7,905 research outputs found
An empirical Bayesian analysis applied to the globular cluster pulsar population
We describe an empirical Bayesian approach to determine the most likely size
of an astronomical population of sources of which only a small subset are
observed above some limiting flux density threshold. The method is most
naturally applied to astronomical source populations at a common distance
(e.g.,stellar populations in globular clusters), and can be applied even to
populations where a survey detects no objects. The model allows for the
inclusion of physical parameters of the stellar population and the detection
process. As an example, we apply this method to the current sample of radio
pulsars in Galactic globular clusters. Using the sample of flux density limits
on pulsar surveys in 94 globular clusters published by Boyles et al., we
examine a large number of population models with different dependencies. We
find that models which include the globular cluster two-body encounter rate,
, are strongly favoured over models in which this is not a factor. The
optimal model is one in which the mean number of pulsars is proportional to
. This model agrees well with earlier work by Hui et al.
and provides strong support to the idea that the two-body encounter rate
directly impacts the number of neutron stars in a cluster. Our model predicts
that the total number of potentially observable globular cluster pulsars in the
Boyles et al. sample is 1070, where the uncertainties signify
the 95% confidence interval. Scaling this result to all Galactic globular
clusters, and to account for radio pulsar beaming, we estimate the total
population to be 2280.Comment: 8 pages, 6 figures, 3 tables, corrected a few minor formatting errors
which have also been submitted as an erratum to MNRA
Parasitism in Calves
There is little information concerning the extent of parasitic infestation in cattle, particularly subclinical infestations. Relatively few experimental studies of parasitic infestations in cattle have been made. This is due, in part at least, to the high cost of cattle as experimental animals and the space required for conducting experiments on them. Internal parasites have long been recognized as a hazard in the production of sheep and swine but comparatively few livestock raisers realize that in many instances parasites cause serious losses in cattle. Even when parasites are present in sufficient numbers to produce clinical symptoms, these symptoms are often confused with and ascribed to other conditions. This is particularly true in those areas where parasitism occurs in conjunction with malnutrition
Characteristic value determination from small samples
The paper deals with the characteristic value determination from relatively small samples. When the distribution and its parameters of a random variable are known, the characteristic value is deterministic quantity. However, in practical problems the parameters of distribution are unknown and can only be estimated from random samples. Therefore the characteristic value is by itself a random variable. The estimates of characteristic values are strongly dependant on the distribution of random variable. In the paper we show the analytical solution for characteristic value determination from random samples of normal and lognormal random variables. The confirmation of analytical results is accomplished by the use of computer simulations. For Gumbel, and Weibull distribution the characteristic value estimates are obtained numerically by combination of simulations and bisection method. In the paper the numerical results are presented for 5% characteristic values with 75% confidence interval, which is in accord with the majority of European building standards. The proposed approach is demonstrated on the data of experimentally obtained bending strengths of finger jointed wooden beams. (C) 2006 Elsevier Ltd. All rights reserved
Group Theory of Chiral Photonic Crystals with 4-fold Symmetry: Band Structure and S-Parameters of Eight-Fold Intergrown Gyroid Nets
The Single Gyroid, or srs, nanostructure has attracted interest as a
circular-polarisation sensitive photonic material. We develop a group
theoretical and scattering matrix method, applicable to any photonic crystal
with symmetry I432, to demonstrate the remarkable chiral-optical properties of
a generalised structure called 8-srs, obtained by intergrowth of eight
equal-handed srs nets. Exploiting the presence of four-fold rotations, Bloch
modes corresponding to the irreducible representations E- and E+ are identified
as the sole and non-interacting transmission channels for right- and
left-circularly polarised light, respectively. For plane waves incident on a
finite slab of the 8-srs, the reflection rates for both circular polarisations
are identical for all frequencies and transmission rates are identical up to a
critical frequency below which scattering in the far field is restricted to
zero grating order. Simulations show the optical activity of the lossless
dielectric 8-srs to be large, comparable to metallic metamaterials,
demonstrating its potential as a nanofabricated photonic material
Group Theory of Circular-Polarization Effects in Chiral Photonic Crystals with Four-Fold Rotation Axes, Applied to the Eight-Fold Intergrowth of Gyroid Nets
We use group or representation theory and scattering matrix calculations to
derive analytical results for the band structure topology and the scattering
parameters, applicable to any chiral photonic crystal with body-centered cubic
symmetry I432 for circularly-polarised incident light. We demonstrate in
particular that all bands along the cubic [100] direction can be identified
with the irreducible representations E+/-,A and B of the C4 point group. E+ and
E- modes represent the only transmission channels for plane waves with wave
vector along the ? line, and can be identified as non-interacting transmission
channels for right- (E-) and left-circularly polarised light (E+),
respectively. Scattering matrix calculations provide explicit relationships for
the transmission and reflectance amplitudes through a finite slab which
guarantee equal transmission rates for both polarisations and vanishing
ellipticity below a critical frequency, yet allowing for finite rotation of the
polarisation plane. All results are verified numerically for the so-called
8-srs geometry, consisting of eight interwoven equal-handed dielectric Gyroid
networks embedded in air. The combination of vanishing losses, vanishing
ellipticity, near-perfect transmission and optical activity comparable to that
of metallic meta-materials makes this geometry an attractive design for
nanofabricated photonic materials
English-learning infantsâ perception of word stress patterns
Adult speakers of different free stress languages (e.g., English, Spanish) differ both in their sensitivity to lexical stress and in their processing of suprasegmental and vowel quality cues to stress. In a head-turn preference experiment with a familiarization phase, both 8-month-old and 12-month-old English-learning infants discriminated between initial stress and final stress among lists of Spanish-spoken disyllabic nonwords that were segmentally varied (e.g. [Ënila, Ëtuli] vs [luËta, puËki]). This is evidence that English-learning infants are sensitive to lexical stress patterns, instantiated primarily by suprasegmental cues, during the second half of the first year of life
- âŠ