Theory of magnetically powered jets

The magnetic theory for the production of jets by accreting objects is reviewed with emphasis on outstanding problem areas. An effort is made to show the connections behind the occasionally diverging nomenclature in the literature, to contrast the different points of view about basic mechanisms, and to highlight concepts for interpreting the results of numerical simulations. The role of dissipation of magnetic energy in accelerating the flow is discussed, and its importance for explaining high Lorentz factors. The collimation of jets to the observed narrow angles is discussed, including a critical discussion of the role of `hoop stress'. The transition between disk and outflow is one of the least understood parts of the magnetic theory; its role in setting the mass flux in the wind, in possible modulations of the mass flux, and the uncertainties in treating it realistically are discussed. Current views on most of these problems are still strongly influenced by the restriction to 2 dimensions (axisymmetry) in previous analytical and numerical work; 3-D effects likely to be important are suggested. An interesting problem area is the nature and origin of the strong, preferably highly ordered magnetic fields known to work best for jet production. The observational evidence for such fields and their behavior in numerical simulations is discussed. I argue that the presence or absence of such fields may well be the `second parameter' governing not only the presence of jets but also the X-ray spectra and timing behavior of X-ray binaries.Comment: 29 pages. Publication delays offered the opportunity for further corrections, an expansion of sect 4.2, and one more Fig. To appear in Belloni, T. (ed.): The Jet Paradigm - From Microquasars to Quasars, Lect. Notes Phys. 794 (2009

State of nature 2019

State of Nature 2019 presents an overview of how the country’s wildlife is faring, looking back over nearly 50 years of monitoring to see how nature has changed in the UK, its Crown Dependencies and Overseas Territories. As well as this long-term view, we focus on what has happened in the last decade, and so whether things are getting better or worse for nature. In addition, we have assessed the pressures that are acting on nature, and the responses being made, collectively, to counter these pressures

Associations of peer acceptance and perceived popularity with bullying and victimization in early adolescence

Item does not contain fulltextThis study examined the role of peer acceptance and perceived popularity in bullying and victimization in early adolescent peer groups. Peer acceptance is the degree to which adolescents are well liked by their peers; perceived popularity indicates visibility, dominance, and prestige. It was hypothesized that acceptance negatively predicts bullying, whereas popularity positively predicts bullying, and that both acceptance and popularity negatively predict victimization. Interactions between acceptance and popularity were tested as well. Participants were 1,207 13-to 14-year-old adolescents who completed sociometric assessments in their classrooms. Both bullying and victimization were predicted by the interaction between acceptance and perceived popularity. Bullying was positively associated with popularity, and this association was stronger for adolescents with lower levels of acceptance. Victimization was negatively associated with popularity, and this association was also stronger for adolescents with lower levels of acceptance. Linear and curvilinear main effects and interactions were found; some were further moderated by gender. The discussion focused on the role of individual status and group dynamic factors in the occurrence of aggression among early adolescents

Critical thickness of high structural quality SrTi03 films grown on orthorhombic (101) DySc03

Strained epitaxial SrTiO3 films were grown on orthorhombic (101) DyScO3 substrates by reactive molecular-beam epitaxy. The epitaxy of this substrate/film combination is cube on cube with a pseudocubic out-of-plane (001) orientation. The strain state and structural perfection of films with thicknesses ranging from 50 to 1000 were examined using x-ray scattering. The critical thickness at which misfit dislocations was introduced was between 350 and 500. These films have the narrowest rocking curves (full width at half maximum) ever reported for any heteroepitaxial oxide film (0.0018 degrees). Only a modest amount of relaxation is seen in films exceeding the critical thicknesses even after postdeposition annealing at 700 degrees C in 1 atm of oxygen. The dependence of strain relaxation on crystallographic direction is attributed to the anisotropy of the substrate. These SrTiO3 films show structural quality more typical of semiconductors such as GaAs and silicon than perovskite materials; their structural relaxation behavior also shows similarity to that of compound semiconductor films. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3037216