The lower boundary of the accretion column in magnetic cataclysmic variables

Using a parameterised function for the mass loss at the base of the post-shock region, we have constructed a formulation for magnetically confined accretion flows which avoids singularities, such as the infinity in density, at the base associated with all previous formulations. With the further inclusion of a term allowing for the heat input into the base from the accreting white dwarf we are able also to obtain the hydrodynamic variables to match the conditions in the stellar atmosphere. (We do not, however, carry out a mutually consistent analysis for the match). Changes to the emitted X-ray spectra are negligible unless the thickness of mass leakage region at the base approaches or exceeds one percent of the height of the post-shock region. In this case the predicted spectra from higher-mass white dwarfs will be harder, and fits to X-ray data will predict lower white-dwarf masses than previous formulations.Comment: 13 pages, 6 figures, accepted for publication in MNRA

First X-ray observations of the polar CE Gru

We report the detection of the polar CE Gru in X-rays for the first time. We find evidence for a dip seen in the hard X-ray light curve which we attribute to the accretion stream obscuring the accretion region in the lower hemisphere of the white dwarf. The X-ray spectrum can be fitted using only a shock model: there is no distinct soft X-ray component. We suggest that this is because the reprocessed component is cool enough so that it is shifted into the UV. We determine a mass for the white dwarf of ~1.0Msun.Comment: Accepted for publication in MNRAS, 6 page

Open Learning Network: the evidence of OER impact

Much of the initial work on Open Educational Resources (OER) has inevitably concentrated on how to produce the resources themselves and to establish the idea in the community. It is now eight years since the term OER was first used and more than ten years since the concept of open content was described and a greater focus is now emerging on the way in which OER can influence policy and change the way in which educational systems help people learn. The Open University UK and Carnegie Mellon University are working in partnership on the OLnet (Open Learning Network), funded by The William and Flora Hewlett Foundation with the aims to search out the evidence for use and reuse of OER and to establish a network for information sharing about research in the field. This means both gathering evidence and developing approaches for how to research and understand ways to learn in a more open world, particularly linked to OER, but also looking at other influences

Logical Reduction of Metarules

International audienceMany forms of inductive logic programming (ILP) use metarules, second-order Horn clauses, to define the structure of learnable programs and thus the hypothesis space. Deciding which metarules to use for a given learning task is a major open problem and is a trade-off between efficiency and expressivity: the hypothesis space grows given more metarules, so we wish to use fewer metarules, but if we use too few metarules then we lose expressivity. In this paper, we study whether fragments of metarules can be logically reduced to minimal finite subsets. We consider two traditional forms of logical reduction: subsumption and entailment. We also consider a new reduction technique called derivation reduction, which is based on SLD-resolution. We compute reduced sets of metarules for fragments relevant to ILP and theoretically show whether these reduced sets are reductions for more general infinite fragments. We experimentally compare learning with reduced sets of metarules on three domains: Michalski trains, string transformations, and game rules. In general, derivation reduced sets of metarules outperform subsumption and entailment reduced sets, both in terms of predictive accuracies and learning times

Traffic fatalities and economic growth

The authors examine the impact of income growth on the death rate due to traffic fatalities, as well as on fatalities per motor vehicle and on the motorization rate (vehicles/population) using panel data from 1963-99 for 88 countries. Specifically, they estimate fixed effects models for fatalities/population, vehicles/population, and fatalities/vehicles and use these models to project traffic fatalities and the stock of motor vehicles to 2020.The relationship between motor vehicle fatality rate and per capita income at first increases with per capita income, reaches a peak, and then declines. This is because at low income levels the rate of increase in motor vehicles outpaces the decline in fatalities per motor vehicle. At higher income levels, the reverse occurs. The income level at which per capita traffic fatalities peaks is approximately $8,600 in 1985 international dollars. This is within the range of income at which other externalities, such as air and water pollution, have been found to peak. Projections of future traffic fatalities suggest that the global road death toll will grow by approximately 66 percent between 2000 and 2020. This number, however, reflects divergent rates of change in different parts of the world-a decline in fatalities in high-income countries of approximately 28 percent versus an increase in fatalities of almost 92 percent in China and 147 percent in India. The authors also predict that the fatality rate will rise to approximately 2 per 10,000 persons in developing countries by 2020, while it will fall to less than 1 per 10,000 in high-income countries.Economic Theory&Research,Environmental Economics&Policies,Fiscal&Monetary Policy,Roads&Highways,Economic Conditions and Volatility,Inter-Urban Roads and Passenger Transport,Roads&Highways,Economic Theory&Research,Environmental Economics&Policies,Inequality

Why have traffic fatalities declined in industrialized countries ? Implications for pedestrians and vehicle occupants

This paper examines whether the relationship between traffic fatalities and per capita income is the same for different classes of road users and investigates the factors underlying the decline in fatalities per vehicle kilometer traveled (VKT) observed in high-income countries over recent decades. Formal models of traffic fatalities are developed for vehicle occupants and pedestrians. Reduced-form approximations to these models are estimated using panel data for 32 high-income countries over 1964-2002. The results suggest that the downward-sloping portion of the curve relating traffic fatalities per capita to per capita income is due primarily to improved pedestrian safety. The more detailed models shed light on some factors influencing pedestrian fatalities per VKT, but much of the reduction in pedestrian fatalities remains unexplained. Increased motorization and a reduction in the proportion of young drivers in the population, however, clearly played a role. In contrast to pedestrian fatalities, occupant fatalities do not show a significant decline with income. What does explain declines in occupant fatalities per VKT are reductions in alcohol abuse and improved medical services, and a reduction in young drivers. The importance of demographic factors suggests that in countries where young persons (between 15 and 24 years of age) comprise an increasing share of the driving population, adopting policies to improve young driver education and reduce speeds will be crucial.

The Emission from Post-Shock Flows in mCVs

We re-examine the vertical structure of the post-shock flow in the accretion region of mCVs, and the X-ray emission as a function of height. We then predict X-ray light curves and phase-resolved spectra, taking into account the vertical structure, examine the implications and check whether the predicted heights are compatible with observation.Comment: 6 pages, to be published in the Proc of the Symp. to mark the 60th birthday of Brian Warne

Multi-epoch spectroscopy and XMM-Newton observations of RX J2115-58

We present phase resolved optical spectroscopy and X-ray light curves of the asynchronous polar RX J2115-58 as they change on the beat period.Comment: To be published in Proc Cape Town Workshop on mCVs, Dec 2002, 2 page