The ultimate outcome of black hole - neutron star mergers

We present a simple, semi--analytical description for the final stages of mergers of black hole (BH) -- neutron star (NS) systems. Such systems are of much interest as gravitational wave sources and gamma--ray burst progenitors. Numerical studies show that in general the neutron star is not disrupted at the first phase of mass transfer. Instead, what remains of the neutron star is left on a wider, eccentric, orbit. We consider the evolution of such systems as they lose angular momentum via gravitational radiation and come into contact for further phases of mass transfer. During each mass transfer event the neutron star mass is reduced until a critical value where mass loss leads to a rapid increase in the stellar radius. At this point Roche lobe overflow shreds what remains of the neutron star, most of the mass forming a disc around the black hole. Such a disc may be massive enough to power a gamma--ray burst. The mass of the neutron star at the time of disruption (and therefore the disc mass) is largely independent of the initial masses of the black hole and neutron star, indicating that BH--NS star mergers may be standard candles.Comment: MNRAS, in pres

Neutron star binaries and long duration gamma-ray bursts

Cosmological long-duration gamma-ray bursts (LGRBs) are thought to originate from the core collapse to black holes of stripped massive stars. Those with sufficient rotation form a centrifugally-supported torus whose collapse powers the GRB. We investigate the role of tidal locking within a tight binary as a source of the necessary angular momentum. We find that the binary orbit must be no wider than a few solar radii for a torus to form upon core collapse. Comparing this criterion to the observed population of binaries containing two compact objects suggests that rotation may have been important in the formation of up to 50% of the observed systems. As these systems created a neutron star and not a black hole they presumably did not produce highly luminous GRBs. We suggest instead that they make the subset of GRBs in the relatively local universe which have much lower luminosity.Comment: 7 pages, accepted for publication in MNRA

Progenitors of Long Gamma-ray Bursts

Pinpointing the progenitors of long duration gamma-ray bursts (LGRBs) remains an extremely important question, although it is now clear that at least a fraction of LGRBs originate in the core collapse of massive stars in type Ic supernovae, the pathways to the production of these stars, and their initial masses, remain uncertain. Rotation is thought to be vital in the creation of LGRBs, and it is likely that black hole creation is also necessary. We suggest that these two constraints can be met if the GRB progenitors are very massive stars (>20 solar masses) and are formed in tight binary systems. Using simple models we compare the predictions of this scenario with observations and find that the location of GRBs on their host galaxies are suggestive of main-sequence masses in excess of 20 solar masses, while 50% of the known compact binary systems may have been sufficiently close to have had the necessary rotation rates for GRB creation. Thus, massive stars in compact binaries are a likely channel for at least some fraction of LGRBs.Comment: To appear in "Gamma-ray bursts: Prospects for GLAST", AIP Conference proceedings 906, Editors M. Axelsson and F Ryd

Livestock Farming Systems in the Northern Tablelands of NSW: An Economic Analysis

The Northern Tablelands region of New South Wales covers an area of approximately 3.12 million hectares including 2.11 million hectares occupied by some 2300 agricultural establishments producing agricultural commodities valued at more than $220 million. Sheep and wool production and cattle production are the dominant agricultural enterprises. In this Report, a whole-farm model of a representative livestock farming system in the Northern Tablelands is developed. Whole-farm economic models of the relevant farming system are a useful first step in understanding the nature of the biological and economic constraints facing producers in their decision making in relation to their choices of inputs and outputs. Such models are also useful in relation to more general concerns such as the expected impacts of investments in new technologies applicable to grazing systems, or of external events such as drought conditions or a depreciation in the exchange rate. A whole farm budget for a representative farm includes a statement of assets and liabilities, based upon estimates of the various capital items including land, livestock and plant and machinery and farm structures. There is also an annual operating budget that includes the cash income and costs associated with each of the farm enterprises as well as the fixed costs incurred for running the farm over the year to derive the farm cash income. Allowances for depreciation and interest costs are deducted from farm cash income to determine farm operating surplus. No family labor allowance is subtracted, so the resulting farm operating surplus represents a return on owner-operated labor, management and farm assets. Pasture costs are not apportioned to the specific animal enterprises and therefore appear as separate negative gross margins. Similarly, supplementary feeding costs and fodder conservation activities are listed as a separate negative gross margin. A representative farm model of the Northern Tablelands livestock farming system was developed based on ABS and ABARE data on the relevant industries, from simulations with a linear programming model, and from discussions with local graziers and extension officers. The farm comprises 920 ha of which about half is native pasture and about half is introduced pasture. This farm runs a flock of 1,108 first-cross ewes, a flock of 1,732 Merino wethers and a 127 cow herd producing 18 month old steers suitable for the heavy feeder steer market. Using average prices and costs over an extended period of time, the annual operating budget for the farm shows a total gross margin of$86,191 and total overhead costs for the year of $24,720. This results in a farm cash income of$61,471 and a farm operating surplus of $37,471 after depreciation and interest costs. The statement of assets and liabilities shows total assets of the farm to be$1,498,060 and liabilities of $100,000 which equates to an equity level of 93.3 per cent. The farm operating surplus achieved on this model farm as a percentage of the owner's equity is 2.7 per cent. This represents a return on operator and family labor, management and equity. Low returns to equity are typical of Australian broadacre agriculture. Other scenarios examined included whole-farm budgets based on 2002 actual market prices and on 2003 expected prices. Given the relatively high prices for sheep relative to cattle in these years, the representative farm would be more profitable running 1,558 first-cross ewes and 3,595 Merino wethers. Such an enterprise mix would achieve a farm total gross margin of$165,736. After overhead costs, depreciation and interest costs there would be a farm operating surplus of $111,818. Based on equity totaling$1,472,870, this operating surplus would represent a business return on operator labor, management and equity of 8.1 per cent. However, while the Northern Tablelands representative farm model would suggest that greater profits could be achieved from changing enterprises as commodity prices change, in practice various biological lags, infrastructure, financial and management constraints prevent regular changes in farm enterprises. In fact, diversification amongst a variety of farm enterprises between various sheep and cattle enterprises as evidenced in the Northern Tablelands is one management response to this commodity price variability. A hypothetical new improved-pasture technology suggested by researchers, involving the selection of pasture varieties with improved winter pasture growth, was examined using the whole-farm model. If the existing 450 ha of improved pasture was replaced by a new variety that gave a 10 per cent increase in winter pasture growth, this would result in a 4.9 per cent increase in farm total gross margin. This corresponds to an increase in farm cash income of 6.9 per cent. These improvements in the profitability of the representative farm would be achieved by increasing the investment in first-cross ewes and in cows producing heavy feeder steers (by 3.5 per cent and 7.8 per cent respectively) and by decreasing the Merino wether enterprise from 1,732 to 1,672 wethers. This indicates that the prime lamb and cow enterprises, under the current assumptions of the model, are better able to utilize the farm resources available given an increase in winter pasture growth. The main conclusions from the analysis are that: Returns to equity are quite low in the Northern Tablelands livestock farming system; variable commodity prices, largely determined in world markets, result in variable levels of profitability of the farming system over time; The optimal farm plan is quite sensitive to small changes in the relative prices of the different outputs produced; In practice farm plans do not change very much as prices change, with most farms maintaining a range of cattle and sheep enterprises; Thus a "representative year" is a more realistic basis for assessing potential changes in farm plans; and new technologies can potentially have large impacts of farm profits and on the mix of resources used and outputs produced.Industrial Organization, Production Economics,

Commensurability oscillations due to pinned and drifting orbits in a two-dimensional lateral surface superlattice

We have simulated conduction in a two-dimensional electron gas subject to a weak two-dimensional periodic potential, $V_x \cos(2\pi x/a) + V_y \cos(2\pi y/a)$. The usual commensurability oscillations in $\rho_{xx}(B)$ are seen with $V_x$ alone. An increase of $V_y$ suppresses these oscillations, rather than introducing the additional oscillations in $\rho_{yy}(B)$ expected from previous perturbation theories. We show that this behavior arises from drift of the guiding center of cyclotron motion along contours of an effective potential. Periodic modulation in the magnetic field can be treated in the same way.Comment: 3 pages text, 4 eps figures, revte

Predicting morphotropic phase boundary locations and transition temperatures in Pb- and Bi-based perovskite solid solutions from crystal chemical data and first-principles calculations

Using data obtained from first-principles calculations, we show that the position of the morphotropic phase boundary (MPB) and transition temperature at MPB in ferroelectric perovskite solutions can be predicted with quantitative accuracy from the properties of the constituent cations. We find that the mole fraction of PbTiO$_3$ at MPB in Pb(B$'$B$''$)O$_3$-PbTiO$_3$, BiBO$_3$-PbTiO$_3$ and Bi(B$'$B$''$)O$_3$-PbTiO$_3$ exhibits a linear dependence on the ionic size (tolerance factor) and the ionic displacements of the B-cations as found by density functional theory calculations. This dependence is due to competition between the local repulsion and A-cation displacement alignment interactions. Inclusion of first-principles displacement data also allows accurate prediction of transiton temperatures at the MPB. The obtained structure-property correlations are used to predict morphotropic phase boundaries and transition temperatures in as yet unsynthesized solid solutions.Comment: Accepted for publication in J. Appl. Phy

Successful conservative management of a colorenal fistula complicating percutaneous cryoablation of renal tumors: a case report.

RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are.INTRODUCTION: Colorenal fistula is a rare phenomenon and may complicate percutaneous cryoablation of renal cell carcinoma. Treatment remains controversial. CASE PRESENTATION: A 62-year-old Caucasian man presented with pneumaturia and left flank pain six weeks following ultrasound-guided percutaneous cryoablation of two recurrent lesions in the left kidney 14 years after partial left nephrectomy for a left renal cell carcinoma. A computed tomography scan eight weeks after cryoablation revealed a cryoablated mass with adjacent stranding and adherent descending colon as well as bubbles of gas in the area of stranding, the left collecting system, and the bladder. These features were consistent with a colorenal fistula at the site of previous ablation. Successful resolution of the fistula, both clinical and radiological, was achieved following a complete conservative non-interventional out-patient approach. No ureteric stent or surgical intervention was employed. CONCLUSIONS: In the absence of severe symptoms or sepsis, complete conservative management of a colorenal fistula complicating percutaneous cryoablation of renal tumors should be considered prior to interventional stenting or resectional surgery

Fewer, Not Fairer

In 2019, police across Dallas County asked the District Attorney to prosecute fewer marijuana cases than the year before. This report examines whether the racial disparity in those cases improved at the same time. Fewer, Not Fairer shows that while the number of referrals declined, police were still more likely to refer a Black person for marijuana prosecution than a non-Black person. However, some cities achieved more fairness when their police departments almost entirely stopped requesting marijuana prosecutions altogether.https://scholar.smu.edu/deasoncenter/1006/thumbnail.jp