53,537 research outputs found
Some effects of small-scale metallicity variations in cooling flows
In an attempt to reconcile recent spectral data with predictions of the
standard cooling flow model, it has been suggested that the metals in the
intracluster medium (ICM) might be distributed inhomogeneously on small scales.
We investigate the possible consequences of such a situation within the
framework of the cooling flow scenario. Using the standard isobaric cooling
flow model, we study the ability of such metallicity variations to
preferentially suppress low-temperature line emission in cooling flow spectra.
We then use simple numerical simulations to investigate the temporal and
spatial evolution of the ICM when the metals are distributed in such a fashion.
Simulated observations are used to study the constraints real data can place on
conditions in the ICM. The difficulty of ruling out abundance variations on
small spatial scales with current observational limits is emphasized. We find
that a bimodal distribution of metals may give rise to interesting effects in
the observed abundance profile, in that apparent abundance gradients with
central abundance drops and off-centre peaks, similar to those seen recently in
some clusters, are produced. Different elements behave in different fashion as
governed by the temperature dependence of their equivalent widths. Our overall
conclusion is that, whilst this process alone seems unlikely to be able to
account for the sharp reduction in low temperature emission lines seen in
current spectral data, a contribution at some level is possible and difficult
to rule out. The possibility of small-scale metallicity variations should be
considered when analysing high resolution cluster X-ray spectra.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
Research on gravitational mass sensors Quarterly progress report no. 1, 15 Oct. 1964 - 14 Jan. 1965
Lunar orbiter and deep space probe gravitational sensor for determining mass distribution of moon and asteroid
Research on gravitational mass sensors quarterly progress report no. 2, 15 jan. - 14 apr. 1965
Cruciform sensor head, phase shift tuned amplifier, and air bearing support for sensor for detecting presence of small moving mass through gravitational interaction
Broughton Archipelago Clam Terrace Survey : final report
During a 1995 aerial video survey of the coastline
of Johnstone Strait, an unusual shoreline feature
was noted and termed “clam terraces” (inset)
because of the terrace-type morphology and the
apparent association with high clam productivity
on the sandflats. Typical alongshore lengths of the
terrace ridges are 20-50m, and across-shore widths
are typically 20-40m.
An area with an especially high density of clam
terraces was noted in the Broughton Archipelago,
between Broughton and Gilford Islands of
southeastern Queen Charlotte Strait. Clam terraces
in this area were inventoried from the aerial video
imagery to quantify their distribution. The terraces accounted for over 14 km of shoreline and
365 clam terraces were documented.
A three-day field survey by a coastal geomorphologist, archeologist and marine biologist was
conducted to document the features and determine their origin. Nine clam terraces were
surveyed. The field observations confirmed that: the ridges are comprised of boulder/cobblesized
material, ridge crests are typically in the range of 1-1.5m above chart datum, sandflats are
comprised almost entirely of shell fragments (barnacles and clams) and sandflats have very high
shellfish production. There are an abundance of shell middens in the area (over 175) suggesting
that the shellfish associated with the terraces were an important food source of aboriginal
peoples.
The origin of the ridges is unknown; they appear to be a relict feature in that they are not actively
being modified by present-day processes. The ridges may be a relict sea-ice feature, although the
mechanics of ridge formation is uncertain. Sand accumulates behind the ridge because the supply
rate of the shell fragments exceeds the dispersal rate in these low energy environments.
The high density areas of clam terraces correspond to high density areas of shell middens, and it
is probable that the clam terraces were subjected to some degree of modification by aboriginal
shellfish gatherers over the thousands of years of occupation in the region. (Document contains 39 pages
Orthotic management of cerebral palsy : recommendations from a consensus conference
An international multidisciplinary group of healthcare professionals and researchers participated in a consensus conference on the management of cerebral palsy, convened by the International Society for Prosthetics and Orthotics. Participants reviewed the evidence and considered contemporary thinking on a range of treatment options including physical and occupational therapy, and medical, surgical and orthotic interventions. The quality of many of the reviewed papers was compromised by inadequate reporting and lack of transparency, in particular regarding the types of patients and the design of the interventions being evaluated. Substantial evidence suggests that ankle-foot orthoses (AFOs) that control the foot and ankle in stance and swing phases can improve gait efficiency in ambulant children (GMFCS levels I-III). By contrast, little high quality evidence exists to support the use of orthoses for the hip, spine or upper limb. Where the evidence for orthosis use was not compelling consensus was reached on recommendations for orthotic intervention. Subsequent group discussions identified recommendations for future research. The evidence to support using orthoses is generally limited by the brevity of follow-up periods in research studies; hence the extent to which orthoses may prevent deformities developing over time remains unclear. The full report of the conference can be accessed free of charge at www.ispoint.org
Optimization of the derivative expansion in the nonperturbative renormalization group
We study the optimization of nonperturbative renormalization group equations
truncated both in fields and derivatives. On the example of the Ising model in
three dimensions, we show that the Principle of Minimal Sensitivity can be
unambiguously implemented at order of the derivative expansion.
This approach allows us to select optimized cut-off functions and to improve
the accuracy of the critical exponents and . The convergence of the
field expansion is also analyzed. We show in particular that its optimization
does not coincide with optimization of the accuracy of the critical exponents.Comment: 13 pages, 9 PS figures, published versio
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