718 research outputs found
Effect of Al on the sharpness of the MgSiO_3 perovskite to post-perovskite phase transition
By means of static ab-initio computations we investigate the influence of Al on the recently discovered perovskite to post-perovskite phase transition in MgSiO_3. We examine three substitution mechanisms for Al in the two structures: MgSi ā AlAl; SiSiO ā AlAlā”; and Si ā AlH. The substitutions introducing oxygen vacancies (highly unfavorable, energetically) and water (favorable) both lower the 0 Kelvin transition pressure, whereas charge coupled substitution increases it relative to 105 GPa for pure MgSiO_3. From the transition pressures for 0, 6.25, and 100 mol% charge coupled Al_2O_3 incorporation and simple solution theories, we estimate the phase diagram of Al-bearing MgSiO_3 at low Al concentrations. Assuming the Clapeyron slope is independent of Al concentration, we find the perovskite-to-post-perovskite transition region to span 127ā140 GPa, at 6.25 mol% Al_2O_3. When the upper pressure limit is bounded by the core-mantle boundary, the phase coexistence region has width 150 km
Computer studies of hybrid-slotted working sections with minimum interference at subsonic speeds
A series of computations on tunnel boundary-interference effects for hybrid-slotted working sections was performed using the WALINT code. The slots were modeled as lines of porosity with linear crossflow characteristics. The basic shape evaluated was for a rectangular section with height-to-width ratio = 0.835 and its companion in the duplex mode (half model testing) with height-to-width ratio = 0.6. A best overall basic configuration was determined with seven slots on each wall with open area ratio on each wall of 17.5%. For both full-span and half-model testing, the optimum solution required closing all but two slots on each of the half-walls parallel to the plane of the wing (equivalent to four slots on the full floor and ceiling). The results are presented here for the best configurations and are shown to be within the figure-of-merit range of + or - 0.04 in upwash, and + or - 0.1 in curvature for the Mach number range 0.6 to 0.85. Blockage effects are shown to be small
First-principles phase diagram calculations for the HfCāTiC, ZrCāTiC, and HfCāZrC solid solutions
We report first-principles phase diagram calculations for the binary systems HfCāTiC, TiCāZrC, and HfCāZrC. Formation energies for superstructures of various bulk compositions were computed with a plane-wave pseudopotential method. They in turn were used as a basis for fitting cluster expansion Hamiltonians, both with and without approximations for excess vibrational free energies. Significant miscibility gaps are predicted for the systems TiCāZrC and HfCāTiC, with consolute temperatures in excess of 2000 K. The HfCāZrC system is predicted to be completely miscibile down to 185 K. Reductions in consolute temperature due to excess vibrational free energy are estimated to be ~7%, ~20%, and ~0%, for HfCāTiC, TiCāZrC, and HfCāZrC, respectively. Predicted miscibility gaps are symmetric for HfCāZrC, almost symmetric for HfCāTiC and asymmetric for TiCāZrC
Centaurus A at Ultra-High Energies
We review the importance of Centaurus A in high energy astrophysics as a
nearby object with many of the properties expected of a major source of very
high energy cosmic rays and gamma-rays. We examine observational techniques and
the results so far obtained in the energy range from 200 GeV to above 100 EeV
and attempt to fit those data with expectations of Centaurus A as an
astrophysical source from VHE to UHE energies.Comment: 11 pages, 4 figures, accepted for publication in PAS
Probing AGN jet precession with LISA
The precession of astrophysical jets produced by active-galactic nuclei is
likely related to the dynamics of the accretion disks surrounding the central
supermassive black holes (BHs) from which jets are launched. The two main
mechanisms that can drive jet precession arise from Lense-Thirring precession
and tidal torquing. These can explain direct and indirect observations of
precessing jets; however, such explanations often utilize crude approximations
of the disk evolution and observing jet precession can be challenging with
electromagnetic facilities. Simultaneously, the Laser Interferometer Space
Antenna (LISA) is expected to measure gravitational waves from the mergers of
massive binary BHs with high accuracy and probe their progenitor evolution. In
this paper, we connect the LISA detectability of binary BH mergers to the
possible jet precession during their progenitor evolution. We make use of a
semi-analytic model that self-consistently treats disk-driven BH alignment and
binary inspiral and includes the possibility of disk breaking. We find that
tidal torquing of the accretion disk provides a wide range of jet precession
timescales depending on the binary separation and the spin direction of the BH
from which the jet is launched. Efficient disk-driven BH alignment results in
shorter timescales of yr which are correlated with higher LISA
signal-to-noise ratios. Disk breaking results in the longest possible times of
yrs, suggesting a deep interplay between the disk critical
obliquity (i.e. where the disk breaks) and jet precession. Studies such as ours
will help to reveal the cosmic population of precessing jets that are
detectable with gravitational waves
COMPTEL upper limits for Seyfert galaxies
The gammaāray emission of Seyfert galaxies has fallen far short of preāGRO expectations. No single object of this class has been detected by either COMPTEL or EGRET, and OSSE has detected only a fraction of the Seyferts expected. To derive a more stringent upper limit to the emission from these objects in the energy ranges 0.75 to 1 and 1 to 3 MeV, we have summed a large number of COMPTEL observations acquired during Phase 1 of the GRO mission. From a total of 47 observations of 23 individual Xāray selected Seyfert galaxies, we derive preliminary upper limits of 8Ć10ā8 photons/(cm2 s keV) in the 0.75ā1 MeV band and 1Ć10ā8 photons/(cm2 s keV) in the 1ā3 MeV band
New optical polarization measurements of the Crab pulsar
The Crab nebula and its pulsar have been observed for about 3 hours with the
high-speed photo-polarimeter OPTIMA in January 2002 at the Calar Alto 3.5m
telescope. The Crab pulsar intensity and polarization are determined at all
phases of rotation with higher statistical accuracy than ever. Therefore, we
were able to separate the so-called 'off-pulse' phase emission (with an
intensity of about 1.2% compared to the main peak, assumed to be present at all
phases) from the pulsed emission and show the 'net' polarization of the pulsed
structures. Recent theoretical results indicate that the measured optical
polarization of the Crab pulsar is similar to expectations from a two-pole
caustic emission model or a striped pulsar wind model.Comment: AIP Conference Proceedings "Astrophysical Sources of High Energy
Particles and Radiation", eds. T. Bulik et al. (NY:AIP), Volume 801, 2005,
pp. 306-31
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