The LISA Gravitational Wave Foreground: A Study of Double White Dwarfs

Double white dwarfs are expected to be a source of confusion-limited noise for the future gravitational wave observatory LISA. In a specific frequency range, this 'foreground noise' is predicted to rise above the instrumental noise and hinder the detection of other types of signals, e.g., gravitational waves arising from stellar mass objects inspiraling into massive black holes. In many previous studies only detached populations of compact object binaries have been considered in estimating the LISA gravitational wave foreground signal. Here, we investigate the influence of compact object detached and Roche-Lobe Overflow Galactic binaries on the shape and strength of the LISA signal. Since >99% of remnant binaries which have orbital periods within the LISA sensitivity range are white dwarf binaries, we consider only these binaries when calculating the LISA signal. We find that the contribution of RLOF binaries to the foreground noise is negligible at low frequencies, but becomes significant at higher frequencies, pushing the frequency at which the foreground noise drops below the instrumental noise to >6 mHz. We find that it is important to consider the population of mass transferring binaries in order to obtain an accurate assessment of the foreground noise on the LISA data stream. However, we estimate that there still exists a sizeable number (~11300) of Galactic double white dwarf binaries which will have a signal-to-noise ratio >5, and thus will be potentially resolvable with LISA. We present the LISA gravitational wave signal from the Galactic population of white dwarf binaries, show the most important formation channels contributing to the LISA disc and bulge populations and discuss the implications of these new findings.Comment: ApJ accepted. 28 pages, 11 figures (low resolution), 5 tables, some new references and changed content since last astro-ph versio

Informed consent comprehension in African research settings

ObjectivePrevious reviews on participants' comprehension of informed consent information have focused on developed countries. Experience has shown that ethical standards developed on Western values may not be appropriate for African settings where research concepts are unfamiliar. We undertook this review to describe how informed consent comprehension is defined and measured in African research settings.MethodsWe conducted a comprehensive search involving five electronic databases: Medline, Embase, Global Health, EthxWeb and Bioethics Literature Database (BELIT). We also examined African Index Medicus and Google Scholar for relevant publications on informed consent comprehension in clinical studies conducted in sub-Saharan Africa. 29 studies satisfied the inclusion criteria; meta-analysis was possible in 21 studies. We further conducted a direct comparison of participants' comprehension on domains of informed consent in all eligible studies.ResultsComprehension of key concepts of informed consent varies considerably from country to country and depends on the nature and complexity of the study. Meta-analysis showed that 47% of a total of 1633 participants across four studies demonstrated comprehension about randomisation (95% CI 13.9–80.9%). Similarly, 48% of 3946 participants in six studies had understanding about placebo (95% CI 19.0–77.5%), while only 30% of 753 participants in five studies understood the concept of therapeutic misconception (95% CI 4.6–66.7%). Measurement tools for informed consent comprehension were developed with little or no validation. Assessment of comprehension was carried out at variable times after disclosure of study information. No uniform definition of informed consent comprehension exists to form the basis for development of an appropriate tool to measure comprehension in African participants.ConclusionsComprehension of key concepts of informed consent is poor among study participants across Africa. There is a vital need to develop a uniform definition for informed consent comprehension in low literacy research settings in Africa. This will be an essential step towards developing appropriate tools that can adequately measure informed consent comprehension. This may consequently suggest adequate measures to improve the informed consent procedure.ObjectifLes normes éthiques élaborées selon les valeurs occidentales ne sont peut-être pas appropriées au contexte africain où les concepts de recherche ne sont pas familiers. Cette revue décrit comment la compréhension du consentement éclairé est définie et mesurée dans les cadres de recherche africains.MéthodesDes recherches ont été effectuées sur Medline, Embase, Global Health, EthxWeb, base de données de la Bioéthique Littérature, Index Medicus African et Google Scholar pour des publications pertinentes sur la compréhension du consentement éclairé dans les études cliniques menées en Afrique sub-saharienne. 29 études répondaient aux critères d'inclusion; une méta-analyse a été possible pour 21 études. La compréhension des participants sur les domaines du consentement éclairé dans toutes les études admissibles a été comparée directement.RésultatsLa compréhension des concepts clés du consentement éclairé varie considérablement selon les pays et dépend de la nature et de la complexité de l’étude. La méta-analyse a montré que 47% des participants ont compris la randomisation (IC95%: 13,9 - 80,9%), 48% ont compris le placebo (IC95%: 19,0 - 77,5%), 30% ont compris le concept de méprise thérapeutique (IC95%: 4,6 - 66,7%). Les outils de mesure de la compréhension du consentement éclairé étaient développés avec peu ou pas de validation.ConclusionsLa compréhension des concepts clés du consentement éclairé est faible en Afrique. Il y a une nécessité vitale d’élaborer une définition uniforme pour la compréhension du consentement éclairé dans les cadres de recherche avec un faible niveau d'alphabétisation en Afrique.ObjetivoLos estándares éticos desarrollados basándose en valores occidentales podrían no ser apropiados para emplazamientos Africanos en donde los conceptos de investigación no son familiares. En esta revisión se describe como la comprensión del consentimiento informado se define y mide en un centro de investigación Africano.MétodosSe buscaron publicaciones relevantes sobre la comprensión del consentimiento informado en estudios clínicos en África subsahariana en Medline, Embase, Global Health, EthxWeb, Bioethics Literature Database, African Index Medicus y Google Scholar. 29 estudios satisfacían los criterios de inclusión y el metaanálisis era posible para 21. La comprensión del consentimiento informado por parte de los participantes se comparó directamente en todos los estudios elegibles.ResultadosLa comprensión de conceptos claves del consentimiento informado varió de forma considerable entre países, y dependía de la naturaleza y de la complejidad del estudio. El meta-análisis mostró que un 47% entendía la aleatorización (IC 95% 13.9-80.9%); un 48% entendía el placebo (IC 95% 19.0-77.5%); y un 30% entendió el concepto terapéutico errado (IC 95% 4.6-66.7%). Las herramientas para medir la comprensión del consentimiento informado se desarrollaron con poca o ninguna validación.ConclusionesEn África, la comprensión de conceptos claves del consentimiento informado es pobre. Existe una necesidad vital de desarrollar una definición uniforme para la comprensión del consentimiento informado en lugares con bajos niveles de alfabetización en África

Evolution of Angular Momentum Distribution during Star Formation

If the angular momentum of the molecular cloud core were conserved during the star formation process, a new-born star would rotate much faster than its fission speed. This constitutes the angular momentum problem of new-born stars. In this paper, the angular momentum transfer in the contraction of a rotating magnetized cloud is studied with axisymmetric MHD simulations. Owing to the large dynamic range covered by the nested-grid method, the structure of the cloud in the range from 10 AU to 0.1 pc is explored. First, the cloud experiences a run-away collapse, and a disk forms perpendicularly to the magnetic field, in which the central density increases greatly in a finite time-scale. In this phase, the specific angular momentum j of the disk decreases to $\simeq 1/3$ of the initial cloud. After the central density of the disk exceeds $\sim 10^{10}{\rm cm}^{-3}$, the infall on to the central object develops. In this accretion stage, the rotation motion and thus the toroidal magnetic field drive the outflow. The angular momentum of the central object is transferred efficiently by the outflow as well as the effect of the magnetic stress. In 7000 yr from the core formation, the specific angular momentum of the central $0.17M_\odot$ decreases a factor of 10^{-4} from the initial value (i.e. from $10^{20}{\rm cm^2 s^{-1}}$ to $10^{16}{\rm cm^2 s^{-1}}$).Comment: 15 pages, 2 figures, Astrophysical Journal Letters in pres

Defect-Mediated Emulsification in Two Dimensions

We consider two dimensional dispersions of droplets of isotropic phase in a liquid with an XY-like order parameter, tilt, nematic, and hexatic symmetries being included. Strong anchoring boundary conditions are assumed. Textures for a single droplet and a pair of droplets are calculated and a universal droplet-droplet pair potential is obtained. The interaction of dispersed droplets via the ordered phase is attractive at large distances and repulsive at short distances, which results in a well defined preferred separation for two droplets and topological stabilization of the emulsion. This interaction also drives self-assembly into chains. Preferred separations and energy barriers to coalescence are calculated, and effects of thermal fluctuations and film thickness are discussed.Comment: revtex4, 13 pages, 12 figure

Comparison of forest attributes derived from two terrestrial lidar systems.

Abstract Terrestrial lidar (TLS) is an emerging technology for deriving forest attributes, including conventional inventory and canopy characterizations. However, little is known about the influence of scanner specifications on derived forest parameters. We compared two TLS systems at two sites in British Columbia. Common scanning benchmarks and identical algorithms were used to obtain estimates of tree diameter, position, and canopy characteristics. Visualization of range images and point clouds showed clear differences, even though both scanners were relatively high-resolution instruments. These translated into quantifiable differences in impulse penetration, characterization of stems and crowns far from the scan location, and gap fraction. Differences between scanners in estimates of effective plant area index were greater than differences between sites. Both scanners provided a detailed digital model of forest structure, and gross structural characterizations (including crown dimensions and position) were relatively robust; but comparison of canopy density metrics may require consideration of scanner attributes

Dark cloud cores and gravitational decoupling from turbulent flows

We test the hypothesis that the starless cores may be gravitationally bound clouds supported largely by thermal pressure by comparing observed molecular line spectra to theoretical spectra produced by a simulation that includes hydrodynamics, radiative cooling, variable molecular abundance, and radiative transfer in a simple one-dimensional model. The results suggest that the starless cores can be divided into two categories: stable starless cores that are in approximate equilibrium and will not evolve to form protostars, and unstable pre-stellar cores that are proceeding toward gravitational collapse and the formation of protostars. The starless cores might be formed from the interstellar medium as objects at the lower end of the inertial cascade of interstellar turbulence. Additionally, we identify a thermal instability in the starless cores. Under par ticular conditions of density and mass, a core may be unstable to expansion if the density is just above the critical density for the collisional coupling of the gas and dust so that as the core expands the gas-dust coupling that cools the gas is reduced and the gas warms, further driving the expansion.Comment: Submitted to Ap

Wall slip and flow of concentrated hard-sphere colloidal suspensions

We present a comprehensive study of the slip and flow of concentrated colloidal suspensions using cone-plate rheometry and simultaneous confocal imaging. In the colloidal glass regime, for smooth, non-stick walls, the solid nature of the suspension causes a transition in the rheology from Herschel-Bulkley (HB) bulk flow behavior at large stress to a Bingham-like slip behavior at low stress, which is suppressed for sufficient colloid-wall attraction or colloid-scale wall roughness. Visualization shows how the slip-shear transition depends on gap size and the boundary conditions at both walls and that partial slip persist well above the yield stress. A phenomenological model, incorporating the Bingham slip law and HB bulk flow, fully accounts for the behavior. Microscopically, the Bingham law is related to a thin (sub-colloidal) lubrication layer at the wall, giving rise to a characteristic dependence of slip parameters on particle size and concentration. We relate this to the suspension's osmotic pressure and yield stress and also analyze the influence of van der Waals interaction. For the largest concentrations, we observe non-uniform flow around the yield stress, in line with recent work on bulk shear-banding of concentrated pastes. We also describe residual slip in concentrated liquid suspensions, where the vanishing yield stress causes coexistence of (weak) slip and bulk shear flow for all measured rates