717 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 â AlAl; SiSiO â AlAlâĄ; and Si â 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 127â140 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 HfCâTiC, ZrCâTiC, and HfCâZrC solid solutions
We report first-principles phase diagram calculations for the binary systems HfCâTiC, TiCâZrC, and HfCâZrC. 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 TiCâZrC and HfCâTiC, with consolute temperatures in excess of 2000 K. The HfCâZrC 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 HfCâTiC, TiCâZrC, and HfCâZrC, respectively. Predicted miscibility gaps are symmetric for HfCâZrC, almost symmetric for HfCâTiC and asymmetric for TiCâZrC
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
COMPTEL upper limits for Seyfert galaxies
The gammaâray emission of Seyfert galaxies has fallen far short of preâGRO 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 Xâray selected Seyfert galaxies, we derive preliminary upper limits of 8Ă10â8 photons/(cm2 s keV) in the 0.75â1 MeV band and 1Ă10â8 photons/(cm2 s keV) in the 1â3 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
COMPTEL Observations of AGN at MeV-Energies
The COMPTEL experiment aboard CGRO, exploring the previously unknown sky at
MeV-energies, has so far detected 10 Active Galactic Nuclei (AGN): 9 blazars
and the radio galaxy Centaurus A. No Seyfert galaxy has been found yet. With
these results COMPTEL has opened the field of extragalactic Gamma-ray astronomy
in the MeV-band.Comment: 4 pages, 2 figures including 1 color plot, to appear in the
Proceedings of the 3rd INTEGRAL Workshop "The Extreme Universe", held in
Taormina, Italy, 14-18 September 199
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