Extremal properties of (epi)Sturmian sequences and distribution modulo 1

Starting from a study of Y. Bugeaud and A. Dubickas (2005) on a question in distribution of real numbers modulo 1 via combinatorics on words, we survey some combinatorial properties of (epi)Sturmian sequences and distribution modulo 1 in connection to their work. In particular we focus on extremal properties of (epi)Sturmian sequences, some of which have been rediscovered several times

Extrasolar comets : the origin of dust in exozodiacal disks?

Comets have been invoked in numerous studies as a potentially important source of dust and gas around stars, but none has studied the thermo-physical evolution, out-gassing rate, and dust ejection of these objects in such stellar systems. We investigate the thermo-physical evolution of comets in exo-planetary systems in order to provide valuable theoretical data required to interpret observations of gas and dust. We use a quasi 3D model of cometary nucleus to study the thermo-physical evolution of comets evolving around a single star from 0.1 to 50 AU, whose homogeneous luminosity varies from 0.1 to 70 solar luminosities. This paper provides mass ejection, lifetimes, and the rate of dust and water gas mass productions for comets as a function of the distance to the star and stellar luminosity. Results show significant physical changes to comets at high stellar luminosities. The models are presented in such a manner that they can be readily applied to any planetary system. By considering the examples of the Solar System, Vega and HD 69830, we show that dust grains released from sublimating comets have the potential to create the observed (exo)zodiacal emission. We show that observations can be reproduced by 1 to 2 massive comets or by a large number of comets whose orbits approach close to the star. Our conclusions depend on the stellar luminosity and the uncertain lifetime of the dust grains. We find, as in previous studies, that exozodiacal dust disks can only survive if replenished by a population of typically sized comets renewed from a large and cold reservoir of cometary bodies beyond the water ice line. These comets could reach the inner regions of the planetary system following scattering by a (giant) planet.Comment: 21 pages, 10 figure

Scattering of small bodies by planets: a potential origin for exozodiacal dust ?

High levels of exozodiacal dust are observed around a growing number of main sequence stars. The origin of such dust is not clear, given that it has a short lifetime against both collisions and radiative forces. Even a collisional cascade with km-sized parent bodies, as suggested to explain outer debris discs, cannot survive sufficiently long. In this work we investigate whether the observed exozodiacal dust could originate from an outer planetesimal belt. We investigate the scattering processes in stable planetary systems in order to determine whether sufficient material could be scattered inwards in order to retain the exozodiacal dust at its currently observed levels. We use N-body simulations to investigate the efficiency of this scattering and its dependence on the architecture of the planetary system. The results of these simulations can be used to assess the ability of hypothetical chains of planets to produce exozodi in observed systems. We find that for older (>100Myr) stars with exozodiacal dust, a massive, large radii (>20AU) outer belt and a chain of tightly packed, low-mass planets would be required in order to retain the dust at its currently observed levels. This brings into question how many, if any, real systems possess such a contrived architecture and are therefore capable of scattering at sufficiently high rates to retain exozodi dust on long timescales

Thermochronological evolution of calcite formation at the potential Yucca Mountain repository site, Nevada: Part 2 fluid inclusion analyses and UPb dating

The presence of two-phase fluid inclusions in thin secondary mineral crusts at the potential Yucca Mountain nuclear waste repository has raised questions regarding the origin, timing, and temperature of past fluid flow through the repository horizon. The geologically recent passage of fluids with high temperatures would call into question the suitability of the site for the storage of high level nuclear waste. This study determined the thermal history of fluid flow through the site using fluid inclusion analyses and constrained the timing of thermal fluids by dating silica minerals spatially associated with the fluid inclusions using U-Pb techniques. Results provide a detailed time-temperature history of fluid migration through primary and secondary pore spaces during the past 8 to 9 million years. One hundred and fifty-five samples were collected in the unsaturated zone from the C-shaped Exploratory Studies Facility (ESF), the ECRB cross drift which crosses the potential repository horizon, and exploratory alcoves. Detailed petrographic and paragenetic studies indicated that the oldest secondary minerals consisted of heterogeneously distributed calcite with lesser chalcedony, quartz, opal, and fluorite. The oldest secondary minerals were overgrown by intermediate bladed calcite. The youngest secondary minerals include chemically distinct Mgenriched, growth-zoned sparry calcite (MGSC) and intergrown U-enriched opal. Fluid inclusion petrography indicated that 50 % of the samples (n = 78) contained fluid inclusion assemblages with two-phase fluid inclusions, and that assemblages of liquid-only fluid inclusions represent \u3e 96% of all fluid inclusions within the secondary minerals. Assemblages of two-phase inclusions also contain liquid-only inclusions that did not nucleate a vapor-bubble owing to formation at relatively low temperatures. Virtually all two-phase fluid inclusions occur in paragenetically old calcite; rare two-phase inclusion assemblages were observed in old fluorite (n = 3) and quartz (n = 2). Rare two-phase fluid inclusions were observed in early-intermediate calcite; sparse, irregularly shaped liquid-only inclusions form the only fluid inclusion assemblages observed in late-intermediate minerals and young MGSC. Homogenization temperatures for calcite across the site are generally 45 - 60 °C, but higher temperatures reaching 83 °C were recorded in the north portal and ramp of the ESF and cooler temperatures of ~ 35 - 45 °C were recorded in the intensely fracture zone. Samples from lithophysal cavities in the ESF and ECRB contain multiple populations of two-phase inclusions. Inclusion temperatures are highest in early calcite (\u3e 45 °C) and cooler in paragenetically younger early calcite, indicating cooling with time. The cooler temperatures coincide with temperatures recorded in the intensely fractured zone and indicate that secondary minerals in the intensely fractured zone began to precipitate later than secondary minerals in other locations. Freezing point depressions determined for inclusions range from -0.2 to -1.6 °C indicating trapping of a low salinity fluid. A small number of fluid inclusions in fluorite and quartz were identified and evaluated. Four inclusions in these minerals homogenized at temperatures higher than those recorded for calcite (91 ° - 95 °C) . Two approaches were used to constrain the timing of thermal fluids at Yucca Mountain. First, the age of MGSC was determined, and it provides a minimum age for fluids with elevated temperatures owing to the presence of only liquid-only inclusions in MGSC. Results indicate that MGSC began to precipitate across the site between 2.90 ± 0.06 Ma and about 1.95 ± 0.06 Ma, and MGSC has continued to precipitate to within the last half million years. These ages constrain fluids with elevated temperatures to have accessed the site more than about 2.90 Ma. Second, more precise temporal constraints were determined for samples in which datable opal or chalcedony occur in the intermediate or older parts of the mineral crusts, or are spatially related to 2-phase fluid inclusions. Such ages indicate that two-phase fluid inclusions are older than 5.32 ± 0.02 Ma, and that fluids with elevated temperatures were present at Yucca Mountain before this time. Results from this study are consistent with a model of descending meteoric water that infiltrated the cooling tuff sequence, became heated, and precipitated secondary minerals within the vadose zone. Fluid inclusions indicate that fluids with elevated temperatures were present during the early history of Yucca Mountain. Sparse, liquid-only fluid inclusions in late intermediate to young calcite indicate that secondary minerals were precipitated from low temperature fluids during the past 5 million years. This study demonstrates that the hypothesis of geologically recent upwelling hydrothermal fluids is untenable and should not disqualify Yucca Mountain as a potential nuclear waste storage site

An Optimized Monetary Policy Rule for ToTEM

The authors propose a monetary policy rule for the Terms-of-Trade Economic Model (ToTEM), the Bank of Canada's new projection and policy-analysis model for the Canadian economy. They consider simple instrument rules such as Taylor-type and inflation-forecast-based rules. The proposed rule minimizes a loss function that reflects the assumed preferences of the monetary authority over inflation and output, as well as over the variability of its instrument. The authors also investigate how robust the proposed rule is with respect to a particular realization of shocks that differs from the historical distribution used to find the optimized rule.Economic models; Monetary policy framework; Transmission of monetary policy

Child labor, education, and children's rights

Child labor is widespread, and bad for development, both that of the individual child, and of the society and economy in which she, or he lives. If allowed to persist to the current extent,child labor will prevent the achievement of the Millennium Development Goals of halving poverty, and achieving Education for All. Nearly all of the world's governments have ratified international human rights conventions, which call for the elimination of child labor, and the provision of universal primary education. Fulfilling these commitments is of critical importance for development. This paper reviews the international legal framework relating to child labor, and access to education, and, provides a statistical portrait of child labor and education participation. It looks at why children work from the perspective of household decision-making. Various policy options are considered, including those which improve the incentives to education relative to labor, remove constraints to schooling, and increase education participation through legislation. Conclusions are drawn in the final section.Children and Youth,Child Labor,Health Monitoring&Evaluation,Street Children,Youth and Governance

The Effects of Preschool Attendance on Kindergarten Success

The goal of this research was to inform and educate parents on the positive effects of preschool. This study was conducted with nine kindergarten students and their parents in a public elementary school. The data collected involved parent surveys, kindergarten checklists, kindergarten behavior logs, and meeting notes from parent conferences. The results were taken from the same nine kindergarten students whose parents returned the parental letter giving permission to be part of this study. The results showed students who had attended preschool had more positive behavior marks, mastered more skills on the kindergarten checklist, and were more socially advanced than those who did not attend preschool. Results from the parent surveys and conferencing showed that parents were open to hearing the pros and cons of preschool attendance, and eight of the nine parents said they would send their children to preschool after hearing the data

Mammal utilisation of artificial water sources in the central Kruger National Park: contemporary seasonal patterns and implications for climate change scenarios

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, August 2016Monitoring the status and abundance of mammals, as well as establishing threats to biodiversity in different areas, is an essential management requirement in protected areas. Monitoring mammal species can assist in determining species interactions, patterns of behaviour and is important for further research, policy and management strategies. Water provision has implications for the preservation of wildlife, and is thus a management concern. Numerous studies monitoring mammal water utilisation patterns have employed traditional data collection methods, which are restricted primarily to diurnal observation during specific time intervals. Given the projected future impacts of global climate change on regional water availability, it is essential to investigate current water usage by mammals in the Kruger National Park (KNP), so as to better ascertain likely future water requirements under climate change scenarios. The use of remote photography for scientific observation, investigation and monitoring has many potential benefits, and an innovative and relatively new method through which one can observe mammal water source visitation patterns, is through the use webcams. There has been comparatively little research on mammal water requirements and visitation patterns at water provisioning sites using remote photography as a data collection method. Further to this, there is a gap in our knowledge concerning how daily climate variables (viz. temperature and rainfall) and astronomical conditions control water source visitation patterns at the finer temporal scale. This research primarily contributes to understanding contemporary water source visitation patterns and how this will influence future management decisions. At a broader scale, determining recent visitation patterns is critical in the context of projected future climatic changes and the associated water requirements for mammals of KNP. Webcam images were obtained for the period March 2012 - March 2014, captured at two artificial water sources in the central KNP. A clear divide is exhibited between herbivore and carnivore visitation patterns, with herbivores exhibiting exclusively diurnal patterns and carnivores’ nocturnal patterns. Significant relationships with Tavg intervals demonstrate that the majority of herbivores are shifting their visitation periods earlier in the day per 5°C increase in mean daily temperature, while the majority of carnivores are shifting their visitation periods later in the night per 5°C increase in mean daily temperature, however there is variability of species responses across the two study sites. Under the highest Tavg interval (30°C - 35°C) impala, warthog, southern giraffe, African buffalo and plains zebra exhibit a shift to earlier visitation by 1 – 6 hours, forcing them to utilise the water sources outside of their preferred temporal range. The influence of the timing of rainfall events indicates that the mean number of individual species sightings is significantly larger on days before rainfall compared to days after rainfall. The study highlights that waterdependent herbivores utilise the artificial water sources in relative proportion to their abundance in the central KNP, while water-independent herbivores are avoiding these artificial water sources. The findings of this research could be used to supplement current water provisioning guidelines and plan for water provisioning efforts in future.TG201