2,442 research outputs found
Large scale topography of Io
To investigate the large scale topography of the Jovian satellite Io, both limb observations and stereographic techniques applied to landmarks are used. The raw data for this study consists of Voyager 1 images of Io, 800x800 arrays of picture elements each of which can take on 256 possible brightness values. In analyzing this data it was necessary to identify and locate landmarks and limb points on the raw images, remove the image distortions caused by the camera electronics and translate the corrected locations into positions relative to a reference geoid. Minimizing the uncertainty in the corrected locations is crucial to the success of this project. In the highest resolution frames, an error of a tenth of a pixel in image space location can lead to a 300 m error in true location. In the lowest resolution frames, the same error can lead to an uncertainty of several km
Quasar optical variability: searching for interband time delays
Aims. The main purpose of this paper is to study time delays between the
light variations in different wavebands for a sample of quasars. Measuring a
reliable time delay for a large number of quasars may help constraint the
models of their central engines. The standard accretion disk irradiation model
predicts a delay of the longer wavelengths behind the shorter ones, a delay
that depends on the fundamental quasar parameters. Since the black hole masses
and the accretion rates are approximately known for the sample we use, one can
compare the observed time delays with the expected ones.
Methods. We applied the interpolation cross-correlation function (ICCF)
method to the Giveon et al. sample of 42 quasars, monitored in two (B and R)
colors, to find the time lags represented by the ICCF peaks. Different tests
were performed to assess the influence of photometric errors, sampling, etc.,
on the final result.
Results. We found that most of the objects show a delay in the red light
curve behind the blue one (a positive lag), which on average for the sample is
about +4 days (+3 for the median), although the scatter is significant. These
results are broadly consistent with the reprocessing model, especially for the
well-sampled objects. The normalized time-lag deviations do not seem to
correlate significantly with other quasar properties, including optical, radio,
or X-ray measurables. On the other hand, many objects show a clear negative
lag, which, if real, may have important consequences for the variability
models.Comment: 5 pages, 4 figures, accepted in A&
Generator Matrix Elements For II
Basis states and generator matrix elements are given for the generic
representation of in an basis.Comment: Latex, 17 pages, one figure using epsf published in J. Phys. A: Math.
Gen 28 (1995) 2581-258
The Effects of High Liquid Water Content on Thunderstorm Charging
Charge transfer to a riming graupel target during interactions with ice crystals has been investigated in the laboratory. When liquid water contents sufficiently high to cause wet growth are achieved, the charge transfer falls to values which are insignificant to thunderstorm electrification. The implications of this null result to a recent analysis of thunderstorm-charging processes by Wiliams et al. (1991) are discussed
Is there an association between low dose aspirin and anemia (without overt bleeding)?: narrative review
<p>Abstract</p> <p>Background</p> <p>Overt bleeding associated with low dose aspirin (LDA) is well-recognized, little attention is given to the possibility of association between LDA and occult bleeding, although this is known to occur in healthy volunteers. LDA is used increasingly in primary and secondary prevention of a number of medical conditions, many of which are common in older people, as is anemia. Anemia in older people is associated with adverse outcomes including disability, morbidity and mortality. The purpose of this study was to review the evidence that LDA might cause anemia without overt bleeding.</p> <p>Methods</p> <p>An extensive narrative review was carried out. Electronic searching (including database links) and reference lists of reports were used to identify studies reporting on use of aspirin â€325 mg/day and anemia or change in hemoglobin (Hb) without overt bleeding. Data were extracted from reports of trials, adverse drug reactions (ADRs) and prevalence studies of adults aged â„18 years, published since 1980.</p> <p>Results</p> <p>There are few relevant data, with considerable heterogeneity among trial designs, duration, and patient characteristics in studies of LDA. In five randomised trials (n = 5879) in (mostly secondary) prevention, the majority of patients were men without peptic ulcer disease aged 50-70 years and no consistent association between LDA and change in Hb was found. In two smaller studies (n = 609) of primary prevention in healthy patients aged â„70 years, there was a small but statistically significant fall in Hb with LDA. Observational studies, and data from trials in which use of LDA was not a primary focus of the study, were inconclusive.</p> <p>Conclusions</p> <p>It is not clear whether there is an association between LDA and anemia in the absence of overt bleeding, but there may be an association between LDA and fall in Hb in (a subset of) older patients. The available evidence has significant limitations, which are discussed; studies including more older patients, and publication of individual patient data, would help clarify this important matter.</p
Hole polaron formation and migration in olivine phosphate materials
By combining first principles calculations and experimental XPS measurements,
we investigate the electronic structure of potential Li-ion battery cathode
materials LiMPO4 (M=Mn,Fe,Co,Ni) to uncover the underlying mechanisms that
determine small hole polaron formation and migration. We show that small hole
polaron formation depends on features in the electronic structure near the
valence-band maximum and that, calculationally, these features depend on the
methodology chosen for dealing with the correlated nature of the
transition-metal d-derived states in these systems. Comparison with experiment
reveals that a hybrid functional approach is superior to GGA+U in correctly
reproducing the XPS spectra. Using this approach we find that LiNiPO4 cannot
support small hole polarons, but that the other three compounds can. The
migration barrier is determined mainly by the strong or weak bonding nature of
the states at the top of the valence band, resulting in a substantially higher
barrier for LiMnPO4 than for LiCoPO4 or LiFePO4
Perceptions of university assessment and feedback among post-16 school pupils
The transition between school and university can be problematic for students. Understanding studentsâ expectations about the system they are entering into is crucial in effecting a smooth transition. The school system involves small classes, often with teachers who know their students well. In contrast university involves large class sizes and a degree of anonymity for the student. In the UK National Student Survey responses for universities show that students are generally less happy about feedback relative to most other issues. The current study investigates school studentsâ expectations about assessment and feedback practices at university. Students were surveyed across Great Britain in 2006 and 2013. There were some significantly different responses from students between the two years of study in how coursework would be handled, but there were also many areas of consistency, demonstrating a clear pattern of expectation. For example, in both years of study 80 % of students expected personal verbal feedback on their university assignments. This study discusses how student expectations may be based on their school experiences rather than considering the reality of the university system they are entering into. Misconceptions may be affecting studentsâ responses to the NSS surveys on assessment and feedback
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