4,315 research outputs found
Direct and sequential radiative three-body reaction rates at low temperatures
We investigate the low-temperature reaction rates for radiative capture
processes of three particles. We compare direct and sequential capture
mechanisms and rates using realistic phenomenological parametrizations of the
corresponding photodissociation cross sections.Energy conservation prohibits
sequential capture for energies smaller than that of the intermediate two-body
structure. A finite width or a finite temperature allows this capture
mechanism. We study generic effects of positions and widths of two- and
three-body resonances for very low temperatures. We focus on nuclear reactions
relevant for astrophysics, and we illustrate with realistic estimates for the
-- and -- radiative capture
processes. The direct capture mechanism leads to reaction rates which for
temperatures smaller than 0.1 GK can be several orders of magnitude larger than
those of the NACRE compilation.Comment: To be published in European Physical Journal
Pattern formation from consistent dynamical closures of uniaxial nematic liquid crystals
Pattern formation in uniaxial polymeric liquid crystals is studied for
different dynamic closure approximations. Using the principles of mesoscopic
non-equilibrium thermodynamics in a mean-field approach, we derive a
Fokker-Planck equation for the single-particle non-homogeneous distribution
function of particle orientations and the evolution equations for the second
and fourth order orientational tensor parameters. Afterwards, two dynamic
closure approximations are discussed, one of them considering the relaxation of
the fourth order orientational parameter and leading to a novel expression for
the free-energy like function in terms of the scalar order parameter.
Considering the evolution equation of the density of the system and values of
the interaction parameter for which isotropic and nematic phases coexist, our
analysis predicts that patterns and traveling waves can be produced in
lyotropic uniaxial nematics even in the absence of external driving.Comment: 34 pages, 7 figure
Prevalence of non-aureus Staphylococcus species causing intramammary infections in Canadian dairy herds
Non-aureus staphylococci (NAS), the microorganisms most frequently isolated from bovine milk worldwide, are a heterogeneous group of numerous species. To establish their importance as a group, the distribution of individual species needs to be determined. In the present study, NAS intramammary infection (IMI) was defined as a milk sample containing ≥1,000 cfu/mL in pure or mixed culture that was obtained from a cohort of cows assembled by the Canadian Bovine Mastitis Research Network. Overall, 6,213 (6.3%) of 98,233 quarter-milk samples from 5,149 cows and 20,305 udder quarters were associated with an NAS IMI. Of the 6,213 phenotypically identified NAS isolates, 5,509 (89%) were stored by the Canadian Bovine Mastitis Research Network Mastitis Pathogen Collection and characterized using partial sequencing of the rpoB housekeeping gene, confirming 5,434 isolates as NAS. Prevalence of each NAS species IMI was estimated using Bayesian models, with presence of a specific NAS species as the outcome. Overall quarter-level NAS IMI prevalence was 26%. The most prevalent species causing IMI were Staphylococcus chromogenes (13%), Staphylococcus simulans (4%), Staphylococcus haemolyticus (3%), Staphylococcus xylosus (2%), and Staphylococcus epidermidis (1%). The prevalence of NAS IMI as a group was highest in first-parity heifers and was evenly distributed throughout cows in parities ≥2. The IMI prevalence of some species such as S. chromogenes, S. simulans, and S. epidermidis differed among parities. Overall prevalence of NAS IMI was 35% at calving, decreased over the next 10 d, and then gradually increased until the end of lactation. The prevalence of S. chromogenes, Staphylococcus gallinarum, Staphylococcus cohnii, and Staphylococcus capitis was highest at calving, whereas the prevalence of S. chromogenes, S. haemolyticus, S. xylosus, and S. cohnii increased during lactation. Although the overall prevalence of NAS IMI was similar across barn types, the prevalence of S. simulans, S. xylosus, S. cohnii, Staphylococcus saprophyticus, S. capitis, and Staphylococcus arlettae IMI was higher in tie-stall barns; the prevalence of S. epidermidis IMI was lowest; and the prevalence of S. chromogenes and Staphylococcus sciuri IMI was highest in bedded-pack barns. Staphylococcus simulans, S. epidermidis, S. xylosus, and S. cohnii IMI were more prevalent in herds with intermediate to high bulk milk somatic cell count (BMSCC) and S. haemolyticus IMI was more prevalent in herds with high BMSCC, whereas other common NAS species IMI were equally prevalent in all 3 BMSCC categories. Distribution of NAS species IMI differed among the 4 regions of Canada. In conclusion, distribution differed considerably among NAS species IMI; therefore, accurate identification (species level) is essential for studying NAS epidemiology
The dynamic exponent of the Ising model on negatively curved surfaces
We investigate the dynamic critical exponent of the two-dimensional Ising
model defined on a curved surface with constant negative curvature. By using
the short-time relaxation method, we find a quantitative alteration of the
dynamic exponent from the known value for the planar Ising model. This
phenomenon is attributed to the fact that the Ising lattices embedded on
negatively curved surfaces act as ones in infinite dimensions, thus yielding
the dynamic exponent deduced from mean field theory. We further demonstrate
that the static critical exponent for the correlation length exhibits the mean
field exponent, which agrees with the existing results obtained from canonical
Monte Carlo simulations.Comment: 14 pages, 3 figures. to appear in J. Stat. Mec
Two massive star-forming regions at early evolutionary stages
We report sensitive ATCA radio-continuum observations toward IRAS 15596-5301
and 16272-4837, two luminous objects (> 2x10^4 Lsun) thought to represent
massive star-forming regions in early stages of evolution (due to previously
undetected radio emission at the 1-sigma level of 2 mJy per beam). Also
reported are 1.2-millimeter continuum and a series of molecular-line
observations made with the SEST telescope. For IRAS 15596-5301, the
observations reveal the presence of three distinct compact radio-continuum
sources associated with a dense molecular core. We suggest that this core
contains a cluster of B stars which are exciting compact HII regions that are
in pressure equilibrium with the dense molecular surroundings. No radio
continuum emission was detected from IRAS 16272-4837 (3-sigma limit of 0.2
mJy). However, a dense molecular core has been detected. The high luminosity
and lack of radio emission from this massive core suggests that it hosts an
embedded young massive protostar that is still undergoing an intense accretion
phase. This scenario is supported by the observed characteristics of the line
profiles and the presence of a bipolar outflow detected from observations of
the SiO emission. We suggest that IRAS 16272-4837 is a bona fide massive star-
forming region in a very early evolutionary stage, being the precursor of an
ultra compact HII region.Comment: 25 pages, 9 figures, accepted for publication in The Astrophysical
Journa
On the Hamilton-Jacobi Theory for Singular Lagrangian Systems
We develop a Hamilton-Jacobi theory for singular lagrangian systems using the
Gotay-Nester-Hinds constraint algorithm. The procedure works even if the system
has secondary constraints.Comment: 36 page
Self-organized network evolution coupled to extremal dynamics
The interplay between topology and dynamics in complex networks is a
fundamental but widely unexplored problem. Here, we study this phenomenon on a
prototype model in which the network is shaped by a dynamical variable. We
couple the dynamics of the Bak-Sneppen evolution model with the rules of the
so-called fitness network model for establishing the topology of a network;
each vertex is assigned a fitness, and the vertex with minimum fitness and its
neighbours are updated in each iteration. At the same time, the links between
the updated vertices and all other vertices are drawn anew with a
fitness-dependent connection probability. We show analytically and numerically
that the system self-organizes to a non-trivial state that differs from what is
obtained when the two processes are decoupled. A power-law decay of dynamical
and topological quantities above a threshold emerges spontaneously, as well as
a feedback between different dynamical regimes and the underlying correlation
and percolation properties of the network.Comment: Accepted version. Supplementary information at
http://www.nature.com/nphys/journal/v3/n11/suppinfo/nphys729_S1.htm
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