34,184 research outputs found
The response of self-graviting protostellar discs to slow reduction in cooling timescale: the fragmentation boundary revisited
A number of previous studies of the fragmentation of self-gravitating
protostellar discs have modeled radiative cooling with a cooling timescale
(t_{cool}) parameterised as a simple multiple (beta_{cool}) of the local
dynamical timescale. Such studies have delineated the `fragmentation boundary'
in terms of a critical value of beta_{cool} (beta_{crit}), where the disc
fragments if beta_{cool} < beta_{crit}. Such an approach however begs the
question of how in reality a disc could ever be assembled with beta_{cool} <
beta_{crit}. Here we adopt the more realistic approach of gradually reducing
beta_{cool}, as might correspond to changes in thermal regime due to secular
changes in the disc density profile. We find that when beta_{cool} is gradually
reduced (on a timescale longer than t_{cool}), the disc is stabilised against
fragmentation, compared with models in which beta_{cool} is reduced rapidly. We
therefore conclude that a disc's ability to remain in a self-regulated,
self-gravitating state (without fragmentation) is partly dependent on its
thermal history, as well as its current cooling rate. Nevertheless, a slow
reduction in t_{cool} appears only to lower the fragmentation boundary by about
a factor two in t_{cool} and thus only permits maximum alpha values
(parameterising the efficiency of angular momentum transfer in the disc) that
are about a factor two higher than determined hitherto. Our results therefore
do not undermine the notion of a fundamental upper limit to the heating rate
that can be delivered by gravitational instabilities before the disc is subject
to fragmentation. An important implication of this work, therefore, is that
self-gravitating discs can enter into the regime of fragmentation via secular
evolution and it is not necessary to invoke rapid (impulsive) events to trigger
fragmentation.Comment: accepted for publication in MNRA
Constraints on the formation mechanism of the planetary mass companion of 2MASS 1207334-393254
In this paper we discuss the nature and the possible formation scenarios of
the companion of the brown dwarf 2MASS 1207334-393254. We initially discuss the
basic physical properties of this object and conclude that, although from its
absolute mass (), it is a planetary object, in terms of its mass
ratio and of its separation with respect to the primary brown dwarf, it
is consistent with the statistical properties of binaries with higher primary
mass. We then explore the possible formation mechanism for this object. We show
that the standard planet formation mechanism of core accretion is far too slow
to form this object within 10 Myr, the observed age of the system. On the other
hand, the alternative mechanism of gravitational instability (proposed both in
the context of planet and of binary formation) may, in principle, work and form
a system with the observed properties.Comment: 5 pages, MNRAS in pres
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Dietary manipulation of broiler breeder growth through the feeding of conjugated linoleic acid
Prodromal symptoms in knee osteoarthritis: a nested case-control study using data from the Osteoarthritis Initiative.
OBJECTIVE: In order to gain a better understanding of the timing of emergent symptoms of osteoarthritis, we sought to investigate the existence, duration and nature of a prodromal symptomatic phase preceding incident radiographic knee osteoarthritis (ROA). DESIGN: Data were from the incidence cohort of the Osteoarthritis Initiative (OAI) public use datasets. Imposing a nested case-control design, ten control knees were selected for each case of incident tibiofemoral ROA between 2004 and 2010 from participants aged 45-79 years. Candidate prodromal symptoms were Western Ontario & McMaster Universities Osteoarthritis Index (WOMAC) and Knee injury and Osteoarthritis Outcome Score (KOOS) subscale scores and individual items, available up to 4 years prior to the time of incident ROA. Multi-level models were used to estimate the length of the prodromal phases. RESULTS: The prodromal phase for subscale scores ranged from 29 months (KOOS Other Symptoms) to 37 months (WOMAC Pain). Pain and difficulty on activities associated with higher dynamic knee loading were associated with longer prodromal phases (e.g., pain on twisting/pivoting (39 months, 95% confidence interval: 13, 64) vs pain on standing (25 months: 7, 42)). CONCLUSIONS: Our analysis found that incident ROA is preceded by prodromal symptoms lasting at least 2-3 years. This has potential implications for understanding phasic development and progression of osteoarthritis and for early recognition and management
An Investigation into the Geometry of Seyfert Galaxies
We present a new method for the statistical investigation into the
distributions of the angle beta between the radio axis and the normal to the
galactic disk for a sample of Seyfert galaxies. We discuss how further
observations of the sample galaxies can strengthen the conclusions. Our data
are consistent with the hypothesis that AGN jets are oriented randomly in
space, independent of the position of the plane of the galaxy. By making the
simple assumption that the Standard Model of AGN holds, with a universal
opening angle of the thick torus of phi_c, we demonstrate a statistical method
to obtain an estimate of phi_c. Our data are not consistent with the
simple-minded idea that Seyfert 1s and Seyfert 2s are differentiated solely by
whether or not our line of sight lies within some fixed angle of the jet axis.
Our result is significant on the 2 sigma level and can thus be considered only
suggestive, not conclusive. A complete sample of Seyfert galaxies selected on
an isotropic property is required to obtain a conclusive result.Comment: 13 pages, Tex, 5 Postscript figures. Accepted Ap
Quality of care for children with severe disease in the Democratic Republic of the Congo
BACKGROUND: Despite the almost universal adoption of Integrated Management of Childhood Illness (IMCI) guidelines for the diagnosis and treatment of sick children under the age of five in low- and middle-income countries, child mortality remains high in many settings. One possible explanation of the continued high mortality burden is lack of compliance with diagnostic and treatment protocols. We test this hypothesis in a sample of children with severe illness in the Democratic Republic of the Congo (DRC). METHODS: One thousand one hundred eighty under-five clinical visits were observed across a regionally representative sample of 321 facilities in the DRC. Based on a detailed list of disease symptoms observed, patients with severe febrile disease (including malaria), severe pneumonia, and severe dehydration were identified. For all three disease categories, treatments were then compared to recommended case management following IMCI guidelines. RESULTS: Out of 1180 under-five consultations observed, 332 patients (28%) had signs of severe febrile disease, 189 patients (16%) had signs of severe pneumonia, and 19 patients (2%) had signs of severe dehydration. Overall, providers gave the IMCI-recommended treatment in 42% of cases of these three severe diseases. Less than 15% of children with severe disease were recommended to receive in-patient care either in the facility they visited or in a higher-level facility. CONCLUSIONS: These results suggest that adherence to IMCI protocols for severe disease remains remarkably low in the DRC. There is a critical need to identify and implement effective approaches for improving the quality of care for severely ill children in settings with high child mortality
Flexible Buffer Materials to Reduce Contact Resistance in Thermal Insulation Measurements
Thermal insulation test methods approach their lower limits as thermal resistance falls below 0.1 m2â‹…K/W. This is the minimum value specified in ASTM C 518 (ASTM International, 2010b) while ASTM C 177 (ASTM International, 2010a) proposes about 0.06 m2â‹…K/W. Nevertheless these are the test methods, along with their ISO equivalents, required by Australasian building codes and directed at many products and materials with thermal resistance on the low side of 0.1 m2â‹…K/W. Alternatives, such as ASTM E 1530 (ASTM International, 2011), cover much lower resistances but require carefully prepared small specimens and very-high contact pressures and are therefore largely unsuitable for both technical and compliance reasons. For these low resistances, the insulation test methods face large errors because of interface resistance between specimen and the apparatus hot and cold plates. Staying with C 518, the problem can be avoided by using direct measurement of the test specimen surface temperatures, but this is difficult, has its own accuracy issues, and is often impractical for commercial laboratories. This technique is generally used in conjunction with interface materials such as flexible foam between the specimen and the hot and cold plates, to enhance contact and also provide an access path for temperature sensors. The alternative prospect of using these interface materials to ensure good specimen contact has been studied, in conjunction with a simple two-step thermal resistance determination based on the difference between presence and absence of the test specimen.
This article presents results of a study using this difference approach for the measurement of 12 highly conducting materials, including sheets of aluminum, phenolic, HDPE, MgO, bonded rubber and cork granules, PMMA, and compressed wood fiber. For each material, repeated measurements have been performed with four different interface or “buffer” materials: PVC, silicone, EVA, and nitrile. Silicone sponge provides the most uniform results, consistent with a measurably lower hysteresis. The difference technique yielded a lower indicated thermal resistance than direct measurement by between 0.003 and 0.01 m2⋅K/W, with some variation depending on the specimen surface characteristics and to a lesser extent on the choice of buffer. Larger differences were associated with bowed, uneven or roughly surfaced specimens. The difference-technique results have greater variability, but they may be seen as better estimates of the actual specimen resistance, as contact resistance is much lower for soft-surface interfaces. An interface resistance of up to 0.01 m2⋅K/W is large enough to be of significance in many thermal measurements
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