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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Spectrophotometric observations of symbiotic stars and related objects

Calibrated optical spectrophotometric observations of 16 symbiotic and symbiotic-like objects are presented. The objects observed include Z And, T CrB, CH Cyg, CI Cyg, V1016 Cyg, V1329 Cyg, AG Dra, YY Her, RS Oph, XX Oph, AG Peg, AX Per, CL Sco, HM Sge, AS 289, and M1-2. Integrated emission-line intensities are tabulated for comparison with ultraviolet and infrared data, as well as with previous optical studies. The reddening to each of the objects is derived by assuming that Balmer lines are emitted in their case B recombination ratios. However, the values so derived are often systematically higher than reddening estimates from the ultraviolet 2200 A feature. Comparisons with the available data from other wavelength ranges are noted.

Blair, W. P.↗
NASA NTRS1983

The composition of asteroid 2 Pallas and its relation to primitive meteorites

High resolution spectroscopic observations of asteroid 2 Pallas from 1.7-3.5 microns are reported. These data are combined with previous measurements from 0.4-1.7 microns to interpret Pallas' surface mineralogy. Evidence is found for low-Fe(2+) hydrated silicates, opaque components, and low Fe(2+) anhydrous silicates. This assemblage is very similar to carbonaceous chondrite matrix material such as is found in type CI and CM meteorites, but it has been subjected to substantial aqueous alteration and there is a major extraneous anhydrous silicate component. This composition is compared to that of asteroid 1 Ceres. Although there are substantial differences in their broad band spectral reflectances it appears that both asteroids are genetically related to know carbonaceous chondrites.

Larson, H. P.↗
NASA NTRS1983

On the low-lying states of TiN

A series of CAS SCF and multi-reference CI calculations are used to describe the lowest states of TiN. The bonding in all states is described as a triple bond involving the Ti 3d orbitals. The system has some ionic character as seen from both population analysis and dipole moment. The origins of the excited states are discussed.

Bauschlicher, C. W., Jr.↗
NASA NTRS1984

The chemistry and origin of the ordinary chondrites Implications from refractory-lithophile and siderophile elements

Thirty-eight ordinary chondrites (17 H, 20 L, and 1 LL) have been analyzed for major and selected trace elements. These data indicate that the lithophile elements Mg, Ca, Al, Cr, and V normalized to Si are in higher abundance in the H than in the L chondrites. The siderophile elements Ni, Co, and Fe show very good correlation within, as well as between, the two major ordinary chondrite groups. Twenty-four of the analyses are of Antarctic finds, while ten are samples of falls. Comparing the Antarctic data with the fall data reveals no evidence that any of the elements studied here have been mobilized by terrestrial weathering processes. Within the H and L chondrite groups there is little chemical variation, indicating that the source of these samples is remarkably homogeneous. Equilibrium condensate fractionation from a nebula of CI composition can result in the observed ordinary chondrite compositions. The fractionation of metal at about 1440 K (and 0.001 atm) into high and low iron groups, followed by a gas-solid fractionation at about 1380 K with the H group losing more solids than the L, will produce the observed H and L compositions and intragroup trends.

Fulton, C. R.↗
NASA NTRS1984

On the low-lying states of TiC

The ground and low-lying excited states of TiC are investigated using a CASSCF-externally contracted CI approach. The calculations yield a 3Sigma(+) ground state, but the 1Sigma(+) state is only 780/cm higher and cannot be ruled out. The low-lying states have some triple bond character. The nature of the bonding and origin of the states are discussed.

Bauschlicher, C. W., Jr.↗
NASA NTRS1984

Hydrogen atom migration in the oxidation of aldehydes - O(3P) + H2CO

An ab initio study of hydrogen atom migration in methylenebis(oxy)H2CO2(3B2) to form triplet formic acid HCOOH (3A1) is reported. From HF, MCHF, and CI calculated energy barriers, the activation energy is estimated to be no less than 30 kcal/mol. It is concluded that the hydrogen migration channel is not accessible in recent room temperature experiments on the O(3P) + H2CO reaction.

Dupuis, M.↗
NASA NTRS1984

Hydrogen atom abstraction from aldehydes - OH + H2CO and O + H2CO

The essential features of the potential energy surfaces governing hydrogen abstraction from formaldehyde by oxygen atom and hydroxyl radical have been characterized with ab inito multiconfiguration Hartree-Fock (MCHF) and configuration interaction (CI) wave functions. The results are consistent with a very small activation energy for the OH + H2CO reaction, and an activation energy of a few kcal/mol for the O + H2CO reaction. In the transition state structure of both systems, the attacking oxygen atom is nearly collinear with the attacked CH bond.

Dupuis, M.↗
NASA NTRS1984

Transmission electron microscopy of the 'LOW-CA' hydrated interplanetary dust particle

Transmission electron microscopy of a hydrated interplanetary dust particle indicates that it consists largely of a poorly crystalline phyllosilicate containing Fe, Mg and Al with an interlayer spacing of 10 to 12 A and so is distinct from the major phyllosilicate in CI and CM carbonaceous chondrites. The silicate is probably an Fe- and Mg-rich smectite or mica. Submicron, spherical to euhedral pyrrhotite and pentlandite are prominent. Unusual, low-Ni pentlandite is also common and typically occurs as rectangular platelets. Unlike many chondritic interplanetary dust particles, olivine is rare and pyroxene was not observed. Other less abundant phases are magnetite, chromite, and an unidentified phase containing Fe, Mg, Si, Ca, and Mn. This particle differs from a hydrated micrometeorite described previously by Brownlee (1978), indicating there are mineralogically different subsets of hydrated interplanetary dust particles. Despite gross similarities in mineralogy between the particle and the carbonaceous chondrites, they show appreciable differences in detail.

Tomeoka, K.↗
NASA NTRS1985

A model for core formation in the early Earth

Two basic types exogenous models were proposed to account for siderophile and chalcophile element abundances in the Earth's upper mantle. The first model requires that the Earth be depleted in volatiles and that, after a core formation event which extracted the most siderophile elements into the core, additional noble siderophile elements (Pt, Ir, Au) were added as a late veneer and mixed into the mantle. The second model postulates a reduced Earth with approximately CI elemental abundances in which a primary core forming event depleted all siderophile elements in the mantle. The plausibility of models which require fine scale mixing of chondritic material into the upper mantle is analyzed. Mixing in liquids is more efficient, but large degrees of silicate partial melting will facilitate the separation of magma from residual solids. Any external events affecting the upper mantle of the Earth should also be evident in the Moon; but siderophile and chalcophile element abundance patterns inferred for the mantles of the Earth and Moon differ. There appear to be significant physical difficulties associated with chondritic veneer models.

Jones, J. H.↗
NASA NTRS1985

Relationships between chondritic meteorites and planets

Chondrites formed in the solar nebula prior to the formation of planets; they probably constituted the bulk of preplanetary solids in the inner solar system. In the highly reduced enstatite chondrites 10% of Si is metallic; in the highly oxidized CM and CI chondrites 20-30% of Fe is in the +3 state. The high density of Mercury implies that nebular Fe was reduced, whereas the low density of Mars indicates that a large fraction was oxidized. Most rare gases in the terrestrial planets seem to have accreted trapped in grains; Venusian interelement ratios resemble those in enstatite chondrites; Earth and Mars ratios are more like those in ordinary or carbonaceous chondrites. These observations imply that enstatite chondrites formed near the Sun (near Venus?), the carbonaceous chondrites formed far from the Sun, the ordinary chondrites at an intermediate location. Compositional data on Mercury, comets, an asteroidal fragment, and the moons of Mars are badly needed.

Wasson, J. T.↗
NASA NTRS1985

Nebular volatile fractionations associated with chondrule formation

Chondrules are ubiquitous constituents of primitive solar system matter, indicating that chondrule formation was an important and widespread process in the early history of the solar system. If chondrules formed from fine grained CI like precursors, some volatile fractionation must have accompanied chondrule formation. This is not likely related to a separation of chondrules from gas (and dust, fine enough to be coupled to gas) shortly after formation, a process required by most models of chondrule formation. If chondrules are continuously formed in and removed from a region of the solar nebula, the chemical environment in this region will change. Vaporization of solid increases both oxygen and sulfur in the gas relative to H. Depending on the oxygen/sulfur ratio, which may vary widely as a function of precursor composition and temperature, two cases can be distinguished: (1) sulfur requires reaction with solid to condense, and thus may become much more enriched than 0; (2) if a significant amount of silicate is vaporized together with sulfides and does not recondense, changes in H/O outweigh S enrichment. In combination, these two effects may be able to account for much of the chemical variation among various types of chondrites. Opaque, fine grained matrix in unequilibrated chondrites, which is thought to derive from dust, has indeed been found to have a volatile element abundance complementary to chondrules.

Kracher, A.↗
NASA NTRS1985

Minor-element signature of relic olivine grains in deep-sea particles - A match with forsterites from C2 meteorites

The results of high-precision electron microprobe analyses on relic forsterites in 13 different deep sea particles (DSP) are described, comparing their 'minor' element (Mn, Cr, Ca, Ti, Al) signatures with those of olivines found in several types of meteorites. The results show that the DSP olivines do not match those from terrestrial and lunar rocks and achondritic meteoroties, match poorly those of the carbonaceous Allende (CV3) and Orgeuil (CI) meteorites, but are well matched with olivines from C2 meteorites, including the unusual Belgica 7904 meteorite.

Steele, I. M.↗
NASA NTRS1985

Calculation of pressure broadening parameters for the CO-He system at low temperatures

Theoretical pressure broadening parameters were computed for the 0-1 and 1-2 rotational transitions of CO in He at very low temperatures and compared with the recent experimental measurements at 4.2 K. The interaction potential was taken from extensive SCF-CI calculations, molecular collision dynamics were described by essentially exact converged close coupling calculations, and pressure broadening cross sections were obtained from the collisional S matrices within the accurate Fano-Ben Reuven framework. Resonances at low collision energies give rise to an increase in the thermally averaged cross sections at low temperatures. Although previous calculations for this system at higher temperatures (77-300 K) were in good accord with experiment, at 4.2 K predicted values are about two times larger than experiment; possible sources of this discrepancy are discussed.

Green, S.↗
NASA NTRS1985

The lower electronic states of ClOO - A computational investigation

Eight doublet and eight quartet states of ClOO were investigated by ab initio CI techniques. The potential energy surfaces of the four lowest energy doublet states of both A-double-prime and A-prime symmetry indicate that only the 1 2A-double-prime state is bound. In contrast to the model provided by the HO2 radical, all of the excited doublet states investigated were repulsive with respect to dissociation to Cl + O2 and metastable or bound with respect to dissociation to ClO + O. The transitions to the excited states investigated span the visible and near UV spectral regions, but the transition moments indicate that they are very weak. Since the photolysis products are the same as those of the rapid thermal dissociation, photolysis is not expected to be an important atmospheric process. The soft bending potential for the 1 2A-prime state and the shape of the 1 4A-double-prime state in the entrance channel of the ClO + O yields Cl + O2 reaction provide a qualitative explanation for the underprediction of the low temperature reaction rate by previous trajectory calculations.

Jafri, J. A.↗
NASA NTRS1985

Molecular processes in a high temperature shock layer

The development of techniques for the calculation of electron capture widths, electronic wave functions, cross sections and rates needed for the description of the dissociative recombination (DR) of molecular ions with electrons were described. The cross sections and rates were calculated by using harmonic oscillator wave functions for the ion and a delta function approximation for the continuum vibrational wave function in the repulsive dissociative channel. In order to obtain DR cross sections of quantitative accuracy, a computer program which solves the one dimensional nuclear motion wave equation was revised to calculate the cross sections and rates. The program and the new results are described. Included is a discussion of large windows found in the dissociative recombination cross sections from excited ion vibrational levels. These windows have not been previously reported in the literature. The magnitude of the DR cross sections for several dissociative routes are sensitive to the location of the crossing of the neutral and ion potential curves. Studies of the effects of basis set and CI wave function size on vertical excitation energies are described. Preliminary studies on N2 and O2 using large scale wave functions are also reported.

Guberman, S. L.↗
NASA NTRS1984

On the extension of the MCSCF/CI method

Research conducted during this period was focused on two main areas: (1) bonding in transition metal oxides; and (2) adsorption of CO on Al and Ni. In both of these theoretical studies a major interest was to obtain a better understanding of the nature of the bonding in transition metal containing systems. The studies used self consistent field (SCF), multi-configuration self cosistent field (MCSCF) and configuration interaction (CI) methods in the treatment of the transition metal oxides and only the SCF method in the adsorption studies. The reports of three principle investigators who contributed to this work during the tenure of the project are presented along with associated published papers.

Bauschlicher, C., Jr.↗
NASA NTRS1986

Integral processing in beyond-Hartree-Fock calculations

The increasing rate at which improvements in processing capacity outstrip improvements in input/output performance of large computers has led to recent attempts to bypass generation of a disk-based integral file. The direct self-consistent field (SCF) method of Almlof and co-workers represents a very successful implementation of this approach. This paper is concerned with the extension of this general approach to configuration interaction (CI) and multiconfiguration-self-consistent field (MCSCF) calculations. After a discussion of the particular types of molecular orbital (MO) integrals for which -- at least for most current generation machines -- disk-based storage seems unavoidable, it is shown how all the necessary integrals can be obtained as matrix elements of Coulomb and exchange operators that can be calculated using a direct approach. Computational implementations of such a scheme are discussed.

Taylor, P. R.↗
NASA NTRS1986

Laser microprobe study of cosmic dust (IDPs) and potential source materials

The study of cosmic dust or interplanetary dust particles (IDP) can provide vital information about primitive materials derived primarily from comets and asteroids along with a small unknown fraction from the nearby interstellar medium. The study of these particles can enhance our understanding of comets along with the decoding of the history of the early solar system. In addition the study of the cosmic dust for IDP particles can assist in the elucidation of the cosmic history of the organogenic elements which are vital to life processes. Studies to date on these particles have shown that they are complex, heterogeneous assemblages of both amorphous and crystalline components. In order to understand the nature of these particles, any analytical measurements must be able to distinguish between the possible sources of these particles. A study was undertaken using a laser microprobe interfaced to a quadrupole mass spectrometer for the analysis of the volatile components present in cosmic dust particles, terrestrial contaminants present in the upper atmosphere, and primitive carbonaceous chondrites. From the study of the volatiles released from the carbonaceous materials it is hoped that one could distinguish between components and sources in the IDP particles analyzed. The technique is briefly described and results for the CI, CM, and CV chondrites and cosmic dust particle W7027B8 are presented.

Gibson, E. K., Jr.↗