5,523 research outputs found
Zinc-oxygen primary cell yields high energy density
Zinc-oxygen primary cell yields high energy density for battery used as an auxiliary power source in space vehicle systems. Maximum reliability and minimum battery weight is achieved by using a stacking configuration of 23 series-connected modules with 6 parallel-connected cells per module
Performance prediction of point-based three-dimensional volumetric measurement systems
Point-based three-dimensional volumetric measurement systems are defined as multi-view vision systems which reconstruct a three-dimensional scene by first identifying key points on the views and then performing the reconstruction. Examples of these are defocusing digital particle image velocimetry (DDPIV) (Pereira et al 2000 Exp. Fluids 29 S78â84) and 3D particle tracking velocimetry (3DPTV) (Papantoniou and Maas 1990 5th Int. Symp. on the Application of Laser Techniques in Fluid Mechanics) which reconstruct clouds of flow tracers in order to estimate flow velocities. The reconstruction algorithms in these systems are variations of an epipolar line search. This paper presents a generalized error analysis of such methods, both in reconstruction precision (error in the reconstructed scene) and reconstruction quality (number of ambiguities or 'ghosts' produced)
Compressible Flows in Fluidic Oscillators
We present qualitative observations on the internal flow characteristics of
fluidic oscillator geometries commonly referred to as sweeping jets in active
flow control applications. This is part of the fluid dynamics videos.Comment: Videos include
GRID PRICING: VALUING CATTLE QUALITY INFORMATION
Grid pricing is increasingly prominent in cattle markets. This study compares selling 11,703 head of fed cattle using grid, live, and dressed weight pricing. Cattle sold on a grid had price variability twice that of live or dressed. Average pricing inefficiency by not selling cattle on a grid exceeded $30/head.Demand and Price Analysis, Livestock Production/Industries,
Apollo 17 EVA-1 and EVA-2 Task Decomposition: Planning for Artemis and Future Mars Missions
A decomposition of the Apollo 17 mission extravehicular activities (EVA) tasks can be used to prepare for Artemis and future Mars missions. A categorized minute by minute breakdown of the astronauts activites could be used to plan future EVAs and determine which scientific tasks or equipment may be prioritized. This is especially relevant in this critical stage for the upcoming Atemis missions and science activity planning. The infographics generated from the decomposition provide a higher level view of actual EVAs and could aid in making future EVAs more efficient and successful
Simulating a White Dwarf-dominated Galactic Halo
Observational evidence has suggested the possibility of a Galactic halo which
is dominated by white dwarfs (WDs). While debate continues concerning the
interpretation of this evidence, it is clear that an initial mass function
(IMF) biased heavily toward WD precursors (1 < m/Msol < 8), at least in the
early Universe, would be necessary in generating such a halo. Within the
framework of homogeneous, closed-box models of Galaxy formation, such biased
IMFs lead to an unavoidable overproduction of carbon and nitrogen relative to
oxygen (as measured against the abundance patterns in the oldest stars of the
Milky Way). Using a three-dimensional Tree N-body smoothed particle
hydrodynamics code, we study the dynamics and chemical evolution of a galaxy
with different IMFs. Both invariant and metallicity-dependent IMFs are
considered. Our variable IMF model invokes a WD-precursor-dominated IMF for
metallicities less than 5% solar (primarily the Galactic halo), and the
canonical Salpeter IMF otherwise (primarily the disk). Halo WD density
distributions and C,N/O abundance patterns are presented. While Galactic haloes
comprised of ~5% (by mass) of WDs are not supported by our simulations, mass
fractions of ~1-2% cannot be ruled out. This conclusion is consistent with the
present-day observational constraints.Comment: accepted for publication in MNRA
Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo
We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red
dwarfs, stars just massive enough to burn Hydrogen. The authors of this search
(Bahcall, Flynn, Gould \& Kirhakos 1994) found that red dwarfs make up less
than 6\% of the galactic halo. First, we extrapolate this result to include
brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a
mass function. Then the total mass of red dwarfs and brown dwarfs
is 18\% of the halo. This result is consistent with microlensing results
assuming a popular halo model. However, using new stellar models and parallax
observations of low mass, low metallicity stars, we obtain much tighter bounds
on low mass stars. We find the halo red dwarf density to be of the halo,
while our best estimate of this value is 0.14-0.37\%. Thus our estimate of the
halo mass density of red dwarfs drops to 16-40 times less than the reported
result of Bahcall et al (1994). For a mass function, this suggests
a total density of red dwarfs and brown dwarfs of 0.25-0.67\% of the
halo, \ie , (0.9-2.5)\times 10^9\msun out to 50 kpc. Such a low result would
conflict with microlensing estimates by the \macho\ group (Alcock \etal
1995a,b).Comment: 13 pages, 2 figures. Figure one only available via fax or snail-mail
To be published in ApJL. fig. 2 now available in postscript. Some minor
changes in dealing with disk forground. Some cosmetic changes. Updated
reference
Chemical Abundance Constraints on White Dwarfs as Halo Dark Matter
We examine the chemical abundance constraints on a population of white dwarfs
in the Halo of our Galaxy. We are motivated by microlensing evidence for
massive compact halo objects (Machos) in the Galactic Halo, but our work
constrains white dwarfs in the Halo regardless of what the Machos are. We focus
on the composition of the material that would be ejected as the white dwarfs
are formed; abundance patterns in the ejecta strongly constrain white dwarf
production scenarios. Using both analytical and numerical chemical evolution
models, we confirm that very strong constraints come from Galactic Pop II and
extragalactic carbon abundances. We also point out that depending on the
stellar model, significant nitrogen is produced rather than carbon. The
combined constraints from C and N give from
comparison with the low C and N abundances in the Ly forest. We note,
however, that these results are subject to uncertainties regarding the
nucleosynthesis of low-metallicity stars. We thus investigate additional
constraints from D and He, finding that these light elements can be kept
within observational limits only for \Omega_{WD} \la 0.003 and for a white
dwarf progenitor initial mass function sharply peaked at low mass (2).
Finally, we consider a Galactic wind, which is required to remove the ejecta
accompanying white dwarf production from the galaxy. We show that such a wind
can be driven by Type Ia supernovae arising from the white dwarfs themselves,
but these supernovae also lead to unacceptably large abundances of iron. We
conclude that abundance constraints exclude white dwarfs as Machos. (abridged)Comment: Written in AASTeX, 26 pages plus 4 ps figure
Magnitude bias of microlensed sources towards the Large Magellanic Cloud
There are lines of evidence suggesting that some of the observed microlensing
events in the direction of the Large Magellanic Cloud (LMC) are caused by
ordinary star lenses as opposed to dark Machos in the Galactic halo. Efficient
lensing by ordinary stars generally requires the presence of one or more
additional concentrations of stars along the line of sight to the LMC disk. If
such a population behind the LMC disk exists, then the source stars (for
lensing by LMC disk objects) will be drawn preferentially from the background
population and will show systematic differences from LMC field stars. One such
difference is that the (lensed) source stars will be farther away than the
average LMC field stars, and this should be reflected in their apparent
baseline magnitudes. We focus on red clump stars: these should appear in the
color-magnitude diagram at a few tenths of a magnitude fainter than the field
red clump. Suggestively, one of the two near-clump confirmed events,
MACHO-LMC-1, is a few tenths of magnitude fainter than the clump.Comment: To appear in ApJ Letters. Shortened to match the accepted version, 8
pages plus 1 ps figur
Direct Detection of Giant Close-In Planets Around the Source Stars of Caustic-Crossing Microlensing Events
We propose a direct method to detect close-in giant planets orbiting stars in
the Galactic bulge. This method uses caustic-crossing binary microlensing
events discovered by survey teams monitoring the bulge to measure light from a
planet orbiting the source star. When the planet crosses the caustic, it is
more magnified than the source star; its light is magnified by two orders of
magnitude for Jupiter size planets. If the planet is a giant close to the star,
it may be bright enough to make a significant deviation in the light curve of
the star. Detection of this deviation requires intensive monitoring of the
microlensing light curve using a 10-meter class telescope for a few hours after
the caustic. This is the only method yet proposed to directly detect close-in
planets around stars outside the solar neighborhood.Comment: 4 pages, 2 figures. Submitted to ApJ Letter
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