4,970 research outputs found
Dependence of Gas Phase Abundances in the ISM on Column Density
Sightlines through high- and intermediate-velocity clouds allow measurements
of ionic gas phase abundances, A, at very low values of HI column density,
N(HI). Present observations cover over 4 orders of magnitude in N(HI).
Remarkably, for several ions we find that the A vs N(HI) relation is the same
at high and low column density and that the abundances have a relatively low
dispersion (factors of 2-3) at any particular N(HI). Halo gas tends to have
slightly higher values of A than disk gas at the same N(HI), suggesting that
part of the dispersion may be attributed to the environment. We note that the
dispersion is largest for NaI; using NaI as a predictor of N(HI) can lead to
large errors. Important implications of the low dispersions regarding the
physical nature of the ISM are: (a) because of clumping, over sufficiently long
pathlengths N(HI) is a reasonable measure of the_local_ density of_most_ of the
H atoms along the sight line; (b) the destruction of grains does not mainly
take place in catastrophic events such as strong shocks, but is a continuous
function of the mean density; (c) the cycling of the ions becoming attached to
grains and being detached must be rapid, and the two rates must be roughly
equal under a wide variety of conditions; (d) in gas that has a low average
density the attachment should occur within denser concentrations
Fast scan infrared detection and measuring instrument monthly progress report, oct. 1-31, 1964
Optics, scan, detector, and product design of fast scan infrared detection and measuring instrumen
CP and related phenomena in the context of Stellar Evolution
We review the interaction in intermediate and high mass stars between their
evolution and magnetic and chemical properties. We describe the theory of
Ap-star `fossil' fields, before touching on the expected secular diffusive
processes which give rise to evolution of the field. We then present recent
results from a spectropolarimetric survey of Herbig Ae/Be stars, showing that
magnetic fields of the kind seen on the main-sequence already exist during the
pre-main sequence phase, in agreement with fossil field theory, and that the
origin of the slow rotation of Ap/Bp stars also lies early in the pre-main
sequence evolution; we also present results confirming a lack of stars with
fields below a few hundred gauss. We then seek which macroscopic motions
compete with atomic diffusion in determining the surface abundances of AmFm
stars. While turbulent transport and mass loss, in competition with atomic
diffusion, are both able to explain observed surface abundances, the interior
abundance distribution is different enough to potentially lead to a test using
asterosismology. Finally we review progress on the turbulence-driving and
mixing processes in stellar radiative zones.Comment: Proceedings of IAU GA in Rio, JD4 on Ap stars; 10 pages, 7 figure
Roadmap on the theoretical work of BinaMIcS
We review the different theoretical challenges concerning magnetism in
interacting binary or multiple stars that will be studied in the BinaMIcS
(Binarity and Magnetic Interactions in various classes of Stars) project during
the corresponding spectropolarimetric Large Programs at CFHT and TBL. We
describe how completely new and innovative topics will be studied with BinaMIcS
such as the complex interactions between tidal flows and stellar magnetic
fields, the MHD star-star interactions, and the role of stellar magnetism in
stellar formation and vice versa. This will strongly modify our vision of the
evolution of interacting binary and multiple stars.Comment: 2 pages, proceeding of IAUS 302 Magnetic fields throughout stellar
evolution, correct list of author
In situ ozone data for evaluation of the laser absorption spectrometer ozone remote sensor: 1979 southeastern Virginia urban plume study summer field program
Ozone data from the 1979 Southeastern Virginia Urban Study (SEV-UPS) field program are presented. The SEV-UPS was conducted for evaluation of an ozone remote sensor, the Laser Absorption Spectrometer. During the measurement program, remote-sensor evaluation was in two areas; (1) determination of the remote sensor's accuracy, repeatability, and operational characteristics, and (2) demonstration of the application of remotely sensed ozone data in air-quality studies. Data from six experiments designed to provide in situ ozone data for evaluation of the sensor in area 1, above, are presented. Experiments consisted of overflights of a test area with the remote sensor aircraft while in situ measurements with a second aircraft and selected surface stations provided correlative ozone data within the viewing area of the remote sensor
In situ correlative measurements for the ultraviolet differential absorption lidar and the high spectral resolution lidar air quality remote sensors: 1980 PEPE/NEROS program
In situ correlative measurements were obtained with a NASA aircraft in support of two NASA airborne remote sensors participating in the Environmental Protection Agency's 1980persistent elevated pollution episode (PEPE) and Northeast regional oxidant study (NEROS) field program in order to provide data for evaluating the capability of two remote sensors for measuring mixing layer height, and ozone and aerosol concentrations in the troposphere during the 1980 PEPE/NEROS program. The in situ aircraft was instrumented to measure temperature, dewpoint temperature, ozone concentrations, and light scattering coefficient. In situ measurements for ten correlative missions are given and discussed. Each data set is presented in graphical and tabular format aircraft flight plans are included
Dust heating by the interstellar radiation field in models of turbulent molecular clouds
We have calculated the radiation field, dust grain temperatures, and far
infrared emissivity of numerical models of turbulent molecular clouds. When
compared to a uniform cloud of the same mean optical depth, most of the volume
inside the turbulent cloud is brighter, but most of the mass is darker. There
is little mean attenuation from center to edge, and clumping causes the
radiation field to be somewhat bluer. There is also a large dispersion,
typically by a few orders of magnitude, of all quantities relative to their
means. However, despite the scatter, the 850 micron emission maps are well
correlated with surface density. The fraction of mass as a function of
intensity can be reproduced by a simple hierarchical model of density
structure.Comment: 32 pages, 14 figures, submitted to Ap
Unified derivation of phase-field models for alloy solidification from a grand-potential functional
In the literature, two quite different phase-field formulations for the
problem of alloy solidification can be found. In the first, the material in the
diffuse interfaces is assumed to be in an intermediate state between solid and
liquid, with a unique local composition. In the second, the interface is seen
as a mixture of two phases that each retain their macroscopic properties, and a
separate concentration field for each phase is introduced. It is shown here
that both types of models can be obtained by the standard variational procedure
if a grand-potential functional is used as a starting point instead of a
free-energy functional. The dynamical variable is then the chemical potential
instead of the composition. In this framework, a complete analogy with
phase-field models for the solidification of a pure substance can be
established. This analogy is then exploited to formulate quantitative
phase-field models for alloys with arbitrary phase diagrams. The precision of
the method is illustrated by numerical simulations with varying interface
thickness.Comment: 36 pages, 1 figur
Calculating Cross Sections of Composite Interstellar Grains
Interstellar grains may be composite collections of particles of distinct
materials, including voids, agglomerated together. We determine the various
optical cross sections of such composite grains, given the optical properties
of each constituent, using an approximate model of the composite grain. We
assume it consists of many concentric spherical layers of the various
materials, each with a specified volume fraction. In such a case the usual Mie
theory can be generalized and the extinction, scattering, and other cross
sections determined exactly.
We find that the ordering of the materials in the layering makes some
difference to the derived cross sections, but averaging over the various
permutations of the order of the materials provides rapid convergence as the
number of shells (each of which is filled by all of the materials
proportionately to their volume fractions) is increased. Three shells, each
with one layer of a particular constituent material, give a very satisfactory
estimate of the average cross section produced by larger numbers of shells.
We give the formulae for the Rayleigh limit (small size parameter) for
multi-layered spheres and use it to propose an ``Effective Medium Theory''
(EMT), in which an average optical constant is taken to represent the ensemble
of materials.
Multi-layered models are used to compare the accuracies of several EMTs
already in the literature.Comment: 29 pages, 6 figures, accepted for publication in the Astrophysical
Journal (part 1, scheduled in Vol. 526, #1, Nov. 20
Lower limb muscle activation and kinematics modifications of young healthy adults while pushing a variable resistance sled
Introduction: The XPO Trainer used in this research is a novel device which provides low rolling resistance at low speeds with an immediate and automatic proportional increase in resistance with increased speed. Purpose: To examine the impact of using the XPO Trainer on gait and neuromuscular activation at low and high speeds in young, seemingly healthy adults. Materials and Methods: This work consisted of 48 healthy adults (age: 25.07 ± 3.46 years, weight: 148.2 ± 25.96 lbs, height: 66.57 ± 4.14 inches). Each participant wore accelerometers/gyroscopes sensors around each wrist and ankle, chest, and low back and surface electromyography (EMG) electrodes on their dominant leg over the quadriceps (QUAD), hamstring (HAM), anterior tibialis (TA), and gastrocnemius (GA). To initiate the tasks, participants walked then ran 40 feet with and without the XPO Trainer sled. Subjects did a total of 3 trials per tasks (total of 12) with one minute of rest between tasks to reduce fatigue factor. The data from the EMG and Mobility Lab sensors were then processed and compared through the SPSS 24 system for a repeated-measures ANOVA. Results: EMG- The QUAD muscle exhibited a significantly higher muscle activation (p ≤ .05) between walk (45.39 ± 24.43) and walk push (74.40 ± 56.73) tasks. Gait Parameters- There was a significant modification (p ≤ .05) between the different gait variables and tasks, including cadence, gait speed, stride length and trunk velocity while pushing the sled. Conclusion/Clinical Relevance: With the XPO Trainer being a novel device, it is important to understand how it affects the activation and response for muscles during different activities before using it as a training tool. Understanding the effect this particular sled can provide on the different components of the (temporospatial) gait parameters and muscle activation is valuable for a clinically appropriate application to specific populations.Supported by Texas Woman's University Small Grant Program
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