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At least 973 records · Page 54
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NASA NTRS1995

Optical spectroscopy of carbon-rich proto-planetary nebulae

In this paper, we present a medium-resolution (3 A) spectroscopic study of six proto-planetary nebulae (PPN), post-asymptotic giant branch (AGB) objects with large infrared excesses. All six are found to display the spectra of G supergiants. However, they also show molecular carbon features, C2 and in most cases C3, and strong absorption lines due to s-process elements. Other evidences of a carbon-rich nature are found in published molecular-line millimeter emissions (CO and HCN) and 3.3 micrometer features attributed to polycyclic aromatic hydrocarbons. These properties are all in accord with what one would expect in a post-AGB star in which carbon-rich materials formed in thermal pulses is dredged up to the surface of a mass-losing object. A correlation is found between the presence of molecular C2 absorption and the presence of the unidentified 21 micrometer emission feature. This strengthens the suggestion that carbon is a major component of the molecule producing this unidentified feature. Four additional proto-planetary nebulae which share some of these properties are also discussed.

Hrivnak, Bruce J.↗
NASA NTRS1995

RY Scuti: Infrared and radio observations of the mass-loss wind of a massive binary star system

We report infrared (IR) imaging, IR photometry, IR spectroscopy, optical/IR photopolarimetry, and Very Large Array (VLA) radio observations of the peculiar binary star RY Scuti. These observations provide an unprecedented view of the detailed spatial structure of the equatorial mass-loss wind of a massive, luminous, 'overcontact' binary system. The binary star (0.43 AU separation) is surrounded by a flattened equatorial disk with an outer radius of approximately = 3 x 10(exp 16) cm (2000 AU) that emits strongly in the IR and radio. The inside of the disk is ionized and emits free-free radiation from hydrogen and 12.8 micrometers forbidden-line emission from (Ne II); the outside of the disk emits thermal radiation from silicate dust. Radio continuum emission is also produced in a compact H II region surrounding the binary. The dust may have a polycyclic aromatic hydrocarbon (PAH) component. We use a rudimentary geometric model in which the thermal IR and radio emission from the disk are assumed to arise in a pair of concentric toroidal rings to estimate the physical properties of the disk. The mean radius of the ionized gas toroid is approximately = 1.3 x 10(exp 16) cm (870 AU), and the mean radius of the dust toroid is approximately = 2.2 x 10(exp 16) cm (1470 AU). RY Scuti has a small intrinsic polarization, with the electric vector perpendicular to the equatorial disk, that is probably caused by electron scattering from hot gas close to the central binary. We conclude that neon in the nebula is overabundant with respect to hydrogen and helium by a factor of between 1.6 and 10. Our IR/radio image data suggest that the circumstellar disk is part of an extensive radiation driven mass-loss outflow that is strongly confined to the equatorial plane of the binary system. The sharp spatial separation of the outer dust torous from the inner ionized gas torus confirms earlier suggestions that dust formation in the circumstellar ejecta of very hot stars must occur in regions that are shielded from the hard radiation field.

Gehrz, R. D.↗
NASA NTRS1995

The neutral atomic phases of the interstellar medium

We calculate the thermal equilibrium gas temperature of the diffuse interstellar medium. Our method incorporates a new photoelectric heating rate from small grains and polycyclic aromatic hydrocarbons (PAHs) that accounts for a size distribution of particles extending from 100 to 3 A radius. We also include a detailed treatment of the ionization rates and heating due to the soft X-ray background and due to cosmic rays. Phase diagrams (thermal pressure P versus hydrogen density n) are presented for gas that is illuminated by local interstellar far-ultraviolet (FUV) and X-ray radiation fields. A stable two-phase medium is produced with thermal pressure in the range P/k approximately = to 10(exp 3-4) K/cc. We demonstrate that photoelectric heating from PAHs dominates in the warm neutral phase (WNM) and cold neutral phase (CNM). If the C II (158 micrometers cooling per hydrogen nucleus in the solar neighborhood represents an average value for the Galaxy, we predict L(sub CII) approximately = to 7 x 10(exp 7) solar luminosities from the CNM in the Galaxy, comparable to that observed by the Cosmic Background Explorer (COBE). We discuss the dependence of the results on absorbing column density, gas phase abundances, dust abundances and metallicity, FUV field, and the X-ray radiation field. These results will be useful in modeling the multiphase structure of high-velocity clouds in the halo, the interstellar matter (ISM) at other galactocentric radii, and the ISM in external galaxies and galactic nuclei.

Wolfire, M. G.↗
NASA NTRS1995

Ultraviolet extinction to 10.8 inverse microns

UV extinction curves that represent observed variations i diffuse clouds (Zeta Oph, Xi Per, and Sigma Sco) have been derived by combining IUE and Voyager data and have been modeled by a multicomponent size distribution of silicate and carbonaceous giants. We have used theoretical model atmospheres as our reddening-free standards, and, in the case of Zeta Oph, also compare with the use of a stellar standard. An attempt was made to classify the UV spectra via theoretical spectral indices, but still missing line opacity appears to limit the use of this method at the present. The modeling includes five possible grain ingrediants, largge (0.015-0.25 micrometers) and small (0.005 micrometers) silicate grains, similarly large and small carbonaceous grains, and polycyclic aromatic hydrocarbons (PAHs). The observed extinction curves are fitted by the model via nonlinear Chi-square-minimization, varying the fractional amount of cosmic Si and C that are locked up in each igredient. The observed variation from 'steep' to 'flat' UV extinction curves is found to correspond to a modest model variation in the lower limit of the power-law size distribution of the large grains. Graphite is found to be necessary grain ingredient that cannot be replaced by the laboratory amorphous carbon materials. Carbonaceous mantles on the larger silicate grains provide no good fits to the observed curves. Using absorption properties of neutral laboratory PAHs limits PAHs to a few percent of the cosmic C abundance for acceptable fits that also give values for R(sub V) close to the observed values.

Aannestad, Per A.↗
NASA NTRS1995

Evolution of the biosynthesis of the branched-chain amino acids

The origins of the biosynthetic pathways for the branched-chain amino acids cannot be understood in terms of the backwards development of the present acetolactate pathway because it contains unstable intermediates. We propose that the first biosynthesis of the branched-chain amino acids was by the reductive carboxylation of short branched chain fatty acids giving keto acids which were then transaminated. Similar reaction sequences mediated by nonspecific enzymes would produce serine and threomine from the abundant prebiotic compounds glycolic and lactic acids. The aromatic amino acids may also have first been synthesized in this way, e.g. tryptophan from indole acetic acid. The next step would have been the biosynthesis of leucine from alpha-ketoisovalerc acid. The acetolactate pathway developed subsequently. The first version of the Krebs cycle, which was used for amino acid biosynthesis, would have been assembled by making use fo the reductive carboxylation and leucine biosynthesis enzymes, and completed with the development of a single new enzyme, succinate dehydrogenase. This evolutionary scheme suggests that there may be limitations to inferring the origins of metabolism by a simple back extrapolation of current pathways.

Keefe, Anthony D.↗
NASA NTRS1995

Polyimides That Contain Cyclobutene-3,4-Dione Moieties

Linear aromatic polyimides containing cyclobutene-3,4,-dione moieties synthesized in experiments. Polymers exhibit high thermal and thermo-oxidative stabilities, making them useful in structural components exposed to hot and/or oxidative environments, as in jet engines and on outer surfaces of supersonic aircraft. Electrons in cyclobutene-3,4-dione moieties readily excited to states of higher energy upon exposure to ultraviolet light, giving rise to electronic properties with potential utility in photovoltaic, photoconductive, and/or photoemissive devices.

St. Clair, Terry L.↗
NASA NTRS1995

Polyimides Made From PDMDA

Thermally stable linear aromatic polyimides with improved optical, electrical, and processing characteristics made from 3,3'-bis(3,4-dicarboxyphenoxy)diphenylmethane dianhydride (PDMDA). Made in form of powders that can be dissolved to cast thin films. Potentially useful as coating and matrix materials. Films made vary from transparent to pale yellow. Greater degrees of transparency and lower dielectric constants make these polyimides more suitable for use in some electronic and optical applications; e.g., as insulating materials on wires and protective films on solar photovoltaic cells.

St. Clair, Anne K.↗
NASA NTRS1995

The Interstellar Medium

The Interstellar Medium (ISM) forms an integral part of the lifecycle of stars and the galaxy. Stars are formed by gravitational contraction of interstellar clouds. Over their life, stars return much of their mass to the ISM through winds and supernova explosions, resulting in a slow enrichment in heavy elements. Understanding the origin and evolution of the ISM is a key problem within astrophysics. The KAO has made many important contributions to studies of the interstellar medium both on the macro and on the micro scale. In this overview, I will concentrate on two breakthroughs in the last decade in which KAO observations have played a major role: (1) the importance of large Polycyclic Aromatic Hydrocarbon (PAH) molecules for the ISM (section 3) and (2) the study of Photodissociation Regions (PDRs) as an analog for the diffuse ISM at large (section 4). Appropriately, the micro and macro problem are intricately interwoven in these problems. Finally, section 5 reviews the origin of the (CII) emission observed by COBE.

Tielens, Alexander G. G. M.↗
NASA NTRS1995

Airborne and laboratory studies of interstellar PAHs

A brief history of the observations which have led to the hypothesis that polycyclic aromatic hydrocarbons (PAH's) are the carriers of the widespread interstellar emission features near 3050, 1615, '1300' and 890 cm(exp -1) (3.29, 6.2, '7.7', and 11.2 mu m) is presented. The central role of airborne spectroscopy is stressed. The principal reason for the assignment to PAH's was the resemblance of the interstellar emission spectrum to the laboratory absorption spectra of PAH's and PAH-like materials. Since precious little information was available on the properties of PAH's in the forms that are thought to exist under interstellar conditions -isolated and ionized in the emission zones, with the smallest PAH's being dehydrogenated- there was a need for a spectral data base on PAH's taken in these states. Here, the relevant infrared spectroscopic properties of PAH's will be reviewed. These laboratory spectra show that relative band intensities are severely altered and that band frequencies shift. It is shown that these new data alleviate several of the spectroscopic criticisms previously leveled at the hypothesis.

Allamandola, L. J.↗
NASA NTRS1995

Spectral imaging of the Orion Bar at 6.2 and 7.8 microns

We have obtained images of the Orion Bar region through narrow-band filters at 6.2 and 7.8 mu m to determine whether the emission observed at these wavelengths arises from C-C modes in polycyclic aromatic hydrocarbons (PAH's). Morphologically, the distribution of the emissions are similar, but appear different in detail. Quantitative comparisons of these band intensities with previous data at 8.4 and 11.3 mu m indicates that they are indeed consistent with emission from fluorescently excited PAH molecules.

Bregman, Jesse↗
NASA NTRS1995

High rotational CO lines in post-AGB stars and PNe

A significant fraction of a star's initial mass is lost while it is on the Asymptotic Giant Branch (AGB). Mass loss rates range from 10(exp -7) solar mass/yr for early AGB stars to a few 10(exp -4) solar mass/yr for stars at the tip of the AGB. Dust grains condense from the outflow as the gas expands and form a dust shell around the central star. A superwind (approximately 10(exp -4) to 10(exp -3) solar mass/yr) is thought to terminate the AGB phase. In the post-AGB phase, the star evolves to a higher effective temperature, the mass loss decreases (approximately 10(exp -8) solar mass/yr), but the wind velocity increases (approximately 1000 km/s). During this evolution, dust and gas are exposed to an increasingly harsher radiation field and when T(sub eff) reaches about 30,000 K, the nebula is ionized and becomes a planetary nebula (PN). Photons from the central star can create a photodissociation region (PDR) in the expanding superwind. Gas can be heated through the photoelectric effect working on small grains and polycyclic aromatic hydrocarbons (PAH's). This gas can cool via the atomic fine structure lines of O I (63 microns and 145 microns) and C II (158 microns), as well as the rotational lines of CO. In the post-AGB phase, the fast wind from the central star will interact with the material ejected during the AGB phase. The shock caused by this interaction will dissociate and heat the gas. This warm gas will cool through atomic fine structure lines of O I and the rotational lines of (newly formed) CO.

Justtanont, K.↗
NASA NTRS1995

Development and applications of laser-induced incandescence

Several NASA-funded investigations focus on soot processes and radiative influences of soot in diffusion flames given their simplicity, practical significance, and potential for theoretical modeling. Among the physical parameters characterizing soot, soot volume fraction, f(sub v), a function of particle size and number density, is often of chief practical interest in these investigations, as this is the geometrical property that directly impacts radiative characteristics and the temperature field of the flame and is basic to understanding soot growth and oxidation processes. Diffusion flames, however, present a number of challenges to the determination of f(sub v) via traditional extinction measurements. Laser-induced incandescence (LII) possesses several advantages compared to line-of-sight extinction techniques for determination of f(sub v). Since LII is not a line-of-sight technique, similar to fluorescence, it possesses geometric versatility allowing spatially resolved measurements of f(sub v) in real time in nonaxisymmetric systems without using deconvolution techniques. The spatial resolution of LII is determined by the detector and imaging magnification used. Neither absorption by polycyclic aromatic hydrocarbons (PAH's) nor scattering contributes to the signal. Temporal capabilities are limited only by the laser pulse and camera gate duration, with measurements having been demonstrated with 10 ns resolution. Because of these advantages, LII should be applicable to a variety of combustion processes involving both homogeneous and heterogeneous phases. Our work has focussed on characterization of the technique as well as exploration of its capabilities and is briefly described.

Vanderwal, Randy L.↗
NASA NTRS1995

A polyimide molecular composite

A polyimide is a hard and sturdy thermoplastic made from the combination of tetracarboxylic dianhydrides and diamines in a polar aprotic solvent. A series of polyimide blends comprising of a tough moldable, wholly aromatic polyimide and a highly crystalline rigid polyimide were prepared. The flexible polyimide was based on LaRC-SI with a 2% offset, and the rigid polyimide as based on para-phenylenediamine and biphenyltetracarboxylic dinnhydride encapped with pthalic anhydride. The polyimide blends were cast as films and cured at 100, 200, and 300 C in air. These films were characterized by finding their glass-transition temperature, melting temperature, coefficient of thermal expansion, tensile modulus, tensile strength, and percent elongation. These characterizations found out which polyimide blend had exceptional mechanical and thermal properties. Experimental results showed the following: (1) A blend with increasing amounts of flexible polyimide produced a better film; (2) A blend with increasing amounts of rigid polyimide produced a brittle film; (3) LaRC-SI (2% offset) tended to phase separate during film casting; (4) Most of the films produced were brittle; and (5) There was an inability to obtain mechanical and thermal properties due to having brittle film. These conclusions demonstrate the difficulty in producing a polyimide blend comprising two physically distinct polyimides. Combining a polysmide with a poly(amic acid) thermosett could be the next phase of research in polyimide synthesis.

Hart, Aprill J.↗
NASA NTRS1996

Effect of Temperature and Deformation Rate on the Tensile Mechanical Properties of Polyimide Films

In order to study the structure-property relationships of different processed oriented polyimide films, the mechanical properties will be identified by using tensile tester Instron 4505 and structural information such as the 3-dimensional birefringence molecular symmetry axis and 3-dimensional refractive indices will be determined by using wave guide coupling techniques. The monoaxial drawing techniques utilized in this research are very useful for improving the tensile mechanical properties of aromatic polyimide films. In order to obtain high modulus/high strength polyimide films the following two techniques have been employed, cold drawing in which polyimide films are drawn at room temperature at different cross head speeds and hot drawing in which polyimide films are drawn at different temperatures and cross head speeds. In the hot drawing process the polyimide films are drawn at different temperatures until the glass transition temperature (Tg) is reached by using the environmental chamber. All of the mechanical and optical property parameters will be identified for each sample processed by both cold and hot drawing techniques.

Moghazy, Samir F.↗
NASA NTRS1995

Confined-Pyrolysis as an Experimental Method for Hydrothermal Organic Synthesis

A closed pyrolysis system has been developed as a tool for studying the reactions of organic compounds under extreme hydrothermal conditions. Small high pressure stainless steel vessels in which the ratio of sediment or sample to water has been adjusted to eliminate the headspace at peak experimental conditions confines the organic components to the bulk solid matrix and eliminates the partitioning of the organic compounds away from the inorganic components during the experiment. Confined pyrolysis experiments were performed to simulate thermally driven catagenetic changes in sedimentary organic matter using a solids to water ratio of 3.4 to 1. The extent of alteration was measured by monitoring the steroid and triterpenoid biomarkers and polycyclic aromatic hydrocarbon distributions. These pyrolysis experiments duplicated the hydrothermal transformations observed in nature. Molecular probe experiments using alkadienes, alkenes and alkanes in H2O and D2O elucidated the isomerization and hydrogenation reactions of aliphatic and the competing oxidative reactions occurring under hydrothermal conditions. This confined pyrolysis technique is being applied to test experiments on organic synthesis of relevance to chemical evolution for the origin of life.

Leif, Roald N.↗
NASA NTRS1995

The 11 Micron Emissions of Carbon Stars

A new classification scheme of the IRAS LRS carbon stars is presented. It comprises the separation of 718 probable carbon stars into 12 distinct self-similar spectral groupings. Continuum temperatures are assigned and range from 470 to 5000 K. Three distinct dust species are identifiable: SiC, alpha:C-H, and MgS. In addition to the narrow 11 + micron emission feature that is commonly attributed to SiC, a broad 11 + micron emission feature, that is correlated with the 8.5 and 7.7 micron features, is found and attributed to alpha:C-H. SiC and alpha:C-H band strengths are found to correlate with the temperature progression among the Classes. We find a spectral sequence of Classes that reflects the carbon star evolutionary sequence of spectral types, or alternatively developmental sequences of grain condensation in carbon-rich circumstellar shells. If decreasing temperature corresponds to increasing evolution, then decreasing temperature corresponds to increasing C/O resulting in increasing amounts of carbon rich dust, namely alpha:C-H. If decreasing the temperature corresponds to a grain condensation sequence, then heterogeneous, or induced nucleation scenarios are supported. SiC grains precede alpha:C-H and form the nuclei for the condensation of the latter material. At still lower temperatures, MgS appears to be quite prevalent. No 11.3 micron PAH features are identified in any of the 718 carbon stars. However, one of the coldest objects, IRAS 15048-5702, and a few others, displays an 11.9 micron emission feature characteristic of laboratory samples of coronene. That feature corresponds to the C-H out of plane deformation mode of aromatic hydrocarbon. This band indicates the presence of unsaturated, sp(sup 3), hydrocarbon bonds that may subsequently evolve into saturated bonds, sp(sup 2), if, and when, the star enters the planetary nebulae phase of stellar evolution. The effusion of hydrogen from the hydrocarbon grain results in the evolution in wavelength of this 11.9 micron emission feature to the 11.3 micron feature.

Goebel, J. H.↗
NASA NTRS1996

Search for Past Life on Mars: Possible Relict Biogenic Activity in Martian Meteorite ALH84001

Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest the PAHs are indigenous to the meteorite. High resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and Fe-monosulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.

McKay, David S.↗
NASA NTRS1995

The 11 Micron Emissions of Cabon Stars

A new classification scheme of the IRAS LRS carbon stars is presented. It comprises the separation of 718 probable carbon stars into 12 distinct self-similar spectral groupings. Continuum temperatures are assigned and range from 470 to 5000 K. Three distinct dust species are identifiable: SiC, alpha:C-H, and MgS. In addition to the narrow 11 + micron emission feature that is commonly attributed to SiC, a broad 11 + micron emission feature, that is correlated with the 8.5 and 7.7 micron features, is found and attributed to alpha:C-H. SiC and alpha:C-H band strengths are found to correlate with the temperature progression among the Classes. We find a spectral sequence of Classes that reflects the carbon star evolutionary sequence of spectral types, or alternatively developmental sequences of grain condensation in carbon-rich circumstellar shells. If decreasing temperature corresponds to increasing evolution, then decreasing temperature corresponds to increasing CIO resulting in increasing amounts of carbon rich dust, namely alpha:C-H. If decreasing the temperature corresponds to a grain condensation sequence, then heterogeneous, or induced nucleation scenarios are supported. SiC grains precede alpha:C-H and form the nuclei for the condensation of the latter material. At still lower temperatures, MgS appears to be quite prevalent. No 11.3 micron PAH features are identified in any of the 718 carbon stars. However, one of the coldest objects, IRAS 15048-5702, and a few others, displays an 11.9 micron emission feature characteristic of laboratory samples of coronene. That feature corresponds to the C-H out of plane deformation mode of aromatic hydrocarbon. This band indicates the presence of unsaturated, sp(sup 3), hydrocarbon bonds that may subsequently evolve into saturated bonds, sp(sup 2), if, and when, the star enters the planetary nebulae phase of stellar evolution. The effusion of hydrogen from the hydrocarbon grain results in the evolution in wavelength of this 11.9 micron emission feature to the 11.3 micron feature.

Goebel, J. H.↗