Pair diffusion, hydrodynamic interactions, and available volume in dense fluids

We calculate the pair diffusion coefficient D(r) as a function of the distance r between two hard-sphere particles in a dense monodisperse suspension. The distance-dependent pair diffusion coefficient describes the hydrodynamic interactions between particles in a fluid that are central to theories of polymer and colloid dynamics. We determine D(r) from the propagators (Green's functions) of particle pairs obtained from discontinuous molecular dynamics simulations. At distances exceeding 3 molecular diameters, the calculated pair diffusion coefficients are in excellent agreement with predictions from exact macroscopic hydrodynamic theory for large Brownian particles suspended in a solvent bath, as well as the Oseen approximation. However, the asymptotic 1/r distance dependence of D(r) associated with hydrodynamic effects emerges only after the pair distance dynamics has been followed for relatively long times, indicating non-negligible memory effects in the pair diffusion at short times. Deviations of the calculated D(r) from the hydrodynamic models at short distances r reflect the underlying many-body fluid structure, and are found to be correlated to differences in the local available volume. The procedure used here to determine the pair diffusion coefficients can also be used for single-particle diffusion in confinement with spherical symmetry.Comment: 6 pages, 5 figure

O stars effective temperature and HII regions ionization parameter gradients in the Galaxy

Extensive photoionization model grids are computed for single star HII regions using stellar atmosphere models from the WM-basic code. Mid-IR emission line intensities are predicted and diagnostic diagrams of [NeIII]/[NeII] and [SIV]/[SIII] excitation ratio are build, taking into account the metallicities of both the star and the HII region. The diagrams are used in conjunction with galactic HII region observations obtained with the ISO Observatory to determine the effective temperature Teff of the exciting O stars and the mean ionization parameter U. Teff and U are found to increase and decrease, respectively, with the metallicity of the HII region represented by the [Ne/Ne_sol] ratio. No evidence is found for gradients of Teff or U with galactocentric distance Rgal. The observed excitation sequence with Rgal is mainly due to the effect of the metallicity gradient on the spectral ionizing shape, upon which the effect of an increase in Teff with Z is superimposed. We show that not taking properly into account the effect of metallicity on the ionizing shape of the stellar atmosphere would lead to an apparent decrease of Teff with Z and an increase of Teff with Rgal.Comment: Accepted in Ap

The Effectiveness of Health Screening

Using a matched insurant-general practitioner panel data set, we estimated the effect of a general health-screening program on individuals’ health status and health care cost. To account for selection into treatment, we used regional variations in the intensity of exposure to supply-determined screening recommendations as an instrumental variable. We found that screening participation substantially increased inpatient and outpatient health care costs for up to two years after treatment. In the medium term, we found cost savings in the outpatient sector, whereas in the long run, no statistically significant effects of screening on either health care cost component could be discerned. In summary, screening participation increases health care costs. Since we did not find any statistically significant effect of screening participation on insurants’ health status at any point in time, we do not recommend a general health-screening program. However, given that we found some evidence for cost-saving potentials for the sub-sample of younger insurants, we suggest more targeted screening programs.Health screening, health care costs, sick leave, mortality

The Effectiveness of Health Screening

Using a matched insurant-general practitioner panel data set, we estimated the effect of a general health-screening program on individuals’ health status and health care cost. To account for selection into treatment, we used regional variations in the intensity of exposure to supply-determined screening recommendations as an instrumental variable. We found that screening participation substantially increased inpatient and outpatient health care costs for up to two years after treatment. In the medium term, we found cost savings in the outpatient sector, whereas in the long run, no statistically significant effects of screening on either health care cost component could be discerned. In summary, screening participation increases health care costs. Since we did not find any statistically significant effect of screening participation on insurants’ health status at any point in time, we do not recommend a general health-screening program. However, given that we found some evidence for cost-saving potentials for the sub-sample of younger insurants, we suggest more targeted screening programs.Health screening, health care costs, sick leave, mortality

Alteration and Oxidiation of an Olivine Lamprophyre Dike from Southern Utah, USA: An Analog for Mars

We report on oxidized basaltic dike intrusions on the Colorado Plateau as analog for Martian basalt oxidation

Racial Disparities in Functional Limitations Among Hispanic Women in the United States

This paper assesses whether there are race differences in functional health among Hispanic women in the United States; ascertains whether the race differences in functional health vary by age; and examines the extent to which race differences in functional health are attributable to key dimensions of demographic, geographic, and socioeconomic heterogeneity. The analysis is based on 15 years of aggregated data from the National Health Interview Survey. Both U.S.- and foreign-born black and other race Hispanic women display a higher level of functional limitations than their white Hispanic counterparts. There is little evidence that such health differences widen with age. U.S.-born black Hispanic women, however, suffer from a high burden of functional limitations across the adult age range. This research speaks to the need for greater attention to racial differences in health among Hispanics, and particularly so within the U.S.-born segment of this rapidly aging population

Nanoporous Membranes of Densely Packed Carbon Nanotubes Formed by Lipid-Mediated Self-Assembly

Nanofiltration technology faces the competing challenges of achieving high fluid flux through uniformly narrow pores of a mechanically and chemically stable filter. Supported dense-packed 2D-crystals of single-walled carbon nanotube (CNT) porins with ∼1 nm wide pores could, in principle, meet these challenges. However, such CNT membranes cannot currently be synthesized at high pore density. Here, we use computer simulations to explore lipid-mediated self-assembly as a route toward densely packed CNT membranes, motivated by the analogy to membrane-protein 2D crystallization. In large-scale coarse-grained molecular dynamics (MD) simulations, we find that CNTs in lipid membranes readily self-assemble into large clusters. Lipids trapped between the CNTs lubricate CNT repacking upon collisions of diffusing clusters, thereby facilitating the formation of large ordered structures. Cluster diffusion follows the Saffman-Delbrück law and its generalization by Hughes, Pailthorpe, and White. On longer time scales, we expect the formation of close-packed CNT structures by depletion of the intervening shared annular lipid shell, depending on the relative strength of CNT-CNT and CNT-lipid interactions. Our simulations identify CNT length, diameter, and end functionalization as major factors for the self-assembly of CNT membranes

Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization

The spectra of the first galaxies and quasars in the Universe should be strongly absorbed shortward of their rest-frame Lyman-alpha wavelength by neutral hydrogen (HI) in the intervening intergalactic medium. However, the Lyman-alpha line photons emitted by these sources are not eliminated but rather scatter until they redshift out of resonance and escape due to the Hubble expansion of the surrounding intergalactic HI. We calculate the resulting brightness distribution and the spectral shape of the diffuse Lyman-alpha line emission around high redshift sources, before the intergalactic medium was reionized. Typically, the Lyman-alpha photons emitted by a source at z=10 scatter over a characteristic angular radius of order 15 arcseconds around the source and compose a line which is broadened and redshifted by about a thousand km/s relative to the source. The scattered photons are highly polarized. Detection of the diffuse Lyman-alpha halos around high redshift sources would provide a unique tool for probing the neutral intergalactic medium before the epoch of reionization. On sufficiently large scales where the Hubble flow is smooth and the gas is neutral, the Lyman-alpha brightness distribution can be used to determine the cosmological mass densities of baryons and matter.Comment: 21 pages, 5 Postscript figures, accepted by ApJ; figures 1--3 corrected; new section added on the detectability of Lyman alpha halos; conclusions update

HeII Recombination Lines From the First Luminous Objects

The hardness of the ionizing continuum from the first sources of UV radiation plays a crucial role in the reionization of the intergalactic medium (IGM). While usual stellar populations have soft spectra, mini-quasars or metal-free stars with high effective temperatures may emit hard photons, capable of doubly ionizing helium and increasing the IGM temperature. Absorption within the source and in the intervening IGM will render the ionizing continuum of high-redshift sources inaccessible to direct observation. Here we show that HeII recombination lines from the first luminous objects are potentially detectable by the Next Generation Space Telescope. Together with measurements of the hydrogen Balmer alpha emission line, this detection can be used to infer the ratio of HeII to HI ionizing photons. A measurement of this ratio would shed light on the nature and emission mechanism of the first luminous sources, with important astrophysical consequences for the reheating and reionization of the IGM.Comment: ApJ published version. Due to an error in one of the references, the strength of the 1640 A line was underestimated in a previous version; this line is now brighter by a factor of 1