963 research outputs found
Simulation of stellar instabilities with vastly different timescales using domain decomposition
Strange mode instabilities in the envelopes of massive stars lead to shock
waves, which can oscillate on a much shorter timescale than that associated
with the primary instability. The phenomenon is studied by direct numerical
simulation using a, with respect to time, implicit Lagrangian scheme, which
allows for the variation by several orders of magnitude of the dependent
variables. The timestep for the simulation of the system is reduced appreciably
by the shock oscillations and prevents its long term study. A procedure based
on domain decomposition is proposed to surmount the difficulty of vastly
different timescales in various regions of the stellar envelope and thus to
enable the desired long term simulations. Criteria for domain decomposition are
derived and the proper treatment of the resulting inner boundaries is
discussed. Tests of the approach are presented and its viability is
demonstrated by application to a model for the star P Cygni. In this
investigation primarily the feasibility of domain decomposition for the problem
considered is studied. We intend to use the results as the basis of an
extension to two dimensional simulations.Comment: 15 pages, 10 figures, published in MNRA
Assessments of fodder values of 3 indigenous and 1 exotic woody plant species in the highlands of central Ethiopia
Leaves and twigs of indigenous woody plant species are used as a source of supplemental animal feed in the mountainous landscapes of central Ethiopia. A study was carried out from 2004 to 2006 to assess the nutritional value of three indigenous and one exotic species, based on the chemical composition, tannin contents, in vitro dry matter digestibility, and digestible energy. The species studied were Hagenia abyssinica (Bruce) J.F. Gmel., Dombeya torrida (J.F. Gmel.) P. Bamps, Buddleja polystachya Fres., and Chamaecytisus palmensis (Christ) Bisby & K. Nicholls. The first three are indigenous, and the last one is an exotic species. The Na content of the foliage and flower bud in the four species was much lower than the minimum requirement for ruminants, while other micro- and macronutrients were within the recommended range of nutrient concentrations in animal feeds. On the other hand, the crude protein content of the foliage and flower bud in the four fodder species was higher than the minimum required level. The foliage and flower bud in vitro dry matter digestibility of H. abyssinica and C. palmensis was 70% and 71%, respectively. The digestible energy of the foliage of H. abyssinica and C. palmensis was significantly higher than the digestible energy of D. torrida and B. polystachya. Therefore, the foliage and flower bud of most of those species can be used as sources of supplemental fodder with a proper feeding management scheme
Instabilities of captured shocks in the envelopes of massive stars
The evolution of strange mode instabilities into the non linear regime has
been followed by numerical simulation for an envelope model of a massive star
having solar chemical composition, M=50M_sun, T_eff=10^4K and L=1.17*10^6
L_sun. Contrary to previously studied models, for these parameters shocks are
captured in the H-ionisation zone and perform rapid oscillations within the
latter. A linear stability analysis is performed to verify that this behaviour
is physical. The origin of an instability discovered in this way is identified
by construction of an analytical model. As a result, the stratification turns
out to be essential for instability. The difference to common stratification
instabilities, e.g., convective instabilities, is discussed.Comment: 16 pages, 6 figures, accepted for publication in MNRA
Hard x-ray photon-in-photon-out spectroscopy with lifetime resolution – of XAS, XES, RIXSS and HERFD
Spectroscopic techniques that aim to resolve the electronic configuration and local coordination of a central
atom by detecting inner-shell radiative decays following photoexcitation using hard X-rays are presented. The
experimental setup requires an X-ray spectrometer based on perfect crystal Bragg optics. The possibilities arising from
non-resonant (X-Ray Emission Spectroscopy - XES) and resonant excitation (Resonant Inelastic X-Ray Scattering
Spectroscopy – RIXSS, High-Energy-Resolution Fluorescence Detected (HERFD) XAS) are discussed when the
instrumental energy broadenings of the primary (beamline) monochromator and the crystal spectrometer for x-ray
emission detection are on the order of the core hole lifetimes of the intermediate and final electronic states. The small
energy bandwidth in the emission detection yields line-sharpened absorption features. In transition metal compounds,
electron-electron interactions as well as orbital splittings and fractional population can be revealed. Combination with
EXAFS spectroscopy enables to extent the k-range beyond unwanted absorption edges in the sample that limit the
EXAFS range in conventional absorption spectroscopy
Comparison of the transformation of organic matter flux through a raised bog and a blanket bog
This study has proposed that organic matter transfer and transformation into and through a peatland is dominated by preferential loss of carbohydrates and the retention of lignin-like molecules. Here we used elemental analysis and thermogravimetric analysis to analyse the biomass, litter, peat soil profile, particulate organic matter, and dissolved organic matter fluxes sampled from a continental raised bog in comparison a maritime blanket bog. The macromolecular composition and thermodynamic analysis showed that in the raised bog there had been little or no transformation of the organic matter and the accumulation was rapid with comparatively little transformation with only 13% loss of cellulose by 1 m depth compared to 92% removal of cellulosic material in the blanket bog. The lack of transformation is reflected in a difference in long term carbon accumulation rates between raised and blanket bog sites. We propose that raised bogs, with their lack of a stream outfall, have high stable water tables that mean the pore water become thermodynamically closed and reactions cease higher in the peat profile than in a blanket bog where sloping sites mean a frequent flushing of pore water and discharge of water leading to fluctuating water tables, flushing of reaction products and pore spaces remaining open
Spin-polarized electronic structure of the core-shell ZnO/ZnO:Mn nanowires probed by x-ray absorption and emission spectroscopy
The combination of x-ray spectroscopy methods complemented with theoretical
analysis unravels the coexistence of paramagnetic and antiferromagnetic phases
in the Zn_0.9Mn_0.1O shell deposited onto array of wurtzite ZnO nanowires. The
shell is crystalline with orientation toward the ZnO growth axis, as
demonstrated by X-ray linear dichroism. EXAFS analysis confirmed that more than
90% of Mn atoms substituted Zn in the shell while fraction of secondary phases
was below 10%. The value of manganese spin magnetic moment was estimated from
the Mn K{\beta} X-ray emission spectroscopy to be 4.3{\mu}B which is close to
the theoretical value for substitutional Mn_Zn. However the analysis of L_2,3
x-ray magnetic circular dichroism data showed paramagnetic behaviour with
saturated spin magnetic moment value of 1.95{\mu}B as determined directly from
the spin sum rule. After quantitative analysis employing atomic multiplet
simulations such difference was explained by a coexistence of paramagnetic
phase and local antiferromagnetic coupling of Mn magnetic moments. Finally,
spin-polarized electron density of states was probed by the spin-resolved Mn
K-edge XANES spectroscopy and consequently analyzed by band structure
calculations.Comment: Supplementary information available at
http://www.rsc.org/suppdata/ja/c3/c3ja50153a/c3ja50153a.pdf J. Anal. At.
Spectrom., 201
On the rotational dynamics of magnetically threaded disks around neutron stars
We investigate the rotational dynamics of disk accretion around a strongly magnetized neutron star with an aligned dipole field. The magnetospheric field is assumed to thread the disk plasma both inside and outside the corotation radius. As a result of disk-star interaction, the magnetic torque on the disk affects the structure of accretion flow to yield the observed spin- up or spin- down rates for a source of given fastness, magnetic field strength, and mass accretion rate. Within the model we obtain a prescription for the dynamical viscosity of such magnetically modified solutions for a Keplerian disk. We then use this prescription to find a model solution for the rotation rate profile throughout the entire disk, including the non-Keplerian inner disk. We find that the non-Keplerian angular velocity transition region is not necessarily narrow for a source of given spin state. The boundary layer approximation, as in the standard magnetically threaded disk model, holds only in the case of dynamical viscosity decreasing all the way to the innermost edge of the disk. These results are applied to several observed disk-fed X-ray pulsars that have exhibited quasi-periodic oscillations (QPOs). The QPO frequencies provide a constraint on the fastness parameter and enable one to determine uniquely the width of the angular velocity transition zone for each source within model assumptions. We discuss the implications of these results on the value of the critical fastness parameter for a magnetized star in spin equilibrium. Applications of our model are also made with relevant parameters from recent numerical simulations of quasi-stationary disk - magnetized star interactions
Galactic Twins of the Ring Nebula Around SN1987A and a Possible LBV-like Phase for Sk-69 202
Some core-collapse supernovae show clear signs of interaction with dense
circumstellar material that often appears to be non-spherical. Circumstellar
nebulae around supernova progenitors provide clues to the origin of that
asymmetry in immediate pre-supernova evolution. Here I discuss outstanding
questions about the formation of the ring nebula around SN1987A and some
implications of similar ring nebulae around Galactic B supergiants. Several
clues hint that SN1987A's nebula may have been ejected in an LBV-like event,
rather than through interacting winds in a transition from a red supergiant to
a blue supergiant.Comment: 2 pages, to appear in procedings of "Massive stars: fundamental
parameters and circumstellar interactions", conference in honor of Virpi
Niemela's 70th birthda
Long-range interactions in the effective low energy Hamiltonian of Sr2IrO4: a core level resonant inelastic x-ray scattering study
We have investigated the electronic structure of Sr2IrO4 using core level
resonant inelastic x-ray scattering. The experimental spectra can be well
reproduced using ab initio density functional theory based multiplet ligand
field theory calculations, thereby validating these calculations. We found that
the low-energy, effective Ir t2g orbitals are practically degenerate in energy.
We uncovered that covalency in Sr2IrO4, and generally in iridates, is very
large with substantial oxygen ligand hole character in the Ir t2g Wannier
orbitals. This has far reaching consequences, as not only the onsite
crystal-field energies are determined by the long range crystal-structure, but,
more significantly, magnetic exchange interactions will have long range
distance dependent anisotropies in the spin direction. These findings set
constraints and show pathways for the design of d^5 materials that can host
compass-like magnetic interactions
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