548 research outputs found
Microstructural characterization of AISI 431 martensitic stainless steel laser-deposited coatings
High cooling rates during laser cladding of stainless steels may alter the microstructure and phase constitution of the claddings and consequently change their functional properties. In this research, solidification structures and solid state phase transformation products in single and multi layer AISI 431 martensitic stainless steel coatings deposited by laser cladding at different processing speeds are investigated by optical microscopy, Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), orientation imaging microscopy (OIM), ternary phase diagram, Schaeffler and TTT diagrams. The results of this study show how partitionless solidification and higher solidification rates alter the microstructure and phase constitution of martensitic stainless steel laser deposited coatings. In addition, it is shown that while different cladding speeds have no effect on austenite–martensite orientation relationship in the coatings, increasing the cladding speed has resulted in a reduction of hardness in deposited coatings which is in contrast to the common idea about obtaining higher hardness values at higher cladding speeds.
Particle transfer and fusion cross-section for Super-heavy nuclei in dinuclear system
Within the dinuclear system (DNS) conception, instead of solving
Fokker-Planck Equation (FPE) analytically, the Master equation is solved
numerically to calculate the fusion probability of super-heavy nuclei, so that
the harmonic oscillator approximation to the potential energy of the DNS is
avoided. The relative motion concerning the energy, the angular momentum, and
the fragment deformation relaxations is explicitly treated to couple with the
diffusion process, so that the nucleon transition probabilities, which are
derived microscopically, are time-dependent. Comparing with the analytical
solution of FPE, our results preserve more dynamical effects. The calculated
evaporation residue cross sections for one-neutron emission channel of Pb-based
reactions are basically in agreement with the known experimental data within
one order of magnitude.Comment: 19 pages, plus 6 figures, submitted to Phys. Rev.
Extended Emission from Short Gamma-Ray Bursts Detected with SPI-ACS/INTEGRAL
The short duration (T90 < 2 s) gamma-ray bursts (GRBs) detected in the
SPI-ACS experiment onboard the INTEGRAL observatory are investigated. Averaged
light curves have been constructed for various groups of events, including
short GRBs and unidentified short events. Extended emission has been found in
the averaged light curves of both short GRBs and unidentified short events. It
is shown that the fraction of the short GRBs in the total number of SPI-ACS
GRBs can range from 30 to 45%, which is considerably larger than has been
thought previously.Comment: 27 pages, 10 figure
Two types of softening detected in X-ray afterglows of Swift bursts: internal and external shock origins?
The softening process observed in the steep decay phase of early X-ray
afterglows of Swift bursts has remained a puzzle since its discovery. The
softening process can also be observed in the later phase of the bursts and its
cause has also been unknown. Recently, it was suggested that, influenced by the
curvature effect, emission from high latitudes would shift the Band function
spectrum from higher energy band to lower band, and this would give rise to the
observed softening process accompanied by a steep decay of the flux density.
The curvature effect scenario predicts that the terminating time of the
softening process would be correlated with the duration of the process. In this
paper, based on the data from the UNLV GRB group web-site, we found an obvious
correlation between the two quantities. In addition, we found that the
softening process can be divided into two classes: the early type softening
() and the late type softening ().
The two types of softening show different behaviors in the duration vs.
terminating time plot. In the relation between the variation rates of the flux
density and spectral index during the softening process, a discrepancy between
the two types of softening is also observed. According to their time scales and
the discrepancy between them, we propose that the two types are of different
origins: the early type is of internal shock origin and the late type is of
external shock origin. The early softening is referred to the steep decay just
following the prompt emission, whereas the late decay typically conceives the
transition from flat decay to late afterglow decay. We suspect that there might
be a great difference of the Lorentz factor in two classes which is responsible
for the observed discrepancy.Comment: 20 pages, 5 figures, 2 tables, Accepted for Publication to Journal of
Cosmology and Astroparticle Physics (JCAP
ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse
We first present spatially resolved ALMA and VLA continuum observations of
the early-M red supergiant Antares to search for the presence of a chromosphere
at radio wavelengths. We resolve the free-free emission of the Antares
atmosphere at 11 unique wavelengths between 0.7 mm (ALMA band 8) and 10 cm (VLA
S band). The projected angular diameter is found to continually increase with
increasing wavelength, from a low of 50.7 mas at 0.7 mm up to a diameter of 431
mas at 10 cm, which corresponds to 1.35 and 11.6 times the photospheric angular
diameter, respectively. All four ALMA measurements show that the shape of the
atmosphere is elongated, with a flattening of 15% at a similar position angle.
The disk-averaged gas temperature of the atmosphere initially rises from a
value of 2700 K at 1.35 (i.e., 0.35 above the
photosphere) to a peak value of 3800 K at 2.5 , after which it
then more gradually decreases to 1650 K at 11.6 . The rise in gas
temperature between 1.35 and 2.5 is evidence for
a chromospheric temperature rise above the photosphere of a red supergiant. We
detect a clear change in the spectral index across the sampled wavelength
range, with the flux density between 0.7 mm
and 1.4 cm, which we associate with chromosphere-dominated emission, while the
flux density between 4.3 cm and 10 cm, which we
associate with wind-dominated emission. We then perform nonlocal thermal
equilibrium modeling of the far-ultraviolet radiation field of another early-M
red supergiant, Betelgeuse, and find that an additional hot (i.e., K)
chromospheric photoionization component with a much smaller filling factor must
also exist throughout the chromospheres of these stars.Comment: 12 pages, 6 figures, accepted for publication in A&
The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk
We study the relationship between age, metallicity, and alpha-enhancement of
FGK stars in the Galactic disk. The results are based upon the analysis of
high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore
the limitations of the observed dataset, i.e. the accuracy of stellar
parameters and the selection effects that are caused by the photometric target
preselection. We find that the colour and magnitude cuts in the survey suppress
old metal-rich stars and young metal-poor stars. This suppression may be as
high as 97% in some regions of the age-metallicity relationship. The dataset
consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr,
Galactocentric distances from 6 kpc to 9.5 kpc, and vertical distances from the
plane 0 < |Z| < 1.5 kpc. On this basis, we find that i) the observed
age-metallicity relation is nearly flat in the range of ages between 0 Gyr and
8 Gyr; ii) at ages older than 9 Gyr, we see a decrease in [Fe/H] and a clear
absence of metal-rich stars; this cannot be explained by the survey selection
functions; iii) there is a significant scatter of [Fe/H] at any age; and iv)
[Mg/Fe] increases with age, but the dispersion of [Mg/Fe] at ages > 9 Gyr is
not as small as advocated by some other studies. In agreement with earlier
work, we find that radial abundance gradients change as a function of vertical
distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In
addition, we show that the inner disk is not only more alpha-rich compared to
the outer disk, but also older, as traced independently by the ages and Mg
abundances of stars.Comment: accepted for publication in A&
Reorganization Energy for Internal Electron Transfer in Multicopper Oxidases.
We have calculated the reorganization energy for the intramolecular electron transfer between the reduced type 1 copper site and the peroxy intermediate of the trinuclear cluster in the multicopper oxidase CueO. The calculations are performed at the combined quantum mechanics and molecular mechanics (QM/MM) level, based on molecular dynamics simulations with tailored potentials for the two copper sites. We obtain a reorganization energy of 91-133 kJ/mol, depending on the theoretical treatment. The two Cu sites contribute by 12 and 22 kJ/mol to this energy, whereas the solvent contribution is 34 kJ/mol. The rest comes from the protein, involving small contributions from many residues. We have also estimated the energy difference between the two electron-transfer states and show that the reduction of the peroxy intermediate is exergonic by 43-87 kJ/mol, depending on the theoretical method. Both the solvent and the protein contribute to this energy difference, especially charged residues close to the two Cu sites. We compare these estimates with energies obtained from QM/MM optimizations and QM calculations in a vacuum and discuss differences between the results obtained at various levels of theory
Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. Detailed abundance analysis of OGLE-2008-BLG-209S
AIMS. Our aims are twofold. First we aim to evaluate the robustness and
accuracy of stellar parameters and detailed elemental abundances that can be
derived from high-resolution spectroscopic observations of microlensed dwarf
and subgiant stars. We then aim to use microlensed dwarf and subgiant stars to
investigate the abundance structure and chemical evolution of the Milky Way
Bulge. [ABRIDGED] METHODS. We present a detailed elemental abundance analysis
of OGLE-2008-BLG-209S, the source star of a new microlensing event towards the
Bulge, for which we obtained a high-resolution spectrum with the MIKE
spectrograph on the Magellan Clay telescope. We have performed four different
analyses of OGLE-2008-BLG-209S. [ABRIDGED] We have also re-analysed three
previous microlensed dwarf stars OGLE-2006-BLG-265S, MOA-2006-BLG-099S, and
OGLE-2007-BLG-349S with the same method. This homogeneous data set, although
small, enables a direct comparison between the different stellar populations.
RESULTS. We find that OGLE-2008-BLG-209S is a subgiant star that has a
metallicity of [Fe/H] ~-0.33. It possesses [alpha/Fe] enhancements similar to
what is found for Bulge giant stars at the same metallicity, and what also is
found for nearby thick disc stars at the same metallicity. In contrast, the
previous three microlensing dwarf stars have very high metallicities,
[Fe/H]>+0.4, and more solar-like abundance ratios, i.e. [alpha/Fe]~0. The
decrease in the [alpha/Fe] ratio with [Fe/H] is the typical signature of
enrichment from low and intermediate mass stars. We furthermore find that the
results for the four Bulge stars, in combination with results from studies of
giant stars in the Bulge, seem to favour a secular formation scenario for the
Bulge.Comment: Accepted for publication in A&A, 17 pages, online table will be
available in published version, or by contacting the first autho
Multi-wavelength observations of the energetic GRB 080810: detailed mapping of the broadband spectral evolution
GRB 080810 was one of the first bursts to trigger both Swift and the Fermi
Gamma-ray Space Telescope. It was subsequently monitored over the X-ray and
UV/optical bands by Swift, in the optical by ROTSE and a host of other
telescopes and was detected in the radio by the VLA. The redshift of z= 3.355
+/- 0.005 was determined by Keck/HIRES and confirmed by RTT150 and NOT. The
prompt gamma/X-ray emission, detected over 0.3-10^3 keV, systematically softens
over time, with E_peak moving from ~600 keV at the start to ~40 keV around 100
s after the trigger; alternatively, this spectral evolution could be identified
with the blackbody temperature of a quasithermal model shifting from ~60 keV to
~3 keV over the same time interval. The first optical detection was made at 38
s, but the smooth, featureless profile of the full optical coverage implies
that this originated from the afterglow component, not the pulsed/flaring
prompt emission.
Broadband optical and X-ray coverage of the afterglow at the start of the
final X-ray decay (~8 ks) reveals a spectral break between the optical and
X-ray bands in the range 10^15 - 2x10^16 Hz. The decay profiles of the X-ray
and optical bands show that this break initially migrates blueward to this
frequency and then subsequently drifts redward to below the optical band by
~3x10^5 s. GRB 080810 was very energetic, with an isotropic energy output for
the prompt component of 3x10^53 erg and 1.6x10^52 erg for the afterglow; there
is no evidence for a jet break in the afterglow up to six days following the
burst.Comment: 15 pages, 9 figures, 4 in colour. Accepted for publication in MNRA
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