5,464 research outputs found
In situ GISAXS study of the growth of Pd on MgO(001)
The morphology of growing Pd nano-particles on MgO(001) surfaces have been
investigated in situ, during growth, by grazing incidence small angle x-ray
scattering, for different substrate temperatures. The 2D patterns obtained are
quantitatively analyzed, and the average morphological parameters (shape, size)
deduced. Above 650 K, the aggregates adopt their equilibrium shape of truncated
octahedron, and the interfacial energy is deduced.Comment: 10 pages, 1 Table, 2 Figure
Recommended from our members
Maximizing the Strength of Fused-Deposition ABS Plastic Parts
Fused-Deposition (FD) creates parts using robotic extrusion of set.D.i-liquid .polymer
fiber, which molecularly bonds with neighboring fibers via thermal-dlffuslo.n bonding. T~e
strength ofthe. part depends on the bulk polymer strength, themesostructure ~flber layout, vOid
geom~try, extent of fiber bonding), and thefiber-t~-fiber ~ond strength. The ~nfluence of these
factors on the mechanical strength of FD-ABS.plasttc parts IS reported.along with the.FD process
variable settingsfor maximum strength. Substantial increases in transverse strength are achieved
at the optirnal settings and additional increases can be achieved by post..fabrication annealing.
Keywords: Stratasys, fused-deposition, ABSplastic, functional parts, strength, mesostructure, polymer diffusion.Mechanical Engineerin
Modelling CO emission from hydrodynamic simulations of nearby spirals, starbursting mergers, and high-redshift galaxies
We model the intensity of emission lines from the CO molecule, based on
hydrodynamic simulations of spirals, mergers, and high-redshift galaxies with
very high resolutions (3pc and 10^3 Msun) and detailed models for the
phase-space structure of the interstellar gas including shock heating, stellar
feedback processes and galactic winds. The simulations are analyzed with a
Large Velocity Gradient (LVG) model to compute the local emission in various
molecular lines in each resolution element, radiation transfer and opacity
effects, and the intensity emerging from galaxies, to generate synthetic
spectra for various transitions of the CO molecule. This model reproduces the
known properties of CO spectra and CO-to-H2 conversion factors in nearby
spirals and starbursting major mergers. The high excitation of CO lines in
mergers is dominated by an excess of high-density gas, and the high turbulent
velocities and compression that create this dense gas excess result in broad
linewidths and low CO intensity-to-H2 mass ratios. When applied to
high-redshift gas-rich disks galaxies, the same model predicts that their
CO-to-H2 conversion factor is almost as high as in nearby spirals, and much
higher than in starbursting mergers. High-redshift disk galaxies contain giant
star-forming clumps that host a high-excitation component associated to gas
warmed by the spatially-concentrated stellar feedback sources, although CO(1-0)
to CO(3-2) emission is overall dominated by low-excitation gas around the
densest clumps. These results overall highlight a strong dependence of CO
excitation and the CO-to-H2 conversion factor on galaxy type, even at similar
star formation rates or densities. The underlying processes are driven by the
interstellar medium structure and turbulence and its response to stellar
feedback, which depend on global galaxy structure and in turn impact the CO
emission properties.Comment: A&A in pres
The First Fermi-LAT SNR Catalog SNR and Cosmic Ray Implications
Galactic cosmic ray (CRs) sources, classically proposed to be Supernova
Remnants (SNRs), must meet the energetic particle content required by direct
measurements of high energy CRs. Indirect gamma-ray measurements of SNRs with
the Fermi Large Area Telescope (LAT) have now shown directly that at least
three SNRs accelerate protons. With the first Fermi LAT SNR Catalog, we have
systematically characterized the GeV gamma-rays emitted by 279 SNRs known
primarily from radio surveys. We present these sources in a multiwavelength
context, including studies of correlations between GeV and radio size, flux,
and index, TeV index, and age and environment tracers, in order to better
understand effects of evolution and environment on the GeV emission. We show
that previously sufficient models of SNRs' GeV emission no longer adequately
describe the data. To address the question of CR origins, we also examine the
SNRs' maximal CR contribution assuming the GeV emission arises solely from
proton interactions. Improved breadth and quality of multiwavelength data,
including distances and local densities, and more, higher resolution gamma-ray
data with correspondingly improved Galactic diffuse models will strengthen this
constraint.Comment: 8 pages, 10 figures; in Proceedings of the 34th International Cosmic
Ray Conference (ICRC 2015), The Hague (The Netherlands
A new nearby pulsar wind nebula overlapping the RX J0852.0-4622 supernova remnant
Energetic pulsars can be embedded in a nebula of relativistic leptons which
is powered by the dissipation of the rotational energy of the pulsar. The
object PSR J0855-4644 is an energetic and fast-spinning pulsar (Edot =
1.1x10^36 erg/s, P=65 ms) discovered near the South-East rim of the supernova
remnant (SNR) RX J0852.0-4622 (aka Vela Jr) by the Parkes multibeam survey. The
position of the pulsar is in spatial coincidence with an enhancement in X-rays
and TeV gamma-rays, which could be due to its putative pulsar wind nebula
(PWN).
The purpose of this study is to search for diffuse non-thermal X-ray emission
around PSR J0855-4644 to test for the presence of a PWN and to estimate the
distance to the pulsar. An X-ray observation was carried out with the
XMM-Newton satellite to constrain the properties of the pulsar and its nebula.
The absorption column density derived in X-rays from the pulsar and from
different regions of the rim of the SNR was compared with the absorption
derived from the atomic (HI) and molecular (12CO) gas distribution along the
corresponding lines of sight to estimate the distance of the pulsar and of the
SNR.
The observation has revealed the X-ray counterpart of the pulsar together
with surrounding extended emission thus confirming the existence of a PWN. The
comparison of column densities provided an upper limit to the distance of the
pulsar PSR J0855-4644 and the SNR RX J0852.0-4622 (d<900 pc). Although both
objects are at compatible distances, we rule out that the pulsar and the SNR
are associated. With this revised distance, PSR J0855-4644 is the second most
energetic pulsar, after the Vela pulsar, within a radius of 1 kpc and could
therefore contribute to the local cosmic-ray e-/e+ spectrum.Comment: 10 pages, 9 Figures. Accepted for publication in A&
- …