7,224 research outputs found
Non Linear Modeling of Mixed Ionic Electronic Conductors
A nonlinear model for the study of Mixed Ionic Electronic Conductors (MIEC) is presented in this paper. The model is time dependent and takes into account electrical carriers motion, the electrical behavior of the MIEC-metal interface and the kinetics of the chemistry occurring at the MIEC surface. By applying a small potential input complex impedances are computed
Tracking and data systems support for the Helios project. Volume 2: DSN support of Project Helios April 1975 - May 1976
Deep Space Network activities in the development of the Helios B mission from planning through entry of Helios 2 into first superior conjunction (end of Mission Phase II) are summarized. Network operational support activities for Helios 1 from first superior conjunction through entry into third superior conjunction are included
Paleohydrology of Some Ogallala (Neogene) Streams in the Southern Panhandle of Nebraska
Stratification and estimated paleoflow conditions for valley-fill deposits suggest that Ogallala Group streams in western Nebraska were similar to modern streams of south-central Alberta.
Ogallala stratification includes medium-scale (0.5 to 2.0 m thick) trough crossbedded sand and gravel, tabular indistinctly horizontally bedded and imbricated gravel, and horizontally bedded sand and pebbly sand. Valley fills are 15 to 55 m thick and 800 to 1800 m wide at the top. Some are in bedrock-floored channels resembling the “inner channels” of Shepherd and Schumm (1974).
Gradients for three well exposed paleovalley floors range from 0.0014 to 0.0020 (m/m) after tectonic correction. This compares with 0.00135 for the modem North Platte River Valley in Nebraska. The average intermediate diameters of the 10 largest clasts from tabular gravel beds found at 17 sites varied between 0.077 and 0.15 m. The average median intermediate diameter for gravel from four well exposed tabular gravel beds is 0.024 m.
Consistent paleodepth estimates of about 2 m correlate well with the scale of cross-stratification observed in the valley fills. Paleovelocities are estimated at about 2 m/sec, and Froude numbers of about 0.4 are consistent with a lower flow regime in the stability field of dunes. Two-dimensional specific in-channel paleodischarges were 3 to 4 m2/sec. Total paleodischarge estimates based on slope-discharge relationships for gravel-bed rivers range from 340 to 1240 m3/sec and are comparable to average annual peak discharges on the North Platte River reported 80 to 90 years ago.
Ogallala streams were probably dominated by macroforms similar to the “crescent-shaped bars” of the North Saskatchewan River. Deposition also took place on longitudinal bars in deeper channels. Shallow upper-flow regime transport and deposition is recorded by horizontally bedded sand and pebbly sand
Paleohydrology of Some Ogallala (Neogene) Streams in the Southern Panhandle of Nebraska
Stratification and estimated paleoflow conditions for valley-fill deposits suggest that Ogallala Group streams in western Nebraska were similar to modern streams of south-central Alberta.
Ogallala stratification includes medium-scale (0.5 to 2.0 m thick) trough crossbedded sand and gravel, tabular indistinctly horizontally bedded and imbricated gravel, and horizontally bedded sand and pebbly sand. Valley fills are 15 to 55 m thick and 800 to 1800 m wide at the top. Some are in bedrock-floored channels resembling the “inner channels” of Shepherd and Schumm (1974).
Gradients for three well exposed paleovalley floors range from 0.0014 to 0.0020 (m/m) after tectonic correction. This compares with 0.00135 for the modem North Platte River Valley in Nebraska. The average intermediate diameters of the 10 largest clasts from tabular gravel beds found at 17 sites varied between 0.077 and 0.15 m. The average median intermediate diameter for gravel from four well exposed tabular gravel beds is 0.024 m.
Consistent paleodepth estimates of about 2 m correlate well with the scale of cross-stratification observed in the valley fills. Paleovelocities are estimated at about 2 m/sec, and Froude numbers of about 0.4 are consistent with a lower flow regime in the stability field of dunes. Two-dimensional specific in-channel paleodischarges were 3 to 4 m2/sec. Total paleodischarge estimates based on slope-discharge relationships for gravel-bed rivers range from 340 to 1240 m3/sec and are comparable to average annual peak discharges on the North Platte River reported 80 to 90 years ago.
Ogallala streams were probably dominated by macroforms similar to the “crescent-shaped bars” of the North Saskatchewan River. Deposition also took place on longitudinal bars in deeper channels. Shallow upper-flow regime transport and deposition is recorded by horizontally bedded sand and pebbly sand
A short response-time atomic source for trapped ion experiments
Ion traps are often loaded from atomic beams produced by resistively heated
ovens. We demonstrate an atomic oven which has been designed for fast control
of the atomic flux density and reproducible construction. We study the limiting
time constants of the system and, in tests with , show we can
reach the desired level of flux in 12s, with no overshoot. Our results indicate
that it may be possible to achieve an even faster response by applying an
appropriate one-off heat treatment to the oven before it is used.Comment: 5 pages, 7 figure
Bounds on the mass and abundance of dark compact objects and black holes in dwarf spheroidal galaxy halos
We establish new dynamical constraints on the mass and abundance of compact
objects in the halo of dwarf spheroidal galaxies. In order to preserve
kinematically cold the second peak of the Ursa Minor dwarf spheroidal (UMi
dSph) against gravitational scattering, we place upper limits on the density of
compact objects as a function of their assumed mass. The mass of the dark
matter constituents cannot be larger than 1000 solar masses at a halo density
in UMi's core of 0.35 solar masses/pc^3. This constraint rules out a scenario
in which dark halo cores are formed by two-body relaxation processes. Our
bounds on the fraction of dark matter in compact objects with masses >3000
solar masses improve those based on dynamical arguments in the Galactic halo.
In particular, objects with masses solar masses can comprise no
more than a halo mass fraction . Better determinations of the
velocity dispersion of old overdense regions in dSphs may result in more
stringent constraints on the mass of halo objects. For illustration, if the
preliminary value of 0.5 km/s for the secondary peak of UMi is confirmed,
compact objects with masses above solar masses could be excluded
from comprising all its dark matter halo.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter
Evaporation of Compact Young Clusters near the Galactic Center
We investigate the dynamical evolution of compact young clusters (CYCs) near
the Galactic center (GC) using Fokker-Planck models. CYCs are very young (< 5
Myr), compact (< 1 pc), and only a few tens of pc away from the GC, while they
appear to be as massive as the smallest Galactic globular clusters (~10^4
Msun). A survey of cluster lifetimes for various initial mass functions,
cluster masses, and galactocentric radii is presented. Short relaxation times
due to the compactness of CYCs, and the strong tidal fields near the GC make
clusters evaporate fairly quickly. Depending on cluster parameters, mass
segregation may occur on a time scale shorter than the lifetimes of most
massive stars, which accelerates the cluster's dynamical evolution even more.
When the difference between the upper and lower mass boundaries of the initial
mass function is large enough, strongly selective ejection of lighter stars
makes massive stars dominate even in the outer regions of the cluster, so the
dynamical evolution of those clusters is weakly dependent on the lower mass
boundary. The mass bins for Fokker-Planck simulations were carefully chosen to
properly account for a relatively small number of the most massive stars. We
find that clusters with a mass <~ 2x10^4 Msun evaporate in <~ 10 Myr. A simple
calculation based on the total masses in observed CYCs and the lifetimes
obtained here indicates that the massive CYCs comprise only a fraction of the
star formation rate (SFR) in the inner bulge estimated from Lyman continuum
photons and far-IR observations.Comment: 20 pages in two-column format, accepted for publication in Ap
Efficient tight-binding Monte Carlo structural sampling of complex materials
While recent work towards the development of tight-binding and ab-initio
algorithms has focused on molecular dynamics, Monte Carlo methods can often
lead to better results with relatively little effort. We present here a
multi-step Monte Carlo algorithm that makes use of the possibility of quickly
evaluating local energies. For the thermalization of a 1000-atom configuration
of {\it a}-Si, this algorithm gains about an order of magnitude in speed over
standard molecular dynamics. The algorithm can easily be ported to a wide range
of materials and can be dynamically optimized for a maximum efficiency.Comment: 5 pages including 3 postscript figure
Stellar luminosity functions of rich star clusters in the Large Magellanic Cloud
We show the results of deep V and I HST photometry of 6 rich star clusters in
the Large Magellanic Cloud with different ages and metallicities. The number of
stars with measured magnitudes in each cluster varies from about 3000 to 10000.
We build stellar density and surface brightness profiles for the clusters and
extract half-light radii and other structural parameters for each. We also
obtain luminosity functions, Phi (Mv), down to Mv ~ 6 (m/msun > 0.6), and
investigate their dependence with distance from the cluster centre well beyond
their half-light radius. In all clusters we find a systematic increase in the
luminosity function slope with radial distance from the centre. Among the
clusters displaying significant mass segregation are the two youngest in the
sample: NGC 1805 and NGC 1818. For these two clusters we obtain present-day
mass functions. The NGC 1818 mass function is in excellent agreement with that
derived by other authors, also using HST data. They young cluster mass function
slopes differ, that of NCG 1805 being systematically steeper than NGC 1818.
Since these are very young stellar systems (age < 40 Myrs), these variations
may reflect the initial conditions rather than evolution due to internal
dynamics.Comment: 10 pages, 24 figure
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