Dynamic and Stagnating Plasma Flow Leading to Magnetic Flux Tube Collimation

Highly collimated, plasma-filled magnetic flux tubes are frequently observed on galactic, stellar and laboratory scales. We propose that a single, universal magnetohydrodynamic pumping process explains why such collimated, plasma-filled magnetic flux tubes are ubiquitous. Experimental evidence from carefully diagnosed laboratory simulations of astrophysical jets confirms this assertion and is reported here. The magnetohydrodynamic process pumps plasma into a magnetic flux tube and the stagnation of the resulting flow causes this flux tube to become collimated.Comment: to be published in PRL; color figures on electronic versio

Imaging of Ultraluminous Infrared Galaxies in the Near-UV

We present the first ground-based U' (3410 angstroms) images of Ultraluminous Infrared Galaxies (ULIGs). Strong U' emission (median total M_U' = -20.8) is seen in all systems and in some cases the extended tidal features (both the smooth stellar distribution and compact star-forming features) contribute up to 60-80% of the total flux. The star-forming regions in both samples are found to have ages based on spectral synthesis models in the range 10-100 Myrs, and most differences in color between them can be attributed to the effects of dust reddening. Additionally, it is found that star-formation in compact knots in the tidal tails is most prominent in those ULIGs which have double nuclei, suggesting that the star-formation rate in the tails peaks prior to the actual coalescence of the galaxy nuclei and diminishes quickly thereafter. Similar to results at other wavelengths, the observed star formation at U' can only account for a small fraction of the known bolometric luminosity of the ULIGs. Azimuthally averaged radial light profiles at U' are characterized by a sersic law with index n=2, which is intermediate between an exponential disk and an r^(-1/4) law and closely resembles the latter at large radii. The implications of this near-ultraviolet imaging for optical/near-infrared observations of high redshift counterparts of ULIGs are discussed.Comment: 30 pages, 4 tables, and 9 figures, 2 of which are JPEGs. To appear in the August, 2000 edition of the Astronomical Journa

Assessment of an electronic voting system within the tutorial setting: a randomised controlled trial (ISRCTN54535861)

Background: Electronic voting systems have been used in various educational settings with little measurement of the educational impact on students. The goal of this study was to measure the effects of the inclusion of an electronic voting system within a small group tutorial. Method: A prospective randomised controlled trial was run at the Royal Adelaide Hospital, a teaching hospital in Adelaide, Australia. 102 students in their first clinical year of medical school participated in the study where an electronic voting system was introduced as a teaching aid into a standard tutorial. Long-term retention of knowledge and understanding of the topics discussed in the tutorials was measured and student response to the introduction of the electronic voting system was assessed. Results: Students using the electronic voting system had improved long-term retention of understanding of material taught in the tutorial. Students had a positive response to the use of this teaching aid. Conclusion: Electronic voting systems can provide a stimulating learning environment for students and in a small group tutorial may improve educational outcomes.Edward J. Palmer, Peter G. Devitt, Neville J. De Young and David Morri

On the exclusion of intra-cluster plasma from AGN-blown bubbles

Simple arguments suggest that magnetic fields should be aligned tangentially to the surface of an AGN-blown bubble. If this is the case, charged particles from the fully ionised intra-cluster medium (ICM) will be prevented, ordinarily, from crossing the boundary by the Lorentz force. However, recent observations indicate that thermal material may occupy up to 50% of the volume of some bubbles. Given the effect of the Lorentz force, the thermal content must then be attributed to one, or a combination, of the following processes: i) the entrainment of thermal gas into the AGN outflow that inflated the bubble; ii) rapid diffusion across the magnetic field lines at the ICM/bubble interface; iii) magnetic reconnection events which transfer thermal material across the ICM/bubble boundary. Unless the AGN outflow behaves as a magnetic tower jet, entrainment may be significant and could explain the observed thermal content of bubbles. Alternatively, the cross-field diffusion coefficient required for the ICM to fill a typical bubble is roughly 10^16 cm^2 s^-1, which is anomalously high compared to predictions from turbulent diffusion models. Finally, the mass transfer rate due to magnetic reconnection is uncertain, but significant for plausible reconnection rates. We conclude that entrainment into the outflow and mass transfer due to magnetic reconnection events are probably the most significant sources of thermal content in AGN-blown bubbles.Comment: Accepted for publication in MNRAS, 8 pages, 1 figur

Density functional study of the actinide nitrides

The full potential all electron linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method, as implemented in the suite of software WIEN2K, has been used to systematically investigate the structural, electronic, and magnetic properties of the actinide compounds AnN (An = Ac, Th, Pa, U, Np, Pu, Am). The theoretical formalism used is the generalized gradient approximation to density functional theory (GGA-DFT) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Each compound has been studied at six levels of theory: non-magnetic (NM), non-magnetic with spin-orbit coupling (NM+SOC), ferromagnetic (FM), ferromagnetic with spin-orbit coupling (FM+SOC), anti-ferromagnetic (AFM), and anti-ferromagnetic with spin-orbit coupling (AFM+SOC). The structural parameters, bulk moduli, densities of states, and charge distributions have been computed and compared to available experimental data and other theoretical calculations published in the literature. The total energy calculations indicate that the lowest energy structures of AcN, ThN, and PaN are degenerate at the NM+SOC, FM+SOC, and AFM+SOC levels of theory with vanishing total magnetic moments in the FM+SOC and AFM+SOC cases, making the ground states essentially non-magnetic with spin-orbit interaction. The ground states of UN, NpN, PuN, and AmN are found to be FM+SOC at the level of theory used in the present computations. The nature of the interactions between the actinide metals and nitrogen atom, and the implications on 5f electron delocalization and localization are discussed in detail.Comment: 5 tables, 12 figure

Signatures of Alfven waves in the polar coronal holes as seen by EIS/Hinode

Context. We diagnose the properties of the plume and interplume regions in a polar coronal hole and the role of waves in the acceleration of the solar wind. Aims. We attempt to detect whether Alfven waves are present in the polar coronal holes through variations in EUV line widths. Methods. Using spectral observations performed over a polar coronal hole region with the EIS spectrometer on Hinode, we study the variation in the line width and electron density as a function of height. We use the density sensitive line pairs of Fe xii 186.88 A & 195.119 A and Fe xiii 203.82 A & 202.04 A . Results. For the polar region, the line width data show that the nonthermal line-of-sight velocity increases from 26 km/s at 1000 above the limb to 42 km/s some 15000 (i.e. 110,000 km) above the limb. The electron density shows a decrease from 3:3 10^9 cm^-3 to 1:9 10^8 cm^-3 over the same distance. Conclusions. These results imply that the nonthermal velocity is inversely proportional to the quadratic root of the electron density, in excellent agreement with what is predicted for undamped radially propagating linear Alfven waves. Our data provide signatures of Alfven waves in the polar coronal hole regions, which could be important for the acceleration of the solar wind.Comment: 5 pages, 11 figures. Astronomy and Astrophysics Letters (accepted) http://www.aanda.org/articles/aa/pdf/forth/aa12242-09.pd

Calcium puffs are generic InsP₃-activated elementary calcium signals and are downregulated by prolonged hormonal stimulation to inhibit cellular calcium responses

Elementary Ca2+ signals, such as "Ca2+ puffs", which arise from the activation of inositol 1,4,5-trisphosphate receptors, are building blocks for local and global Ca2+ signalling. We characterized Ca2+ puffs in six cell types that expressed differing ratios of the three inositol 1,4,5-trisphosphate receptor isoforms. The amplitudes, spatial spreads and kinetics of the events were similar in each of the cell types. The resemblance of Ca2+ puffs in these cell types suggests that they are a generic elementary Ca2+ signal and, furthermore, that the different inositol 1,4,5-trisphosphate isoforms are functionally redundant at the level of subcellular Ca2+ signalling. Hormonal stimulation of SH-SY5Y neuroblastoma cells and HeLa cells for several hours downregulated inositol 1,4,5-trisphosphate expression and concomitantly altered the properties of the Ca2+ puffs. The amplitude and duration of Ca2+ puffs were substantially reduced. In addition, the number of Ca2+ puff sites active during the onset of a Ca2+ wave declined. The consequence of the changes in Ca2+ puff properties was that cells displayed a lower propensity to trigger regenerative Ca2+ waves. Therefore, Ca2+ puffs underlie inositol 1,4,5-trisphosphate signalling in diverse cell types and are focal points for regulation of cellular responses