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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 361 records · Page 20
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NASA NTRS1991

Detection of silicates in the Beta Pictoris disk

A broad 10-micron emission feature has been detected within 3 arcsec of the star Beta Pic which is known to have a circumstellar, possibly protoplanetary, disk. The observations were made using four narrow-band filters in the MSFC bolometer array at the IRTF. The emission feature closely resembles silicate features observed in Comets Halley and Kohoutek and in a variety of Galactic sources. The feature is strong, with the total flux density emitted by dust at 10.3 microns being at least twice the estimated continuum at that wavelength, and must be emitted by particles with characteristic radii less than 10 microns.

Telesco, C. M.↗
NASA NTRS1990

Nebular properties of proto-planetary nebulae

Recent ground-based observations of cool IRAS sources have led to the discovery of many candidates for protoplanetary nebulae (PPN). These objects have cool dust shells and molecular envelopes reminiscent of the circumstellar envelopes of asymptotic giant branch (AGB) stars. Observations of PPN confirm that the circumstellar envelope ejected during the AGB phase dominates the infrared continuum of post-AGB objects. It is suggested that an infrared sequence can be traced throughout the evolutionary phases from AGB to planetary nebulae.

Kwok, Sun↗
NASA NTRS1990

Optical studies of proto-planetary nebulae

A group of cool protoplanetary nebulae (PPN) candidates with bright optical counterparts have been studied. IRAS and ground-based data have been combined for about a dozen of these, and double-peaked flux distribution have been found and fitted with a PPN model. They each possess the spectral types expected for objects in this evolutionary phase. For an additional dozen new, noncataloged sources under similar study, carbon-rich spectra confirms their post-AGB nature.

Hrivnak, Bruce J.↗
NASA NTRS1991

Abundances in red giant stars - Nitrogen isotopes in carbon-rich molecular envelopes

Results are presented of millimeter- and submillimeter-wave observations of HCN and HCCCN that were made of the circmustellar envelopes of eight carbon stars, including the two protoplanetary nebulae CRL 618 and CRL 2688. The observations yield a measure of the double ratio (N-14)(C-13)/(N-15)(C-12). Measured C-12/C-13 ratios are used to estimate the N-14/N-15 abundance ratio, with the resulting lower limits in all eight envelopes and possible direct determinations in two envelopes. The two determinations and four of the remaining six lower limits are found to be in excess of the terrestrial value of N-14/N-15 = 272, indicating an evolution of the nitrogen isotope ratio, which is consistent with stellar CNO processing. Observations of thermal SiO (v = 0, J = 2-1) emission show that the Si-29/Si-28 ratio can be determined in carbon stars, and further observations are indicated.

Wannier, P. G.↗
NASA NTRS1991

Clay minerals in primitive meteorites and interplanetary dust 1

Many meteorites and interplanetary dust particles (IDPs) with primitive compositions contain significant amounts of phyllosilicate minerals, which are generally interpreted as evidence of protoplanetary aqueous alteration at an early period of the solar system. These meteorites are chondrites (near solar composition) of the carbonaceous and ordinary varieties. The former are subdivided (according to bulk composition and petrology) into CI, CM, CV, CO, CR, and ungrouped classes. IDPs are extraterrestrial particulates, collected in stratosphere, which have chemical compositions indicative of a primitive origin; they are typically distinct from the primitive meteorites. Characterization of phyllosilicates in these materials is a high priority because of the important physico-chemical information they hold. The most common phyllosilicates present in chondritic extraterrestrial materials are serpentine-group minerals, smectites, and micas. We discuss these phyllosilicates and describe the interpretation of their occurrence in meteorites and IDPs and what this indicates about history of their parent bodies, which are probably the hydrous asteroids.

Zolensky, M. E.↗
NASA NTRS1991

A reexamination of early numerical simulations of planetary accretion

The intermediate stage of protoplanetary accretion, where sub-km sized bodies accrete in 500-1000 km diameter bodies, is critical in determining the character of subsequent evolution. Greenberg et al. discovered that, at least through the formation of 500 km planetary embryos, the small particle dominate the population and control the velocity distribution, keeping velocities much smaller and setting up the possibility of runaway accretion, where the largest body is able to accumulate quickly a much greater mass and separate itself from the continuum of the size distribution. However, some controversy remains as to the exact methods used by Greenberg et al. I reviewed that planet formation model in detail to address these criticisms and better explain the algorithm used in the study. In particular, the algorithms for mutual velocity stirring among planetesimals and mass shifting between adjacent diameter bins are rederived in detail. I also examined the likelihood of artificial acceleration of the runaway growth phase of the intermediate stage of planetesimal formation.

Kolvoord, R. A.↗
NASA NTRS1991

Accretion rates of protoplanets

The rate at which planetesimals in a uniform surface density disk collide with, and are (assumed to be) accreted by, a massive protoplanet is calculated. The collision cross-section of a protoplanet is enhanced relative to its geometric cross-section due to its gravitational focusing of planetesimal trajectories. The gravitation enhancement (eccentricities and inclinations) decrease. For large random velocity planetesimals, encounters are sufficiently rapid (less than or approximately = 5 percent of and orbital period) that F(sub g) is well approximated by the two-body particle in a box formula, which neglects the gravitational effect of the Sun. As planetesimal velocities decrease, F(sub g) increases to approximately twice the two-body value, and then rises less rapidly than the two-body value, eventually dropping below it and asymptotically approaching a constant for sufficiently small random velocities. A scaling argument is presented that generalizes the results to protoplanets of arbitrary mass, radius, and orbital semimajor axis. Gravitational scatterings by a protoplanet prevent random velocities of the planetesimals within its accretion zone from becoming to small. When gravitational stirring is included, the maximum plausible value of the gravitation enhancement factor for rock protoplanets 1 Au from the Sun is F(sub g) of approximately 1000. If one protoplanet dominates gravitational scatterings in a given region of a protoplanetary disk, then it was found that the planetesimal inclinations are excited much less rapidly than eccentricities, in contrast to the two-body approximation, in which energy is roughly equipartitioned between eccentric and inclined random motions. The resulting skewed velocity dispersion allows for a more rapid rate of protoplanet growth.

Greenzweig, Yuval↗
NASA NTRS1991

Generation and behavior of solar system magnetic fields

A range of problems aimed primarily at elucidating the character and consequences of magnetic-field generation in the solar system and at testing ideas against known properties of natural magnetic fields are covered. Among the specific subjects currently under investigation are the magnetohydrodynamic (MHD) character of the protosolar nebula, including the generation and behavior of magnetic fields, the electrical conductivity of dusty nebular gas, transient magnetodynamic and electrodynamic nebular phenomena, and the generation and dynamical behavior of planetary magnetic fields. During the past year, the focus of this research was on disk magnetic field generation and transient MHD heating events possibly associated with the protoplanetary nebula.

Levy, Eugene H.↗
NASA NTRS1990

Turbulent transport in the solar nebula

This paper describes the current state of an ongoing project to simulate turbulent flow in a solar nebula, which is the flattened disk of dust and gas out of which a solar system forms. The goal of this project is to determine a model for the transport of mass and angular momentum in the nebula. The nebula flow exhibits compressibility, thermal conduction, viscosity, internal heating through viscous dissipation, a stable shear due to Keplerian rotation, and a gravitational acceleration in the vertical direction which is linear with altitude. These properties combine to give flow patterns not seen in terrestrial applications. Primordial solar systems are known to exist and are presumably undergoing an evolution similar to the early stages of our own solar system; for example, the IRAS infrared telescope has discovered such a protoplanetary system around the star Vega. Solar nebula evolution is the subject of much research in the astrophysical community. In the long run, researchers hope to gain a better understanding of planetary formation and the processes which dissipate the solar nebula with time.

Thompson, K. W.↗
NASA NTRS1992

Cometary origin of carbon and water on the terrestrial planets

An early high-temperature phase of the protosolar accretion disk is implied by at least three different telltales in chondrites and confirmed by peculiarities in the dust grains of Comet Halley. The existence of this high-temperature phase implies a large accretion rate hence a massive early disk. This clarifies the origin of the Kuiper Belt and of the Oort cloud, those two cometary populations of different symmetry that subsist today. Later, when the dust sedimented and was removed from the thermal equilibrium with the gas phase, a somewhat lower temperature of the disk explains the future planets' densities as well as the location beyond 2.6 AU of the carbonaceous chondrite chemistry. This lower temperature remains however large enough to require an exogenous origin for all carbon and all water now present in the earth. The later orbital diffusion of planetesimals, which is required by protoplanetary growth, is needed to explain the origin of the terrestrial biosphere (atmosphere, oceans, carbonates and organic compounds) by a veneer mostly made of comets.

Delsemme, A. H.↗
NASA NTRS1992

Near-infrared observations of the proto-planetary nebula AFGL 618

Near-IR images and spectroscopy of the bipolar protoplanetary nebula AFGL 618 are presented. Emission from molecular hydrogen is detected in the 0.90-1.34-micron spectrum. A visual extinction of 3.4 +4.0/-2.0 mag to the H2-emitting region is derived from molecular hydrogen line ratios. Models of collisionally excited and fluorescent H2 emission are compared with the data: the near-IR H2 spectrum is found to be dominated by emission from collisionally excited molecules at Tex of about 2000 K, but a component of fluorescent emission is also present. It is shown that this type of near-IR spectrum is an excellent tool for discriminating relatively low levels of fluorescent H2 emission from a strong collisionally excited component. Images in the H- and K-bandpasses are analyzed using models which assume single scattering of photons from the central objects. The models indicate that the bipolar axis is inclined to the plane of the sky by about 45 deg. The emission in the H- and K-bands is found to be consistent with single scattering of photons by dust with a distribution such that the dust density increases with increasing stellar latitude and is nearly zero at the poles.

Latter, William B.↗
NASA NTRS1992

Comets: Role and importance to exobiology

The transfer of organic compounds from interstellar space to the outskirts of a protoplanetary disk, their accretion into cometary objects, and the transport of the latter into the inner solar system by orbital diffusion throw a new light on the central problem of exobiology. It suggests the existence of a cosmic mechanism, working everywhere, that can supply prebiotic compounds to ubiquitous rocky planets, in search of the proper environment to start life in many places in the Universe. Under the heading of chemistry of the cometary nucleus, the following topics are covered: radial homogeneity of the nucleus; the dust-to-ice ratio; nature of the dust grains; origin of the dust in comets; nature of the volatile fraction; the CO distribution in comet Halley; dust contribution to the volatile fraction; elemental balance sheet of comet Halley; quantitative molecular analysis of the volatile fraction; and isotopic ratios. Under the heading of exogenous origin of carbon on terrestrial planets the following topics are covered: evidence for a high-temperature phase; from planetesimals to planets; a veneer of volatile and organic material; and cometary contribution.

Delsemme, Armand H.↗
NASA NTRS1992

Carbon petrology in cometary dust

Chondritic porous (CP) interplanetary dust particles (IDP's) are collected in the Earth's stratosphere. There exists an extensive database on major and minor element chemistry, stable isotopes, noble gas abundances and mineralogy of many CP IDP's, as well as infrared and Raman spectroscopic properties. For details on the mineralogy, chemistry and physical properties of IDP's, I refer to the reviews by Mackinnon and Rietmeijer (1987), Bradley et al. (1988) and Sandford (1987). Texture, mineralogy (Mackinnon and Rietmeijer, 1987) and chemistry (Schramm et al., 1989; Flynn and Sutton, 1991) support the notion that CP IDP's are a unique group of ultrafine-grained extraterrestiral materials that are distinct from any known meteorite class. Their fluffy, or porous, morphology suggests that CP IDP's probably endured minimal alteration by protoplanetary processes since their formation. It is generally accepted that CP IDP's are solid debris from short-period comets. The evidence is mostly circumstantial but this notion gained significant support based on the comet Halley dust data (Brownlee, 1990). In this paper, I will accept that CP IDP's are indeed cometary dust. The C/Si ratio in CP IDP's is 3.3 times higher than in CI carbonaceous chondrites (Schramm et al. 1989). The intraparticle carbon distribution is heteorogeneous (Rietmeijer and McKay, 1986). Carbon occurs both in oxidized and reduced forms. Analytical electron microscope (AEM) and Raman spectroscopic analyses have shown the presence of several carbon forms in CP IDP's but the data are scattered in the literature. Carbons in cometary CP IDP's are among the most pristine Solar System carbons available for laboratory study. Similar to a recently developed petrological model for the diversity of layer silicates in CP IDP's (Zolensky, 1991) that is useful to constrain in situ aqueous alteration in comets (Rietmeijer and Mackinnon, 1987a), I here present the first effort to develop a petrological concept of carbons in CP IDP's. This concept is useful to constrain comet evolution. I also present the philosophical constraint facing Earth Scientists in studies of protoplanets that require a new approach to cometary dust studies.

Rietmeijer, Frans J. M.↗
NASA NTRS1992

Submillimeter line observations of the proto-planetary nebula CRL 2688

The protoplanetary nebula CRL 2688 with 14-arcsec resolution was observed, and the envelope in CO, (C-13)O, CS, and H(C-13)N was mapped. The submillimeter lines show strong emission from two components of the envelope: a cold, dense, optically thick slow wind and a warm, optically thin fast wind. In the maps, the slow wind appears compact and circularly symmetric. A mass-loss rate in the fast wind of 0.00002 solar mass/yr is estimated, if the wind was continuous. The C-12/C-13 ratio in the fast wind is 5, compared to about 20 in the slow wind. The value in the HCN/CO abundance ratio may be lower in the fast wind than in the slow wind. It is argued that the velocity gradients are not caused by rotation, but rather by asymmetries in the envelope associated with the fast wind.

Jaminet, P. A.↗
NASA NTRS1993

Detection of accreting gas toward HD 45677 - A newly recognized, Herbig Be proto-planetary system

We report the detection of high-velocity, accreting gas toward the BE(e) star with IR excess and bipolar nebula, HD 45677. High-velocity (+200 to +400 km/s), variable column density gas is visible in all IUE spectra from 1979 to 1992 in transitions of Si II, C II, Al III, Fe III, Si IV, and C IV. Low-velocity absorption profiles from low oscillator strength transitions of Si II, Fe II, and Zn II exhibit double-peaked absorption profiles similar to those previously reported in optical spectra of FU Orionis objects. The UV absorption data, together with previously reported analyses of the IR excess and polarization of this object, suggest that HD 45677 is a massive, Herbig Be star with an actively accreting circumstellar, protoplanetary disk.

Grady, C. A.↗
NASA NTRS1993

Orbital simulations of satellite escape/capture and the origin of satellites such as Triton

We investigate satellite escape/capture in the context of the restricted, circular three body problem as applied to the Sun, Neptune, and Triton. We have computed a large number of coplanar prograde and retrograde orbital simulations over a range of initial distances and velocities. The satellite starts at superior conjunction within approximately 2 Hill radii of Neptune and has a velocity orthogonal to the Sun-planet line. Orbits with these initial conditions can be reflected with respect to time, so an escape is simply the reverse of a capture. We numerically integrate the equations of motion to compute the satellite's position until it escapes, collides with Neptune, or after 100 planetary years fails to escape, when computations cease. The initial distance x and velocity v in the restricted problem uniquely define the Jacobi constant C, a conserved energy-like quantity. Plots of the simulation outcomes in the prograde and retrograde C, x phase spaces reveal distinct zones in which temporary satellites approach the planet closely enough that permanent capture can be effected by gas drag with a protoplanetary nebula or by collision with a pre-existing satellite. Single and double close-flybys constitute the most common possible capture orbits. Long term multiple flyby orbits occur near the stability limits between bound and unbound orbits, and are more common among retrograde captures.

Benner, Lance A. M.↗
NASA NTRS1992

Physical properties of gas disks around shell stars with and without dust

Analysis of archival IRAS and IUE data has resulted in: (1) identification of 8 new A star proto-planetary candidates; (2) detection of a mass outflow event around Beta Pic (subsequently confirmed by the 1991 July HST observation); and (3) confirmation of the suggestion by Waters et al. (1988) that 51 Oph is a protoplanetary system similar to beta Pic with the detection of high density, high velocity, collisionally ionized accreting gas in the line of sight toward this star.

Grady, Carol A.↗
NASA NTRS1993

Gas-grain energy transfer in solar nebula shock waves: Implications for the origin of chondrules

Meteoritic chondrules provide evidence for the occurrence of rapid transient heating events in the protoplanetary nebula. Astronomical evidence suggests that gas dynamic shock waves are likely to be excited in protostellar accretion disks by processes such as protosolar mass ejections, nonaxisymmetric structures in an evolving disk, and impact on the nebula surface of infalling 'clumps' of circumstellar gas. Previous detailed calculations of gas-grain energy and momentum transfer have supported the possibility that such shock waves could have melted pre-existing chondrule-sized grains. The main requirement for grains to reach melting temperatures in shock waves with plausibly low Mach numbers is that grains existed in dust-rich zones (optical depth greater than 1) where radiative cooling of a given grain can be nearly balanced by radiation from surrounding grains. Localized dust-rich zones also provide a means of explaining the apparent small spatial scale of heating events. For example, the scale size of at least some optically thick dust-rich zones must have been relatively small (less than 10 kilometers) to be consistent with petrologic evidence for accretion of hot material onto cold chondrules. The implied number density of mm-sized grains for these zones would be greater than 30 m(exp -3). In this paper, we make several improvements of our earlier calculations to include radiation self-consistently in the shock jump conditions, and we include heating of grains due to radiation from the shocked gas. In addition, we estimate the importance of momentum feedback of dust concentrations onto the shocked gas which would tend to reduce the efficiency of gas dynamic heating of grains in the center of the dust cloud.

Hood, L. L.↗