8,034 research outputs found
Modellling the benthic impact of coastal discharges
The discharge of effluent into coastal waters can lead to environmental degradation, and
proactive management of this practice is therefore desirable. This requires a capacity to predict environmental impacts. Benthic, impacts can be detected in the redox state of the upper part of the sediment column. A mechanistic model has therefore been developed that describes vertical gradients of sedimentary redox intensity (Eh).
The model is essentially a one dimensional multi-component reaction-transport model. Bioturbation is represented using a simple Fickian analogue, and irrigation is modelled as a non-local exchange process. Degradation of organic matter is characterised as six sequential metabolic reactions. Monod formulations combined with an inhibition scheme are used to model the utilisation of electron acceptors. 'Various other reactions are also included. The model equations are discretised. using finite differences, and solved for both steady state and transient cases. The redox intensity is calculated from, the predicted distribution of oxidants and reductants using a simple Nerstian approach. Consequently, only a limited interpretation can be placed on the calculated Eh profile.
A theoretical investigation of the model response demonstrates that both high data requirements and uncertainty in parameter values impedes the predictive application of the redox model. Hence, whilst the model can describe the development of vertical redox, gradients, and therefore has some diagnostic capacity, it does not represent a viable tool that would aid in the proactive management of coastal resources. An alternative modelling approach is, however, suggested that would use predicted carbon fluxes to give a qualitative expression of benthic impac
NICMOS and VLBA observations of the gravitational lens system B1933+503
NICMOS observations of the complex gravitational lens system B1933+503 reveal
infrared counterparts to two of the inverted spectrum radio images. The
infrared images have arc-like structures. The corresponding radio images are
also detected in a VLBA map made at 1.7 GHz with a resolution of 6 mas. We fail
to detect two of the four inverted radio spectrum components with the VLBA even
though they are clearly visible in a MERLIN map at the same frequency at a
different epoch. The absence of these two components could be due to rapid
variability on a time-scale less than the time delay, or to broadening of the
images during propagation of the radio waves through the ISM of the lensing
galaxy to an extent that they fall below the surface brightness detectability
threshold of the VLBA observations. The failure to detect the same two images
with NICMOS is probably due to extinction in the ISM of the lensing galaxy.Comment: 5 pages, 4 figures, submitted to MNRA
Growth to early adulthood following extremely preterm birth: the EPICure study.
OBJECTIVE: To investigate growth trajectories from age 2.5 to 19 years in individuals born before 26 weeks of gestation (extremely preterm; EP) compared with term-born controls. METHODS: Multilevel modelling of growth data from the EPICure study, a prospective 1995 birth cohort of 315 EP participants born in the UK and Ireland and 160 term-born controls recruited at school age. Height, weight, head circumference and body mass index (BMI) z-scores were derived from UK standards at ages 2.5, 6, 11 and 19 years. RESULTS: 129 (42%) EP children were assessed at 19 years. EP individuals were on average 4.0 cm shorter and 6.8 kg lighter with a 1.5 cm smaller head circumference relative to controls at 19 years. Relative to controls, EP participants grew faster in weight by 0.06 SD per year (95% CI 0.05 to 0.07), in head circumference by 0.04 SD (95% CI 0.03 to 0.05), but with no catch-up in height. For the EP group, because of weight catch-up between 6 and 19 years, BMI was significantly elevated at 19 years to +0.32 SD; 23.4% had BMI >25 kg/m2 and 6.3% >30 kg/m2 but these proportions were similar to those in control subjects. EP and control participants showed similar pubertal development in early adolescence, which was not associated with height at 19 years in either study group. Growth through childhood was related to birth characteristics and to neonatal feeding practices. CONCLUSIONS: EP participants remained shorter and lighter and had smaller head circumferences than reference data or controls in adulthood but had elevated BMI
Assembling thefacebook: Using heterogeneity to understand online social network assembly
Online social networks represent a popular and diverse class of social media
systems. Despite this variety, each of these systems undergoes a general
process of online social network assembly, which represents the complicated and
heterogeneous changes that transform newly born systems into mature platforms.
However, little is known about this process. For example, how much of a
network's assembly is driven by simple growth? How does a network's structure
change as it matures? How does network structure vary with adoption rates and
user heterogeneity, and do these properties play different roles at different
points in the assembly? We investigate these and other questions using a unique
dataset of online connections among the roughly one million users at the first
100 colleges admitted to Facebook, captured just 20 months after its launch. We
first show that different vintages and adoption rates across this population of
networks reveal temporal dynamics of the assembly process, and that assembly is
only loosely related to network growth. We then exploit natural experiments
embedded in this dataset and complementary data obtained via Internet
archaeology to show that different subnetworks matured at different rates
toward similar end states. These results shed light on the processes and
patterns of online social network assembly, and may facilitate more effective
design for online social systems.Comment: 13 pages, 11 figures, Proceedings of the 7th Annual ACM Web Science
Conference (WebSci), 201
A Bayesian Analogue of Gleason's Theorem
We introduce a novel notion of probability within quantum history theories
and give a Gleasonesque proof for these assignments. This involves introducing
a tentative novel axiom of probability. We also discuss how we are to interpret
these generalised probabilities as partially ordered notions of preference and
we introduce a tentative generalised notion of Shannon entropy. A Bayesian
approach to probability theory is adopted throughout, thus the axioms we use
will be minimal criteria of rationality rather than ad hoc mathematical axioms.Comment: 14 pages, v2: minor stylistic changes, v3: changes made in-line with
to-be-published versio
CFHT AO Imaging of the CLASS Gravitational Lens System B1359+154
We present adaptive optics imaging of the CLASS gravitational lens system
B1359+154 obtained with the Canada-France-Hawaii Telescope (CFHT) in the
infrared K band. The observations show at least three brightness peaks within
the ring of lensed images, which we identify as emission from multiple lensing
galaxies. The results confirm the suspected compound nature of the lens, as
deduced from preliminary mass modeling. The detection of several additional
nearby galaxies suggests that B1359+154 is lensed by the compact core of a
small galaxy group. We attempted to produce an updated lens model based on the
CFHT observations and new 5-GHz radio data obtained with the MERLIN array, but
there are too few constraints to construct a realistic model at this time. The
uncertainties inherent with modeling compound lenses make B1359+154 a
challenging target for Hubble constant determination through the measurement of
differential time delays. However, time delays will offer additional
constraints to help pin down the mass model. This lens system therefore
presents a unique opportunity to directly measure the mass distribution of a
galaxy group at intermediate redshift.Comment: 12 pages including 3 figures; ApJL accepte
Gravitational lensing statistics with extragalactic surveys. II. Analysis of the Jodrell Bank-VLA Astrometric Survey
We present constraints on the cosmological constant from
gravitational lensing statistics of the Jodrell Bank-VLA Astrometric Survey
(JVAS). Although this is the largest gravitational lens survey which has been
analysed, cosmological constraints are only comparable to those from optical
surveys. This is due to the fact that the median source redshifts of JVAS are
lower, which leads to both relatively fewer lenses in the survey and a weaker
dependence on the cosmological parameters. Although more approximations have to
be made than is the case for optical surveys, the consistency of the results
with those from optical gravitational lens surveys and other cosmological tests
indicate that this is not a major source of uncertainty in the results.
However, joint constraints from a combination of radio and optical data are
much tighter. Thus, a similar analysis of the much larger Cosmic Lens All-Sky
Survey should provide even tighter constraints on the cosmological constant,
especially when combined with data from optical lens surveys.
At 95% confidence, our lower and upper limits on ,
using the JVAS lensing statistics information alone, are respectively -2.69 and
0.68. For a flat universe, these correspond to lower and upper limits on
\lambda_{0} of respectively -0.85 and 0.84. Using the combination of JVAS
lensing statistics and lensing statistics from the literature as discussed in
Quast & Helbig (Paper I) the corresponding values are
-1.78 and 0.27. For a flat universe, these correspond to lower and upper limits
on of respectively -0.39 and 0.64.Comment: LaTeX, 9 pages, 18 PostScript files in 6 figures. Paper version
available on request. Data available from
http://gladia.astro.rug.nl:8000/ceres/data_from_papers/papers.htm
Academic attainment and special educational needs in extremely preterm children at 11 years of age : the EPICure Study
Aim: To assess academic attainment and special educational needs (SEN) in extremely preterm (EP) children in middle childhood.
Methods: Of 307 EP (=25 weeks) survivors born in the UK and Ireland in 1995, 219 (71%) were re-assessed at 11 years, with a comparison group of 153 classmates born at term, using standardised tests of cognitive ability and academic attainment and teacher reports of school performance and special educational needs (SEN). Multiple imputation was used to correct for selective dropout.
Results: EP children had significantly lower scores than classmates for cognitive ability (-20 points; 95%CI: -23,-17), reading (-18 points; -22,-15) and mathematics (-27 points; -31,-23). Twenty-nine (13%) EP children attended special school. In mainstream schools, 105 (57%) EP children had SEN (OR: 10; 6, 18) and 103 (55%) required SEN resource provision (OR: 10; 5, 18). Teachers rated 50% of EP children with attainment below the average range compared with 5% of classmates (OR: 18; CI: 8, 41). EP children who are entered for mainstream education an academic year early due to preterm birth had similar academic attainment but required more SEN support (OR: 2; 1.1,3.8).
Conclusions: EP survivors remain at high risk for learning impairments and poor academic attainment in middle childhood. A significant proportion require full-time specialist education and over half of those attending mainstream schools require additional health or educational resources in order to access the national curriculum. The prevalence and impact of SEN is likely to increase as these children approach the transition to secondary school.
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Laser Guide Star for Large Segmented-Aperture Space Telescopes, Part I: Implications for Terrestrial Exoplanet Detection and Observatory Stability
Precision wavefront control on future segmented-aperture space telescopes
presents significant challenges, particularly in the context of high-contrast
exoplanet direct imaging. We present a new wavefront control architecture that
translates the ground-based artificial guide star concept to space with a laser
source aboard a second spacecraft, formation flying within the telescope
field-of-view. We describe the motivating problem of mirror segment motion and
develop wavefront sensing requirements as a function of guide star magnitude
and segment motion power spectrum. Several sample cases with different values
for transmitter power, pointing jitter, and wavelength are presented to
illustrate the advantages and challenges of having a non-stellar-magnitude
noise limited wavefront sensor for space telescopes. These notional designs
allow increased control authority, potentially relaxing spacecraft stability
requirements by two orders of magnitude, and increasing terrestrial exoplanet
discovery space by allowing high-contrast observations of stars of arbitrary
brightness.Comment: Submitted to A
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