^{+0.14}
km. We point out that detections of future events will further improve these
constraints. Moreover, we show that a future event with a signature of a prompt
collapse of the merger remnant will establish even stronger constraints on the
NS radius from above and the maximum mass M_max of NSs from above. These
constraints are particularly robust because they only require a measurement of
the chirp mass and a distinction between prompt and delayed collapse of the
merger remnant, which may be inferred from the electromagnetic signal or even
from the presence/absence of a ringdown gravitational-wave (GW) signal. This
prospect strengthens the case of our novel method of constraining NS
properties, which is directly applicable to future GW events with accompanying
electromagnetic counterpart observations. We emphasize that this procedure is a
new way of constraining NS radii from GW detections independent of existing
efforts to infer radius information from the late inspiral phase or postmerger
oscillations, and it does not require particularly loud GW events.Comment: 7 pages, 5 figures, accepted for publication in ApJ
Imprints of neutrino-pair flavor conversions on nucleosynthesis in ejecta from neutron-star merger remnants
The remnant of neutron star mergers is dense in neutrinos. By employing
inputs from one hydrodynamical simulation of a binary neutron star merger
remnant with a black hole of in the center, dimensionless spin
parameter and an accretion torus of , the neutrino emission
properties are investigated as the merger remnant evolves. Initially, the local
number density of is larger than that of everywhere above
the remnant. Then, as the torus approaches self-regulated equilibrium, the
local abundance of neutrinos overcomes that of antineutrinos in a funnel around
the polar region. The region where the fast pairwise flavor conversions can
occur shrinks accordingly as time evolves. Still, we find that fast flavor
conversions do affect most of the neutrino-driven ejecta. Assuming that fast
flavor conversions lead to flavor equilibration, a significant enhancement of
nuclei with mass numbers is found as well as a change of the lanthanide
mass fraction by more than a factor of a thousand. Our findings hint towards a
potentially relevant role of neutrino flavor oscillations for the prediction of
the kilonova (macronova) lightcurves and motivate further work in this
direction.Comment: 16 pages, 12 figures, minor modifications to match the published
versio
Effects of LESA in Three-Dimensional Supernova Simulations with Multi-Dimensional and Ray-by-Ray-plus Neutrino Transport
A set of eight self-consistent, time-dependent supernova (SN) simulations in
three spatial dimensions (3D) for 9 solar-mass and 20 solar-mass progenitors is
evaluated for the presence of dipolar asymmetries of the electron lepton-number
emission as discovered by Tamborra et al. and termed lepton-number emission
self-sustained asymmetry (LESA). The simulations were performed with the
Aenus-Alcar neutrino/hydrodynamics code, which treats the energy- and
velocity-dependent transport of neutrinos of all flavors by a two-moment scheme
with algebraic M1 closure. For each of the progenitors, results with fully
multi-dimensional (FMD) neutrino transport and with ray-by-ray-plus (RbR+)
approximation are considered for two different grid resolutions. While the 9
solar-mass models develop explosions, the 20 solar-mass progenitor does not
explode with the employed version of simplified neutrino opacities. In all 3D
models we observe the growth of substantial dipole amplitudes of the
lepton-number (electron neutrino minus antineutrino) flux with stable or slowly
time-evolving direction and overall properties fully consistent with the LESA
phenomenon. Models with RbR+ transport develop LESA dipoles somewhat faster and
with temporarily higher amplitudes, but the FMD calculations exhibit cleaner
hemispheric asymmetries with a far more dominant dipole. In contrast, the RbR+
results display much wider multipole spectra of the neutrino-emission
anisotropies with significant power also in the quadrupole and higher-order
modes. Our results disprove speculations that LESA is a numerical artifact of
RbR+ transport. We also discuss LESA as consequence of a dipolar convection
flow inside of the nascent neutron star and establish, tentatively, a
connection to Chandrasekhar's linear theory of thermal instability in spherical
shells.Comment: 20 pages, 9 figures; revised version accepted by ApJ; new Figs. 6,7,
and new panels in Fig.8 added; Sects. 4,5,6 considerably extended in reply to
referee question
Impact of weak interactions of free nucleons on the r-process in dynamical ejecta from neutron-star mergers
We investigate beta-interactions of free nucleons and their impact on the
electron fraction (Y_e) and r-process nucleosynthesis in ejecta characteristic
of binary neutron star mergers (BNSMs). For that we employ trajectories from a
relativistic BNSM model to represent the density-temperature evolutions in our
parametric study. In the high-density environment, positron captures decrease
the neutron richness at the high temperatures predicted by the hydrodynamic
simulation. Circumventing the complexities of modelling three-dimensional
neutrino transport, (anti)neutrino captures are parameterized in terms of
prescribed neutrino luminosities and mean energies, guided by published results
and assumed as constant in time. Depending sensitively on the adopted
neutrino-antineutrino luminosity ratio, neutrino processes increase Y_e to
values between 0.25 and 0.40, still allowing for a successful r-process
compatible with the observed solar abundance distribution and a significant
fraction of the ejecta consisting of r-process nuclei. If the electron neutrino
luminosities and mean energies are relatively large compared to the
antineutrino properties, the mean Y_e might reach values >0.40 so that neutrino
captures seriously compromise the success of the r-process. In this case, the
r-abundances remain compatible with the solar distribution, but the total
amount of ejected r-material is reduced to a few percent, because the
production of iron-peak elements is favored. Proper neutrino physics, in
particular also neutrino absorption, have to be included in BNSM simulations
before final conclusions can be drawn concerning r-processing in this
environment and concerning observational consequences like kilonovae, whose
peak brightness and color temperature are sensitive to the
composition-dependent opacity of the ejecta.Comment: 12 pages, 9 figures; submitted to MNRA
Mechanochemical Synthesis of Cu2MgSn3S8 and Ag2MgSn3S8
Two new thiospinels of the type AI2BIICIV3SVI8 were successfully synthesized via a mechanochemical route using binary sulfides and sulfur. Cu2MgSn3S8 and Ag2MgSn3S8 are the first AI2BIICIV3SVI8 compounds with magnesium as divalent cation. The crystal structures of Cu2MgSn3S8 and Ag2MgSn3S8 were refined in the cubic space group Fd3m using Xâray powder diffraction. According to UV/Vis measurements, a direct optical bandgap of ca. 1.65 eV was determined for both Cu2MgSn3S8 and Ag2MgSn3S8. Temperatureâdependent magnetic susceptibility measurements of the Cu2MgSn3S8 sample indicate diamagnetism. A 119Sn MĂśssbauer spectrum confirms the tetravalent state of tin, underlining the electronâprecise description.TU Berlin, Open-Access-Mittel - 202
Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers
We present the first comprehensive study of r-process element nucleosynthesis
in the ejecta of compact binary mergers (CBMs) and their relic black-hole
(BH)-torus systems. The evolution of the BH-accretion tori is simulated for
seconds with a Newtonian hydrodynamics code including viscosity effects,
pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment
closure scheme for the transport of electron neutrinos and antineutrinos. The
investigated cases are guided by relativistic double neutron star (NS-NS) and
NS-BH merger models, producing ~3-6 Msun BHs with rotation parameters of A~0.8
and tori of 0.03-0.3 Msun. Our nucleosynthesis analysis includes the dynamical
(prompt) ejecta expelled during the CBM phase and the neutrino and viscously
driven outflows of the relic BH-torus systems. While typically ~20-25% of the
initial accretion-torus mass are lost by viscously driven outflows,
neutrino-powered winds contribute at most another ~1%, but neutrino heating
enhances the viscous ejecta significantly. Since BH-torus ejecta possess a wide
distribution of electron fractions (0.1-0.6) and entropies, they produce heavy
elements from A~80 up to the actinides, with relative contributions of A>130
nuclei being subdominant and sensitively dependent on BH and torus masses and
the exact treatment of shear viscosity. The combined ejecta of CBM and BH-torus
phases can reproduce the solar abundances amazingly well for A>90. Varying
contributions of the torus ejecta might account for observed variations of
lighter elements with 40<Z<56 relative to heavier ones, and a considerable
reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly
asymmetric NS-BH mergers, might explain the composition of heavy-element
deficient stars.Comment: 30 pages, 22 figures; revised version, accepted by MNRAS; appendix
added with test results for neutrino transpor
Nucleosynthesis in dynamical and torus ejecta of compact binary mergers
We present a comprehensive study of r-process element nucleosynthesis in the
ejecta of compact binary mergers (CBMs) and their relic black-hole (BH)-torus
systems. The evolution of the BH-accretion tori is simulated for seconds with a
Newtonian hydrodynamics code including viscosity effects, pseudo-Newtonian
gravity for rotating BHs, and an energy-dependent two-moment closure scheme for
the transport of electron neutrinos and antineutrinos. The investigated cases
are guided by relativistic double neutron star (NS-NS) and NS-BH merger models,
producing ~3-6 Msun BHs with rotation parameters of A~0.8 and tori of 0.03-0.3
Msun. Our nucleosynthesis analysis includes the dynamical (prompt) ejecta
expelled during the CBM phase and the neutrino and viscously driven outflows of
the relic BH-torus systems. While typically ~20-25% of the initial
accretion-torus mass are lost by viscously driven outflows, neutrino-powered
winds contribute at most another ~1%, but neutrino heating enhances the viscous
ejecta significantly. Since BH-torus ejecta possess a wide distribution of
electron fractions (0.1-0.6) and entropies, they produce heavy elements from
A~80 up to the actinides, with relative contributions of A>130 nuclei being
subdominant and sensitively dependent on BH and torus masses and the exact
treatment of shear viscosity. The combined ejecta of CBM and BH-torus phases
can reproduce the solar abundances amazingly well for A>90. Varying
contributions of the torus ejecta might account for observed variations of
lighter elements with 40<Z<56 relative to heavier ones, and a considerable
reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly
asymmetric NS-BH mergers, might explain the composition of heavy-element
deficient stars.Comment: 7 pages, 4 figures, only changed title compared to previous version,
accepted for publication in Proceedings of Science (Nuclei in the Cosmos
XIII, Debrecen
NADA-FLD: A General Relativistic, Multi-dimensional Neutrino-hydrodynamics Code Employing Flux-limited Diffusion
We present the new code NADA-FLD to solve multi-dimensional
neutrino-hydrodynamics in full general relativity (GR) in spherical polar
coordinates. The energy-dependent neutrino transport assumes the flux-limited
diffusion (FLD) approximation and evolves the neutrino energy densities
measured in the frame comoving with the fluid. Operator splitting is used to
avoid multi-dimensional coupling of grid cells in implicit integration steps
involving matrix inversions. Terms describing lateral diffusion and advection
are integrated explicitly using the Allen-Cheng or the Runge-Kutta-Legendre
method, which remain stable even in the optically thin regime. We discuss
several toy-model problems in one and two dimensions to test the basic
functionality and individual components of the transport scheme. We also
perform fully dynamic core-collapse supernova (CCSN) simulations in spherical
symmetry. For a Newtonian model we find good agreement with the M1 code ALCAR,
and for a GR model we reproduce the main effects of GR in CCSNe already found
by previous works.Comment: 28 pages, 11 figures; accepted by MNRAS; supplemented by RKL method,
resolution tests, and appendix with detailed derivations of FLD flux in
response to referee comment
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