Astromaterial Science and Nuclear Pasta [HEAP]

The heavens contain a variety of materials that range from conventional to extraordinary and extreme. For this colloquium, 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 density, such as extraordinary mechanical strength, or alternatively be organized in ways similar to more conventional materials. The study of astromaterials may suggest ways to improve terrestrial materials. Likewise, advances in the science of conventional materials may allow new insights into astromaterials. We discuss Coulomb crystals in the interior of cold white dwarfs and in the crust of neutron stars and review the limited observations of how stars freeze. We apply astromaterial science to the generation of gravitational waves. According to Einstein’s Theory of General Relativity accelerating masses radiate gravitational waves. However, very strong materials may be needed to vigorously accelerate large masses in order to produce continuous gravitational waves that are observable in present detectors. We review large-scale molecular dynamics simulations of the breaking stress of neutron star crust that suggest it is the strongest material known, some ten billion times stronger than steel. Nuclear pasta is an example of a soft astromaterial. It is expected near the base of the neutron star crust at densities of ten to the fourteen grams per cubic centimeter. Competition between nuclear attraction and Coulomb repulsion rearrange neutrons and protons into complex non-spherical shapes such as flat plates (lasagna) or thin rods (spaghetti). We review semi-classical molecular dynamics simulations of nuclear pasta. We illustrate some of the shapes that are possible and discuss transport properties including shear viscosity and thermal and electrical conductivities.

Read this paper on arXiv…

M. Caplan and C. Horowitz
Tue, 14 Jun 16

Comments: 13 pages, 7 figures

The sequence to hydrogenate coronene cations: A journey guided by magic numbers [CL]

The understanding of hydrogen attachment to carbonaceous surfaces is essential to a wide variety of research fields and technologies such as hydrogen storage for transportation, precise localization of hydrogen in electronic devices and the formation of cosmic H2. For coronene cations as prototypical Polycyclic Aromatic Hydrocarbon (PAH) molecules, the existence of magic numbers upon hydrogenation was uncovered experimentally. Quantum chemistry calculations show that hydrogenation follows a site-specific sequence leading to the appearance of cations having 5, 11, or 17 hydrogen atoms attached, exactly the magic numbers found in the experiments. For these closed-shell cations, further hydrogenation requires appreciable structural changes associated with a high transition barrier. Controlling specific hydrogenation pathways would provide the possibility to tune the location of hydrogen attachment and the stability of the system. The sequence to hydrogenate PAHs, leading to PAHs with magic numbers of H atoms attached, provides clues to understand that carbon in space is mostly aromatic and partially aliphatic in PAHs. PAH hydrogenation is fundamental to assess the contribution of PAHs to the formation of cosmic H2.

Read this paper on arXiv…

S. Cazaux, L. Boschman, N. Rougeau, et. al.
Wed, 9 Mar 16

Comments: Published in Scientific reports from Nature publishing group

A Preliminary Jupiter Model [EPA]

In anticipation of new observational results for Jupiter’s axial moment of inertia and gravitational zonal harmonic coefficients from the forthcoming Juno orbiter, we present a number of preliminary Jupiter interior models. We combine results from ab initio computer simulations of hydrogen-helium mixtures, including immiscibility calculations, with a new nonperturbative calculation of Jupiter’s zonal harmonic coefficients, to derive a self-consistent model for the planet’s external gravity and moment of inertia. We assume helium rain modified the interior temperature and composition profiles. Our calculation predicts zonal harmonic values to which measurements can be compared. Although some models fit the observed (pre-Juno) second- and fourth-order zonal harmonics to within their error bars, our preferred reference model predicts a fourth-order zonal harmonic whose absolute value lies above the pre-Juno error bars. This model has a dense core of about 12 Earth masses, and a hydrogen-helium-rich envelope with approximately 3 times solar metallicity.

Read this paper on arXiv…

W. Hubbard and B. Militzer
Wed, 17 Feb 16

Comments: 18 figures, 3 tables, in press, Astrophysical Journal

CO2 hydrate dissociation at low temperatures – formation and annealing of ice Ic [CL]

Dissociation of gas hydrates below 240 K leads to the formation of a metastable form of water ice, so called cubic ice (Ic). Through its defective nature and small particle size the surface film composed of such material is incapable of creating any significant diffusion barrier. Above 160 K, cubic ice gradually transforms to the stable hexagonal (Ih) form on laboratory time scales. The annealing, coupled with a parallel decomposition of gas hydrates, accelerates as temperature rises but already above 190 K the first process prevails, transforming cubic stacking sequences in-to ordinary Ih ice within a few minutes. Remaining stacking faults are removed through very slow isothermal annealing or after heating up above 240 K. The role of the proportion of cubic stacking on the decomposition rate is discussed. A better understanding of the dissociation kinetics at low temperatures is particularly im-portant for the critical evaluation of existing hypotheses that consider clathrates as a potential medium that actively participate in geological processes or is able to store gases (e.g. CH4, CO2 or Xe) in environments like comets, icy moons (i. e. Titan, Europa, Enceladus) or on Mars. Here, we present kinetics studies on the dissociation of CO2 clathrates at isothermal and isobaric conditions between 170 and 190K and mean Martian surface pressure. We place special attention to the formed ice and demonstrate its influence on the dissociation rates with a combination of neutron diffraction studies (performed on D20 at ILL/Grenoble) and cryo-SEM. More detailed crystallo-graphic information has been acquired via a flexible stacking-fault model capable of revealing the time evolution of the defect structure of ice Ic in terms of stacking probabilities and crystal size.

Read this paper on arXiv…

A. Falenty, T. Hansen and W. Kuhs
Wed, 28 Oct 15

Comments: Unpublished contribution to the 7th International Conference on Gas Hydrates (ICGH-7), Edinburgh, UK, 17-21 July 2011 (was only available to the conference participants)

Towards graphene-based detectors for dark matter directional detection [CL]

Dark matter detectors with directional sensitivity have the capability to distinguish dark matter induced nuclear recoils from isotropic backgrounds, thus providing a smoking gun signature for dark matter in the Galactic halo. Here we propose a conceptually novel class of high directional sensitivity dark matter detectors utilizing graphene-based van der Waals heterostructures. The advantages over conventional low pressure gas time projection chamber-based directional detectors are discussed in detail. A practical implementation using graphene/hexagonal boron nitride and graphene/molybdenum disulfide heterostructures is presented together with an overwhelming amount of experimental evidence in strong support of its feasibility.

Read this paper on arXiv…

S. Wang
Wed, 30 Sep 15

Comments: 5 pages, 1 figure

Microwave Loss Reduction in Cryogenically Cooled Conductors [IMA]

Measurements of microwave attenuation at room temperature and 4.2 K have been performed on some conductors commonly used in receiver input circuits. The reduction in loss on cooling is substantial, particularly for copper and plated gold, both of which showed a factor of 3 loss reduction. Copper passivated with benzotriazole shows the same loss as without passivation. The residual resistivity ratio between room temperature and 4.2 K, deduced from the measurements using the classical skin effect formula, was smaller than the measured DC value to a degree consistent with conduction in the extreme anomalous skin effect regime at cryogenic temperatures. The measurements were made in the 5-10 GHz range. The materials tested were: aluminum alloys 1100-T6 and 6061-O, C101 copper, benzotriazole treated C101 copper, and brass plated with electroformed copper, Pur-A-Gold 125-Au soft gold, and BDT200 bright gold.

Read this paper on arXiv…

R. Finger and A. Kerr
Fri, 18 Sep 15

Comments: 9 pages

Crystal chemistry of three-component white dwarfs and neutron star crusts: phase stability, phase stratification, and physical properties [HEAP]

A systematic search for multicomponent crystal structures is carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are “bred” by a genetic algorithm, and optimized at constant pressure under the assumption of linear response (Thomas-Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the $T=0$ bulk phase diagrams, five of which are complicated multinary structures not before predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferences of systems with pairwise Yukawa interactions, including and extending to the regime of low density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravitating, hydrostatically stable white dwarf, with fixed overall composition. To our knowledge, this is the first attempt to incorporate complex multinary phases into the equilibrium phase layering diagram and mass-radius-composition dependence, both of which are reported for He-C-O and C-O-Ne white dwarfs. Finite thickness interfacial phases (“interphases”) show up at the boundaries between single-component bcc crystalline regions, some of which have lower lattice symmetry than cubic. A second application — quasi-static settling of heavy nuclei in white dwarfs — builds on our equilibrium phase layering method. Tests of this nonequilibrium method reveal extra phases which play the role of transient host phases for the settling species.

Read this paper on arXiv…

T. Engstrom, N. Yoder and V. Crespi
Mon, 24 Aug 15

Comments: 11 pages, 4 figures, 1 table. Submitted to ApJ