13,834 research outputs found
Zirconium, Barium, Lanthanum and Europium Abundances in Open Clusters
We present an analysis of the s-process elements Zr, Ba, and La and the
r-process element Eu in a sample of 50 stars in 19 open clusters. Stellar
abundances of each element are based on measures of a minimum of two lines per
species via both equivalent width and spectrum synthesis techniques. We
investigate cluster mean neutron-capture abundance trends as a function of
cluster age and location in the Milky Way disk and compare them to results
found in other studies in the literature. We find a statistically significant
trend of increasing cluster [Ba/Fe] as a function of decreasing cluster age, in
agreement with recent findings for other open cluster samples, supporting the
increased importance of low-mass asymptotic giant branch stars to the
generation of s-process elements. However, the other s-process elements,
[La/Fe] and [Zr/Fe], do not show similar dependences, in contrast to
theoretical expectations and the limited observational data from other studies.
Conversely, cluster [Eu/Fe] ratios show a slight increase with increasing
cluster age, although with marginal statistical significance. Ratios of
[s/r]-process abundances, [Ba/Eu] and [La/Eu], however, show more clearly the
increasing efficiency of s-process relative to r-process enrichment in open
cluster chemical evolution, with significant increases among younger clusters.
Last, cluster neutron-capture element abundances appear to be independent of
Galactocentric distance. We conclude that a homogeneous analysis of a larger
sample of open clusters is needed to resolve the apparent discrepant
conclusions between different studies regarding s-process element abundance
trends with age to better inform models of galactic chemical evolution.Comment: 24 pages, 13 figures, 10 tables; published in The Astronomical
Journa
Noise Power Spectrum Scene-Dependency in Simulated Image Capture Systems
The Noise Power Spectrum (NPS) is a standard measure for image capture system noise. It is derived traditionally from captured uniform luminance patches that are unrepresentative of pictorial scene signals. Many contemporary capture systems apply non- linear content-aware signal processing, which renders their noise scene-dependent. For scene-dependent systems, measuring the NPS with respect to uniform patch signals fails to characterize with accuracy: i) system noise concerning a given input scene, ii) the average system noise power in real-world applications. The scene- and-process-dependent NPS (SPD-NPS) framework addresses these limitations by measuring temporally varying system noise with respect to any given input signal. In this paper, we examine the scene-dependency of simulated camera pipelines in-depth by deriving SPD-NPSs from fifty test scenes. The pipelines apply either linear or non-linear denoising and sharpening, tuned to optimize output image quality at various opacity levels and exposures. Further, we present the integrated area under the mean of SPD-NPS curves over a representative scene set as an objective system noise metric, and their relative standard deviation area (RSDA) as a metric for system noise scene-dependency. We close by discussing how these metrics can also be computed using scene-and-process- dependent Modulation Transfer Functions (SPD-MTF)
The Spectrum of Crab Nebula X-Rays to 120 Kev
Counting rate and pulse height distribution spectral data of Crab Nebula telemetered from balloon detector
On the effects of the Dvali-Gabadadze-Porrati braneworld gravity on the orbital motion of a test particle
In this paper we explicitly work out the secular perturbations induced on all
the Keplerian orbital elements of a test body to order O(e^2) in the
eccentricity e by the weak-field long-range modifications of the usual
Newton-Einstein gravity due to the Dvali-Gabadadze-Porrati (DGP) braneworld
model. The Gauss perturbative scheme is used. It turns out that the argument of
pericentre and the mean anomaly are affected by secular rates which are
independent of the semimajor axis of the orbit of the test particle. The first
nonvaishing eccentricity-dependent corrections are of order O(e^2). For
circular orbits the Lue-Starkman (LS) effect on the pericentre is obtained.
Some observational consequences are discussed for the Solar System planetary
mean longitudes lambda which would undergo a 1.2\cdot 10^-3 arcseconds per
century braneworld secular precession. According to recent data analysis over
92 years for the EPM2004 ephemerides, the 1-sigma formal accuracy in
determining the Martian mean longitude amounts to 3\cdot 10^-3 milliarcseconds,
while the braneworld effect over the same time span would be 1.159
milliarcseconds. The major limiting factor is the 2.6\cdot 10^-3 arcseconds per
century systematic error due to the mismodelling in the Keplerian mean motion
of Mars. A suitable linear combination of the mean longitudes of Mars and Venus
may overcome this problem. The formal, 1-sigma obtainable observational
accuracy would be \sim 7%. The systematic error due to the present-day
uncertainties in the solar quadrupole mass moment, the Keplerian mean motions,
the general relativistic Schwarzschild field and the asteroid ring would amount
to some tens of percent.Comment: LaTex2e, 23 pages, 5 tables, 1 figure, 37 references. Second-order
corrections in eccentricity explicitly added. Typos corrected. References
update
Collapse of Kaluza-Klein Bubbles
Kaluza-Klein theory admits ``bubble" configurations, in which the
circumference of the fifth dimension shrinks to zero on some compact surface. A
three parameter family of such bubble initial data at a moment of time-symmetry
(some including a magnetic field) has been found by Brill and Horowitz,
generalizing the (zero-energy) ``Witten bubble" solution. Some of these data
have negative total energy. We show here that all the negative energy bubble
solutions start out expanding away from the moment of time symmetry, while the
positive energy bubbles can start out either expanding or contracting. Thus it
is unlikely that the negative energy bubbles would collapse and produce a naked
singularity.Comment: 6 pages, plain LaTeX, UMDGR-94-08
Bridging the Gap Between Imaging Performance and Image Quality Measures
Imaging system performance measures and Image Quality Metrics (IQM) are reviewed from a systems engineering perspective, focusing on spatial quality of still image capture systems. We classify IQMs broadly as: Computational IQMs (CPIQM), Multivariate Formalism IQMs (MF-IQM), Image Fidelity Metrics (IF-IQM), and Signal Transfer Visual IQMs (STV-IQM). Comparison of each genre finds STV-IQMs well suited for capture system quality evaluation: they incorporate performance measures relevant to optical systems design, such as Modulation Transfer Function (MTF) and Noise-Power Spectrum (NPS); their bottom, modular approach enables system components to be optimised separately. We suggest that correlation between STV IQMs and observer quality scores is limited by three factors: current MTF and NPS measures do not characterize scene-dependent performance introduced by imaging system non-linearities; contrast sensitivity models employed do not account for contextual masking effects; cognitive factors are not considered. We hypothesise that implementation of scene and process-dependent MTF (SPD-MTF) and NPS (SPD-NPS) measures should mitigate errors originating from scene dependent system performance. Further, we propose implementation of contextual contrast detection and discrimination models to better represent low-level visual performance in image quality analysis. Finally, we discuss image quality optimization functions that may potentially close the gap between contrast detection/discrimination and quality
Quantum field theory on a growing lattice
We construct the classical and canonically quantized theories of a massless
scalar field on a background lattice in which the number of points--and hence
the number of modes--may grow in time. To obtain a well-defined theory certain
restrictions must be imposed on the lattice. Growth-induced particle creation
is studied in a two-dimensional example. The results suggest that local mode
birth of this sort injects too much energy into the vacuum to be a viable model
of cosmological mode birth.Comment: 28 pages, 2 figures; v.2: added comments on defining energy, and
reference
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