The Physics of Star Cluster Formation and Evolution

© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00689-4.Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.Peer reviewe

EFFECTS OF PLASTIC DEFORMATION ON IONIC CONDUCTIVITY

On décrit les principaux effets de la déformation plastique sur la conductivité des halogénures alcalins à basse température. Ils sont interprétés comme suit : 1. La forte conductivité pendant la déformation est due au mouvement des dislocations chargées sous l'influence simultanée de la contrainte et du champ électrique appliqué. 2. L'accroissement de la conductivité après déformation qui est suivi par une décroissance (effet Gyulai-Hartly) est dû à la destruction des paires lacunes-impuretés ou à celle des amas. Ceci est lié au balayage des charges par les dislocations en mouvement. 3. La conductivité finale (état stationnaire) est plus faible qu'avant déformation parce que les lacunes sont liées aux dislocations et que celles-ci sont entourées par des charges d'espace compensatrices qui sont des régions de conductivité réduite. Pour rendre compte des observations il est nécessaire de supposer que l'effet n'est pas tant dû aux dislocations normales qu'aux débris.The principal effects of plastic deformation on the low-temperature conductivity of alkali halide crystals are summarised and interpreted as follows : 1. The high conductivity during deformation is due to the movement of charged dislocations under the forces exerted simultaneously by the stress and the applied electric field. 2. The increased conductivity following deformation which subsequently decays (Gyulai-Hartly effect) is most commonly due to the breaking-up of impurity-vacancy pairs or clusters. This is related to the sweep-up of charge by moving dislocations. 3. The final steady-state conductivity is less than that before deformation because vacancies are bound to dislocations and the dislocations are surrounded by compensating charge clouds which are regions of reduced conductivity. To account for the observations it is necessary to suppose that the effect is due not so much to normal dislocations as to dislocation debris

A TECHNIQUE FOR THE GROWTH OF HIGH QUALITY SINGLE CRYSTALS OF ICE

Une nouvelle technique est décrite pour la fabrication de monocristaux de glace ayant une faible densité de dislocations. Aucun germe cristallin n'est nécessaire et l'eau est gelée de bas en haut de manière à opérer sous vide ou sous atmosphère contrôlée. Des cristaux de 45 mm de diamètre et de 80 mm de long ont ainsi été fabriqués. La densité de dislocations déterminée par topographie X à l'aide du rayonnement synchrotron est inférieure à 100 cm-2.A new technique is described for the growth of single crystals of ice of low dislocation density. No seed crystal is required and the water is frozen from the bottom upwards so that it is possible to work with a vacuum or controlled atmosphere above it. Crystals of 45 mm diameter and 80 mm long have been grown and found by synchrotron radiation topography to have dislocation densities less than about 100 cm-2

THE STUDY OF DISLOCATION GLIDE IN ICE BY SYNCHROTRON RADIATION X-RAY TOPOGRAPHY

En utilisant un faisceau de rayons X provenant d'une source synchrotron, des séquences de topographies ont été obtenues ; elles montrent des images de dislocations dans un monocristal de glace pour des temps de pose inférieurs à une minute. Le mouvement des dislocations sous contrainte a été étudié dans un très large domaine de déformation plastique. Les topographies donnent des renseignements sur les vitesses des dislocations, le processus de multiplication et la nature du glissement dans les plans basals et prismatiques.Using the X-ray beam from a synchrotron radiation source, sequences of topographs have been obtained showing dislocations in single crystals of ice with exposure times of less than 1 min. Dislocation motion under stress has been studied in the very early stages of plastic deformation. The topographs reveal details about dislocation velocities, the process of multiplication and the nature of glide on both basal and prismatic planes

Risk factors for increased BTEX exposure in four Australian cities

Benzene, toluene, ethylbenzene and xylenes (BTEX) are common volatile organic compounds (VOCs) found in urban airsheds. Elevated levels of VOCs have been reported in many airsheds at many locations, particularly those associated with industrial activity, wood heater use and heavy traffic. Exposure to some VOCs has been associated with health risks. There have been limited investigations into community exposures to BTEX using personal monitoring to elucidate the concentrations to which members of the community may be exposed and the main contributors to that exposure. In this cross sectional study we investigated BTEX exposure of 204 non-smoking, non-occupationally exposed people from four Australian cities. Each participant wore a passive BTEX sampler over 24 h on five consecutive days in both winter and summer and completed an exposure source questionnaire for each season and a diary for each day of monitoring. The geometric mean (GM) and range of daily BTEX concentrations recorded for the study population were benzene 0.80 (0.04-23.8 ppb); toluene 2.83 (0.03-2120 ppb); ethylbenzene 0.49 (0.03-119 ppb); and xylenes 2.36 (0.04-697 ppb). A generalised linear model was used to investigate significant risk factors for increased BTEX exposure. Activities and locations found to increase personal exposure included vehicle repair and machinery use, refuelling of motor vehicles, being in an enclosed car park and time spent undertaking arts and crafts. A highly significant difference was found between the mean exposures in each of the four cities, which may be explained by differences in fuel composition, differences in the mix and density of industry, density of motor vehicles and air pollution meteorology