Population III Gamma Ray Bursts

We discuss a model of Poynting-dominated gamma-ray bursts from the collapse of very massive first generation (pop. III) stars. From redshifts of order 20, the resulting relativistic jets would radiate in the hard X-ray range around 50 keV and above, followed after roughly a day by an external shock component peaking around a few keV. On the same timescales an inverse Compton component around 75 GeV may be expected, as well as a possible infra-red flash. The fluences of these components would be above the threshold for detectors such as Swift and Fermi, providing potentially valuable information on the formation and properties of what may be the first luminous objects and their black holes in the high redshift Universe.Comment: 12 pages; Apj, subm. 12/10/2009; accepted 04/12/201

Resolving Conflict through Explicit Bargaining

This article analyzes the impact of conciliatory initiatives on conflict resolution in two-party bargaining. It specifically develops and tests a theory of unilateral initiatives derived from Osgood\u27s (1962) notion of Graduated and Reciprocated Initiatives in Tension Reduction (GRIT). The major propositions of the theory indicate that, given a pattern of mutual resistance or hostility, unilateral initiatives and tit-for-tat retaliation in response to punitive action will produce more conciliation and less hostility by an opponent. To test the theory, a bargaining setting was created in a laboratory experiment in which parties exchanged offers and counteroffers on an issue across a number of rounds while also having the option to engage in punitive action against one another. The results indicated that (1) unilateral initiatives produced more concession making and less hostility than a reciprocity strategy, and (2) tit-for-tat retaliation heightened hostility initially but reduced it over time. The article suggests some general, abstract conditions under which two parties in conflict can produce conciliation and reach agreements without the intervention of third parties

Finite Element Modelling of Mechanical Phenomena Connected to the Technological Process of Continuous Casting of Steel

A finite element method algorithm is presented that enables numerical simulation of real phenomena that take place during an industrial process of continuous casting of steel. The algorithm takes into account all known kinds of nonlinearities: material nonlinearity connected to the nonlinear temperature dependence of material properties, large deformations from the process of material forming and contacts between the slab and rollers of the strand. Received results describe the sensitivity of the product to crack initiation, not only during the process of continuous casting itself but also in the finished and cooled slab

Effects of Rotation on the Minimum Mass of Primordial Progenitors of Pair Instability Supernovae

The issue of which stars may reach the conditions of electron/positron pair formation instability is of importance to understand the final evolution both of the first stars and of contemporary stars. The criterion to enter the pair instability regime in density and temperature is basically controlled by the mass of the oxygen core. The main sequence masses that produce a given oxygen core mass are, in turn, dependent on metallicity, mass loss, and convective and rotationally-induced mixing. We examine the evolution of massive stars to determine the minimum main sequence mass that can encounter pair-instability effects, either a pulsational pair instability (PPISN) or a full-fledged pair-instability supernova (PISN). We concentrate on zero-metallicity stars with no mass loss subject to the Schwarzschild criterion for convective instability, but also explore solar metallicity and mass loss and the Ledoux criterion. As expected, for sufficiently strong rotationally-induced mixing, the minimum main sequence mass is encountered for conditions that induce effectively homogeneous evolution such that the original mass is converted almost entirely to helium and then to oxygen. For this case, we find that the minimum main sequence mass is ~40 Msun to encounter PPISN and ~65 Msun to encounter a PISN. When mass-loss is taken into account those mass limits become ~50 Msun for PPISN and ~80 Msun for PISN progenitors. The implications of these results for the first stars and for contemporary supernovae is discussed.Comment: 23 pages, 8 figure

The Supernova Channel of Super-AGB Stars

We study the late evolution of solar metallicity stars in the transition region between white dwarf formation and core collapse. This includes the super-asymptotic giant branch (super-AGB, SAGB) stars, which have massive enough cores to ignite carbon burning and form an oxygen-neon (ONe) core. The most massive SAGB stars have cores that may grow to the Chandrasekhar mass because of continued shell-burning. Their cores collapse, triggering a so called electron capture supernovae (ECSN). From stellar evolution models we find that the initial mass range for SAGB evolution is 7.5 ... 9.25\msun. We perform calculations with three different stellar evolution codes to investigate the sensitivity of this mass range to some of the uncertainties in current stellar models. The mass range significantly depends on the treatment of semiconvective mixing and convective overshooting. To consider the effect of a large number of thermal pulses, as expected in SAGB stars, we construct synthetic SAGB models that include a semi-analytical treatment of dredge-up, hot-bottom burning, and thermal pulse properties. This synthetic model enables us to compute the evolution of the main properties of SAGB stars from the onset of thermal pulses until the core reaches the Chandrasekhar mass or is uncovered by the stellar wind. Thereby, we determine the stellar initial mass ranges that produce ONe-white dwarfs and electron-capture supernovae. The latter is found to be 9.0 ... 9.25\msun for our fiducial model, implying that electron-capture supernovae would constitute about 4% of all supernovae in the local universe. Our synthetic approach allows us to explore the uncertainty of this number imposed by uncertainties in the third dredge-up efficiency and ABG mass loss rate. We find for ECSNe a upper limit of ~20% of all supernovae (abridged).Comment: 13 pages, 16 figures, submitted to ApJ, uses emulateap

On the Sensitivity of Massive Star Nucleosynthesis and Evolution to Solar Abundances and to Uncertainties in Helium Burning Reaction Rates

We explore the dependence of pre-supernova evolution and supernova nucleosynthesis yields on the uncertainties in helium burning reaction rates. Using the revised solar abundances of Lodders (2003) for the initial stellar composition, instead of those of Anders & Grevesse (1989), changes the supernova yields and limits the constraints that those yields place on the 12C(a,g)16O reaction rate. The production factors of medium-weight elements (A = 16-40) were found to be in reasonable agreement with observed solar ratios within the current experimental uncertainties in the triple alpha reaction rate. Simultaneous variations by the same amount in both reaction rates or in either of them separately, however, can induce significant changes in the central 12C abundance at core carbon ignition and in the mass of the supernova remnant. It therefore remains important to have experimental determinations of the helium burning rates so that their ratio and absolute values are known with an accuracy of 10% or better.Comment: Accepted for publication by the Astrophysical Journa

A COMPARISON OF ACHIEVEMENT LEVELS IN MATHEMATICS AND SCIENCE, AND CURRENT ATTITUDES, OF SECONDARY STUDENTS IN A SIX-PERIOD DAILY SCHEDULE, WITH THOSE OF STUDENTS IN A ROTATE-EIGHT BLOCK SCHEDULE

In part one of this study, secondary students¿ mathematics and science achievement levels in a Six-Period Daily (SPD) schedule were compared with those in a Rotate-Eight Block (REB) schedule (eight macroperiods in a two day rotation). In part two, alumni were surveyed to compare current opinions of the schedules¿ effectiveness overall and on two subscales. Archival test data and demographic information were obtained on two graduated classes in a selected suburban Midwestern high school, enrollment grades 9 to 12 of approximately 1000 students. Stratified random samples of 50 students from each class were selected based upon treatment, academic ability, ethnicity, and gender. Grade Point Averages (GPAs) and Missouri Assessment Program (MAP) test scores in mathematics and science were examined through univariate three-way analyses of variance (ANOVA) of the differences in the post-treatment means. Where initial equivalence was not found, ANOVA was used to study effects for subgroups. All main effects and interactions were tested. Gender was taken into account by equalizing numbers across subgroups to the extent possible. No statistically significant results or trends based on treatment were discovered as main effects or interactions in part one. The ¿achievement gap¿ between African-American and Caucasian students was confirmed in all achievement measures except science GPA, where only ability, not ethnicity or treatment, was found to be of significance as a main effect. Though not of statistical significance, a pattern favoring low ability REB subgroups and high ability SPD subgroups was noted. Analysis of survey results indicated that groups and subgroups differed significantly in scores for effectiveness of the schedules overall, and for the classroom activities subscale. Groups and subgroups consistently rated the effectiveness of the SPD higher. On only one measure did any subgroup rate the REB higher than the SPD: Caucasian males rated effectiveness of classroom activities slightly higher in the REB. The largest opinion differences were exhibited between African-American males and Caucasian males. African-American males rated the SPD classroom activities higher than did any other subgroup, and the REB lower (at exactly neutral) than did any other subgroup