The Influence of the Degree of Heterogeneity on the Elastic Properties of Random Sphere Packings

The macroscopic mechanical properties of colloidal particle gels strongly depend on the local arrangement of the powder particles. Experiments have shown that more heterogeneous microstructures exhibit up to one order of magnitude higher elastic properties than their more homogeneous counterparts at equal volume fraction. In this paper, packings of spherical particles are used as model structures to computationally investigate the elastic properties of coagulated particle gels as a function of their degree of heterogeneity. The discrete element model comprises a linear elastic contact law, particle bonding and damping. The simulation parameters were calibrated using a homogeneous and a heterogeneous microstructure originating from earlier Brownian dynamics simulations. A systematic study of the elastic properties as a function of the degree of heterogeneity was performed using two sets of microstructures obtained from Brownian dynamics simulation and from the void expansion method. Both sets cover a broad and to a large extent overlapping range of degrees of heterogeneity. The simulations have shown that the elastic properties as a function of the degree of heterogeneity are independent of the structure generation algorithm and that the relation between the shear modulus and the degree of heterogeneity can be well described by a power law. This suggests the presence of a critical degree of heterogeneity and, therefore, a phase transition between a phase with finite and one with zero elastic properties.Comment: 8 pages, 6 figures; Granular Matter (published online: 11. February 2012

Generation of Porous Particle Structures using the Void Expansion Method

The newly developed "void expansion method" allows for an efficient generation of porous packings of spherical particles over a wide range of volume fractions using the discrete element method. Particles are randomly placed under addition of much smaller "void-particles". Then, the void-particle radius is increased repeatedly, thereby rearranging the structural particles until formation of a dense particle packing. The structural particles' mean coordination number was used to characterize the evolving microstructures. At some void radius, a transition from an initially low to a higher mean coordination number is found, which was used to characterize the influence of the various simulation parameters. For structural and void-particle stiffnesses of the same order of magnitude, the transition is found at constant total volume fraction slightly below the random close packing limit. For decreasing void-particle stiffness the transition is shifted towards a smaller void-particle radius and becomes smoother.Comment: 9 pages, 8 figure

The Megamaser Cosmology Project. III. Accurate Masses of Seven Supermassive Black Holes in Active Galaxies with Circumnuclear Megamaser Disks

Observations of H$_2$O masers from circumnuclear disks in active galaxies for the Megamaser Cosmology Project allow accurate measurement of the mass of supermassive black holes (BH) in these galaxies. We present the Very Long Baseline Interferometry (VLBI) images and kinematics of water maser emission in six active galaxies: NGC~1194, NGC~2273, NGC~2960 (Mrk~1419), NGC~4388, NGC~6264 and NGC~6323. We use the Keplerian rotation curves of these six megamaser galaxies, plus a seventh previously published, to determine accurate enclosed masses within the central $\sim0.3$ pc of these galaxies, smaller than the radius of the sphere of influence of the central mass in all cases. We also set lower limits to the central mass densities of between 0.12 and 60 $\times 10^{10} M_{\odot}$~pc$^{-3}$. For six of the seven disks, the high central densities rule out clusters of stars or stellar remnants as the central objects, and this result further supports our assumption that the enclosed mass can be attributed predominantly to a supermassive black hole. The seven BHs have masses ranging between 0.76 and 6.5$\times$10$^7 M_{\odot}$. The BH mass errors are $\approx11$\%, dominated by the uncertainty of the Hubble constant. We compare the megamaser BH mass determination with other BH mass measurement techniques. The BH mass based on virial estimation in four galaxies is consistent with the megamaser BH mass given the latest empirical value of $\langle f \rangle$, but the virial mass uncertainty is much greater. MCP observations continue and we expect to obtain more maser BH masses in the future.Comment: 18 pages, 4 figures. This paper has been submitted to ApJ. An updated version of this paper will be posted when it gets accepte

Generation of folk song melodies using Bayes transforms

The paper introduces the `Bayes transform', a mathematical procedure for putting data into a hierarchical representation. Applicable to any type of data, the procedure yields interesting results when applied to sequences. In this case, the representation obtained implicitly models the repetition hierarchy of the source. There are then natural applications to music. Derivation of Bayes transforms can be the means of determining the repetition hierarchy of note sequences (melodies) in an empirical and domain-general way. The paper investigates application of this approach to Folk Song, examining the results that can be obtained by treating such transforms as generative models

Placental Transport of Zidovudine in the Rhesus Monkey

Objective: This study was undertaken to characterize the pharmacokinetics of zidovudine (ZDV) and ZDV-glucuronide (ZDVG) in the material and :fetal circulations of the rhesus monkey

SY Cancri, a case for unstable mass transfer?

Intermediate resolution (0.5-1 Angs) optical spectroscopy of the cataclysmic variable SY Cnc reveals the spectrum of the donor star. Our data enable us to resolve the orbital motion of the donor and provide a new orbital solution, binary mass ratio and spectral classification. We find that the donor star has spectral type G8+-2 V and orbits the white dwarf with P=0.3823753 +- 0.0000003 day, K2=88.0 +-2.9 km/s and V sin i=75.5 +- 6.5 km/s. Our values are significantly different from previous works and lead to q=M2/M1=1.18 +- 0.14. This is one of the highest mass ratios known in a CV and is very robust because it is based on resolving the rotational broadening over a large number of metallic absorption lines. The donor could be a slightly evolved main-sequence or descendant from a massive star which underwent an episode of thermal-timescale mass transfer.Comment: accepted for publication in MNRAS, 5 pages, 5 figure

Quantum Diffusion and Eigenfunction Delocalization in a Random Band Matrix Model

We consider Hermitian and symmetric random band matrices $H$ in $d \geq 1$ dimensions. The matrix elements $H_{xy}$, indexed by $x,y \in \Lambda \subset \Z^d$, are independent, uniformly distributed random variables if \abs{x-y} is less than the band width $W$, and zero otherwise. We prove that the time evolution of a quantum particle subject to the Hamiltonian $H$ is diffusive on time scales $t\ll W^{d/3}$. We also show that the localization length of an arbitrarily large majority of the eigenvectors is larger than a factor $W^{d/6}$ times the band width. All results are uniform in the size \abs{\Lambda} of the matrix.Comment: Minor corrections, Sections 4 and 11 update

Reading Self-Perceived Ability, Enjoyment and Achievement: A Genetically Informative Study of Their Reciprocal Links Over Time

Extant literature has established a consistent association between aspects of reading motivation, such as enjoyment and self-perceived ability, and reading achievement, in that more motivated readers are generally more skilled readers. However, the developmental etiology of this relation is yet to be investigated. The present study explores the development of the motivation–achievement association and its genetic and environmental underpinnings. Applying cross-lagged design in a sample of 13,825 twins, we examined the relative contribution of genetic and environmental factors to the association between reading enjoyment and self-perceived ability and reading achievement. Children completed a reading comprehension task and self-reported their reading enjoyment and perceived ability twice in middle childhood: when they were 9 –10 and 12 years old. Results showed a modest reciprocal association over time between reading motivation (enjoyment and perceived ability) and reading achievement. Reading motivation at age 9 –10 statistically predicted the development of later achievement, and similarly, reading achievement at age 9 –10 predicted the development of later motivation. This reciprocal association was observed beyond the stability of the variables and their contemporaneous correlation and was largely explained by genetic factors

Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing Galaxies Survey: A Candidate Protocluster at Redshift z~8

Theoretical and numerical modeling of dark-matter halo assembly predicts that the most luminous galaxies at high redshift are surrounded by overdensities of fainter companions. We test this prediction with HST observations acquired by our Brightest of Reionizing Galaxies (BoRG) survey, which identified four very bright z~8 candidates as Y-dropout sources in four of the 23 non-contiguous WFC3 fields observed. We extend here the search for Y-dropouts to fainter luminosities (M_* galaxies with M_AB\sim-20), with detections at >5sigma confidence (compared to >8sigma confidence adopted earlier) identifying 17 new candidates. We demonstrate that there is a correlation between number counts of faint and bright Y-dropouts at >99.84% confidence. Field BoRG58, which contains the best bright z\sim8 candidate (M_AB=-21.3), has the most significant overdensity of faint Y-dropouts. Four new sources are located within 70arcsec (corresponding to 3.1 comoving Mpc at z=8) from the previously known brighter z\sim8 candidate. The overdensity of Y-dropouts in this field has a physical origin to high confidence (p>99.975%), independent of completeness and contamination rate of the Y-dropout selection. We modeled the overdensity by means of cosmological simulations and estimate that the principal dark matter halo has mass M_h\sim(4-7)x10^11Msun (\sim5sigma density peak) and is surrounded by several M_h\sim10^11Msun halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M_h>2x10^14Msun galaxy cluster by z=0. Follow-up observations with ground and space based telescopes are required to secure the z\sim8 nature of the overdensity, discover new members, and measure their precise redshift.Comment: Minor revision: ApJ accepted [17 pages (emulateapj style), 7 figures, 2 tables