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C. J. Horowitz

Publications and source records attributed to C. J. Horowitz.

At least 73 records · Page 4Linked to original sources
arXiv2017

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 $10^{14}$ g/cm$^3$. 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.

astro-ph.HE↗
arXiv2017

Quantum Nuclear Pasta and Nuclear Symmetry Energy

Complex and exotic nuclear geometries are expected to appear naturally in dense nuclear matter found in the crust of neutron stars and supernovae environment collectively referred to as nuclear pasta. The pasta geometries depend on the average baryon density, proton fraction and temperature and are critically important in the determination of many transport properties of matter in supernovae and the crust of neutron stars. Using a set of self-consistent microscopic nuclear energy density functionals we present the first results of large scale quantum simulations of pasta phases at baryon densities $0.03 \leq ρ\leq 0.10$ fm$^{-3}$, proton fractions $0.05 \leq Y_p \leq 0.40$, and zero temperature. The full quantum simulations, in particular, allow us to thoroughly investigate the role and impact of the nuclear symmetry energy on pasta configurations. We use the Sky3D code that solves the Skyrme Hartree-Fock equations on a three-dimensional Cartesian grid. For the nuclear interaction we use the state of the art UNEDF1 parametrization, which was introduced to study largely deformed nuclei, hence is suitable for studies of the nuclear pasta. Density dependence of the nuclear symmetry energy is simulated by tuning two purely isovector observables that are insensitive to the current available experimental data. We find that a minimum total number of nucleons $A=2000$ is necessary to prevent the results from containing spurious shell effects and to minimize finite size effects. We find that a variety of nuclear pasta geometries are present in the neutron star crust and the result strongly depends on the nuclear symmetry energy. The impact of the nuclear symmetry energy is less pronounced as the proton fractions increase. Quantum nuclear pasta calculations at $T=0$ MeV are shown to get easily trapped in meta-stable states, and possible remedies to avoid meta-stable solutions are discussed.

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arXiv2016

Nuclear pasta and supernova neutrinos at late times

Nuclear pasta, with nucleons arranged into tubes, sheets, or other complex shapes, is expected in core collapse supernovae (SNe) at just below nuclear density. We calculate the additional opacity from neutrino-pasta coherent scattering using molecular dynamics simulations. We approximately include this opacity in simulations of SNe. We find that pasta slows neutrino diffusion and greatly increases the neutrino signal at late times of 10 or more seconds after stellar core collapse. This signal, for a galactic SN, should be clearly visible in large detectors such as Super-Kamiokande.

astro-ph.HE↗
arXiv2016

Neutrino-nucleon scattering in supernova matter from the virial expansion

We generalize our virial approach to study the neutral current neutrino response of nuclear matter at low densities. In the long-wavelength limit, the virial expansion makes model-independent predictions for neutrino-nucleon scattering rates and the density S_V and spin S_A responses. We find S_A is significantly reduced from one even at low densities. We provide a simple fit S_A^f(n,T,Y_p) of the axial response as a function of density n, temperature T and proton fraction Y_p. This fit reproduces our model independent virial results at low densities and reproduces the Burrows and Sawyer random phase approximation (RPA) results at high densities. Our fit can be incorporated into supernova simulations in a straight forward manner. Preliminary one dimensional supernova simulations suggest that the reduction in the axial response may enhance neutrino heating rates in the gain region during the accretion phase of a core-collapse supernovae.

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arXiv2016

A lower limit on the heat capacity of the neutron star core

We show that observations of the core temperature of transiently-accreting neutron stars combined with observations of an accretion outburst give a lower limit to the neutron star core heat capacity. For the neutron stars in the low mass X-ray binaries KS 1731-260, MXB 1659-29, and XTE J1701-462, we show that the lower limit is a factor of a few below the core heat capacity expected if neutrons and protons in the core are paired, so that electrons provide the dominant contribution to the heat capacity. This limit rules out a core dominated by a quark color-flavor-locked (CFL) phase, which would have a much lower heat capacity. Future observations of or limits on cooling during quiescence will further constrain the core heat capacity.

astro-ph.HE↗
arXiv2016

Enhancement of fusion at near and sub-barrier energies for neutron-rich light nuclei

Measurement of the fusion cross-section for neutron-rich light nuclei is crucial in ascertaining if fusion of these nuclei occurs in the outer crust of a neutron star. We have therefore measured the fusion excitation function at near-barrier energies for the 19O + 12C system and compared the experimental results with the fusion excitation function of 18O + 12C and 16O + 12C. In the experiment a beam of 19O, produced via the 18O(d,p) reaction, was incident on a 12C target at energies near the Coulomb barrier. Evaporation residues produced in fusion of 18,19O ions with 12C target nuclei were detected with good geometric efficiency and identified by measuring their energy and time-of-flight. A significant enhancement in the fusion probability of 19O ions with a 12C target as compared to 18O ions is observed. The significantly larger cross-sections observed at near barrier energies are not predicted by a static model of fusion for 19O + 12C indicating that dynamics play an important role in the fusion of neutron-rich light nuclei.

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arXiv2016

Quantum Simulations of Nuclei and Nuclear Pasta with the Multi-resolution Adaptive Numerical Environment for Scientific Simulations

Neutron star and supernova matter at densities just below the nuclear matter saturation density is expected to form a lattice of exotic shapes. These so-called nuclear pasta phases are caused by Coulomb frustration. Their elastic and transport properties are believed to play an important role for thermal and magnetic field evolution, rotation and oscillation of neutron stars. Furthermore, they can impact neutrino opacities in core-collapse supernovae. In this work, we present proof-of-principle 3D Skyrme Hartree-Fock (SHF) simulations of nuclear pasta with the Multi-resolution ADaptive Numerical Environment for Scientific Simulations (MADNESS). We perform benchmark studies of $^{16} \mathrm{O}$, $^{208} \mathrm{Pb}$ and $^{238} \mathrm{U}$ nuclear ground states and calculate binding energies via 3D SHF simulations. Results are compared with experimentally measured binding energies as well as with theoretically predicted values from an established SHF code. The nuclear pasta simulation is initialized in the so-called waffle geometry as obtained by the Indiana University Molecular Dynamics (IUMD) code. The size of the unit cell is 24\:fm with an average density of about $ρ= 0.05 \:\mathrm{fm}^{-3}$, proton fraction of $Y_p = 0.3$ and temperature of $T=0\:$MeV. Our calculations reproduce the binding energies and shapes of light and heavy nuclei with different geometries. For the pasta simulation, we find that the final geometry is very similar to the initial waffle state. In the present pasta calculations spin-orbit forces are not included but will be added in the future. Within the MADNESS framework, we can successfully perform calculations of inhomogeneous nuclear matter. By using pasta configurations from IUMD it is possible to explore different geometries and test the impact of self-consistent calculations on the latter.

astro-ph.SR↗
arXiv2016

Effect of topological defects on "nuclear pasta" observables

[Background] The "pasta" phase of nuclear matter may play an important role in the structure and evolution of neutron stars. Recent works suggest nuclear pasta has a high resistivity which could be explained by the presence of long lived topological defects. The defects act as impurities that decrease thermal and electrical conductivity of the pasta. [Purpose] To quantify how topological defects affect transport properties of nuclear pasta and estimate this effect using an impurity parameter $Q_{\text{imp}}$. [Methods] Contrast molecular dynamics simulations of up to 409\,600 nucleons arranged in parallel nuclear pasta slabs (perfect pasta) with simulations of pasta slabs connected by topological defects (impure pasta). From these simulations compare the viscosity and heat conductivity of perfect and impure pasta to obtain an effective impurity parameter $Q_{\text{imp}}$ due to the presence of defects. [Results] Both the viscosity and thermal conductivity calculated for both perfect and impure pasta are anisotropic, peaking along directions perpendicular to the slabs and reaching a minimum close to zero parallel to them. In our 409\,600 nucleon simulation topological defects connecting slabs of pasta reduce both the thermal conductivity and viscosity on average by about 37\%. We estimate an effective impurity parameter due to the defects of order $Q_{\text{imp}}\sim30$. [Conclusions] Topological defects in the pasta phase of nuclear matter have an effect similar to impurities in a crystal lattice. The irregularities introduced by the defects reduce the thermal and electrical conductivities and the viscosity of the system. This effect can be parameterized by a large impurity parameter $Q_{\text{imp}}\sim30$.

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arXiv2015

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.

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arXiv2015

The full weak charge density distribution of 48Ca from parity violating electron scattering

Background: The ground state neutron density of a medium mass nucleus contains fundamental nuclear structure information and is at present relatively poorly known. Purpose: We explore if parity violating elastic electron scattering can provide a feasible and model independent way to determine not just the neutron radius but the full radial shape of the neutron density $ρ_n(r)$ and the weak charge density $ρ_W(r)$ of a nucleus. Methods: We expand the weak charge density of $^{48}$Ca in a model independent Fourier Bessel series and calculate the statistical errors in the individual coefficients that might be obtainable in a model parity violating electron scattering experiment. Results: We find that it is feasible to determine roughly six Fourier Bessel coefficients of the weak charge density of 48Ca within a reasonable amount of beam time. However, it would likely be much harder to determine the full weak density of a significantly heavier nucleus such as 208Pb. Conclusions: Parity violating elastic electron scattering can determine the full weak charge density of a medium mass nucleus in a model independent way. This weak density contains fundamental information on the size, surface thickness, shell oscillations, and saturation density of the neutron distribution in a nucleus. The measured $ρ_W(r)$, combined with the previously known charge density $ρ_{ch}(r)$, will literally provide a detailed textbook picture of where the neutrons and protons are located in an atomic nucleus.

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arXiv2015

Swelling of nuclei embedded in neutron-gas and consequences for fusion

Fusion of very neutron rich nuclei may be important to determine the composition and heating of the crust of accreting neutron stars. We present an exploratory study of the effect of the neutron-gas environment on the structure of nuclei and the consequences for pycnonuclear fusion cross-sections in the neutron drip region. We studied the formation and properties of Oxygen and Calcium isotopes embedded in varying neutron-gas densities. We observe that the formed isotope is the drip-line nucleus for the given effective interaction. Increasing the neutron-gas density leads to the swelling of the nuclear density. We have used these densities to study the effect of this swelling on the fusion cross-sections using the São-Paulo potential. At high neutron-gas densities the cross-section is substantially increased but at lower densities the modification is minimal.

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arXiv2014

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.

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arXiv2014

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 $51200$ and $409600$ nucleons of nuclear pasta. From the output of the MD simulations we characterize the topology and compute two observables, the radial distribution function $g(r)$ and the structure factor $S(q)$, for systems with proton fractions $Y_p=0.10, 0.20, 0.30$ and $0.40$ at about one third of nuclear saturation density and temperatures near $1.0$ MeV. We observe that the two lowest proton fraction systems simulated, $Y_p=0.10$ and $0.20$, equilibrate quickly and form liquid-like structures. Meanwhile, the two higher proton fraction systems, $Y_p=0.30$ and $0.40$, take a longer time to equilibrate and organize themselves in solid-like periodic structures. Furthermore, the $Y_p=0.40$ system is made up of slabs, lasagna phase, interconnected by defects while the $Y_p=0.30$ systems consist of a stack of perforated plates, the nuclear waffle phase. The periodic configurations observed in our MD simulations for proton fractions $Y_p\ge0.30$ have important consequences for the structure factors $S(q)$ 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 $1.0$ MeV and the holes in the plates form an hexagonal lattice at temperatures slightly lower than $1.0$ MeV.

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arXiv2014

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).

astro-ph.HE↗
arXiv2014

Parity violating elastic electron scattering from 27Al and the Qweak measurement

I calculate the parity violating asymmetry A_{pv} for elastic electron scattering from 27Al in order to compare with the Qweak experiment "background" 27Al measurement. I find that Coulomb distortions, and the quadrupole form factor, reduce A_{pv} near the diffraction minimum. The Qweak data can be used to confirm the neutron radius of 27Al, if nuclear structure uncertainties are indeed small, as I suggest, and one can estimate inelastic and impurity contributions. This could provide an important check of the measurement, analysis, and theory.

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arXiv2014

Pulsar Glitches: The Crust may be Enough

Pulsar glitches-the sudden spin-up in the rotational frequency of a neutron star-suggest the existence of an angular-momentum reservoir confined to the inner crust of the neutron star. Large and regular glitches observed in the Vela pulsar have originally constrained the fraction of the stellar moment of inertia that must reside in the solid crust to about 1.4%. However, crustal entrainment-which until very recently has been ignored-suggests that in order to account for the Vela glitches, the fraction of the moment of inertia residing in the crust must increase to about 7%. This indicates that the required angular momentum reservoir may exceed that which is available in the crust. We explore the possibility that uncertainties in the equation of state provide enough flexibility for the construction of models that predict a large crustal thickness and consequently a large crustal moment of inertia. Given that analytic results suggest that the crustal moment of inertia is sensitive to the transition pressure at the crust-core interface, we tune the parameters of the model to maximize the transition pressure, while still providing an excellent description of nuclear observables. In this manner we are able to obtain fractional moments of inertia as large as 7% for neutron stars with masses below 1.6 solar masses. In particular, we find that if the neutron-skin thickness of 208Pb falls within the (0.20-0.26) fm range, large enough transition pressures can be generated to explain the large Vela glitches without invoking an additional angular-momentum reservoir beyond that confined to the solid crust. Our results suggest that the crust may be enough.

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arXiv2014

Weak charge form factor and radius of 208Pb through parity violation in electron scattering

We use distorted wave electron scattering calculations to extract the weak charge form factor F_W(q), the weak charge radius R_W, and the point neutron radius R_n, of 208Pb from the PREX parity violating asymmetry measurement. The form factor is the Fourier transform of the weak charge density at the average momentum transfer q=0.475 fm$^{-1}$. We find F_W(q) =0.204 \pm 0.028 (exp) \pm 0.001 (model). We use the Helm model to infer the weak radius from F_W(q). We find R_W= 5.826 \pm 0.181 (exp) \pm 0.027 (model) fm. Here the exp error includes PREX statistical and systematic errors, while the model error describes the uncertainty in R_W from uncertainties in the surface thickness σof the weak charge density. The weak radius is larger than the charge radius, implying a "weak charge skin" where the surface region is relatively enriched in weak charges compared to (electromagnetic) charges. We extract the point neutron radius R_n=5.751 \pm 0.175 (exp) \pm 0.026 (model) \pm 0.005 (strange) fm$, from R_W. Here there is only a very small error (strange) from possible strange quark contributions. We find R_n to be slightly smaller than R_W because of the nucleon's size. Finally, we find a neutron skin thickness of R_n-R_p=0.302\pm 0.175 (exp) \pm 0.026 (model) \pm 0.005 (strange) fm, where R_p is the point proton radius.

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