1,475 research outputs found
Applications of Machine-Learning Algorithms for Infrared Colour Selection of Galactic Wolf-Rayet Stars
We have investigated and applied machine-learning algorithms for Infrared
Colour Selection of Galactic Wolf-Rayet (WR) candidates. Objects taken from the
GLIMPSE catalogue of the infrared objects in the Galactic plane can be
classified into different stellar populations based on the colours inferred
from their broadband photometric magnitudes (, and from 2MASS, and
the four \textit{Spitzer}/IRAC bands). The algorithms tested in this pilot
study are variants of the -Nearest Neighbours (-NN) approach, which is
ideal for exploratory studies of classification problems where interrelations
between variables and classes are complicated. The aims of this study are (1)
to provide an automated tool to select reliable WR candidates and potentially
other classes of objects, (2) to measure the efficiency of infrared colour
selection at performing these tasks and, (3) to lay the groundwork for
statistically inferring the total number of WR stars in our Galaxy. We report
the performance results obtained over a set of known objects and selected
candidates for which we have carried out follow-up spectroscopic observations,
and confirm the discovery of 4 new WR stars.Comment: Authors' version of published paper, now at MNRAS, 473, 256
Fly-by-light flight control system technology development plan
The results of a four-month, phased effort to develop a Fly-by-Light Technology Development Plan are documented. The technical shortfalls for each phase were identified and a development plan to bridge the technical gap was developed. The production configuration was defined for a 757-type airplane, but it is suggested that the demonstration flight be conducted on the NASA Transport Systems Research Vehicle. The modifications required and verification and validation issues are delineated in this report. A detailed schedule for the phased introduction of fly-by-light system components has been generated. It is concluded that a fiber-optics program would contribute significantly toward developing the required state of readiness that will make a fly-by-light control system not only cost effective but reliable without mitigating the weight and high-energy radio frequency related benefits
CASE STUDY: Feed Intake and Performance of Heifers Sired by High- or Low-Residual Feed Intake Angus Bulls
The objective of this project was to investigate the effects of selecting sires for residual feed intake (RFI) on the performance of their daughters. Bulls with low or high estimated breeding values (EBV) for RFI were selected from the Angus Society of Australia sire summary and mated to Angus cross commercial cows at the Kansas State University Cow-Calf Unit in 2005 and 2006. The average EBV of low- and high-RFI bulls were −0.55 and 0.27 kg DM, respectively. Heifers born in 2006 were tested for feed intake in 2 groups (n = 24, n = 26), and heifers born in 2007 (n = 42) were sent to a commercial bull test facility for feed intake and BW gain tests. Body weights were collected every 14 d and used to calculate midtest BW and ADG. Actual feed intake was regressed on midtest metabolic BW and ADG to calculate an expected feed intake for each heifer. Residual feed intake was calculated by subtracting the expected intake from the actual intake. There were no significant differences between heifers sired by lowor high-RFI EBV bulls in RFI, feed intake, G:F, or BW gain (P \u3e 0.05). Heifers in this study were being developed on a less energy-dense diet than the diet used to rank their sires. Genetic differences in RFI calculated in growing bulls may not have been expressed on the lower plane of nutrition of these developing heifers
CASE STUDY: Feed Intake and Performance of Heifers Sired by High- or Low-Residual Feed Intake Angus Bulls
The objective of this project was to investigate the effects of selecting sires for residual feed intake (RFI) on the performance of their daughters. Bulls with low or high estimated breeding values (EBV) for RFI were selected from the Angus Society of Australia sire summary and mated to Angus cross commercial cows at the Kansas State University Cow-Calf Unit in 2005 and 2006. The average EBV of low- and high-RFI bulls were −0.55 and 0.27 kg DM, respectively. Heifers born in 2006 were tested for feed intake in 2 groups (n = 24, n = 26), and heifers born in 2007 (n = 42) were sent to a commercial bull test facility for feed intake and BW gain tests. Body weights were collected every 14 d and used to calculate midtest BW and ADG. Actual feed intake was regressed on midtest metabolic BW and ADG to calculate an expected feed intake for each heifer. Residual feed intake was calculated by subtracting the expected intake from the actual intake. There were no significant differences between heifers sired by lowor high-RFI EBV bulls in RFI, feed intake, G:F, or BW gain (P \u3e 0.05). Heifers in this study were being developed on a less energy-dense diet than the diet used to rank their sires. Genetic differences in RFI calculated in growing bulls may not have been expressed on the lower plane of nutrition of these developing heifers
organic crystals: superconducting versus antiferromagnetic instabilities in an anisotropic triangular lattice Hubbard model
A Hubbard model at half-filling on an anisotropic triangular lattice has been
proposed as the minimal model to describe conducting layers of
organic materials. The model interpolates between the
square lattice and decoupled chains. The materials
present many similarities with cuprates, such as the presence of unconventional
metallic properties and the close proximity of superconducting and
antiferromagnetic phases. As in the cuprates, spin fluctuations are expected to
play a crucial role in the onset of superconductivity. We perform a
weak-coupling renormalization-group analysis to show that a superconducting
instability occurs. Frustration in the antiferromagnetic couplings, which
arises from the underlying geometrical arrangement of the lattice, breaks the
perfect nesting of the square lattice at half-filling. The spin-wave
instability is suppressed and a superconducting instability predominates. For
the isotropic triangular lattice, there are again signs of long-range magnetic
order, in agreement with studies at strong-coupling.Comment: 4 pages, 5 eps figs, to appear in Can. J. Phys. (proceedings of the
Highly Frustrated Magnetism (HFM-2000) conference, Waterloo, Canada, June
2000
Bosonization and Fermion Liquids in Dimensions Greater Than One
(Revised, with postscript figures appended, corrections and added comments.)
We develop and describe new approaches to the problem of interacting Fermions
in spatial dimensions greater than one. These approaches are based on
generalizations of powerful tools previously applied to problems in one spatial
dimension. We begin with a review of one-dimensional interacting Fermions. We
then introduce a simplified model in two spatial dimensions to study the role
that spin and perfect nesting play in destabilizing Fermion liquids. The
complicated functional renormalization group equations of the full problem are
made tractable in our model by replacing the continuum of points that make up
the closed Fermi line with four Fermi points. Despite this drastic
approximation, the model exhibits physically reasonable behavior both at
half-filling (where instabilities occur) and away from half-filling (where a
Luttinger liquid arises). Next we implement the Bosonization of higher
dimensional Fermi surfaces introduced by Luther and advocated most recently by
Haldane. Bosonization incorporates the phase space and small-angle scattering
.... (7 figures, appended as a postscript file at the end of the TeX file).Comment: 48 text pages, plain TeX, BUP-JBM-
Large-N solutions of the Heisenberg and Hubbard-Heisenberg models on the anisotropic triangular lattice: application to CsCuCl and to the layered organic superconductors -(BEDT-TTF)X
We solve the Sp(N) Heisenberg and SU(N) Hubbard-Heisenberg models on the
anisotropic triangular lattice in the large-N limit. These two models may
describe respectively the magnetic and electronic properties of the family of
layered organic materials -(BEDT-TTF)X. The Heisenberg model is
also relevant to the frustrated antiferromagnet, CsCuCl. We find rich
phase diagrams for each model. The Sp(N) antiferromagnet is shown to have five
different phases as a function of the size of the spin and the degree of
anisotropy of the triangular lattice. The effects of fluctuations at finite-N
are also discussed. For parameters relevant to CsCuCl the ground state
either exhibits incommensurate spin order, or is in a quantum disordered phase
with deconfined spin-1/2 excitations and topological order. The SU(N)
Hubbard-Heisenberg model exhibits an insulating dimer phase, an insulating box
phase, a semi-metallic staggered flux phase (SFP), and a metallic uniform
phase. The uniform and SFP phases exhibit a pseudogap. A metal-insulator
transition occurs at intermediate values of the interaction strength.Comment: Typos corrected, one reference added. 20 pages, 17 figures, RevTeX
3.
Massive stars and the energy balance of the interstellar medium. II. The 35 solar mass star and a solution to the "missing wind problem"
We continue our numerical analysis of the morphological and energetic
influence of massive stars on their ambient interstellar medium for a 35 solar
mass star that evolves from the main sequence through red supergiant and
Wolf-Rayet phases, until it ultimately explodes as a supernova. We find that
structure formation in the circumstellar gas during the early main-sequence
evolution occurs as in the 60 solar mass case but is much less pronounced
because of the lower mechanical wind luminosity of the star. Since on the other
hand the shell-like structure of the HII region is largely preserved, effects
that rely on this symmetry become more important. At the end of the stellar
lifetime 1% of the energy released as Lyman continuum radiation and stellar
wind has been transferred to the circumstellar gas. From this fraction 10% is
kinetic energy of bulk motion, 36% is thermal energy, and the remaining 54% is
ionization energy of hydrogen. The sweeping up of the slow red supergiant wind
by the fast Wolf-Rayet wind produces remarkable morphological structures and
emission signatures, which are compared with existing observations of the
Wolf-Rayet bubble S308. Our model reproduces the correct order of magnitude of
observed X-ray luminosity, the temperature of the emitting plasma as well as
the limb brightening of the intensity profile. This is remarkable, because
current analytical and numerical models of Wolf-Rayet bubbles fail to
consistently explain these features. A key result is that almost the entire
X-ray emission in this stage comes from the shell of red supergiant wind swept
up by the shocked Wolf-Rayet wind rather than from the shocked Wolf-Rayet wind
itself as hitherto assumed and modeled. This offers a possible solution to what
is called the ``missing wind problem'' of Wolf-Rayet bubbles.Comment: 52 pages, 20 figures, 2 tables, accepted for publication in the
Astrophysical Journa
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