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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 955 records · Page 53
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NASA NTRS1987

Star formation in Seyfert galaxies

An analysis of the IRAS data for a sample of classical (optically selected) Seyfert galaxies is presented. The IRAS fluxes at 25 micron, 60 micron, and 100 micron are found to be uncorrelated or only very weakly correlated with the UV/Optical continuum flux and the near and mid IR flux at 3.5 and 10 microns. To investigate the possibility that star formation accounts for the far IR flux, the IRAS measurements for the Seyfert galaxies are compared to IRAS observations of a sample of normal spiral galaxies, and a sample of Starburst galaxies. It is shown that the far IR luminosities and far IR colors of Seyfert galaxies are indistinguishable from those of the Starburst galaxies. Besides, normal galaxies are an order of magnitude less luminous than both the Seyfert and the Starburst galaxies. This indicates that star formation produces the bulk of the far infrared emission in Seyfert galaxies.

Espinosa, J. M. Rodriguez↗
NASA NTRS1987

Integration of altitude and airspeed information into a primary flight display via moving-tape formats: Evaluation during random tracking task

A ground-based aircraft simulation study was conducted to determine the effects on pilot preference and performance of integrating airspeed and altitude information into an advanced electronic primary flight display via moving-tape (linear moving scale) formats. Several key issues relating to the implementation of moving-tape formats were examined in this study: tape centering, tape orientation, and trend information. The factor of centering refers to whether the tape was centered about the actual airspeed or altitude or about some other defined reference value. Tape orientation refers to whether the represented values are arranged in descending or ascending order. Two pilots participated in this study, with each performing 32 runs along seemingly random, previously unknown flight profiles. The data taken, analyzed, and presented consisted of path performance parameters, pilot-control inputs, and electrical brain response measurements.

Abbott, Terence S.↗
NASA NTRS1986

On star formation in stellar systems. I - Photoionization effects in protoglobular clusters

The progressive ionization and subsequent dynamical evolution of nonhomogeneously distributed low-metal-abundance diffuse gas after star formation in globular clusters are investigated analytically, taking the gravitational acceleration due to the stars into account. The basic equations are derived; the underlying assumptions, input parameters, and solution methods are explained; and numerical results for three standard cases (ionization during star formation, ionization during expansion, and evolution resulting in a stable H II region at its equilibrium Stromgren radius) are presented in graphs and characterized in detail. The time scale of residual-gas loss in typical clusters is found to be about the same as the lifetime of a massive star on the main sequence.

Tenorio-Tagle, G.↗
NASA NTRS1986

Shock-tube pyrolysis of acetylene - Sensitivity analysis of the reaction mechanism for soot formation

The impact of thermodynamic parameters on the sensitivity of model predictions of soot formation by shock-tube pyrolysis of acetylene were assessed analytically. The pyrolysis process was treated as having three components: initiation, the initial pyrolysis stages; cyclization, formation of larger molecules and radicals and small aromatic molecules; and polymerization, further growth of aromatic rings. Rate equations are reviewed for each component. Thermodynamic effects were assessed by varying the C2H-H and C2H3-H bond energies and the Ct-(Ct) group additivity value. Any change in the C2H-H bond energy had a significant impact on the temperature and the maximum amount of the soot yield. The findings underscore the necessity of using accurate thermodynamic data for modeling high-temperature chemical kinetics.

Frenklach, M.↗
NASA NTRS1986

Chromospheric dust formation, stellar masers and mass loss

A multistep scenario which describes a plausible mass loss mechanism associated with red giant and related stars is outlined. The process involves triggering a condensation instability in an extended chromosphere, leading to the formation of cool, dense clouds which are conducive to the formation of molecules and dust grains. Once formed, the dust can be driven away from the star by radiation pressure. Consistency with various observed phenomena is discussed.

Stencel, R. E.↗
NASA NTRS1986

Glass formation and crystallization of high lead content PbO-B2O3 compositions

The glass-forming and undercooling ability of PbO-B2O3 melts in the 2PbO.B2O3 to 4PbO.B2O3 composition range were studied. The glass formation propensities were investigated as a function of cooling rate and sample mass. A qualitative investigation of the crystallization process(es) preventing glass formation was made, and it was concluded that under normal circumstances heterogeneous nucleation was the common occurrence. Hence, it was concluded that such compositions are prime candidates for containerless experiments aboard the Space Shuttle.

Weinberg, Michael C.↗
NASA NTRS1986

The heat of formation of Mercury vacancies in Hg(0.8)Cd(0.2)Te

A modified mass loss measurement technique has been employed for the first time for the direct in situ determination of vacancy concentrations in Hg(0.8)Cd(0.2)Te at elevated temperatures. This technique can also be used to establish the pressure-temperature phase diagram for this type of system. The derived mean value for the heat of formation of mercury vacancies in the above alloy is 0.43 eV. Theoretical considerations concerning the vacancy formation in HgTe and in Hg(0.8)Cd(0.2)Te are in qualitative agreement with the experimental value.

Wiedemeier, H.↗
NASA NTRS1986

The molecular content of interacting and isolated galaxies The effect of environment on the efficiency of star formation

Molecular gas observations of merging/interacting and isolated galaxies are presented in order to study the relationship between environment and the efficiency of star formation. The two galaxy samples differ primarily in their IR properties and are quite similar in their molecular gas contents. The ratios of IR luminosity to H2 mass have a mean value of 78 and 12 solar luminosity/solar mass for interacting and isolated galaxies, respectively. The highest star formation efficiencies (SFEs) appear to occur in the merging and interacting pairs. The SFE in merging/interacting galaxies is greater than that found in the spiral arms of M51 and may be roughly proportional to the rate of cloud-cloud collisions in the interacting systems.

Young, Judith S.↗
NASA NTRS1987

Bimodal star formation - Constraints from the solar neighborhood

The chemical evolution resulting from a simple model of bimodal star formulation is investigated, using constraints from the solar neighborhood to set the parameters of the initial mass function and star formation rate. The two modes are an exclusively massive star mode, which forms stars at an exponentially declining rate, and a mode which contains stars of all masses and has a constant star formation rate. Satisfactory agreement with the age-metallicity relation for the thin disk and with the metallicity structure of the thin-disk and spheroid stars is possible only for a small range of parameter values. The preferred model offers a resolution to several of the long-standing problems of galactic chemical evolution, including explanations of the age-metallicity relation, the gas consumption time scale, and the stellar cumulative metallicity distributions.

Wyse, Rosemary F. G.↗
NASA NTRS1987

Core formation in the shergottite parent body and comparison with the earth

Abundances of elements in shergottite, nakhlite, and Chassigny meteorites which originated on a single planet, the shergottite parent body (SPB), were examined with the aim of elucidating the chemical conditions of metal separation and core formation in the SPB and of testing present models of planetary core formation. Using partition coefficients and the SPB mantle composition determined in earlier studies, the abundances of Ag, Au, Co, Ga, Mo, Ni, P, Re, S, and W were modeled, with free parameters being oxygen fugacity, proportion of solid metal formed, proportion of metallic liquid formed, and proportion of silicate that is molten. It is shown that the abundances of all elements (except Mo) could be reproduced using models with these four free parameters. In contrast to the SPB, an equivalent model used to predict element abundances in the earth's mantle was shown by Jones and Drake (1986) to be inadequate; there is at present no hypothesis capable of quantitatively reproducing the elemental abundances of the earth's mantle. The contrast suggests that these two terrestrial planets (assuming that the SPB is Mars) may have accreted or differentiated differently.

Treiman, Allan H.↗
NASA NTRS1987

The role of orbital dynamics and cloud-cloud collisions in the formation of giant molecular clouds in global spiral structures

The role of orbit crowding and cloud-cloud collisions in the formation of GMCs and their organization in global spiral structure is investigated. Both N-body simulations of the cloud system and a detailed analysis of individual particle orbits are used to develop a conceptual understanding of how individual clouds participate in the collective density response. Detailed comparisons are made between a representative cloud-particle simulation in which the cloud particles collide inelastically with one another and give birth to and subsequently interact with young star associations and stripped down simulations in which the cloud particles are allowed to follow ballistic orbits in the absence of cloud-cloud collisions or any star formation processes. Orbit crowding is then related to the behavior of individual particle trajectories in the galactic potential field. The conceptual picture of how GMCs are formed in the clumpy ISMs of spiral galaxies is formulated, and the results are compared in detail with those published by other authors.

Roberts, William W., Jr.↗
NASA NTRS1987

The effects of interactions on spiral galaxies. II - Disk star-formation rates

H-alpha emission-line and IRAS far-IR observations of interacting spiral and irregular galaxies are here used to assess the influence of interactions on their global star-formation rates. Two samples of interacting galaxies were observed: a complete sample of close pairs, and an Arp atlas sample of peculiar systems. When compared to a control sample of single galaxies, both samples of interacting systems exhibit systematically higher levels of H-alpha and infrared emission on average, and a larger dispersion in emission properties. Emission levels in the very active system are much more strongly correlated with the properties of the interaction than with the internal properties of the galaxies themselves. Strong disk emission is almost always accompanied by unusually strong nuclear activity. Simple star-formation burst models can reproduce the observed H-alpha equivalent widths and broadband colors of most of the galaxies. The bursts are relatively short (few times 10 million yr) and rarely involve more than 1-2 percent of a galaxy's total mass.

Kennicutt, Robert C., Jr.↗
NASA NTRS1987

Determining star formation rates in X-ray cluster cooling flows

Many X-ray clusters of galaxies are observed to have cooling flows at their centers. Each of these cooling flows is depositing mass onto a central dominant galaxy at a rate of 10-400 solar masses/yr. With such large accretion rates it seems possible that these accreting galaxies are still being formed through ongoing star formation in their associated cooling flows. In this paper techniques are developed to determine directly the distributions of local star formation rate, mass, gas density, temperature, and velocity from cooling flow X-ray surface brightness data. These techniques take account of the potentially important X-ray emission from star-forming cooling condensations dropping out of the background flow. Surface brightness data with either good or poor energy resolution are considered separately.

White, Raymond E., III↗
NASA NTRS1987

Numerical models of star formation in X-ray cluster cooling flows

A grid of numerical models is used here to study how accretion flows are affected by star formation. The observables associated with these models are investigated, presenting their X-ray surface brightness profiles. An X-ray diagnostic is searched for that, in the absence of spatially resolved X-ray spectral data, can be an unambiguous indicator for ongoing star formation in cooling flows. The spatial distributions of newly formed stars are presented and compared to profiles of the accreting galaxy in each model. Whether the accretion population can form the luminous parts of the accreting galaxy or whether it forms a dark component is considered.

White, Raymond E., III↗
NASA NTRS1987

Bimodal star formation - Constraints from galaxy colors at high redshift

The possibility that at early epochs the light from elliptical galaxies is dominated by stars with an initial mass function (IMF) which is deficient in low-mass stars, relative to the solar neighborhood is investigated. V-R colors for the optical counterparts of 3CR radio sources offer the most severe constraints on the models. Reasonable fits are obtained to both the blue, high-redshift colors and the redder, low-redshift colors with a model galaxy which forms with initially equal star formation rates in each of two IMF modes: one lacking low-mass stars, and one with stars of all masses. The net effect is that the time-integrated IMF has twice as many high-mass stars as the solar neighborhood IMF, relative to low mass stars. A conventional solar neighborhood IMF does not simultaneously account for both the range in colors at high redshift and the redness of nearby ellipticals, with any single star formation epoch. Models with a standard IMF require half the stellar population to be formed in a burst at low redshift z of about 1.

Wyse, Rosemary F. G.↗
NASA NTRS1987

Heterogeneous-nucleation and glass-formation studies of 56Ga2O3-44CaO

Glass formation and heterogeneous crystallization are described for the reluctant-glass-forming 56Ga2O3-44CaO eutectic composition. The times and temperatures for nucleation at various cooling rates and experimental conditions were measured and empirical continuous-cooling-crystallization boundaries were constructed for various heterogeneous nucleation processes. A definition for an empirical critical cooling rate to form a glass from reluctant borderline glass formers is proposed, i.e., the cooling rate that results in glass formation in 95 percent of the quenching experiments.

Ethridge, Edwin C.↗
NASA NTRS1986

Kink band formation in unidirectional composites under compression

The kink band formation in unidirectional composites under compression was studied with the help of optical and scanning electron microscopy. Materials studied included graphite/epoxy, S-2 glass/epoxy, and Kevlar 49/epoxy composites. The kink bands in the graphite/epoxy and glass/epoxy composites were initiated by fiber microbuckling while those in the Kevlar/epoxy composite were triggered by microkinking in the fibers themselves. Fracture surfaces of the graphite/epoxy composites showed fibers broken in bending and fiber segments tilted in the direction of slip. The kink band formation depended not only on the constituent material properties but also on the kinematic constraints imposed. Both primary and secondary kink bands were observed in the graphite/epoxy specimens, but only primary kink bands were present in the other two composites because of higher lateral flexibility of the specimen during testing.

Sohi, M. M.↗
NASA NTRS1987

Efficient near ultraviolet light induced formation of hydrogen by ferrous hydroxide

A possible origin of early hydrogen by UV-induced photoreduction of ferrous ions was investigated by measuring the rate of H2 formation from irradiated FeSO4 solutions as a function of pH and the range of UV sources. It was found that, in addition to the known reaction in acid solution which decreases in yield with increasing pH and requires far-UV light, there is an efficient reaction occurring between pH 6 and 9 which utilizes near-UV light (of a 200-W mercury arc lamp). This latter reaction is a linear function of both the concentration of Fe(2+) and the light intensity. These results support the suggestion of Braterman et al. (1983) that the near-UV-driven photooxidation of ferrous ions may be responsible for the origin of the banded iron formations on the early earth. The efficient photoreaction could also explain the source of reducing equivalents for CO2 reduction.

Borowska, Zofia K.↗