2,809 research outputs found
Spatially extended OH+ emission from the Orion Bar and Ridge
We report the first detection of a Galactic source of OH+ line emission: the
Orion Bar, a bright nearby photon-dominated region. Line emission is detected
over ~1' (0.12 pc), tracing the Bar itself as well as the Southern tip of the
Orion Ridge. The line width of ~4 km/s suggests an origin of the OH+ emission
close to the PDR surface, at a depth of A_V ~0.3-0.5 into the cloud where most
hydrogen is in atomic form. Steady-state collisional and radiative excitation
models require unrealistically high OH+ column densities to match the observed
line intensity, indicating that the formation of OH+ in the Bar is rapid enough
to influence its excitation. Our best-fit OH+ column density of ~1x10^14 cm^-2
is similar to that in previous absorption line studies, while our limits on the
ratios of OH+/H2O+ (>~40) and OH+/H3O+ (>~15) are higher than seen before.
The column density of OH+ is consistent with estimates from a thermo-chemical
model for parameters applicable to the Orion Bar, given the current
uncertainties in the local gas pressure and the spectral shape of the ionizing
radiation field. The unusually high OH+/H2O+ and OH+/H3O+ ratios are probably
due to the high UV radiation field and electron density in this object. In the
Bar, photodissociation and electron recombination are more effective destroyers
of OH+ than the reaction with H2, which limits the production of H2O+. The
appearance of the OH+ lines in emission is the result of the high density of
electrons and H atoms in the Orion Bar, since for these species, inelastic
collisions with OH+ are faster than reactive ones. In addition, chemical
pumping, far-infrared pumping by local dust, and near-UV pumping by Trapezium
starlight contribute to the OH+ excitation. Similar conditions may apply to
extragalactic nuclei where OH+ lines are seen in emission.Comment: Accepted by A&A; 10 pages, 5 figure
The high seas (C’s) of music piracy in information systems : cost, convenience and choice
Systems and technologies used for unauthorised file sharing have received little attention in the Information Systems literature. This paper attempts to fill this gap by presenting a critical, qualitative study on the motivations for using unauthorised file sharing systems. Based on 30 interviews with music consumers, musicians, and the music industry, this paper reports on the decision of music consumers to ‘pirate or purchase’. This paper highlights file sharing from multiple perspectives of users, musicians, and representatives from the music recording industry. Three main themes emerged on the cost, convenience and choice as motivators for unauthorised file sharing.<br /
Use of the MATRIXx Integrated Toolkit on the Microwave Anisotropy Probe Attitude Control System
Recent advances in analytical software tools allow the analysis, simulation, flight code, and documentation of an algorithm to be generated from a single source, all within one integrated analytical design package. NASA's Microwave Anisotropy Probe project has used one such package, Integrated Systems' MATRIXx suite, in the design of the spacecraft's Attitude Control System. The project's experience with the linear analysis, simulation, code generation, and documentation tools will be presented and compared with more traditional development tools. In particular, the quality of the flight software generated will be examined in detail. Finally, lessons learned on each of the tools will be shared
A Hierarchical Filtering-Based Monitoring Architecture for Large-scale Distributed Systems
On-line monitoring is essential for observing and improving the reliability and performance of large-scale distributed (LSD) systems. In an LSD environment, large numbers of events are generated by system components during their execution and interaction with external objects (e.g. users or processes). These events must be monitored to accurately determine the run-time behavior of an LSD system and to obtain status information that is required for debugging and steering applications. However, the manner in which events are generated in an LSD system is complex and represents a number of challenges for an on-line monitoring system. Correlated events axe generated concurrently and can occur at multiple locations distributed throughout the environment. This makes monitoring an intricate task and complicates the management decision process. Furthermore, the large number of entities and the geographical distribution inherent with LSD systems increases the difficulty of addressing traditional issues, such as performance bottlenecks, scalability, and application perturbation.
This dissertation proposes a scalable, high-performance, dynamic, flexible and non-intrusive monitoring architecture for LSD systems. The resulting architecture detects and classifies interesting primitive and composite events and performs either a corrective or steering action. When appropriate, information is disseminated to management applications, such as reactive control and debugging tools.
The monitoring architecture employs a novel hierarchical event filtering approach that distributes the monitoring load and limits event propagation. This significantly improves scalability and performance while minimizing the monitoring intrusiveness. The architecture provides dynamic monitoring capabilities through: subscription policies that enable applications developers to add, delete and modify monitoring demands on-the-fly, an adaptable configuration that accommodates environmental changes, and a programmable environment that facilitates development of self-directed monitoring tasks. Increased flexibility is achieved through a declarative and comprehensive monitoring language, a simple code instrumentation process, and automated monitoring administration. These elements substantially relieve the burden imposed by using on-line distributed monitoring systems. In addition, the monitoring system provides techniques to manage the trade-offs between various monitoring objectives.
The proposed solution offers improvements over related works by presenting a comprehensive architecture that considers the requirements and implied objectives for monitoring large-scale distributed systems. This architecture is referred to as the HiFi monitoring system.
To demonstrate effectiveness at debugging and steering LSD systems, the HiFi monitoring system has been implemented at the Old Dominion University for monitoring the Interactive Remote Instruction (IRI) system. The results from this case study validate that the HiFi system achieves the objectives outlined in this thesis
The High Seas (C\u27s) of Piracy in Information Systems: Cost, convenience and choice
Systems and technologies used for unauthorised file sharing have received little attention in the Information Systems literature. This paper attempts to fill this gap by presenting a critical, qualitative study on the motivations for using unauthorised file sharing systems. Based on 30 interviews with music consumers, musicians, and the music industry, this paper reports on the decision of music consumers to ‘pirate or purchase’. This paper highlights file sharing from multiple perspectives of users, musicians, and representatives from the music recording industry. Three main themes emerged on the cost, convenience and choice as motivators for unauthorised file sharing
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