The thermodynamics of collapsing molecular cloud cores using smoothed particle hydrodynamics with radiative transfer

We present the results of a series of calculations studying the collapse of molecular cloud cores performed using a three-dimensional smoothed particle hydr odynamics code with radiative transfer in the flux-limited diffusion approximation. The opacities and specific heat capacities are identical for each calculation. However, we find that the temperature evolution during the simulations varies significantly when starting from different initial conditions. Even spherically-symmetric clouds with different initial densities show markedly different development. We conclude that simple barotropic equations of state like those used in some previous calculations provide at best a crude approximation to the thermal behaviour of the gas. Radiative transfer is necessary to obtain accurate temperatures.Comment: 8 pages, 9 figures, accepted for publication in MNRA

Star Formation Around Super-Massive Black Holes

The presence of young massive stars orbiting on eccentric rings within a few tenths of a parsec of the supermassive black hole in the Galactic centre is challenging for theories of star formation. The high tidal shear from the black hole should tear apart the molecular clouds that form stars elsewhere in the Galaxy, while transporting the stars to the Galactic centre also appears unlikely during their stellar lifetimes. We present numerical simulations of the infall of a giant molecular cloud that interacts with the black hole. The transfer of energy during closest approach allows part of the cloud to become bound to the black hole, forming an eccentric disc that quickly fragments to form stars. Compressional heating due to the black hole raises the temperature of the gas to 100-1000K, ensuring that the fragmentation produces relatively high stellar masses. These stars retain the eccentricity of the disc and, for a sufficiently massive initial cloud, produce an extremely top-heavy distribution of stellar masses. This potentially repetitive process can therefore explain the presence of multiple eccentric rings of young stars in the presence of a supermassive black hole.Comment: 20 pages includingh 7 figures. "This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science, 321, (22 August 2008), doi:10.1126/science.1160653". Reprints and animations can be found at http://star-www.st-and.ac.uk/~iab1

Improved efficiency of nutrient and water use for high quality field vegetable production using fertigation

Drip-based fertigation may improve the application efficiency of water and nutrients while maintaining or improving marketable yield and quality at harvest and post-harvest. Two plantings of lettuce (Lactuca sativa) were grown in the UK, with six N treatments and two methods of irrigation and N application. The conventional overhead irrigated treatments had all N applied in the base dressing with irrigation scheduled from SMD calculations. The closed loop treatments had nitrogen and irrigation delivered via drip automatically controlled by a sensor and logger system. The work established that water content in the root zone can be monitored in real time using horizontally oriented soil moisture sensors linked to data logging and telemetry, and that these data can be used to automatically trigger drip irrigation for commercially grown field vegetables. When the closed loop irrigation control was combined with fertigation treatments, lettuce crops were grown with savings of up to 60% and 75% of water and nitrogen respectively, compared to standard UK production systems. However, excess supply of N through fertigation rather than solid fertiliser was more detrimental to marketable yield and post harvest quality highlighting that care is needed when selecting N rates for fertigation

Simulating Black Hole White Dwarf Encounters

The existence of supermassive black holes lurking in the centers of galaxies and of stellar binary systems containing a black hole with a few solar masses has been established beyond reasonable doubt. The idea that black holes of intermediate masses ($\sim 1000$ \msun) may exist in globular star clusters has gained credence over recent years but no conclusive evidence has been established yet. An attractive feature of this hypothesis is the potential to not only disrupt solar-type stars but also compact white dwarf stars. In close encounters the white dwarfs can be sufficiently compressed to thermonuclearly explode. The detection of an underluminous thermonuclear explosion accompanied by a soft, transient X-ray signal would be compelling evidence for the presence of intermediate mass black holes in stellar clusters. In this paper we focus on the numerical techniques used to simulate the entire disruption process from the initial parabolic orbit, over the nuclear energy release during tidal compression, the subsequent ejection of freshly synthesized material and the formation process of an accretion disk around the black hole.Comment: 9 pages, 4 figures, Computer Physics Communications, accepted; movie can be found at http://www.faculty.iu-bremen.de/srosswog/; reference correcte

Use of collaborative computer simulation activities by high school science students learning relative motion.

Galileo\u27s contemporaries as well as today\u27s students have difficulty understanding relative motion. It is hypothesized that construction of visual models, resolution of these visual models with numeric models, and, in many cases, rejection of epistemological commitments such as the belief in one true velocity, are necessary for students to form integrated mental models of relative motion events. To investigate students\u27 relative motion problem solving, high school science students were videotaped in classroom and laboratory settings as they performed collaborative predict-observe-explain activities with relative motion computer simulations. The activities were designed to facilitate conceptual change by challenging common alternative conceptions. Half of the students interacted with simulations that provided animated feedback; the other half received numeric feedback. Learning, as measured by a diagnostic test, occurred following both conditions. There was no statistically significant difference between groups on the measure. It is hypothesized that students did not show statistically significant performance differences on the relative motion test because (a) many students were able to solve numeric problems through algorithm use; (b) many numeric condition students were aided in their ability to visualize problems by interaction with the treatment; and (c) the animation condition fostered little learning because the activities were too easy for students to perform. Students\u27 problem solving was examined through analyses of protocols and through statistical analyses of written responses. Evidence supported the following findings: (1) Numeric condition students had more difficulty with the computer activities than animation condition students. (2) Many students in both groups were able to construct accurate mental models of relative motion events. (3) A number of numeric condition students used faulty mechanical algorithms to solve problems. (4) A number of animation condition students used visualization to solve problems, mapping dynamic visual features of the animations onto posttest problems. Thus, there is evidence that presentation of numeric data can foster students\u27 use of mechanical algorithms. Presentation of animations can foster visualization of target problems solved off-line. These results suggest that, in addition to the structure of the simulations, how computer simulations are used may have a great impact on students\u27 cognition

Policy Practitioners’ Accounts of Evidence-Based Policy Making: The Case of Universal Credit

This paper draws on insider accounts from UK Department for Work and Pensions (DWP) officials to analyse the relationship between evidence and policy making at a time of rapid policy development relating to Universal Credit (UC). The paper argues, firstly, that evidence selection within the DWP was constrained by the overarching austerity paradigm, which constituted a Zeitgeist and had a significant bearing on the evidence selection and translation process, sharpening the focus of policy officials and analysts on the primacy of quantitative evidence when advising Ministers. Secondly, while methodological preferences (or an ‘evidence hierarchy’) impacted on evidence selection, this was not as significant as practitioners’ perceived capabilities to handle and develop evidence for policy. These capabilities were linked to departmental structures and constrained by political feasibility. Together, these dimensions constituted a significant filtration mechanism determining the kinds of evidence that were selected for policy development and those omitted, particularly in relation to UC. The paper contributes to debates about the contemporary role of evidence in policymaking and the potential of the relationship between future evidence production and use