243,423 research outputs found
The Synthetic-Oversampling Method: Using Photometric Colors to Discover Extremely Metal-Poor Stars
Extremely metal-poor (EMP) stars ([Fe/H] < -3.0 dex) provide a unique window
into understanding the first generation of stars and early chemical enrichment
of the Universe. EMP stars are exceptionally rare, however, and the relatively
small number of confirmed discoveries limits our ability to exploit these
near-field probes of the first ~500 Myr after the Big Bang. Here, a new method
to photometrically estimate [Fe/H] from only broadband photometric colors is
presented. I show that the method, which utilizes machine-learning algorithms
and a training set of ~170,000 stars with spectroscopically measured [Fe/H],
produces a typical scatter of ~0.29 dex. This performance is similar to what is
achievable via low-resolution spectroscopy, and outperforms other photometric
techniques, while also being more general. I further show that a slight
alteration to the model, wherein synthetic EMP stars are added to the training
set, yields the robust identification of EMP candidates. In particular, this
synthetic-oversampling method recovers ~20% of the EMP stars in the training
set, at a precision of ~0.05. Furthermore, ~65% of the false positives from the
model are very metal-poor stars ([Fe/H] < -2.0 dex). The synthetic-oversampling
method is biased towards the discovery of warm (~F-type) stars, a consequence
of the targeting bias from the SDSS/SEGUE survey. This EMP selection method
represents a significant improvement over alternative broadband optical
selection techniques. The models are applied to >12 million stars, with an
expected yield of ~600 new EMP stars, which promises to open new avenues for
exploring the early universe.Comment: 15 pages, 7 figures, to be submitted to Ap
Wide-field birefringent elements
Birefrigent array consists of two plates with retardation properties nearly independent of direction of incident light over unlimited range of wavelengths. Array can be used as birefrigent color filter. Optical properties of plates may also be changed electro-optically rather than thermally
Introduction to life modeling of thermal barrier coatings
Thermal barrier coatings may be applied to air-cooled turbine section airfoils to insulate such components from hot gases in the engine. The coatings, which typically consist of about 0.01 to 0.04 cm of zirconia-yttria ceramic over about 0.01 cm of NiCrAlY or NiCrAlZr alloy bond coat, allow increased gas temperatures or reduced cooling air flows. This, in turn, leads to marked improvements in engine efficiency and performance. However, certain risks are associated with designing for maximum benefits, and eventually a point is reached where coating loss would immediately jeopardize the underlying component. Therefore, designers must be able to accurately predict the life of a given bill-of-material coating in any particular design. The results of an in-house aeronautics, base research and technology program which is designed to provide the first steps towards developing mission-capable life-prediction models are outlined
An efficient, simple dialyzer
Easily assembled, efficient, countercurrent, sandwich-type barrier dialyzer was developed. Dialyzer contains six blood chambers that provide 500 sq cm membrane area. Design membranes are cuprammonium cellulose film. Unit performance was compared with thirteen other dialyzers
The CMB - Contemporary Measurements and Cosmology
Since the discovery of the Cosmic Microwave Background (CMB) in 1965,
characterization of the CMB anisotropy angular power spectrum has become
somewhat of a holy grail for experimental cosmology. Because CMB anisotropy
measurements are difficult, the full potential of the CMB is only now being
realized. Improvements in experimental techniques and detector technology have
yielded an explosion of progress in the past couple of years resulting in the
ability to use measurements of the CMB to place meaningful constraints on
cosmological parameters. In this review, I discuss the theory behind the CMB
but focus primarily on the experiments, reviewing briefly the history of CMB
anisotropy measurements and focusing on the recent experiments that have
revolutionized this field. Results from these modern experiments are reviewed
and the cosmological implications discussed. I conclude with brief comments
about the future of CMB physics.Comment: 18 pages including 5 ps figures. GR16 proceedings to appear in World
Scientifi
Infinite Nuclear Matter on the Light Front: A Modern Approach to Brueckner Theory
Understanding an important class of experiments requires that light-front
dynamics and related light cone variables k^+ and k_perp be used. If one uses
k^+ as a momentum variable, the corresponding canonical spatial variable is
x^-=x^0-x^3 and the time variable is x^0+x^3. This is the light front (LF)
approach of Dirac. A relativistic light front formulation of nuclear dynamics
is developed and applied to treating infinite nuclear matter in a method which
includes the corelations of pairs of nculeons. This is light front Brueckner
theory.Comment: 7 pages, text of an invited talk presented at the 10th International
Conference on Recent Progress In Many-Body Theories. To be published by World
Scientific as volume 3 of "Series on Advances in Quantum Many-Body Theory",
eds. R.F. Bishop, C.E. Campbell, J.W. Clark and S. Fanton
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