3,750 research outputs found
Astromaterial Science and Nuclear Pasta
We define `astromaterial science' as the study of materials in astronomical
objects that are qualitatively denser than materials on earth. Astromaterials
can have unique properties related to their large density, though they may be
organized in ways similar to more conventional materials. By analogy to
terrestrial materials, we divide our study of astromaterials into hard and soft
and discuss one example of each. The hard astromaterial discussed here is a
crystalline lattice, such as the Coulomb crystals in the interior of cold white
dwarfs and in the crust of neutron stars, while the soft astromaterial is
nuclear pasta found in the inner crusts of neutron stars. In particular, we
discuss how molecular dynamics simulations have been used to calculate the
properties of astromaterials to interpret observations of white dwarfs and
neutron stars. Coulomb crystals are studied to understand how compact stars
freeze. Their incredible strength may make crust "mountains" on rotating
neutron stars a source for gravitational waves that the Laser Interferometer
Gravitational-Wave Observatory (LIGO) may detect. Nuclear pasta is expected
near the base of the neutron star crust at densities of g/cm.
Competition between nuclear attraction and Coulomb repulsion rearranges
neutrons and protons into complex non-spherical shapes such as sheets (lasagna)
or tubes (spaghetti). Semi-classical molecular dynamics simulations of nuclear
pasta have been used to study these phases and calculate their transport
properties such as neutrino opacity, thermal conductivity, and electrical
conductivity. Observations of neutron stars may be sensitive to these
properties, and can be be used to interpret observations of supernova
neutrinos, magnetic field decay, and crust cooling of accreting neutron stars.
We end by comparing nuclear pasta shapes with some similar shapes seen in
biological systems.Comment: 16 pages, 7 figures, added references and revised for clarity,
Reviews of Modern Physics in pres
The Elasticity of Nuclear Pasta
The elastic properties of neutron star crusts are relevant for a variety of
currently observable or near-future electromagnetic and gravitational wave
phenomena. These phenomena may depend on the elastic properties of nuclear
pasta found in the inner crust. We present large scale classical molecular
dynamics simulations where we deform nuclear pasta. We simulate idealized
samples of nuclear pasta and describe their breaking mechanism. We also deform
nuclear pasta that is arranged into many domains, similar to what is known for
the ions in neutron star crusts. Our results show that nuclear pasta may be the
strongest known material, perhaps with a shear modulus of
and breaking strain greater than 0.1.Comment: 5 pages, 2 figures. Submitted to Physical Review Letter
Thermal Fluctuations in Nuclear Pasta
Despite their astrophysical relevance, nuclear pasta phases are relatively
unstudied at high temperatures. We present molecular dynamics simulations of
symmetric nuclear matter with several topologies of `lasagna' at a range of
temperatures to study the pasta-uniform transition. Using the Minkowski
functionals we quantify trends in the occupied volume, surface area, mean
breadth, and Euler characteristic. The amplitude of surface displacements of
the pasta increase with temperature which produce short lived topological
defects such as holes and filaments near melting, resulting in power laws for
increasing surface curvature with temperature. We calculate the static
structure factor and report the shear viscosity and thermal conductivity of
pasta, finding that the shear viscosity is minimized at the melting
temperature. These results may have implications for the thermoelastic
properties of nuclear pasta and finite temperature corrections to the equation
of state at pasta densities.Comment: 12 pages, 8 figure
Pasta Nucleosynthesis: Molecular dynamics simulations of nuclear statistical equilibrium
Background: Exotic non-spherical nuclear pasta shapes are expected in nuclear
matter at just below saturation density because of competition between short
range nuclear attraction and long range Coulomb repulsion. Purpose: We explore
the impact of nuclear pasta on nucleosynthesis, during neutron star mergers, as
cold dense nuclear matter is ejected and decompressed. Methods: We perform
classical molecular dynamics simulations with 51200 and 409600 nucleons, that
are run on GPUs. We expand our simulation region to decompress systems from an
initial density of 0.080 fm^{-3} down to 0.00125 fm^{-3}. We study proton
fractions of Y_P=0.05, 0.10, 0.20, 0.30, and 0.40 at T =0.5, 0.75, and 1.0 MeV.
We calculate the composition of the resulting systems using a cluster
algorithm. Results: We find final compositions that are in good agreement with
nuclear statistical equilibrium models for temperatures of 0.75 and 1 MeV.
However, for proton fractions greater than Y_P=0.2 at a temperature of T = 0.5
MeV, the MD simulations produce non-equilibrium results with large rod-like
nuclei. Conclusions: Our MD model is valid at higher densities than simple
nuclear statistical equilibrium models and may help determine the initial
temperatures and proton fractions of matter ejected in mergers.Comment: 13 page
Simulating the Novel Phase Separation of a Rapid Proton Capture Ash Composition
Nucleosynthesis in the oceans of accreting neutron stars can produce novel
mixtures of nuclides, whose composition is dependent on the exact astrophysical
conditions. Many simulations have now been done to determine the
nucleosynthesis products in the ocean, but the phase separation at the base of
the ocean, which determines the composition of the crust, has not been as well
studied. In this work, we simulate the phase separation of a composition, which
was predicted to produce a crust enriched in light nuclei, in contrast with
past work which predicts that crust is enriched in heavy nuclei. We perform
molecular dynamics simulations of the phase separation of this mixture using
the methods of Horowitz (2007). We find good agreement with
the predictions of Mckinven (2016) for the phase separation
of this mixture. Moreover, this supports their method as a computationally
efficient alternative to molecular dynamics for calculating phase separation
for a wider regime of astrophysical conditions.Comment: 8 pages, 5 figures, Submitted to Phys. Rev.
Nuclear Waffles
The dense neutron-rich matter found in supernovae and neutron stars is
expected to form complex nonuniform phases referred to as nuclear pasta. The
pasta shapes depend on density, temperature and proton fraction and determine
many transport properties in supernovae and neutron star crusts. We use two
recently developed hybrid CPU/GPU codes to perform large scale molecular
dynamics (MD) simulations with and nucleons of nuclear pasta.
From the output of the MD simulations we characterize the topology and compute
two observables, the radial distribution function and the structure
factor , for systems with proton fractions and
at about one third of nuclear saturation density and temperatures near
MeV. We observe that the two lowest proton fraction systems simulated,
and , equilibrate quickly and form liquid-like structures.
Meanwhile, the two higher proton fraction systems, and , take
a longer time to equilibrate and organize themselves in solid-like periodic
structures. Furthermore, the system is made up of slabs, lasagna
phase, interconnected by defects while the systems consist of a
stack of perforated plates, the nuclear waffle phase. The periodic
configurations observed in our MD simulations for proton fractions
have important consequences for the structure factors of protons and
neutrons, which relate to many transport properties of supernovae and neutron
star crust. A detailed study of the waffle phase and how its structure depends
on temperature, size of the simulation and the screening length showed that
finite-size effects appear to be under control and, also, that the plates in
the waffle phase merge at temperatures slightly above MeV and the holes
in the plates form an hexagonal lattice at temperatures slightly lower than
MeV.Comment: 16 pages, 12 figires, 6 tables, small changes with respect to
previous version, Phys Rev C in pres
Actinide crystallization and fission reactions in cooling white dwarf stars
The first solids that form as a cooling white dwarf (WD) starts to
crystallize are expected to be greatly enriched in actinides. This is because
the melting points of WD matter scale as and actinides have the
largest charge . We estimate that the solids may be so enriched in actinides
that they could support a fission chain reaction. This reaction could ignite
carbon burning and lead to the explosion of an isolated WD in a thermonuclear
supernova (SN Ia). Our mechanism could potentially explain SN Ia with
sub-Chandrasekhar ejecta masses and short delay times.Comment: 8 pages, 6 figures total including Appendix, Phys. Rev. Let. in pres
Disordered nuclear pasta, magnetic field decay, and crust cooling in neutron stars
Nuclear pasta, with non-spherical shapes, is expected near the base of the
crust in neutron stars. Large scale molecular dynamics simulations of pasta
show long lived topological defects that could increase electron scattering and
reduce both the thermal and electrical conductivities. We model a possible low
conductivity pasta layer by increasing an impurity parameter Q_{imp}.
Predictions of light curves for the low mass X-ray binary MXB 1659-29, assuming
a large Q_{imp}, find continued late time cooling that is consistent with
Chandra observations. The electrical and thermal conductivities are likely
related. Therefore observations of late time crust cooling can provide insight
on the electrical conductivity and the possible decay of neutron star magnetic
fields (assuming these are supported by currents in the crust).Comment: 5 pages, 2 figure
Parking-garage structures in astrophysics and biophysics
A striking shape was recently observed for the cellular organelle endoplasmic
reticulum consisting of stacked sheets connected by helical ramps. This shape
is interesting both for its biological function, to synthesize proteins using
an increased surface area for ribosome factories, and its geometric properties
that may be insensitive to details of the microscopic interactions. In the
present work, we find very similar shapes in our molecular dynamics simulations
of the nuclear pasta phases of dense nuclear matter that are expected deep in
the crust of neutron stars. There are dramatic differences between nuclear
pasta and terrestrial cell biology. Nuclear pasta is 14 orders of magnitude
denser than the aqueous environs of the cell nucleus and involves strong
interactions between protons and neutrons, while cellular scale biology is
dominated by the entropy of water and complex assemblies of biomolecules.
Nonetheless the very similar geometry suggests both systems may have similar
coarse-grained dynamics and that the shapes are indeed determined by
geometrical considerations, independent of microscopic details. Many of our
simulations self-assemble into flat sheets connected by helical ramps. These
ramps may impact the thermal and electrical conductivities, viscosity, shear
modulus, and breaking strain of neutron star crust. The interaction we use,
with Coulomb frustration, may provide a simple model system that reproduces
many biologically important shapes.Comment: 5 pages, 3 figure
Nuclear fission chain reaction in cooling white dwarf stars
The first solids that form as a white dwarf (WD) starts to crystallize are
expected to be greatly enriched in actinides. Previously [PRL 126, 1311010] we
found that these solids might support a nuclear fission chain reaction that
could ignite carbon burning and provide a new Type Ia supernova (SN Ia)
mechanism involving an {\it isolated} WD. Here we explore this fission
mechanism in more detail and calculate the final temperature and density after
the chain reaction and discuss a number of open physics questions.Comment: 8 pages, 4 figure
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