8,452 research outputs found
Near-threshold -meson production in proton-proton collisions: With or without resonance excitations ?
We present results for the reaction studied by
considering two different scenarios: with and without the inclusion of nucleon
resonance excitations. The recently measured angular distribution by the
COSY-TOF Collaboration at an excess energy of MeV and the energy
dependence of the total cross section data for are used
to calibrate the model parameters. The inclusion of nucleon resonances improves
the theoretical prediction for the energy dependence of the total cross section
in at excess energies MeV. However, it still
underestimates the data by about a factor of two, and remains a problem in
understanding the reaction mechanism.Comment: Fig.5 and text modified, Latex, 4 pages, 8 embedded figures, uses
espcrc1.sty (included), talk presented at PANIC02, Osaka, Japan, 30 September
- 4 October 200
Stellar Nucleosynthesis in the Hyades Open Cluster
We report a comprehensive light element (Li, C, N, O, Na, Mg, and Al)
abundance analysis of three solar-type main sequence (MS) dwarfs and three red
giant branch (RGB) clump stars in the Hyades open cluster using high-resolution
and high signal-to-noise spectroscopy. For each group (MS or RGB), the CNO
abundances are found to be in excellent star-to-star agreement. Our results
confirm that the giants have undergone the first dredge-up and that material
processed by the CN cycle has been mixed to the surface layers. The observed
abundances are compared to predictions of a standard stellar model based on the
Clemson-American University of Beirut (CAUB) stellar evolution code. The model
reproduces the observed evolution of the N and O abundances, as well as the
previously derived 12C/13C ratio, but it fails to predict by a factor of 1.5
the observed level of 12C depletion. Li abundances are derived to determine if
non-canonical extra mixing has occurred in the Hyades giants. The Li abundance
of the giant gamma Tau is in good accord with the predicted level of surface Li
dilution, but a ~0.35 dex spread in the giant Li abundances is found and cannot
be explained by the stellar model. Possible sources of the spread are
discussed; however, it is apparent that the differential mechanism responsible
for the Li dispersion must be unrelated to the uniformly low 12C abundances of
the giants. Na, Mg, and Al abundances are derived as an additional test of our
stellar model. All three elements are found to be overabundant by 0.2-0.5 dex
in the giants relative to the dwarfs. Such large enhancements of these elements
are not predicted by the stellar model, and non-LTE effects significantly
larger (and, in some cases, of opposite sign) than those implied by extant
literature calculations are the most likely cause.Comment: 40 pages, 6 figures, 6 tables; accepted by Ap
s-Process Nucleosynthesis in Advanced Burning Phases of Massive Stars
We present a detailed study of s-process nucleosynthesis in massive stars of
solar-like initial composition and masses 15, 20,25, and 30 Msun. We update our
previous results of s-process nucleosynthesis during the core He-burning of
these stars and then focus on an analysis of the s-process under the physical
conditions encountered during the shell-carbon burning. We show that the recent
compilation of the Ne22(alpha,n)Mg25 rate leads to a remarkable reduction of
the efficiency of the s-process during core He-burning. In particular, this
rate leads to the lowest overproduction factor of Kr80 found to date during
core He-burning in massive stars. The s-process yields resulting from shell
carbon burning turn out to be very sensitive to the structural evolution of the
carbon shell. This structure is influenced by the mass fraction of C12 attained
at the end of core helium burning, which in turn is mainly determined by the
C12(alpha,gamma)O16 reaction. The still present uncertainty in the rate for
this reaction implies that the s-process in massive stars is also subject to
this uncertainty. We identify some isotopes like Zn70 and Rb87 as the
signatures of the s-process during shell carbon burning in massive stars. In
determining the relative contribution of our s-only stellar yields to the solar
abundances, we find it is important to take into account the neutron exposure
of shell carbon burning. When we analyze our yields with a Salpeter Initial
Mass Function, we find that massive stars contribute at least 40% to s-only
nuclei with mass A 90, massive stars
contribute on average ~7%, except for Gd152, Os187, and Hg198 which are ~14%,
\~13%, and ~11%, respectively.Comment: 52 pages, 16 figures, accepted for publication in Ap
High performance Peer-to-Peer distributed computing with application to obstacle problem
International audienceThis paper deals with high performance Peer-to-Peer computing applications. We concentrate on the solution of large scale numerical simulation problems via distributed iterative methods. We present the current version of an environment that allows direct communication between peers. This environment is based on a self-adaptive communication protocol. The protocol configures itself automatically and dynamically in function of application requirements like scheme of computation and elements of context like topology by choosing the most appropriate communication mode between peers. A first series of computational experiments is presented and analyzed for the obstacle problem
Iron Implantation in Presolar Supernova Grains
We consider the potential of measured iron isotopic ratios within presolar grains from supernovae (as discovered in meteorites) for identifying the gas from which the grains condensed. We show that although iron isotopic ratios vary dramatically with radial coordinate in the initial supernova, it seems likely that the concentration of iron that thermally condenses in SiC grains within the supernova interior may be smaller than the concentration that will later be implanted by high-speed grain-gas collisions following the penetration of the reverse shock into the supernova ïŹow. In that case, the Fe isotopic composition is much altered. We propose that the 58Fe richness that is very evident in the three SiC grains analyzed to date is the result of ion implantation during the grainâs rapid radial motion through the shocked and decelerated overlying supernova gas that is 58Fe-rich. We point to other likely applications of this same idea and speculate that only the dominant isotopes of the SiC grains, namely 28Si and 12C, can be safely assumed to be initial thermal condensate. We conclude that a violent period of implantation plus sputtering has overprinted the initial thermal condensate. If correct, this points to a new technique for sampling the velocity mixing within young supernova remnants
Discovering new structural diversity from unexplored fungi
Discovery of anticancer drugs with high efficacy coupled with action at novel target sites is necessary to combat cancer. As part of a multidisciplinary project to identify anticancer leads from diverse natural product resources, our group has been studying fungi from different ecological habitats, including filamentous Ascomycota from terrestrial, freshwater, and symbiotic fungi (fungal endophytes), as a source of novel scaffolds for drug design and development. During the course of my research work, 56 bioactive compounds have been isolated and identified, with 30 of the isolated leads representing new chemical entities. Our lab relies on bioactivity-directed fractionation methodology for the isolation and purification of cytotoxic lead compounds from fungi, in which the bioassay results guide the purification processes. However, one of the inefficient outputs of utilizing this methodology is the re-isolation of previously known compounds, particularly mycotoxins. It is hypothesized that discovery of cytotoxic bioactive compounds with novel structures will be expedited by development and application of a dereplication methodology that has the capability to identify known compounds at the level of the crude extract. A dereplication methodology has been developed and implemented successfully for the identification of fungal secondary metabolites in crude culture extracts using a UPLC-PDA-HRMS-MS/MS method. Finally, the chemical diversity of the isolated compounds was analyzed through principal component analysis
Reassessment of the nutrient intakes and anthropometric measurements of adolescent females after a two-year period
This study reassessed and compared the nutrient intake and body composition of 92 teenage females, aged 14 and 16 years old, who had participated in the S-150 study in 1981 and 1983. The nutrient intakes were determined by using two 24-hour dietary recalls. The body composition was estimated by using the anthropometric measurements of weight, height, arm circumference, and biceps, triceps, subscapula, and ileac skinfold thicknesses. The changes in the percentage of body fat among subjects over the two-year period was also estimated. A comparison between the two different methods of estimating the percentage of body fat in 1983 was performed. The correlation between energy or protein intakes and the percentage of body fat was investigated. Over 15% of the entire sample in 1983 consumed less than twothirds of the RDA for calories, calcium, vitamin A, and ascorbic acid. In certain age-race categories, over 15% of subjects also consumed less than two-thirds of RDA for thiamin and riboflavin. In 1981, nutrients consumed by over 15% of the sample at less than two-thirds of the RDA were calcium, iron, and ascorbic acid. Mean nutrient intakes for the 1983 population from diet alone differed significantly from intakes from diet plus supplements. Mean intakes of calories, protein, calcium, and vitamin A decreased significantly over the period 1981 to 1983
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CD4+ CountâGuided Interruption of Antiretroviral Treatment
BACKGROUND
Despite declines in morbidity and mortality with the use of combination antiretroviral therapy, its effectiveness is limited by adverse events, problems with adherence, and resistance of the human immunodeficiency virus (HIV).
Full Text of Background...
METHODS
We randomly assigned persons infected with HIV who had a CD4+ cell count of more than 350 per cubic millimeter to the continuous use of antiretroviral therapy (the viral suppression group) or the episodic use of antiretroviral therapy (the drug conservation group). Episodic use involved the deferral of therapy until the CD4+ count decreased to less than 250 per cubic millimeter and then the use of therapy until the CD4+ count increased to more than 350 per cubic millimeter. The primary end point was the development of an opportunistic disease or death from any cause. An important secondary end point was major cardiovascular, renal, or hepatic disease.
RESULTS
A total of 5472 participants (2720 assigned to drug conservation and 2752 to viral suppression) were followed for an average of 16 months before the protocol was modified for the drug conservation group. At baseline, the median and nadir CD4+ counts were 597 per cubic millimeter and 250 per cubic millimeter, respectively, and 71.7% of participants had plasma HIV RNA levels of 400 copies or less per milliliter. Opportunistic disease or death from any cause occurred in 120 participants (3.3 events per 100 person-years) in the drug conservation group and 47 participants (1.3 per 100 person-years) in the viral suppression group (hazard ratio for the drug conservation group vs. the viral suppression group, 2.6; 95% confidence interval [CI], 1.9 to 3.7; P<0.001). Hazard ratios for death from any cause and for major cardiovascular, renal, and hepatic disease were 1.8 (95% CI, 1.2 to 2.9; P=0.007) and 1.7 (95% CI, 1.1 to 2.5; P=0.009), respectively. Adjustment for the latest CD4+ count and HIV RNA level (as time-updated covariates) reduced the hazard ratio for the primary end point from 2.6 to 1.5 (95% CI, 1.0 to 2.1).
CONCLUSIONS
Episodic antiretroviral therapy guided by the CD4+ count, as used in our study, significantly increased the risk of opportunistic disease or death from any cause, as compared with continuous antiretroviral therapy, largely as a consequence of lowering the CD4+ cell count and increasing the viral load. Episodic antiretroviral therapy does not reduce the risk of adverse events that have been associated with antiretroviral therapy. (ClinicalTrials.gov number, NCT00027352.
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