Smoothed particle magnetohydrodynamic simulations of protostellar outflows with misaligned magnetic field and rotation axes

We have developed a modified form of the equations of smoothed particle magnetohydrodynamics which are stable in the presence of very steep density gradients. Using this formalism, we have performed simulations of the collapse of magnetised molecular cloud cores to form protostars and drive outflows. Our stable formalism allows for smaller sink particles (< 5 AU) than used previously and the investigation of the effect of varying the angle, {\theta}, between the initial field axis and the rotation axis. The nature of the outflows depends strongly on this angle: jet-like outflows are not produced at all when {\theta} > 30{\deg}, and a collimated outflow is not sustained when {\theta} > 10{\deg}. No substantial outflows of any kind are produced when {\theta} > 60{\deg}. This may place constraints on the geometry of the magnetic field in molecular clouds where bipolar outflows are seen.Comment: Accepted for publication in MNRAS, 13 pages, 14 figures. Animations can be found at http://www.astro.ex.ac.uk/people/blewis/research/outflows_misaligned_fields.htm

Measurements on fully wetted and ventilated ring wing hydrofoils

Force measurements and visual observations were made in a water tunnel on fully wetted and ventilated flows past a family of conical ring wings having a flat plate section geometry. The diameter-chord ratio was varied from one to three, and the total included cone angle was 12 degrees. The fully wetted flows all exhibited separation from the leading edge except for the largest diameter-chord ratio, a result which was in agreement with previous work. The effect of ventilation is to reduce markedly the lift curve slope. Pressure distribution measurements were also made under ventilating conditions for one member of this series. The effect of ventilation over only a portion of the circumference of the ring was also briefly investigated. Large cross forces were developed by such ventilation and some comparisons are made between this method of obtaining control forces and more conventional methods

Transcranial electric stimulation and cognitive training improves face perception

Recently, there has been much interest the effectiveness of cognitive training programmes across a variety of cognitive and perceptual domains. Some evidence suggests that combining training programmes with noninvasive brain stimulation techniques such as transcranial random noise stimulation (tRNS) can enhance training gains, but to date this has only been examined in numerosity and arithmetic tasks. In this study, we examined whether tRNS modulated the effects of a face recognition training programme. Participants completed a face discrimination training task for an hour per day over five days. Each day, training was preceded by twenty minutes of active high frequency tRNS or sham stimulation, targeted at the posterior temporal cortices or the inferior frontal gyri (IFG). Participants who received active stimulation to the posterior temporal cortices showed significant improvement on a facial identity discrimination task (the Cambridge Face Perception Test) after training, whereas those receiving sham or IFG stimulation showed no performance change. There was no evidence of an effect of stimulation on a face memory task (the Cambridge Face Memory Test). These results suggest that tRNS can enhance the effectiveness of cognitive training programmes, but further work is needed to establish whether perceptual gains can be generalised to face memory

Superior shoulder suspensory complex fracture dislocation case report

Background: Acromioclavicular joint dislocation can be more complex than it first appears. The presented case had an unusual combination of injuries to the superior shoulder suspensory complex, which yielded some interesting learning points. Case summary: The injuries were sustained after a fall from a push bike and included acromioclavicular dislocation with coracoid process, clavicle and acromion process fractures. These were identified on the initial X-ray examination, which was followed by computed tomography for surgical planning. The injuries were successfully treated by internal fixation. Conclusion: The unexpected complexity of the injuries could have led to subtle but important findings being overlooked. This case highlights the importance of a thorough search strategy, consideration of injury biomechanics and knowledge of associated injuries

The Properties of Prestellar Discs in Isolated and Multiple Prestellar Systems

We present high-resolution 3D smoothed particle hydrodynamics simulations of the formation and evolution of protostellar discs in a turbulent molecular cloud. Using a piecewise polytropic equation of state, we perform two sets of simulations. In both cases we find that isolated systems undergo a fundamentally different evolution than members of binary or multiple systems. When formed, isolated systems must accrete mass and increase their specific angular momentum, leading to the formation of massive, extended discs, which undergo strong gravitational instabilities and are susceptible to disc fragmentation. Fragments with initial masses of 5.5 M_jup, 7.4 M_jup and 12 M_jup are produced in our simulations. In binaries and small clusters, we observe that due to competition for material from the parent core, members do not accrete significant amounts of high specific angular momentum gas relative to isolated systems. We find that discs in multiple systems are strongly self-gravitating but that they are stable against fragmentation due to disc truncation and mass profile steeping by tides, accretion of high specific angular momentum gas by other members, and angular momentum being redirected into members' orbits. In general, we expect disc fragmentation to be less likely in clusters and to be more a feature of isolated systems.Comment: 15 pages, 21 figures. Accepted for publication in MNRA

Gravitational Collapse in Turbulent Molecular Clouds. I. Gasdynamical Turbulence

Observed molecular clouds often appear to have very low star formation efficiencies and lifetimes an order of magnitude longer than their free-fall times. Their support is attributed to the random supersonic motions observed in them. We study the support of molecular clouds against gravitational collapse by supersonic, gas dynamical turbulence using direct numerical simulation. Computations with two different algorithms are compared: a particle-based, Lagrangian method (SPH), and a grid-based, Eulerian, second-order method (ZEUS). The effects of both algorithm and resolution can be studied with this method. We find that, under typical molecular cloud conditions, global collapse can indeed be prevented, but density enhancements caused by strong shocks nevertheless become gravitationally unstable and collapse into dense cores and, presumably, stars. The occurance and efficiency of local collapse decreases as the driving wave length decreases and the driving strength increases. It appears that local collapse can only be prevented entirely with unrealistically short wave length driving, but observed core formation rates can be reproduced with more realistic driving. At high collapse rates, cores are formed on short time scales in coherent structures with high efficiency, while at low collapse rates they are scattered randomly throughout the region and exhibit considerable age spread. We suggest that this naturally explains the observed distinction between isolated and clustered star formation.Comment: Minor revisions in response to referee, thirteen figures, accepted to Astrophys.

Forming the First Stars in the Universe: The Fragmentation of Primordial Gas

In order to constrain the initial mass function (IMF) of the first generation of stars (Population III), we investigate the fragmentation properties of metal-free gas in the context of a hierarchical model of structure formation. We investigate the evolution of an isolated 3-sigma peak of mass 2x10^6 M_solar which collapses at z_coll=30 using Smoothed Particle Hydrodynamics. We find that the gas dissipatively settles into a rotationally supported disk which has a very filamentary morphology. The gas in these filaments is Jeans unstable with M_J~10^3 M_solar. Fragmentation leads to the formation of high density (n>10^8 cm^-3) clumps which subsequently grow in mass by accreting surrounding gas and by merging with other clumps up to masses of ~10^4 M_solar. This suggests that the very first stars were rather massive. We explore the complex dynamics of the merging and tidal disruption of these clumps by following their evolution over a few dynamical times.Comment: 7 pages, 3 figures, uses emulateapj.sty. Accepted for publication in the Astrophysical Journal Letter