41,903 research outputs found
Numerical Models for the Diffuse Ionized Gas in Galaxies. II. Three-dimensional radiative transfer in inhomogeneous interstellar structures as a tool for analyzing the diffuse ionized gas
Aims: We systematically explore a plausible subset of the parameter space
involving effective temperatures and metallicities of the ionizing stellar
sources, the effects of the hardening of their radiation by surrounding leaky
HII regions with different escape fractions, as well as different scenarios for
the clumpiness of the DIG, and compute the resulting line strength ratios for a
number of diagnostic optical emission lines.
Methods: For the ionizing fluxes we compute a grid of stellar spectral energy
distributions (SEDs) from detailed, fully non-LTE model atmospheres that
include the effects of stellar winds and line blocking and blanketing. To
calculate the ionization and temperature structure in the HII regions and the
diffuse ionized gas we use spherically symmetric photoionization models as well
as state-of-the-art three-dimensional (3D) non-LTE radiative transfer
simulations, considering hydrogen, helium, and the most abundant metals.
Results: We provide quantitative predictions of how the line ratios from HII
regions and the DIG vary as a function of metallicity, stellar effective
temperature, and escape fraction from the HII region. The range of predicted
line ratios reinforces the hypothesis that the DIG is ionized by (filtered)
radiation from hot stars; however, comparison of observed and predicted line
ratios indicates that the DIG is typically ionized with a softer SED than
predicted by the chosen stellar population synthesis model. Even small changes
in simulation parameters like the clumping factor can lead to considerable
variation in the ionized volume. Both for a more homogeneous gas and a very
inhomogeneous gas containing both dense clumps and channels with low gas
density, the ionized region in the dilute gas above the galactic plane can
cease to be radiation-bounded, allowing the ionizing radiation to leak into the
intergalactic medium.Comment: 21 pages, 9 figures, accepted by A&
A mathematical simulation model of the CH-47B helicopter, volume 1
A nonlinear simulation model of the CH-47B helicopter was adapted for use in the NASA Ames Research Center (ARC) simulation facility. The model represents the specific configuration of the ARC variable stability CH-47B helicopter and will be used in ground simulation research and to expedite and verify flight experiment design. Modeling of the helicopter uses a total force approach in six rigid body degrees of freedom. Rotor dynamics are simulated using the Wheatlely-Bailey equations including steady-state flapping dynamics. Also included in the model is the option for simulation of external suspension, slung-load equations of motion
A Class of Free Boundary Problems with Onset of a new Phase
A class of diffusion driven Free Boundary Problems is considered which is
characterized by the initial onset of a phase and by an explicit kinematic
condition for the evolution of the free boundary. By a domain fixing change of
variables it naturally leads to coupled systems comprised of a singular
parabolic initial boundary value problem and a Hamilton-Jacobi equation. Even
though the one dimensional case has been thoroughly investigated, results as
basic as well-posedness and regularity have so far not been obtained for its
higher dimensional counterpart. In this paper a recently developed regularity
theory for abstract singular parabolic Cauchy problems is utilized to obtain
the first well-posedness results for the Free Boundary Problems under
consideration. The derivation of elliptic regularity results for the underlying
static singular problems will play an important role
Feynman versus Bakamjian-Thomas in Light Front Dynamics
We compare the Bakamjian-Thomas (BT) formulation of relativistic few-body
systems with light front field theories that maintain closer contact with
Feynman diagrams. We find that Feynman diagrams distinguish Melosh rotations
and other kinematical quantities belonging to various composite subsystem
frames that correspond to different loop integrals. The BT formalism knows only
the rest frame of the whole composite system, where everything is evaluated.Comment: 5 page
Effects of Neutron Irradiation on Pinning Force Scaling in State-of-the-Art Nb3Sn Wires
We present an extensive irradiation study involving five state-of-the-art
Nb3Sn wires which were subjected to sequential neutron irradiation up to a fast
neutron fluence of 1.6 * 10^22 m^-2 (E > 0.1 MeV). The volume pinning force of
short wire samples was assessed in the temperature range from 4.2 to 15 K in
applied fields of up to 7 T by means of SQUID magnetometry in the unirradiated
state and after each irradiation step. Pinning force scaling computations
revealed that the exponents in the pinning force function differ significantly
from those expected for pure grain boundary pinning, and that fast neutron
irradiation causes a substantial change in the functional dependence of the
volume pinning force. A model is presented, which describes the pinning force
function of irradiated wires using a two-component ansatz involving a
point-pinning contribution stemming from radiation induced pinning centers. The
dependence of this point-pinning contribution on fast neutron fluence appears
to be a universal function for all examined wire types.Comment: 8 page
Developments in quantitative luminescence techniques
Developments in quantitative luminescence technique
Where Do Consumers in Nairobi Purchase their Food and Why Does this Matter? The Need for Investment to Improve Kenya's "Traditional" Food Marketing System
Published by Tegemeo Institute for Agricultural Policy and Developmentfood security, food policy, Kenya, food marketing system, Food Consumption/Nutrition/Food Safety, Q18,
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