The effect of deposition forming by metal inert gas welding and rolling on the properties of steels

Imperial Users onl

On the use of electron-multiplying CCDs for astronomical spectroscopy

Conventional CCD detectors have two major disadvantages: they are slow to read out and they suffer from read noise. These problems combine to make high-speed spectroscopy of faint targets the most demanding of astronomical observations. It is possible to overcome these weaknesses by using electron-multiplying CCDs (EMCCDs). EMCCDs are conventional frame-transfer CCDs, but with an extended serial register containing high-voltage electrodes. An avalanche of secondary electrons is produced as the photon-generated electrons are clocked through this register, resulting in signal amplification that renders the read noise negligible. Using a combination of laboratory measurements with the QUCAM2 EMCCD camera and Monte Carlo modelling, we show that it is possible to significantly increase the signal-to-noise ratio of an observation by using an EMCCD, but only if it is optimised and utilised correctly. We also show that even greater gains are possible through the use of photon counting. We present a recipe for astronomers to follow when setting up a typical EMCCD observation which ensures that maximum signal-to-noise ratio is obtained. We also discuss the benefits that EMCCDs would bring if used with the next generation of extremely large telescopes. Although we mainly consider the spectroscopic use of EMCCDs, our conclusions are equally applicable to imaging.Comment: 18 figures, 3 tables, 18 page

A Self-Occulting Accretion Disk in the SW Sex Star DW UMa

We present the ultraviolet spectrum of the SW Sex star and nova-like variable DW UMa in an optical low state, as observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). The data are well described by a synthetic white dwarf (WD) spectrum with T_eff = 46,000 +/- 1000 K, log g = 7.60 +/- 0.15, v*sin(i) = 370 +/- 100 km/s and Z/Z_solar = 0.47 +/- 0.15. For this combination of T_eff and log g, WD models predict M_WD = 0.48 +/- 0.06 M_solar and R_WD = (1.27 +/- 0.18) * 10^9 cm. Combining the radius estimate with the normalization of the spectral fit, we obtain a distance estimate of d = 830 +/-150 pc. During our observations, DW UMa was approximately 3 magnitudes fainter in V than in the high state. A comparison of our low-state HST spectrum to a high-state spectrum obtained with the International Ultraviolet Explorer shows that the former is much bluer and has a higher continuum level shortward of 1450 A. Since DW UMa is an eclipsing system, this suggests that an optically thick accretion disk rim blocks our view of the WD primary in the high state. If self-occulting accretion disks are common among the SW Sex stars, we can account for (i) the preference for high-inclination systems within the class and (ii) their V-shaped continuum eclipses. Moreover, even though the emission lines produced by a self-obscured disk are generally still double-peaked, they are weaker and narrower than those produced by an unobscured disk. This may allow a secondary line emission mechanism to dominate and produce the single-peaked, optical lines that are a distinguishing characteristic of the SW Sex stars.Comment: 9 pages, including 2 figures; accepted for publication in Astrophysical Journal Letters; New version matches version in press (footnote added to discussion section; figures now use color

Information Aggregation in Exponential Family Markets

We consider the design of prediction market mechanisms known as automated market makers. We show that we can design these mechanisms via the mold of \emph{exponential family distributions}, a popular and well-studied probability distribution template used in statistics. We give a full development of this relationship and explore a range of benefits. We draw connections between the information aggregation of market prices and the belief aggregation of learning agents that rely on exponential family distributions. We develop a very natural analysis of the market behavior as well as the price equilibrium under the assumption that the traders exhibit risk aversion according to exponential utility. We also consider similar aspects under alternative models, such as when traders are budget constrained

An observational test of common-envelope evolution

By analysing and modelling the change in the abundance ratio of $^{12}$C/$^{13}$C and $^{16}$O/$^{17}$O on the surface of the lower mass star of a binary during the common-envelope (CE) phase of evolution, we propose a simple observational test of the CE scenario. The test is based on the infrared measurement of either the $^{12}$C/$^{13}$C or $^{16}$O/$^{17}$O ratio of red dwarfs in post-common envelope binaries (PCEB's). In certain cases (main-sequence red dwarf secondaries in PCEB's without planetary nebulae), as well as determining whether or not accretion has occurred during the CE phase, we can determine the amount of mass accreted during the CE phase and hence the initial mass of the red dwarf component prior to the CE phase. In the other cases considered (low-mass red dwarfs in PCEB's and red dwarf's in PCEB's with planetary nebulae) we can only say whether or not accretion has occurred during the CE phase.Comment: uuencoded compressed postscript. The preprint are also available at URL http://www.ast.cam.ac.uk/preprint/PrePrint.htm

Dog Population Management and Dog Bite Prevention in Rural and Remote Northern Saskatchewan Aboriginal Communities

Communities employ a wide variety of methods to reduce critical encounters and dog population numbers. However, systematic studies evaluating the success of approaches and techniques are currently lacking. Nor has significant consideration of community decision-making processes and policy development, or of the long-term sustainability of these programs been completed. Therefore, to assess the perception of dog-related issues, methods of policy creation and implementation, and true within-community dog demographic characteristics and rate of aggressive encounters a community-based research project was developed. A multiphase, convergent mixed methods study design in four separate northern Saskatchewan communities was implemented to evaluate these concerns. Methods of community-driven policy creation and implementation were recorded, management plans and strategies were monitored, and options were evaluated for successful reduction in dog bites and violent encounters. Community-based participatory methods created exchange and discussion with all levels of society, providing in-depth two-way channels for knowledge translation for researchers and community members. Policy creation and implementation was found to vary significantly between communities. Policies surrounding dog ownership and bite prevention are often dependent upon perceived risks for imminent human-canine aggressive encounters. Regrettably, sustainable interventions require sustained key community partner support and resource access. Community engagement and knowledge translation creates long-term, trusting relationships permitting more in-depth understanding of group choices. In addition, involving community members in research and data collection provides public appreciation of the scope and breadth of community issues and opinions. Enabling and empowering communities entails constant communication and education of all parties. No single model can be effective in all situations. Although enforceable legislation and widespread canine sterilisation are key aspects for community dog issues, comprehensive all-inclusive community education is indispensable. Wide-spread education and communication have the potential to dramatically decrease the number of aggressive dog:human encounters and fulfil goals for dog-human relationships that occur in indigenous communities in Canada

Power Relationships in Information Systems Security Policy Formulation and Implementation

This research argues that organizational power impacts the development and implementation of Information Systems (IS) Security policy. The study was conducted via an in depth case study at the IT department within a large financial organization in the United States. The theoretical foundation for the research was based was Clegg’s (2002) Circuits of Power. A conceptual framework was created utilizing Circuits of Power. This was used to study power relationships and how they might affect the formulation and implementation of IS Security policy in this organization. The case study demonstrated that power relationships have a clear impact on the IS security policy process. Though there is a strong security culture at the organization and a well defined set of processes, an improvement in the process and ensuing security culture is possible by accounting for the effect of power relationships

LS Peg: A Low-Inclination SW Sextantis-Type Cataclysmic Binary with High-Velocity Balmer Emission Line Wings

We present time-resolved spectroscopy and photometry of the bright cataclysmic variable LS Peg (= S193). The Balmer lines exhibit broad, asymmetric wings Doppler-shifted by about 2000 km/s at the edges, while the HeI lines show phase-dependent absorption features strikingly similar to SW Sextantis stars, as well as emission through most of the phase. The CIII/NIII emission blend does not show any phase dependence. From velocities of Halpha emission lines, we determine an orbital period of 0.174774 +/- 0.000003 d (= 4.1946 h), which agrees with Szkody's (1995) value of approximately 4.2 hours. No stable photometric signal was found at the orbital period. A non-coherent quasi-periodic photometric signal was seen at a period of 20.7 +/- 0.3 min. The high-velocity Balmer wings most probably arise from a stream re-impact point close to the white dwarf. We present simulated spectra based on a kinematic model similar to the modified disk-overflow scenario of Hellier & Robinson (1994). The models reproduce the broad line wings, though some other details are unexplained. Using an estimate of dynamical phase based on the model, we show that the phasing of the emission- and absorption-line variations is consistent with that in (eclipsing) SW Sex stars. We therefore identify LS Peg as a low-inclination SW Sex star. Our model suggests i = 30 deg, and the observed absence of any photometric signal at the orbital frequency establishes i < 60 deg. This constraint puts a severe strain on interpretations of the SW Sex phenomenon which rely on disk structures lying slightly out of the orbital plane.Comment: 29 pages, 13 figures, to be published in PASP Feb. 199

A Risk Comparison of Ordinary Least Squares vs Ridge Regression

We compare the risk of ridge regression to a simple variant of ordinary least squares, in which one simply projects the data onto a finite dimensional subspace (as specified by a Principal Component Analysis) and then performs an ordinary (un-regularized) least squares regression in this subspace. This note shows that the risk of this ordinary least squares method is within a constant factor (namely 4) of the risk of ridge regression.Comment: Appearing in JMLR 14, June 201