555 research outputs found
Modelling CO emission from Mira's wind
We have modelled the circumstellar envelope of {\it o} Ceti (Mira) using new
observational constraints. These are obtained from photospheric light scattered
in near-IR vibrational-rotational lines of circumstellar CO molecules at 4.6
micron: absolute fluxes, the radial dependence of the scattered intensity, and
two line ratios. Further observational constraints are provided by ISO
observations of far-IR emission lines from highly excited rotational states of
the ground vibrational state of CO, and radio observations of lines from
rotational levels of low excitation of CO. A code based on the Monte-Carlo
technique is used to model the circumstellar line emission.
We find that it is possible to model the radio and ISO fluxes, as well as the
highly asymmetric radio-line profiles, reasonably well with a spherically
symmetric and smooth stellar wind model. However, it is not possible to
reproduce the observed NIR line fluxes consistently with a `standard model' of
the stellar wind. This is probably due to incorrectly specified conditions of
the inner regions of the wind model, since the stellar flux needs to be larger
than what is obtained from the standard model at the point of scattering, i.e.,
the intermediate regions at approximately 100-400 stellar radii (2"-7") away
from the star. Thus, the optical depth in the vibrational-rotational lines from
the star to the point of scattering has to be decreased. This can be
accomplished in several ways. For instance, the gas close to the star (within
approximately 2") could be in such a form that light is able to pass through,
either due to the medium being clumpy or by the matter being in radial
structures (which, further out, developes into more smooth or shell-like
structures).Comment: 18 pages, 3 figures, accepted for publication in Ap
A test of the power law relationship between gamma-ray burst pulse width ratio and energy expected in fireballs or uniform jets
Recently, under the assumption that the Doppler effect of the
relativistically expanding fireball surface is important, Qin et al. showed
that in most cases the power law relationship between the pulse width and
energy of gamma-ray bursts (GRBs)would exist in a certain energy range. We
check this prediction with two GRB samples which contain well identified
pulses. A power law anti-correlation between the full pulse width and energy
and a power law correlation between the pulse width ratio and energy are seen
in the light curves of the majority (around 65%) of bursts of the two samples
within the energy range of BATSE, suggesting that these bursts are likely to
arise from the emission associated with the shocks occurred on a
relativistically expanding fireball surface. For the rest of the bursts, the
relationships between these quantities were not predicted previously. We
propose to consider other spectral evolutionary patterns or other radiation
mechanisms such as a varying synchrotron or Comptonized spectrum to check if
the observed relationships for these rest bursts can also be accounted for by
the Doppler model. In addition, we find that the upper limits of the width
ratio for the two samples do not exceed 0.9, in agrement with what predicted
previously by the Doppler model. The plateau/power law/plateau and the peaked
features predicted and detected previously by Qin et al. are generally
observed, with the exceptions being noticed only in a few cases. According to
the distinct values of two power law indices of FWHM and ratio and energy, we
divide the bursts into three subsets which are located in different areas of
the two indices plane. We suspect that different locations of the two indices
might correspond to different mechanisms.Comment: 16 pages, 7 figures, MNRAS accepte
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.
SOFIA/EXES Observations of Water Absorption in the Protostar AFGL 2591 at High Spectral Resolution
We present high spectral resolution (~3 km/s) observations of the nu_2
ro-vibrational band of H2O in the 6.086--6.135 micron range toward the massive
protostar AFGL 2591 using the Echelon-Cross-Echelle Spectrograph (EXES) on the
Stratospheric Observatory for Infrared Astronomy (SOFIA). Ten absorption
features are detected in total, with seven caused by transitions in the nu_2
band of H2O, two by transitions in the first vibrationally excited nu_2 band of
H2O, and one by a transition in the nu_2 band of H2{18}O. Among the detected
transitions is the nu_2 1(1,1)--0(0,0) line which probes the lowest lying
rotational level of para-H2O. The stronger transitions appear to be optically
thick, but reach maximum absorption at a depth of about 25%, suggesting that
the background source is only partially covered by the absorbing gas, or that
the absorption arises within the 6 micron emitting photosphere. Assuming a
covering fraction of 25%, the H2O column density and rotational temperature
that best fit the observed absorption lines are N(H2O)=(1.3+-0.3)*10^{19}
cm^{-2} and T=640+-80 K.Comment: 6 pages, 3 figures, 1 table, accepted for publication in ApJ
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.
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
Signature inversion in semi-decoupled bands: Residual interaction between h9/2 protons and i13/2 neutrons
Semi-decoupled bands based on the πh9/2 ⊗ vi13/2 configuration are observed in 162Tm,164Tm and 174Ta. Spins assigned to these bands imply an inversion of the expected signature splitting, which is interpreted as being the result of a residual proton-neutron interactionComisión Interministerial de Ciencia y Tecnología PB95-0533US Dept. of Energy DE-FGOS- 92ER4069
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&
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
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