603 research outputs found
Aircraft electromagnetic compatibility
Illustrated are aircraft architecture, electromagnetic interference environments, electromagnetic compatibility protection techniques, program specifications, tasks, and verification and validation procedures. The environment of 400 Hz power, electrical transients, and radio frequency fields are portrayed and related to thresholds of avionics electronics. Five layers of protection for avionics are defined. Recognition is given to some present day electromagnetic compatibility weaknesses and issues which serve to reemphasize the importance of EMC verification of equipment and parts, and their ultimate EMC validation on the aircraft. Proven standards of grounding, bonding, shielding, wiring, and packaging are laid out to help provide a foundation for a comprehensive approach to successful future aircraft design and an understanding of cost effective EMC in an aircraft setting
NASA/FAA helicopter simulator workshop
A workshop was convened by the FAA and NASA for the purpose of providing a forum at which leading designers, manufacturers, and users of helicopter simulators could initiate and participate in a development process that would facilitate the formulation of qualification standards by the regulatory agency. Formal papers were presented, special topics were discussed in breakout sessions, and a draft FAA advisory circular defining specifications for helicopter simulators was presented and discussed. A working group of volunteers was formed to work with the National Simulator Program Office to develop a final version of the circular. The workshop attracted 90 individuals from a constituency of simulator manufacturers, training organizations, the military, civil regulators, research scientists, and five foreign countries
Part 1: Executive summary
A workshop was convened by the FAA and NASA for the purpose of providing a forum at which leading designers, manufacturers, and users of helicopter simulators could initiate and participate in a development process that would facilitate the formulation of qualification standards by the regulatory agency. Formal papers were presented, special topics were discussed in breakout sessions, and a draft FAA advisory circular defining specifications for helicopter simulators was presented and discussed. A working group of volunteers was formed to work with the National Simulator Program Office to develop a final version of the circular. The workshop attracted 90 individuals from a constituency of simulator manufacturers, training organizations, the military, civil regulators, research scientists, and five foreign countries. A great amount of information was generated and recorded verbatim. This information is presented herein within the limits of accuracy inherent in recording, transcribing, and editing spoken technical material
On the Formation of Galaxy Halos: Comparing NGC 5128 and the Local Group Members
The metallicity distribution function (MDF) for the old red-giant stars in
the halo of NGC 5128, the nearest giant elliptical galaxy, is virtually
identical with the MDF for the old-disk stars in the LMC and also strongly
resembles the halo MDF in M31. These galaxies all have high mean halo
metallicities ( ~ -0.4$) with very small proportions of low-metallicity
stars. These observations reinforce the view that metal-rich halos are quite
normal for large galaxies of all types. Such systems are unlikely to have built
up by accretion of pre-existing, gas-free small satellite galaxies, unless
these satellites had an extremely shallow mass distribution (d log N / d log M
> -1). We suggest that the halo of NGC 5128 is more likely to have assembled
from hierarchical merging of gas-rich lumps in which the bulk of star formation
took place during or after the merger stage.Comment: 10 pages, LaTeX, plus 3 figures in separate postscript files;
Astronomical Journal, in press for December 200
The structure of the hydrated electron. Part 2. A mixed quantum classical molecular dynamics - embedded cluster density functional theory: single-excitation configuration interaction study
Adiabatic mixed quantum/classical molecular dynamics simulations were used to
generate snapshots of the hydrated electron (e-) in liquid water at 300 K.
Water cluster anions that include two complete solvation shells centered on the
e- were extracted from these simulations and embedded in a matrix of fractional
point charges designed to represent the rest of the solvent. Density functional
theory and single-excitation configuration interaction methods were then
applied to these embedded clusters. The salient feature of these hybrid
calculations is significant transfer (ca. 0.18) of the excess electron's charge
density into the O 2p orbitals in OH groups forming the solvation cavity. We
used the results of these calculations to examine the structure of the
molecular orbitals, the density of states, the absorption spectra in the
visible and ultraviolet, the hyperfine coupling (hfc) tensors, and the IR and
Raman spectra of the e-. The calculated hfc tensors were used to compute the
EPR and ESEEM spectra for the e- that compared favorably to the experimental
spectra of trapped e- in alkaline ice. The calculated vibrational spectra of
the e- are consistent with the red-shifted bending and stretching frequencies
observed in resonance Raman experiments. The model also accounts for the VIS
and 190-nm absorption bands of the e-. Thus, our study suggests that to explain
several important experimentally observed properties of the e-, many-electron
effects must be accounted for.Comment: 68 pages, 12 figures + 16 more figures in the supplement (included)
submitted to J Phys Chem
Structural Parameters of the M87 Globular Clusters
We derive structural parameters for ~2000 globular clusters in the giant
Virgo elliptical M87 using extremely deep Hubble Space Telescope images in
F606W (V) and F814W (I) taken with the ACS/WFC. The cluster scale sizes
(half-light radii r_h) and ellipticities are determined from PSF-convolved
King-model profile fitting. We find that the r_h distribution closely resembles
the inner Milky Way clusters, peaking at r_h~2.5 pc and with virtually no
clusters more compact than r_h ~ 1 pc. The metal-poor clusters have on average
an r_h 24% larger than the metal-rich ones. The cluster scale size shows a
gradual and noticeable increase with galactocentric distance. Clusters are very
slightly larger in the bluer waveband V a possible hint that we may be
beginning to see the effects of mass segregation within the clusters. We also
derived a color magnitude diagram for the M87 globular cluster system which
show a striking bimodal distribution.Comment: ApJ accepte
New g'r'i'z' Photometry of the NGC 5128 Globular Cluster System
We present new photometry for 323 of the globular clusters in NGC 5128
(Centaurus A), measured for the first time in the filter system. The
color indices are calibrated directly to standard stars in the
system and are used to establish the fiducial mean colors for the blue and red
(low and high metallicity) globular cluster sequences. We also use
spectroscopically measured abundances to establish the conversion between the
most metallicity-sensitive colors (, ) and metallicity,
[Fe/H].Comment: 14 pages, 9 figures, accepted in A
Globular Cluster Systems in Giant Ellipticals: the Mass/Metallicity Relation
Hubble Space Telescope ACS/WFC data in (B,I) are used to investigate the
globular cluster populations around 6 gE galaxies ~40 Mpc distant. The total
comprises a sample of ~8000 high-probability globular clusters. PSF-convolved
King-model profiles are used to measure their individual total magnitudes,
colors, and effective radii. The classic bimodal form of the GC color-magnitude
distribution shows up unambiguously in all the galaxies, allowing an accurate
definition of the mean colors along each of the two sequences as a function of
magnitude (the mass/metallicity relation or MMR). The blue, metal-poor cluster
sequence shows a clearly defined but nonlinear MMR, changing smoothly from a
near-vertical sequence at low luminosity to an increasingly redward slope at
higher luminosity, while the red, metal-rich sequence is nearly vertical at all
luminosities. All the observed features of the present data agree with the
interpretation that the MMR is created primarily by GC self-enrichment, along
the lines of the quantitative model of Bailin and Harris (2009): The
"threshold" mass at which this effect should become noticeable is near 1
million Solar masses, which is closely consistent with the transition region
that is seen in the data. Correlation of the median half-light radii of the GCs
with other parameters shows that the metal-poor clusters are consistently 17%
larger than those of the metal-rich clusters, at all galactocentric distances
and luminosities. At the same time, cluster size scales with halo location as
r_h ~ R_gc^0.11, indicating that both metallicity and the external tidal
environment play roles in determining the scale size of a given cluster.
Lastly, both the red and blue GC components show metallicity gradients with
galactocentric distance, following Z ~ R_gc^-0.1.Comment: In press for Astrophysical Journal. Complete preprint with higher
quality figures is available at
http://physwww.mcmaster.ca/%7Eharris/Publications.htm
Measuring the cosmological lepton asymmetry through the CMB anisotropy
A large lepton asymmetry in the Universe is still a viable possibility and
leads to many interesting phenomena such as gauge symmetry nonrestoration at
high temperature. We show that a large lepton asymmetry changes the predicted
cosmic microwave background (CMB) anisotropy and that any degeneracy in the
relic neutrino sea will be measured to a precision of 1% or better when the CMB
anisotropy is measured at the accuracy expected to result from the planned
satellite missions MAP and Planck. In fact, the current measurements already
put an upper limit on the lepton asymmetry of the Universe which is stronger
than the one coming from considerations of primordial nucleosynthesis and
structure formation.Comment: 4 pagex LaTex, 1 color postscript figure, uses epsf. Version
submitted to PRL. (Bug in code fixed, new figure, conclusions unchanged
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