1,219 research outputs found
Empirical Abundance Scaling Laws and Implications for the Gamma-Process in Core-Collapse Supernovae
Analyzing the solar system abundances, we have found two empirical abundance
scaling laws concerning the p- and s-nuclei with the same atomic number. The
first scaling is s/p ratios are almost constant over a wide range of the atomic
number, where the p-nculei are lighter than the s-nuclei by two or four
neutrons. The second scaling is p/p ratios are almost constant, where the
second -nuclei are lighter than the first p-nucleus by two neutrons. These
scalings are a piece of evidence that most p-nuclei are dominantly synthesized
by the gamma-process in supernova explosions. The scalings lead to a novel
concept of "universality of gamma-process" that the s/p and p/p ratios of
nuclei produced by individual gamma-processes are almost constant,
respectively. We have calculated the ratios by gamma-process based on
core-collapse supernova explosion models under various astrophysical conditions
and found that the scalings hold for materials produced by individual
gamma-processes independent of the astrophysical conditions assumed. The
universality originates from three mechanisms: the shifts of the gamma-process
layers to keep their peak temperature, the weak s-process in pre-supernovae,
and the independence of the s/p ratios of the nuclear reactions. The results
further suggest an extended universality that the s/p ratios in the
gamma-process layers are not only constant but also centered on a specific
value of 3. With this specific value and the first scaling, we estimate that
the ratios of -process abundance contributions from the AGB stars to the
massive stars are almost 6.7 for the -nuclei of A > 90. We find that large
enhancements of s/p ratios for Ce, Er, and W are a piece of evidence that the
weak s-process actually occurred before SNe.Comment: 35 pages, 15 figure
Re(\gamm,n) cross section close to and above the neutron threshold
The neutron capture cross section of the unstable nucleus Re is
studied by investigating the inverse photodisintegration reaction
Re(,n). The special interest of the {\it s}-process branching
point Re is related to the question of possible {\it s}-process
contributions to the abundance of the {\it r}-process chronometer nucleus
^{187}^{186}\gamma^{186}$Os; the two predicted neutron-capture cross sections
differ by a factor of 2.4; this calls for future theoretical study.Comment: Phys. Rev. C, in pres
Low-Energy Direct Capture in the 16O(alpha,gamma)20Ne Reaction
The cross section of the 16O(a,g)20Ne capture reaction is analyzed at low
energies where the direct capture mechanism is dominant. For temperatures below
T9 = 0.2 the resulting astrophysical reaction rate is about a factor of two
higher than in a recent compilation whereas the energy dependence of the
astrophysical S-factor and the branching ratios to the 20Ne bound states are
very similar to previous calculations. The validity of the widely used detailed
balance theorem for the inverse 20Ne(g,a)16O photodisintegration rate is
confirmed for the special case of high-lying first excited states.Comment: 11 pages, 4 figures, Phys. Rev. C, accepted for publicatio
Risk Factors for High Early Mortality in Patients on Antiretroviral Treatment in a Rural District of Malawi.
OBJECTIVES: Among adults started on antiretroviral treatment (ART) in a rural district hospital (a) to determine the cumulative proportion of deaths that occur within 3 and 6 months of starting ART, and (b) to identify risk factors that may be associated with such mortality. DESIGN AND SETTING: A cross-sectional analytical study set in Thyolo district, Malawi. METHODS: Over a 2-year period (April 2003 to April 2005) mortality within the first 3 and 6 months of starting ART was determined and risk factors were examined. RESULTS: A total of 1507 individuals (517 men and 990 women), whose median age was 35 years were included in the study. There were a total of 190 (12.6%) deaths on ART of which 116 (61%) occurred within the first 3 months (very early mortality) and 150 (79%) during the first 6 months of initiating ART. Significant risk factors associated with such mortality included WHO stage IV disease, a baseline CD4 cell count under 50 cells/mul and increasing grades of malnutrition. A linear trend in mortality was observed with increasing grades of malnutrition (chi for trend = 96.1, P </= 0.001) and decreasing CD4 cell counts (chi for trend = 72.4, P </= 0.001). Individuals who were severely malnourished [body mass index (BMI) < 16.0 kg/m] had a six times higher risk of dying in the first 3 months than those with a normal nutritional status. CONCLUSIONS: Among individuals starting ART, the BMI and clinical staging could be important screening tools for use to identify and target individuals who, despite ART, are still at a high risk of early death
Machine learning to detect marine animals in UAV imagery: effect of morphology, spacing, behaviour and habitat
Machine learning algorithms are being increasingly used to process large volumes of wildlife imagery data from unmanned aerial vehicles (UAVs); however, suitable algorithms to monitor multiple species are required to enhance efficiency. Here, we developed a machine learning algorithm using a low-cost computer. We trained a convolutional neural network and tested its performance in: (1) distinguishing focal organisms of three marine taxa (Australian fur seals, loggerhead sea turtles and Australasian gannets; body size ranges: 0.8–2.5 m, 0.6–1.0 m, and 0.8–0.9 m, respectively); and (2) simultaneously delineating the fine-scale movement trajectories of multiple sea turtles at a fish cleaning station. For all species, the algorithm performed best at detecting individuals of similar body length, displaying consistent behaviour or occupying uniform habitat (proportion of individuals detected, or recall of 0.94, 0.79 and 0.75 for gannets, seals and turtles, respectively). For gannets, performance was impacted by spacing (huddling pairs with offspring) and behaviour (resting vs. flying shapes, overall precision: 0.74). For seals, accuracy was impacted by morphology (sexual dimorphism and pups), spacing (huddling and creches) and habitat complexity (seal sized boulders) (overall precision: 0.27). For sea turtles, performance was impacted by habitat complexity, position in water column, spacing, behaviour (interacting individuals) and turbidity (overall precision: 0.24); body size variation had no impact. For sea turtle trajectories, locations were estimated with a relative positioning error of <50 cm. In conclusion, we demonstrate that, while the same machine learning algorithm can be used to survey multiple species, no single algorithm captures all components optimally within a given site. We recommend that, rather than attempting to fully automate detection of UAV imagery data, semi-automation is implemented (i.e. part automated and part manual, as commonly practised for photo-identification). Approaches to enhance the efficiency of manual detection are required in parallel to the development of effective implementation of machine learning algorithms
Lithium in the Symbiotic Mira V407 Cyg
We report an identification of the lithium resonance doublet LiI 6708A in the
spectrum of V407 Cyg, a symbiotic Mira with a pulsation period of about 745
days. The resolution of the spectra used was R~18500 and the measured
equivalent width of the line is ~0.34A. It is suggested that the lithium
enrichment is due to hot bottom burning in the intermediate mass AGB variable,
although other possible origins cannot be totally ruled out. In contrast to
lithium-rich AGB stars in the Magellanic clouds, ZrO 5551A, 6474A absorption
bands were not found in the spectrum of V407Cyg. These are the bands used to
classify the S-type stars at low-resolution. Although we identified weak ZrO
5718A, 6412A these are not visible in the low-resolution spectra, and we
therefore classify the Mira in V407 Cyg as an M type. This, together with other
published work, suggests lithium enrichment can precede the third dredge up of
s-process enriched material in galactic AGB stars.Comment: 4 pages, 2 figures, to be published in MNRA
Making space for experiences
Leisure and retail providers need to understand the elements of the visitor experience and the way in which they evaluate their satisfaction. This article suggests a holistic prism model of the interaction between the management and the visitor in a leisure space. This is applied to a netnographic study of visitors to a folk festival to illustrate the interconnectiveness of the different attributes causing dissatisfaction. It found that the physical and operational attributes were evaluated not through a checklist of individual features but as hindrances to the visitor's desire to make best use of the time. Visitors also evaluated the experience in the light of their own values and concerns, passing judgement on the values communicated by the management. At the heart of the experience was the enjoyment of choosing from an abundant offer and discovering something new. The main attraction is often only the pretext for enjoying the company of friends so places to meet before and chill-out afterwards are vital to the experience. The distinctiveness of the setting, the food and drink can become the sensory cues which give the event or location its uniqueness. The challenge to retail and leisure organisations is to design these elements of a memorable experience into their offerings
Nucleosynthesis in Massive Stars With Improved Nuclear and Stellar Physics
We present the first calculations to follow the evolution of all stable
nuclei and their radioactive progenitors in stellar models computed from the
onset of central hydrogen burning through explosion as Type II supernovae.
Calculations are performed for Pop I stars of 15, 19, 20, 21, and 25 M_sun
using the most recently available experimental and theoretical nuclear data,
revised opacity tables, neutrino losses, and weak interaction rates, and taking
into account mass loss due to stellar winds. A novel ``adaptive'' reaction
network is employed with a variable number of nuclei (adjusted each time step)
ranging from about 700 on the main sequence to more than 2200 during the
explosion. The network includes, at any given time, all relevant isotopes from
hydrogen through polonium (Z=84). Even the limited grid of stellar masses
studied suggests that overall good agreement can be achieved with the solar
abundances of nuclei between 16O and 90Zr. Interesting discrepancies are seen
in the 20 M_sun model and, so far, only in that model, that are a consequence
of the merging of the oxygen, neon, and carbon shells about a day prior to core
collapse. We find that, in some stars, most of the ``p-process'' nuclei can be
produced in the convective oxygen burning shell moments prior to collapse; in
others, they are made only in the explosion. Serious deficiencies still exist
in all cases for the p-process isotopes of Ru and Mo.Comment: 53 pages, 17 color figures (3 as separate GIF images), slightly
extended discussion and references, accepted by Ap
Photodissociation of p-process nuclei studied by bremsstrahlung induced activation
A research program has been started to study experimentally the
near-threshold photodissociation of nuclides in the chain of cosmic heavy
element production with bremsstrahlung from the ELBE accelerator. An important
prerequisite for such studies is good knowledge of the bremsstrahlung
distribution which was determined by measuring the photodissociation of the
deuteron and by comparison with model calculations. First data were obtained
for the astrophysically important target nucleus 92-Mo by observing the
radioactive decay of the nuclides produced by bremsstrahlung irradiation at
end-point energies between 11.8 MeV and 14.0 MeV. The results are compared to
recent statistical model calculations.Comment: 6 pages, 8 figures, Proceedings Nuclear Physics in Astrophysics II,
May 16-20, 2005, Debrecen, Hungary. The original publication is available at
www.eurphysj.or
Analysis of 26 Barium Stars II. Contributions of s-, r- and p-processes in the production of heavy elements
Barium stars show enhanced abundances of the slow neutron capture (s-process)
heavy elements, and for this reason they are suitable objects for the study of
s-process elements. The aim of this work is to quantify the contributions of
the s-, r- and p-processes for the total abundance of heavy elements from
abundances derived for a sample of 26 barium stars. The abundance ratios
between these processes and neutron exposures were studied. The abundances of
the sample stars were compared to those of normal stars thus identifying the
fraction relative to the s-process main component. The fittings of the sigmaN
curves (neutron capture cross section times abundance, plotted against atomic
mass number) for the sample stars suggest that the material from the companion
asymptotic giant branch star had approximately the solar isotopic composition
as concerns fractions of abundances relative to the s-process main component.
The abundance ratios of heavy elements, hs, ls and s and the computed neutron
exposure are similar to those of post-AGB stars. For some sample stars, an
exponential neutron exposure fits well the observed data, whereas for others, a
single neutron exposure provides a better fit. The comparison between barium
and AGB stars supports the hypothesis of binarity for the barium star
formation. Abundances of r-elements that are part of the s-process path in
barium stars are usually higher than those in normal stars,and for this reason,
barium stars seemed to be also enriched in r-elements, although in a lower
degree than s-elements. No dependence on luminosity classes was found in the
abundance ratios behaviour among the dwarfs and giants of the sample barium
stars.Comment: 30 pages including 24 figures, accepted to A&
- …