Tilted accretion discs in cataclysmic variables: tidal instabilities and superhumps

We investigate the growth of tidal instabilities in accretion discs in a binary star potential, using three dimensional numerical simulations. As expected from analytic work, the disc is prone to an eccentric instability provided that it is large enough to extend to the 3:1 resonance. The eccentric disc leads to positive superhumps in the light curve. It has been proposed that negative superhumps might arise from a tilted disc, but we find no evidence that the companion gravitational tilt instability can grow fast enough in a fluid disc to create a measurable inclination. The origin of negative superhumps in the light curves of cataclysmic variables remains a puzzle.Comment: 7 pages, 7 figures, accepted for publication in MNRA

Phase Transitions and superuniversality in the dynamics of a self-driven particle

We study an active random walker model in which a particle's motion is determined by a self-generated field. The field encodes information about the particle's path history. This leads to either self-attractive or self-repelling behavior. For self-repelling behavior, we find a phase transition in the dynamics: when the coupling between the field and the walker exceeds a critical value, the particle's behavior changes from renormalized diffusion to one characterized by a diverging diffusion coefficient. The dynamical behavior for all cases is surprisingly independent of dimension and of the noise amplitude.Comment: 14 pages, 4 figure

The effects of tidally induced disc structure on white dwarf accretion in intermediate polars

We investigate the effects of tidally induced asymmetric disc structure on accretion onto the white dwarf in intermediate polars. Using numerical simulation, we show that it is possible for tidally induced spiral waves to propagate sufficiently far into the disc of an intermediate polar that accretion onto the central white dwarf could be modulated as a result. We suggest that accretion from the resulting asymmetric inner disc may contribute to the observed X-ray and optical periodicities in the light curves of these systems. In contrast to the stream-fed accretion model for these periodicities, the tidal picture predicts that modulation can exist even for systems with weaker magnetic fields where the magnetospheric radius is smaller than the radius of periastron of the mass transfer stream. We also predict that additional periodic components should exist in the emission from low mass ratio intermediate polars displaying superhumps.Comment: 9 pages, 5 figures, accepted for publication in MNRA

Lifetime cost effectiveness of simvastatin in a range of risk groups and age groups derived from a randomised trial of 20,536 people

<i>Objectives</i>: To evaluate the cost effectiveness of 40 mg simvastatin daily continued for life in people of different ages with differing risks of vascular disease. Design A model developed from a randomised trial was used to estimate lifetime risks of vascular events and costs of treatment and hospital admissions in the United Kingdom. <i>Setting</i>: 69 hospitals in the UK. <i>Participants</i>: 20 536 men and women (aged 40-80) with coronary disease, other occlusive arterial disease, or diabetes. <i>Interventions</i>: 40 mg simvastatin daily versus placebo for an average of 5 years. <i>Main</i> <i>outcome</i> <i>measures</i>: Cost effectiveness of 40 mg simvastatin daily expressed as additional cost per life year gained. Major vascular event defined as non-fatal myocardial infarction or death from coronary disease, any stroke, or revascularisation procedure. Results were extrapolated to younger and older age groups at lower risk of vascular disease than were studied directly, as well as to lifetime treatment. <i>Results</i>: At the April 2005 UK price of £4.87 (€7; $9) per 28 day pack of generic 40 mg simvastatin, lifetime treatment was cost saving in most age groups and vascular disease risk groups studied directly. Gains in life expectancy and cost savings decreased with increasing age and with decreasing risk of vascular disease. People aged 40-49 with 5 year risks of major vascular events of 42% and 12% at start of treatment gained 2.49 and 1.67 life years, respectively. Treatment with statins remained cost saving or cost less than £2500 per life year gained in people as young as 35 years or as old as 85 with 5 year risks of a major vascular event as low as 5% at the start of treatment. <i>Conclusions</i>: Treatment with statins is cost effective in a wider population than is routinely treated at present

Evidence that process simulations reduce anxiety in patients receiving dental treatment: randomized exploratory trial

Process simulations – mental simulations that ask people to imagine the process of completing a task – have been shown to decrease anxiety in students facing hypothetical or psychological threats in the short term. The aim of the present study was to see whether process simulations could reduce anxiety in a sample of the general population attending a dental practice, and whether these effects could be sustained throughout treatment. Participants (N = 75) were randomized to an experimental condition where they were asked to simulate mentally the process of seeing the dentist, or to a control condition where they were asked to simulate mentally the outcome of seeing the dentist. Findings showed that participants in the experimental condition were significantly less anxious both before and after their consultations. Self-efficacy and self-esteem remained unchanged. This study suggests that process simulation is one active ingredient in anxiety treatment programs and further research is required to enhance its effects

Travelling waves in a nonlinear degenerate diffusion model for bacterial pattern formation

We study a reaction diffusion model recently proposed in [5] to describe the spatiotemporal evolution of the bacterium Bacillus subtilis on agar plates containing nutrient. An interesting mathematical feature of the model, which is a coupled pair of partial differential equations, is that the bacterial density satisfies a degenerate nonlinear diffusion equation. It was shown numerically that this model can exhibit quasi-one-dimensional constant speed travelling wave solutions. We present an analytic study of the existence and uniqueness problem for constant speed travelling wave solutions. We find that such solutions exist only for speeds greater than some threshold speed giving minimum speed waves which have a sharp profile. For speeds greater than this minimum speed the waves are smooth. We also characterise the dependence of the wave profile on the decay of the front of the initial perturbation in bacterial density. An investigation of the partial differential equation problem establishes,via a global existence and uniqueness argument, that these waves are the only long time solutions supported by the problem. Numerical solutions of the partial differential equation problem are presented and they confirm the results of the analysis