39 research outputs found
Formation and evolution of compact binaries in globular clusters: II. Binaries with neutron stars
In this paper, the second of a series, we study the stellar dynamical and
evolutionary processes leading to the formation of compact binaries containing
neutron stars (NSs) in dense globular clusters (GCs). For this study, 70 dense
clusters were simulated independently, with a total stellar mass ~2x10^7Msun,
exceeding the total mass of all dense GCs in our Galaxy.
We find that, in order to reproduce the empirically derived formation rate of
low-mass X-ray binaries (LMXBs), we must assume that NSs can be formed via
electron-capture supernovae (ECS) with typical natal kicks smaller than in
core-collapse supernovae. Our results explain the observed dependence of the
number of LMXBs on ``collision number'' as well as the large scatter observed
between different GCs. We predict that the number of quiescent LMXBs in
different GCs should not have a strong metallicity dependence. In our cluster
model the following mass-gaining events create populations of MSPs that do not
match the observations: (i) accretion during a common envelope event with a NS
formed through ECS, and (ii) mass transfer (MT) from a WD donor. Some processes
lead only to a mild recycling. In addition, for MSPs, we distinguish
low-magnetic-field (long-lived) and high-magnetic-field (short-lived)
populations. With this distinction and by considering only those mass-gaining
events that appear to lead to NS recycling, we obtain good agreement of our
models with the numbers and characteristics of observed MSPs in 47 Tuc and
Terzan 5, as well as with the cumulative statistics for MSPs detected in GCs of
different dynamical properties. We find that significant production of merging
double NSs potentially detectable as short gamma-ray bursts occurs only in very
dense, most likely core-collapsed GCs. (abridged)Comment: 25 pages, 7 figures, 12 tables, MNRAS accepte
Binary and Millisecond Pulsars at the New Millennium
We review the properties and applications of binary and millisecond pulsars.
Our knowledge of these exciting objects has greatly increased in recent years,
mainly due to successful surveys which have brought the known pulsar population
to over 1300. There are now 56 binary and millisecond pulsars in the Galactic
disk and a further 47 in globular clusters. This review is concerned primarily
with the results and spin-offs from these surveys which are of particular
interest to the relativity community.Comment: 59 pages, 26 figures, 5 tables. Accepted for publication in Living
Reviews in Relativity (http://www.livingreviews.org
Federal Consumer Protection and the Toy Industry: The Dilemma of Legislative Exclusion
This article will explore how and why an industry of this magnitude was excluded from coverage under the Consumer Product Safety Act and what effect this exclusion has on consumer safety. It will look at the history and present status of federal regulation of the toy industry and conclude with a recommendation for a system more equitable to both consumer and manufacturer
15 years of pond assessment in Britain: results and lessons learned from the work of Pond Conservation
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The ecological niche of Daphnia magna characterized using population growth rate
The concept of an organism's niche is central to ecological theory, but an operational definition is needed that allows both its experimental delineation and interpretation of field distributions of the species. Here we use population growth rate (hereafter, pgr) to de. ne the niche as the set of points in niche space where pgr. 0. If there are just two axes to the niche space, their relationship to pgr can be pictured as a contour map in which pgr varies along the axes in the same way that the height of land above sea level varies with latitude and longitude. In laboratory experiments we measured the pgr of Daphnia magna over a grid of values of pH and Ca2+, and so defined its "laboratory niche'' in pH-Ca2+ space. The position of the laboratory niche boundary suggests that population persistence is only possible above 0.5 mg Ca2+/L and between pH 5.75 and pH 9, though more Ca2+ is needed at lower pH values. To see how well the measured niche predicts the field distribution of D. magna, we examined relevant field data from 422 sites in England and Wales. Of the 58 colonized water bodies, 56 lay within the laboratory niche. Very few of the sites near the niche boundary were colonized, probably because pgr there is so low that populations are vulnerable to extinction by other factors. Our study shows how the niche can be quantified and used to predict field distributions successfully
Fractionated Total Body Irradiation in Bone Marrow Transplantation - An Emphasis on Lung Dosimetry
Access to single-visit IUD insertion at obstetrician-gynecology practices in Ohio: An audit study
Student learning in Australian high schools: Contrasting personological and contextual variables in a longitudinal structural model
Predicting Learning Outcomes in Distance Learning Universities: Perspectives from an Integrated Model
The progressive spread of online academic courses is a result of the flexible and customisable nature of the
related learning process, while some studies on studentsâ achievement in distance learning universities
have underlined retention as a priority issue for future research. Despite the number of studies that have
investigated specific variables related to online learning, there are no systemic reference models that
consider specific online environmental variables, IT competence and outcomes together. This paper offers
an integrated model to test the contribution of different variables in predicting student performance in
online academic courses, building on the literature on the digital learning environment and achievement.
The model, based on the initial Biggsâ 3P learning model, aims to evaluate technical competency and the
ability to self-manage as personal variables; furthermore, it proposes the analysis of a set of perceptions
related to course design. Through the proposed model, a studentâs background, personal variables,
perception of the physical learning environment and perception of the course design can be utilized as
predictors of student performance. Future research should investigate the applicability of the model in
academic distance learning contexts