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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Simultaneous high-P, high-T X ray diffraction study of beta-(Mg,Fe)2SiO4 to 26 GPa and 900 K

Attention is given to the lattice parameters of beta phase (Mg(0.84)Fe(0.16))2SiO4 determined by X-ray diffraction using synchrotron radiation under simultaneous high-pressure and high-temperature conditions. The experiments were conducted up to a pressure of 26 GPa and a temperature of 900 K. High pressures were generated in a Mao-Bell type diamond anvil cell using neon gas as a pressure medium. The sample was heated with an external Ni80Cr20 wire heater. Gold was used as an internal high-pressure calibrant at high temperature. The experimental data indicated the anisotropic behavior of the beta phase at high pressure and temperature, i.e., the c axis is about 35-percent more expansible and about 25-percent more compressible than the a and b axes. A value of 5.1 +/-0.8 was found for the Anderson-Grueneisen parameter. The derived thermodynamic parameters for the beta phase are summarized.

Fei, Yingwei↗
NASA NTRS1992

The unusual K-band characteristics of the gravitational lens system MG 1131 + 0456

The radio morphology of the gravitational lens system MG 1131 + 0456 is that of an 'Einstein ring'. Newly obtained K-prime-band images of the system reveal the presence of an infrared bright object that has a morphology reminiscent of that seen in the radio. There are two bright objects embedded in an elliptical halo; these seem to be the infrared counterparts to the radio morphology. Two nearby objects have no radio counterparts. All four objects have extremely red optical-infrared colors: R - K greater than 6. Few extragalactic objects are this red. A plausible candidate for the source object is an unusually red radio galaxy.

Annis, James↗
NASA NTRS1992

SiC particles from asymptotic giant branch stars - Mg burning and the s-process

The question of whether isotopically anomalous SiC particles found in meteorites originate in AGB stars is addressed. It is shown that if the peak helium shell flash temperatures of massive (6-9 solar masses) stars are about 10 percent larger than they are normally assumed to be, alpha particle reactions with the magnesium will become significant. Then the (Mg-29)(alpha, n)Si-29 reaction produces a large excess of Si-29. With a light element nuclear reaction network, the evolution of the silicon isotopic composition during AGB evolution is calculated. It is found that the experimentally determined correlation between excess Si-29 and excess Si-30 in SiC particles from carbonaceous chondrites can indeed be naturally produced in this way. It is suggested that if the large isotopically anomalous SiC particles carrying nearly pure-process krypton and xenon do indeed originate in AGB stars, those stars were massive and had peak shell flash temperatures near 450 KM.

Brown, Lawrence E.↗
NASA NTRS1992

Moessbauer spectroscopy of Mg(0.9)Fe(0.1)SiO3 perovskite

Ambient pressure Moessbauer spectra of Mg(0.9)Fe-57(0.1)SiO3 perovskite synthesized at pressure-temperature conditions of about 50 GPa and 1700 K show that the iron is entirely high-spin Fe(2+) and appears to be primarily located in the octahedral site within the crystal structure. We observe broad Moessbauer lines, suggesting a distribution of electric-field gradients caused by disorder associated with the Fe ions. Also, the perovskite exhibits magnetic ordering at temperatures lower than 5 K, implying that there is a magnetic contribution to the absolute ('third-law') entropy of this phase.

Jeanloz, Raymond↗
NASA NTRS1992

A theoretical study of Mg(CO2)n(+) and Sr(CO2)n(+) for n = 1 and 2 and Mg2CO2(+)

The structure and binding energies are determined for Mg(CO2)n(+) and Sr(CO2)n(+) for n = 1 and 2. We also consider Mg2(+) and Mg2CO2(+) to compare the binding of CO2 to a single metal ion with the binding to a diatomic ion. The vertical excitation energies are computed for all species. The potential energy curves for the low-lying states of Mg2(+) are reported. The MgCO2(+) results are in good agreement with the experimental results of Duncan and co-workers.

Sodupe, Mariona↗
NASA NTRS1993

Intrinsic fatigue crack growth rates for Al-Li-Cu-Mg alloys in vacuum

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable delta-prime (Al3Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constant K(max) to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent; da/dN varied up to tenfold based on applied Delta-K or Delta-K/E. When compared based on a crack tip cyclic strain or opening displacement parameter, growth rates were equivalent for all alloys except X2095-T8, which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly delta-prime, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear.

Slavik, D. C.↗
NASA NTRS1993

Petrography, mineralogy, and Mg isotope composition of VICTA: A vigarano CaAl4O7-bearing type A inclusion

Thermodynamic calculations predict that Ca-dialuminate (CaAl4O7) condenses from a cooling gas of solar composition after hibonite and before melilite. Although Ca-dialuminate has now been recorded from Ca Al-rich inclusions (CAI's) in at least 9 meteorites, compared to hibonite it is a relatively rare phase. As pointed out by Michel-Levy et al., the absence of Ca-dialuminate from most hibonite-bearing inclusions poses a serious problem for the condensation model of CAI formation. Here we describe an inclusion which contains abundant CA-dialuminate partially altered to a hercynite-rich (FeAl2O4) assemblage. The evidence from VICTA indicates that compared to all other phases in type A inclusions, Ca-dialuminate is the most susceptible to secondary alteration; a feature which may explain its restricted occurrence. Unaltered Ca-dialuminate and melilite in VICTA display excess Mg-26 indicative of in situ decay of Al-26.

Greenwood, R. C.↗
NASA NTRS1993

Al-26-Mg-26 ages of iron meteorites

An exposure age for an iron meteorite can be calculated from measurements of a radioactive nuclide and a stable nuclide that are produced by similar sets of nuclear reactions, provided that the stable nuclide is present with low initial abundance. The standard methods rely on either K-40 (t(sub 1/2) = 1.26 Gy), K-39, and K-41 or on a shorter-lived radionuclide and a stable, noble gas isotope. Widely used pairs of this type include Cl-36/Ar-36 and Al-26/Ne-21. Other pairs that may serve the purpose for iron meteorites contain many stable isotopes besides those of K and the noble gases that are produced partly by cosmic rays. We consider here the calculation of exposure ages, t(sub 26), from measurements of Al-26 (t(sub 1/2) = 0.7 My) and (stable) Mg-26. Ages based on Al-26/Mg-26 ratios, like those based on Cl-36/Ar-36 ratios, are 'buffered' against changes in relative production rates due to shielding because decay of the radioactive nuclide accounts for a good part of the inventory of the stable nuclide.

Herzog, G. F.↗
NASA NTRS1993

The Mg-suite and the highland crust: An unsolved enigma

Most of the rocks returned from the highlands are polymict breccias, pulverized by the massive bombardment. However, some monomict breccias with low siderophile element contents are considered to be 'pristine' rocks that represent the original igneous components making up the highland crust. Three principal pristine constituents make up the lunar highland crust: ferroan anorthosites, the Mg-suite, and KREEP. A discussion of these three constituents is presented.

Taylor, Stuart Ross↗
NASA NTRS1994

Chronology of chrondrule and CAI formation: Mg-Al isotopic evidence

Details of the chondrule and Ca-Al-rich inclusion (CAI) formation during the earliest history of the solar system are imperfectly known. Because CAI's are more 'refractory' than ferromagnesian chondrules and have the lowest recorded initial Sr-87/Sr-86 ratios of any solar system materials, the expectation is that CAI's formed earlier than chondrules. But it is not known, for example, if CAI formation had stopped by the time chondrule formation began. Conventional (absolute) age-dating techniques cannot adequately resolve small age differences (less than 10(exp 6) years) between objects of such antiquity. One approach has been to look at systematic differences in the daughter products of short-lived radionuclides such as Al-26 and I-129. Unfortunately, neither system appears to be 'well-behaved.' One possible reason for this circumstance is that later secondary events have partially reset the isotopic systems, but a viable alternative continues to be large-scale (nebular) heterogeneity in initial isotopic abundances, which would of course render the systems nearly useless as chronometers. In the past two years the nature of this problem has been redefined somewhat. Examination of the Al-Mg isotopic database for all CAI's suggests that the vast majority of inclusions originally had the same initial Al-26/Al-27 abundance ratio, and that the ill-behaved isotopic systematics now observed are the results of later partial reequilibration due to thermal processing. Isotopic heterogeneities did exist in the nebula, as demonstrated by the existence of so-called FUN inclusions in CV3 chondrites and isotopically anomalous hibonite grains in CM2 chondrites, which had little or no live Al-26 at the time of their formation. But, among the population of CV3 inclusions at least, FUN inclusions appear to have been a relatively minor nebular component.

Macpherson, G. J.↗
NASA NTRS1994

Mg IX emission lines in an active region spectrum obtained with the Solar EUV Rocket Telescope and Spectrograph (SERTS)

Theoretical electron-temperature-sensitive Mg IX emission line ratios are presented for R(sub 1) = I(443.96 A)/I(368.06 A), R(sub 2) = I(439.17 A)/I(368.06 A), R(sub 3) = I(443.37 A)/I(368.06 A), R(sub 4) = I(441.22 A)/I(368.06 A), and R(sub 5) = I(448.28 A)/I(368.06 A). A comparison of these with observational data for a solar active region, obtained during a rocket flight by the Solar EUV Rocket Telescope and Spectrograph (SERTS), reveals excellent agreement between theory and observation for R(sub 1) through R(sub 4), with discrepancies that average only 9%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations, and also resolves discrepancies found previously when the theoretical results were compared with solar data from the S082A instrument on board Skylab. However in the case of R(sub 5), the theoretical and observed ratios differ by almost a factor of 2. This may be due to the measured intensity of the 448.28 A line being seriously affected by instrumental effects, as it lies very close to the long wavelength edge of the SERTS spectral coverage (235.46-448.76 A).

Keenan, F. P.↗
NASA NTRS1994

Atomic data from the iron project. 3: Rate coefficients for electron impact excitation of boron-like ions: Ne VI, Mg VIII, Al IX, Si X, S XII, Ar XIV, Ca XVI and Fe XXII

Collison strengths and maxwellian averaged rate coefficients have been calculated for the 105 transitions among all 15 fine structure levels of the 8 LS terms 2s(sup 2) 2 P(P-2(sup 0 sub 1/, 3/2)), 2s2p(sup 2)(P-4(sub 1/2,3/2,5/2), D-2(sub 3/2, 5/2), S-2(sub 1/2), P-2(sub 1/2, 3/2)), 2p(sup 3)(S-4(sup 0)(sub 3/2), D-2(sup 0 sub 3/2, 5/2), P-2(sup 0 sub 1/2, 3/2)) in highly- charged B-like Ne, Mg, Al, Si, S, Ar, Ca and Fe. Rate coefficients have been tabulated at a wide range of temperatures, depending on the ion charge and abundance in plasma sources. Earlier work for O IV has also been extended to include the high temperature range. A brief discussion of the calculations, sample results, and comparison with earlier works is also given. While much of the new data should be applicable to UV spectral diagnostics, the new rates for the important ground state fine structure transition P-2(sup 0 sub 1/2)-P-2(sup 0 sub 3/2) should result in significant revision of the IR cooling rates in plasmas where B-like ions are prominent constituents, since the new rate coefficients are generally higher by several factors compared with the older data.

Zhang, Hong Lin↗
NASA NTRS1995

Mg II h and k profiles in high-liminosity, late-type stars

Using high-resolution spectroscopic data taken with the Goddard High Resolution Spectrographic (GHRS) aboard the Hubble Space Telescope (HST) and with the International Ultraviolet Expolorer (IUE) satellite, we compare the profiles of the Mg II h and k lines seen in stars with spectral types ranging from early K through mid-M and luminosities from giants to supergiants. For all of these stars the lines are broad emission features with a central absorption. When plotted on a velocity scale the absorption features of the h and k lines agree very well in both shape and position, as do the blue wings of the emission component. The red wings of the emission, however, show a pronounced difference, with the k line wing consistently shifted to the red of the h line wing. At present the reason for this discrepancy is unknown, but we suggest several possibilities, including radiative transfer effects and high-speed stellar winds.

Robinson, R. D.↗
NASA NTRS2000

Jet-Like Structures and Wake in Mg I (518 nm) Images of 1999 Leonid Storm Meteors

Small meteoric fragments are ejected at significant transverse velocities from some (up to approx. 8%) fast Leonid meteors. We reach this conclusion using low light intensified image measurements obtained during the 1999 Leonid Multi-Instrument Aircraft Campaign. High spatial resolution, narrow band image measurements of the Mg I emission at 518 nm have been used to clearly identify jet-like features in the meteor head that are the same as first observed in white light. We postulate that these unusual structures are caused by tiny meteoroid fragments (containing metallic grains) being rapidly ejected away from the core meteoroid as the constituent glue evaporates. Marked curvature observed in the jet-like filaments suggest that the parent meteoroids are spinning and as the whirling fragments are knocked away by the impinging air molecules, or by grain-grain collisions in the fragment ensemble, they ablate quickly generating an extended area of structured luminosity up to about 1-2 km from the meteoroid center. Fragments with smaller transverse velocity components are thought to be responsible for the associated beading evident in the wake of these unusual Leonid meteors.

Taylor, M. J.↗
NASA NTRS2001

Mg II Spectral Atlas and Flux Catalog for Late-Type Stars in the Hyades Cluster

In the course of a long-running IUE Guest Observer program, UV spectral images were obtained for more than 60 late-type members of the Hyades Cluster in order to investigate their chromospheric emissions. The emission line fluxes extracted from those observations were used to study the dependence of stellar dynamo activity upon age and rotation (IUE Observations of Rapidly Rotating Low-Mass Stars in Young Clusters: The Relation between Chromospheric Activity and Rotation). However, the details of those measurements, including a tabulation of the line fluxes, were never published. The purpose of the investigation summarized here was to extract all of the existing Hyades long-wavelength Mg II spectra in the IUE public archives in order to survey UV chromospheric emission in the cluster, thereby providing a consistent dataset for statistical and correlative studies of the relationship between stellar dynamo activity, rotation, and age over a broad range in mass.

Simon, Theodore↗
NASA NTRS2001

An Electrochemical Framework to Explain Intergranular Stress Corrosion Cracking in an Al-5.4%Cu-0.5%Mg-0.5%Ag Alloy

A modified version of the Cu-depletion electrochemical framework was used to explain the metallurgical factor creating intergranular stress corrosion cracking susceptibility in an aged Al-Cu-Mg-Ag alloy, C416. This framework was also used to explain the increased resistance to intergranular stress corrosion cracking in the overaged temper. Susceptibility in the under aged and T8 condition is consistent with the grain boundary Cu-depletion mechanism. Improvements in resistance of the T8+ thermal exposure of 5000 h at 225 F (T8+) compared to the T8 condition can be explained by depletion of Cu from solid solution.

Little, D. A.↗