2,065 research outputs found
Influence of coating on the thermal resistance of a Ni-Based superalloy
In this paper, the influence of M-CrAlY polycrystalline coating on the thermal fatigue behavior of a Nickel-base superalloy has been investigated. A special device using a rotating bending machine and two thermal sources has been used to perform thermo-mechanical tests. The two thermal sources have been set to obtain temperature variations between 750 and 1120 °C in the central part of the specimens, with a frequency of 0.1 Hz. The results showed a deleterious effect of the coating on the fatigue resistance. Numerical simulations have been carried out on SAMCEF to determine the thermo-mechanical field of the so-tested specimens. Calculated thermo-mechanical cycles of critical sites are associated with microstructure evolution and damage by cracking observed on the specimens. Damage mechanisms related to the presence of coating are discussed
OVI, NV and CIV in the Galactic Halo: II. Velocity-Resolved Observations with Hubble and FUSE
We present a survey of NV and OVI (and where available CIV) in the Galactic
halo, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the
Hubble Space Telescope (HST) along 34 sightlines. These ions are usually
produced in nonequilibrium processes such as shocks, evaporative interfaces, or
rapidly cooling gas, and thus trace the dynamics of the interstellar medium.
Searching for global trends in integrated and velocity-resolved column density
ratios, we find large variations in most measures, with some evidence for a
systematic trend of higher ionization (lower NV/OVI column density ratio) at
larger positive line-of-sight velocities. The slopes of log[N(NV)/N(OVI)] per
unit velocity range from -0.015 to +0.005, with a mean of
-0.0032+/-0.0022(r)+/-0.0014(sys) dex/(km/s). We compare this dataset with
models of velocity-resolved high-ion signatures of several common physical
structures. The dispersion of the ratios, OVI/NV/CIV, supports the growing
belief that no single model can account for hot halo gas, and in fact some
models predict much stronger trends than are observed. It is important to
understand the signatures of different physical structures to interpret
specific lines of sight and future global surveys.Comment: ApJ in press 43 pages, 22 fig
Multiple emission components in the Cygnus cocoon detected from Fermi-LAT observations
Star-forming regions may play an important role in the life cycle of Galactic
cosmic rays. Gamma-ray observations of Cygnus X have revealed the presence of
an excess of hard-spectrum gamma-ray emission, possibly related to a cocoon of
freshly accelerated particles. Based on ~13 years of observations with the
Fermi-Large Area Telescope (LAT), we performed an improved
spectro-morphological characterisation of the residual emission including the
cocoon. The best-fit model for the cocoon includes two main emission
components: an extended component FCES G78.74+1.56, described by a 2D Gaussian
of extension r_{68} = 4.4^\circ \pm 0.1^\circ\,^{+0.1^\circ}_{-0.1^\circ},
and a central component FCES G80.00+0.50, traced by the distribution of ionised
gas within the borders of the photo-dissociation regions. The two have
significantly different spectra. An additional extended emission component FCES
G78.83+3.57, located on the edge of the central cavities in Cygnus X and with a
spectrum compatible with that of FCES G80.00+0.50, is likely related to the
cocoon. For the two main components, spectra and radial-azimuthal profiles of
the emission can be accounted for in a diffusion-loss framework involving one
single population of non-thermal particles. Particles span the full extent of
FCES G78.74+1.56 as a result of diffusion from a central source, and give rise
to source FCES G80.00+0.50 by interacting with ionised gas in the innermost
region. For this simple diffusion-loss model, viable setups can be very
different in terms of energetics, transport conditions, and timescales
involved, and both hadronic and leptonic scenarios are possible. The solutions
range from long-lasting particle acceleration, possibly in prominent star
clusters such as Cyg OB2 and NGC 6910, to a more recent and short-lived release
of particles within the last 10-100 kyr, likely from a supernova remnant.
(Abridged)Comment: Published in A&A. Fixed typos in metadata. The main results are
available in machine-readable format at
https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/671/A4
The Double Quasar HE1104-1805: a case study for time delay determination with poorly sampled lightcurves
We present a new determination of the time delay of the gravitational lens
system HE1104-1805 ('Double Hamburger') based on a previously unpublished
dataset. We argue that the previously published value of dt_(A-B)=0.73 years
was affected by a bias of the employed method. We determine a new value of
dt_(A-B)=0.85+/-0.05 years (2 sigma confidence level), using six different
techniques based on non interpolation methods in the time domain. The result
demonstrates that even in the case of poorly sampled lightcurves, useful
information can be obtained with regard to the time delay. The error estimates
were calculated through Monte Carlo simulations. With two already existing
models for the lens and using its recently determined redshift, we infer a
range of values of the Hubble parameter: Ho=48+/-4 km/s Mpc^-1 (2 sigma) for a
singular isothermal ellipsoid (SIE) and Ho=62+/-4 km/s Mpc^-1 (2 sigma) for a
constant mass-to-light ratio plus shear model (M/L+gamma). The possibly much
larger errors due to systematic uncertainties in modeling the lens potential
are not included in this error estimate.Comment: 11 pages, 15 figures, accepted by Astronomy and Astrophysic
FIR colours of nearby late-type galaxies in the Herschel Reference Survey
We study the far infrared (60-500 m) colours of late-type galaxies in
the Reference Survey, a K-band selected, volume limited sample of
nearby galaxies. The far infrared colours are correlated with each other, with
tighter correlations for the indices that are closer in wavelength. We also
compare the different colour indices to various tracers of the physical
properties of the target galaxies, such as the surface brightness of the
ionising and non-ionising stellar radiation, the dust attenuation and the
metallicity. The emission properties of the cold dust dominating the far
infrared spectral domain are regulated by the properties of the interstellar
radiation field. Consistent with that observed in nearby, resolved galaxies,
our analysis shows that the ionising and the non-ionising stellar radiation,
including that emitted by the most evolved, cold stars, both contribute to the
heating of the cold dust component. This work also shows that metallicity is
another key parameter characterising the cold dust emission of normal,
late-type galaxies. A single modified black body with a grain emissivity index
=1.5 better fits the observed SPIRE flux density ratios vs.
than =2, although values of 2 are possible
in metal rich, high surface brightness galaxies. Values of
1.5 better represent metal poor, low surface brightness objects. This
observational evidence provides strong constraints for dust emission models of
normal, late type galaxies.Comment: Accepted for publication on A&
Speeding up Simplification of Polygonal Curves using Nested Approximations
We develop a multiresolution approach to the problem of polygonal curve
approximation. We show theoretically and experimentally that, if the
simplification algorithm A used between any two successive levels of resolution
satisfies some conditions, the multiresolution algorithm MR will have a
complexity lower than the complexity of A. In particular, we show that if A has
a O(N2/K) complexity (the complexity of a reduced search dynamic solution
approach), where N and K are respectively the initial and the final number of
segments, the complexity of MR is in O(N).We experimentally compare the
outcomes of MR with those of the optimal "full search" dynamic programming
solution and of classical merge and split approaches. The experimental
evaluations confirm the theoretical derivations and show that the proposed
approach evaluated on 2D coastal maps either shows a lower complexity or
provides polygonal approximations closer to the initial curves.Comment: 12 pages + figure
A resolved analysis of cold dust and gas in the nearby edge-on spiral NGC 891
We investigate the connection between dust and gas in the nearby edge-on
spiral galaxy NGC 891. High resolution Herschel PACS and SPIRE 70, 100, 160,
250, 350, and 500 m images are combined with JCMT SCUBA 850 m
observations to trace the far-infrared/submillimetre spectral energy
distribution (SED). Maps of the HI 21 cm line and CO(J=3-2) emission trace the
atomic and molecular hydrogen gas, respectively. We fit one-component modified
blackbody models to the integrated SED, finding a global dust mass of
8.510 M and an average temperature of 232 K. We
also fit the pixel-by-pixel SEDs to produce maps of the dust mass and
temperature. The dust mass distribution correlates with the total stellar
population as traced by the 3.6 m emission. The derived dust temperature,
which ranges from approximately 17 to 24 K, is found to correlate with the 24
m emission. Allowing the dust emissivity index to vary, we find an average
value of = 1.90.3. We confirm an inverse relation between the dust
emissivity spectral index and dust temperature, but do not observe any
variation of this relationship with vertical height from the mid-plane of the
disk. A comparison of the dust properties with the gaseous components of the
ISM reveals strong spatial correlations between the surface mass densities of
dust and the molecular hydrogen and total gas surface densities. Observed
asymmetries in the dust temperature, and the H-to-dust and total
gas-to-dust ratios hint that an enhancement in the star formation rate may be
the result of larger quantities of molecular gas available to fuel star
formation in the NE compared to the SW. Whilst the asymmetry likely arises from
dust obscuration due to the geometry of the line-of-sight projection of the
spiral arms, we cannot exclude an enhancement in the star formation rate in the
NE side of the disk.Comment: Accepted for publication in A&A. 21 pages, including 13 figures and 4
table
- âŠ