369 research outputs found
r-Process Lanthanide Production and Heating Rates in Kilonovae
r-Process nucleosynthesis in material ejected during neutron star mergers may
lead to radioactively powered transients called kilonovae. The timescale and
peak luminosity of these transients depend on the composition of the ejecta,
which determines the local heating rate from nuclear decays and the opacity.
Kasen et al. (2013, ApJ, 774, 25) and Tanaka & Hotokezaka (2013, ApJ, 775, 113)
pointed out that lanthanides can drastically increase the opacity in these
outflows. We use the new general-purpose nuclear reaction network SkyNet to
carry out a parameter study of r-process nucleosynthesis for a range of initial
electron fractions , initial specific entropies , and expansion
timescales . We find that the ejecta is lanthanide-free for , depending on and . The heating rate is insensitive to
and , but certain, larger values of lead to reduced heating
rates, due to individual nuclides dominating the heating. We calculate
approximate light curves with a simplified gray radiative transport scheme. The
light curves peak at about a day (week) in the lanthanide-free (-rich) cases.
The heating rate does not change much as the ejecta becomes lanthanide-free
with increasing , but the light curve peak becomes about an order of
magnitude brighter because it peaks much earlier when the heating rate is
larger. We also provide parametric fits for the heating rates between 0.1 and
, and we provide a simple fit in , , and to
estimate whether the ejecta is lanthanide-rich or not.Comment: 19 pages, 9 figure
SkyNet: A modular nuclear reaction network library
Almost all of the elements heavier than hydrogen that are present in our
solar system were produced by nuclear burning processes either in the early
universe or at some point in the life cycle of stars. In all of these
environments, there are dozens to thousands of nuclear species that interact
with each other to produce successively heavier elements. In this paper, we
present SkyNet, a new general-purpose nuclear reaction network that evolves the
abundances of nuclear species under the influence of nuclear reactions. SkyNet
can be used to compute the nucleosynthesis evolution in all astrophysical
scenarios where nucleosynthesis occurs. SkyNet is free and open-source and aims
to be easy to use and flexible. Any list of isotopes can be evolved and SkyNet
supports various different types of nuclear reactions. SkyNet is modular so
that new or existing physics, like nuclear reactions or equations of state, can
easily be added or modified. Here, we present in detail the physics implemented
in SkyNet with a focus on a self-consistent transition to and from nuclear
statistical equilibrium (NSE) to non-equilibrium nuclear burning, our
implementation of electron screening, and coupling of the network to an
equation of state. We also present comprehensive code tests and comparisons
with existing nuclear reaction networks. We find that SkyNet agrees with
published results and other codes to an accuracy of a few percent.
Discrepancies, where they exist, can be traced to differences in the physics
implementations.Comment: 39 pages, 11 figures, published in ApJ Supplement Serie
Medium modification of the charged current neutrino opacity and its implications
Previous work on neutrino emission from proto-neutron stars which employed
full solutions of the Boltzmann equation showed that the average energies of
emitted electron neutrinos and antineutrinos are closer to one another than
predicted by older, more approximate work. This in turn implied that the
neutrino driven wind is proton rich during its entire life, precluding
-process nucleosynthesis and the synthesis of Sr, Y, and Zr. This work
relied on charged current neutrino interaction rates that are appropriate for a
free nucleon gas. Here, it is shown in detail that the inclusion of the nucleon
potential energies and collisional broadening of the response significantly
alters this conclusion. Iso-vector interactions, which give rise to the nuclear
symmetry energy, produce a difference between the neutron and proton
single-particle energies and alter the kinematics of the
charged current reactions. In neutron-rich matter, and for a given
neutrino/antineutrino energy, the rate for is
enhanced while is suppressed because the
value for these reactions is altered by , respectively. In the
neutrino decoupling region, collisional broadening acts to enhance both
and cross-sections and RPA corrections decrease the
cross-section and increase the cross-section, but mean field
shifts have a larger effect. Therefore, electron neutrinos decouple at lower
temperature than when the nucleons are assumed to be free and have lower
average energies. The change is large enough to allow for a reasonable period
of time when the neutrino driven wind is predicted to be neutron rich. It is
also shown that the electron fraction in the wind is influenced by the nuclear
symmetry energy.Comment: Version submitted to PRC, 10 pages, 6 figures (Additional discussion
of RPA effects added
Wave heating from proto-neutron star convection and the core-collapse supernova explosion mechanism
Our understanding of the core-collapse supernova explosion mechanism is incomplete. While the favoured scenario is delayed revival of the stalled shock by neutrino heating, it is difficult to reliably compute explosion outcomes and energies, which depend sensitively on the complex radiation hydrodynamics of the post-shock region. The dynamics of the (non-)explosion depend sensitively on how energy is transported from inside and near the proto-neutron star (PNS) to material just behind the supernova shock. Although most of the PNS energy is lost in the form of neutrinos, hydrodynamic and hydromagnetic waves can also carry energy from the PNS to the shock. We show that gravity waves excited by core PNS convection can couple with outgoing acoustic waves that present an appreciable source of energy and pressure in the post-shock region. Using one-dimensional simulations, we estimate the gravity wave energy flux excited by PNS convection and the fraction of this energy transmitted upwards to the post-shock region as acoustic waves. We find wave energy fluxes near 10⁵¹ergs⁻¹ are likely to persist for ∼1s post-bounce. The wave pressure on the shock may exceed 10 per cent of the thermal pressure, potentially contributing to shock revival and, subsequently, a successful and energetic explosion. We also discuss how future simulations can better capture the effects of waves, and more accurately quantify wave heating rates
Probing Extreme-Density Matter with Gravitational Wave Observations of Binary Neutron Star Merger Remnants
We present a proof-of-concept study, based on numerical-relativity
simulations, of how gravitational waves (GWs) from neutron star merger remnants
can probe the nature of matter at extreme densities. Phase transitions and
extra degrees of freedom can emerge at densities beyond those reached during
the inspiral, and typically result in a softening of the equation of state
(EOS). We show that such physical effects change the qualitative dynamics of
the remnant evolution, but they are not identifiable as a signature in the GW
frequency, with the exception of possible black-hole formation effects. The EOS
softening is, instead, encoded in the GW luminosity and phase and is in
principle detectable up to distances of the order of several Mpcs with advanced
detectors and up to hundreds of Mpcs with third generation detectors. Probing
extreme-density matter will require going beyond the current paradigm and
developing a more holistic strategy for modeling and analyzing postmerger GW
signals.Comment: 5 pages, 3 figures. Matches version accepted on ApJ
Dynamical Mass Ejection from Binary Neutron Star Mergers
We present fully general-relativistic simulations of binary neutron star
mergers with a temperature and composition dependent nuclear equation of state.
We study the dynamical mass ejection from both quasi-circular and
dynamical-capture eccentric mergers. We systematically vary the level of our
treatment of the microphysics to isolate the effects of neutrino cooling and
heating and we compute the nucleosynthetic yields of the ejecta. We find that
eccentric binaries can eject significantly more material than quasi-circular
binaries and generate bright infrared and radio emission. In all our
simulations the outflow is composed of a combination of tidally- and
shock-driven ejecta, mostly distributed over a broad angle from
the orbital plane, and, to a lesser extent, by thermally driven winds at high
latitudes. Ejecta from eccentric mergers are typically more neutron rich than
those of quasi-circular mergers. We find neutrino cooling and heating to
affect, quantitatively and qualitatively, composition, morphology, and total
mass of the outflows. This is also reflected in the infrared and radio
signatures of the binary. The final nucleosynthetic yields of the ejecta are
robust and insensitive to input physics or merger type in the regions of the
second and third r-process peaks. The yields for elements on the first peak
vary between our simulations, but none of our models is able to explain the
Solar abundances of first-peak elements without invoking additional first-peak
contributions from either neutrino and viscously-driven winds operating on
longer timescales after the mergers, or from core-collapse supernovae.Comment: 19 pages, 10 figures. We corrected a problem in the formulation of
the neutrino heating scheme and re-ran all of the affected models. The main
conclusions are unchanged. This version also contains one more figure and a
number of improvements on the tex
General Relativistic Three-Dimensional Multi-Group Neutrino Radiation-Hydrodynamics Simulations of Core-Collapse Supernovae
We report on a set of long-term general-relativistic three-dimensional (3D)
multi-group (energy-dependent) neutrino-radiation hydrodynamics simulations of
core-collapse supernovae. We employ a full 3D two-moment scheme with the local
M1 closure, three neutrino species, and 12 energy groups per species. With
this, we follow the post-core-bounce evolution of the core of a nonrotating
- progenitor in full unconstrained 3D and in octant symmetry for
. We find the development of an asymmetric runaway
explosion in our unconstrained simulation. We test the resolution dependence of
our results and, in agreement with previous work, find that low resolution
artificially aids explosion and leads to an earlier runaway expansion of the
shock. At low resolution, the octant and full 3D dynamics are qualitatively
very similar, but at high resolution, only the full 3D simulation exhibits the
onset of explosion.Comment: Accepted to Ap
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