Efficient design of tall tapered feeders

Toowoomba Foundry is seeking help with the design of tall tapered feeders, which supply extra molten iron to poured castings as they cool. There is a problem with the reliability of the feeders, particularly for thicker castings of the newer spheroidal graphite irons. An effective feeder remains molten until the casting has set. The setting of the melt in the feeder is delayed by making it large enough to retain its heat for longer than the casting, and by placing it close to the thermal centre of the casting. So a larger feeder is more reliable. But any metal remaining in the feeder is recycled, and has an associated energy cost. If a feeder is too small, it will set too soon, and the casting will have unwanted holes in it that may require the entire casting to be recycled. Thus there is a tension between making the feeder smaller so as to minimise recycled metal, and having the feeder large enough that the casting is good. Existing design methods use purely conductive models of heat transport. We investigate the relevance of convection in the cooling feeder, and set up a boundary-layer model of flow driven by density differences. We find that convection is a significant factor in the design of a feeder, effectively maintaining constant temperature across it. The height of the feeder is important mainly in providing the driving force for this flow

Electromagnetic Form Factors of Charged and Neutral Kaons

The charged and neutral kaon form factors are calculated as a phenomenological application of the QCD Dyson-Schwinger equations. The results are compared with the pion form factor calculated in the same framework and yield \mbox{$F_{K^\pm}(Q^2) > F_{\pi^\pm}(Q^2)$} on \mbox{$Q^2\in[0,3]$~GeV$^2$}; and a neutral kaon form factor that is similar in form and magnitude to the neutron charge form factor. These results are sensitive to the difference between the kaon and pion Bethe-Salpeter amplitude and the $u$- and $s$-quark propagation characteristics.Comment: 11 Pages, 2 figures, REVTEX, uses epsfig. No chang

The ultraluminous state revisited : fractional variability and spectral shape as diagnostics of super-Eddington accretion

Although we are nearing a consensus that most ultraluminous X-ray sources (ULXs) below 1041 erg s−1 represent stellar mass black holes accreting in a super-Eddington ‘ultraluminous’ accretion state, little is yet established of the physics of this extreme accretion mode. Here, we use a combined X-ray spectral and timing analysis of an XMM–Newton sample of ULXs to investigate this new accretion regime. We start by suggesting an empirical classification scheme that separates ULXs into three classes based on the spectral morphologies observed by Gladstone et al.: a singly peaked broadened disc class, and two-component hard ultraluminous and soft ultraluminous regimes, with the spectra of the latter two classes dominated by the harder and softer component, respectively. We find that at the lowest luminosities (LX < 3 × 1039 erg s−1) the ULX population is dominated by sources with broadened disc spectra, whilst ULXs with two-component spectra are seen almost exclusively at higher luminosities, suggestive of a distinction between ∼Eddington and super-Eddington accretion modes. We find high levels of fractional variability are limited to ULXs with soft ultraluminous spectra, and a couple of the broadened disc sources. Furthermore, the variability in these sources is strongest at high energies, suggesting it originates in the harder of the two spectral components. We argue that these properties are consistent with current models of super-Eddington emission, where a massive radiatively driven wind forms a funnel-like geometry around the central regions of the accretion flow. As the wind provides the soft spectral component this suggests that inclination is the key determinant in the observed two-component X-ray spectra, which is very strongly supported by the variability results if this originates due to clumpy material at the edge of the wind intermittently obscuring our line-of-sight to the spectrally hard central regions of the ULX. The pattern of spectral variability with luminosity in two ULXs that straddle the hard/soft ultraluminous regime boundary is consistent with the wind increasing at higher accretion rates, and thus narrowing the opening angle of the funnel. Hence, this work suggests that most ULXs can be explained as stellar mass black holes accreting at and above the Eddington limit, with their observed characteristics dominated by two variables: accretion rate and inclination

X-ray spectral evolution in the ultraluminous X-ray source M33 X-8

The bright ultraluminous X-ray source (ULX), M33 X-8, has been observed several times by XMM–Newton, providing us with a rare opportunity to ‘flux bin’ the spectral data and search for changes in the average X-ray spectrum with flux level. The aggregated X-ray spectra appear unlike standard sub-Eddington accretion state spectra which, alongside the lack of discernible variability at any energy, argues strongly against conventional two-component, sub-Eddington models. Although the lack of variability could be consistent with disc-dominated spectra, sub-Eddington disc models are not sufficiently broad to explain the observed spectra. Fits with a ∼ Eddington accretion rate slim disc model are acceptable, but the fits show that the temperature decreases with flux, contrary to expectations, and this is accompanied by the appearance of a harder tail to the spectrum. Applying a suitable two-component model reveals that the disc becomes cooler and less advection dominated as the X-ray flux increases, and this is allied to the emergence of an optically thick Comptonization medium. We present a scenario in which this is explained by the onset of a radiatively driven wind from the innermost regions of the accretion disc, as M33 X-8 exceeds the Eddington limit. Furthermore, we argue that the direct evolution of this spectrum with increasing luminosity (and hence radiation pressure) leads naturally to the two-component spectra seen in more luminous ULXs

Continent formation through time

The continental crust is the primary archive of geological history, and is host to most of our natural resources. Thus, the following remain critical questions in Earth Science, and provide an underlying theme to all of the contributions within this volume: when, how and where did the continental crust form? How did it differentiate and evolve through time? How has it has been preserved in the geological record? This introductory review provides a background to these themes, and provides an outline of the contributions contained within this volum

Bose-Einstein condensate collapse: a comparison between theory and experiment

We solve the Gross-Pitaevskii equation numerically for the collapse induced by a switch from positive to negative scattering lengths. We compare our results with experiments performed at JILA with Bose-Einstein condensates of Rb-85, in which the scattering length was controlled using a Feshbach resonance. Building on previous theoretical work we identify quantitative differences between the predictions of mean-field theory and the results of the experiments. Besides the previously reported difference between the predicted and observed critical atom number for collapse, we also find that the predicted collapse times systematically exceed those observed experimentally. Quantum field effects, such as fragmentation, that might account for these discrepancies are discussed.Comment: 4 pages, 2 figure

A spectral-timing model for ULXs in the supercritical regime

Ultraluminous X-ray sources (ULXs) with luminosities lying between ∼3 × 1039 and 2 × 1040 erg s−1 represent a contentious sample of objects as their brightness, together with a lack of unambiguous mass estimates for the vast majority of the central objects, leads to a degenerate scenario where the accretor could be a stellar remnant (black hole or neutron star) or intermediate-mass black hole (IMBH). Recent, high-quality observations imply that the presence of IMBHs in the majority of these objects is unlikely unless the accretion flow somehow deviates strongly from expectation based on objects with known masses. On the other hand, physically motivated models for supercritical inflows can re-create the observed X-ray spectra and their evolution, although have been lacking a robust explanation for their variability properties. In this paper, we include the effect of a partially inhomogeneous wind that imprints variability on to the X-ray emission via two distinct methods. The model is heavily dependent on both inclination to the line of sight and mass accretion rate, resulting in a series of qualitative and semiquantitative predictions. We study the time-averaged spectra and variability of a sample of well-observed ULXs, finding that the source behaviours can be explained by our model in both individual cases as well as across the entire sample, specifically in the trend of hardness-variability power. We present the covariance spectra for these sources for the first time, which shed light on the correlated variability and issues associated with modelling broad ULX spectra

Resonantly damped surface and body MHD waves in a solar coronal slab with oblique propagation

The theory of magnetohydrodynamic (MHD) waves in solar coronal slabs in a zero-$\beta$ configuration and for parallel propagation of waves does not allow the existence of surface waves. When oblique propagation of perturbations is considered both surface and body waves are able to propagate. When the perpendicular wave number is larger than a certain value, the body kink mode becomes a surface wave. In addition, a sausage surface mode is found below the internal cut-off frequency. When non-uniformity in the equilibrium is included, surface and body modes are damped due to resonant absorption. In this paper, first, a normal-mode analysis is performed and the period, the damping rate, and the spatial structure of eigenfunctions are obtained. Then, the time-dependent problem is solved, and the conditions under which one or the other type of mode is excited are investigated.Comment: 19 pages, 9 figures, accepted for publication in Solar Physic