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At least 1,225 records · Page 68
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Planning repair sequences using the AND/OR graph representation of assembly plans

A simple modification is shown in the set of goal nodes of the AND/OR graph that allows its use in planning repairs such as the replacement of a part or a subassembly. An algorithm for the generation of all feasible sequences for disassembly and reassembly of parts that will achieve a repair is shown. This approach has been demonstrated for the example of the repair of space-based satellite equipment.

Homem De Mello, L. S.↗
NASA NTRS1988

Differential oscillator strengths and dipole polarizabilities for transitions of the helium sequence

The dipole radial integral for an initial discrete 1s state and a final continuum state has been calculated under the screened hydrogenic model. In this model, single-electron hydrogenic wave functions are employed, and the initial and the final states are treated by two different effective-charge parameters. Numerical values of differential oscillator strengths for transitions from 1s2 1S to the continuum for the helium sequence ions are obtained. Also calculated are the dipole polarizabilities, which are found to be in excellent agreement with the results of other authors.

Khan, F.↗
NASA NTRS1989

Soft X-ray observations of pre-main-sequence stars in the Chamaeleon dark cloud

X-ray observations of the Chamaeleon I cloud, a star-forming region, are reported. A total of 22 distinct X-ray sources, most associated with previously identified premain sequence stars, are found. The spatial distributions and HR diagrams of the stars are very similar, suggesting that they are coeval. Luminosity functions suggest that the stars have an average X-ray luminosity (Lx) several times that of the Pleiades dwarfs. The value of Lx is significantly correlated with optical magnitude, though no relation between X-ray emission and any photometric or emission line characteristic is present. It is suggested that a Skumanich-type power-law relation may be present over the entire range of stellar ages between 10 to the 6th and 10 to the 10th yr.

Feigelson, Eric D.↗
NASA NTRS1988

Capacity for patterns and sequences in Kanerva's SDM as compared to other associative memory models

The information capacity of Kanerva's Sparse Distributed Memory (SDM) and Hopfield-type neural networks is investigated. Under the approximations used here, it is shown that the total information stored in these systems is proportional to the number connections in the network. The proportionality constant is the same for the SDM and Hopfield-type models independent of the particular model, or the order of the model. The approximations are checked numerically. This same analysis can be used to show that the SDM can store sequences of spatiotemporal patterns, and the addition of time-delayed connections allows the retrieval of context dependent temporal patterns. A minor modification of the SDM can be used to store correlated patterns.

Keeler, James D.↗
NASA NTRS1988

Predicted and observed effects of stacking sequence and delamination size on instability-related delamination growth

A three-dimensional, geometrically nonlinear finite element analysis was used to calculate strain-energy release rate distributions for several laminates which exhibited instability-related delamination growth. Two stacking sequences, (0/90/90/0)6 and (90/0/0/90)6 were considered. The specimens were fabricated with a double layer of 0.013 mm Kapton film located between the fourth and fifth plies, thereby simulating a delamination. Initial delamination sizes were 30, 40, or 60 mm in diameter. The static strain corresponding to delamination growth was determined experimentally. Comparison of the analytical and experimental results indicated that delamination growth was governed by the magnitude of GI.

Whitcomb, John D.↗
NASA NTRS1989

Pre-main-sequence disk accretion in Z Canis Majoris

It is suggested that the pre-main-sequence object Z CMa is a luminous accretion disk, similar in many respects to the FU Orionis variables. Z CMa shows the broad, doubled optical absorption lines expected from a rapidly rotating accretion disk. The first overtone CO absorption detected in Z CMa is blue-shifted, suggesting line formation in a disk wind. Accretion at rates about 0.001 solar mass/yr over 100 yr is required to explain the luminosity of Z CMa. The large amount of material accreted (0.1 solar mass/yr) indicates that Z CMa is in a very early stage of stellar evolution, possibly in an initial phase of massive disk accretion.

Hartmann, L.↗
NASA NTRS1989

Detection of a dozen X-ray-emitting main-sequence B6-A3 stars in Orion

The detection of 12 X-ray-emitting main-sequence B6-A3 stars in the Orion Nebula is reported. The luminosity of these stars in the 0.2-3.5 keV band is considerably in excess of field stars of the same spectral type. Consideration is given to the possibilities that these stars may not actually be the source of the X-rays, but that the emission may be attributable to T Tauri or 'naked' T Tauri star companions, or that these hot stars are indeed the sources of emission and that current theories of X-ray emission from early-type stars must be amended.

Caillault, Jean-Pierre↗
NASA NTRS1988

Cool circumstellar matter around nearby main-sequence stars

Stars are presented which have characteristics similar to Vega and other main-sequence stars with cool dust disks, based on the IRAS Point Source Catalog fluxes. The objects are selected to have a 60-micron/100-micron ratio similar to Vega, Beta Pic, Alpha PsA, and Epsilon Eri, and they are also required to show evidence of extension in the IRAS Working Survey Database. The fluxes are modeled using a blackbody energy distribution. The temperatures derived range from 50 to 650 K. The diameters of the dust disks observed by IRAS are estimated.

Walker, H. J.↗
NASA NTRS1989

Circumstellar material associated with solar-type pre-main-sequence stars - A possible constraint on the timescale for planet building

The observed frequency distribution of near-IR excesses for a sample of 83 stars in the Taurus-Auriga star-forming complex is used to provide an estimate of the number of solar-type premain-sequence (PMS) stars surrounded by circumstellar disks. The results suggest that if all such PMS stars are initially surrounded by disks, then disk lifetimes must range from much less than 3 x 10 to the 6th yr to about 10 to the 7th yr, providing a constraint on the time available for planet building. PMS stars with small near-IR excesses, but significant mid-IR and far-IR excesses, provide some evidence for changes in disk structure with time.

Strom, Karen M.↗
NASA NTRS1989

On the synchronizability and detectability of random PPM sequences

The problem of synchronization and detection of random pulse-position modulation (PPM) sequences is investigated under the assumption of perfect slot synchronization. Maximum-likelihood PPM symbol synchronization and receiver algorithms are derived that make decisions based on both soft as well as hard data; these algorithms are seen to be easily implementable. Bounds were derived on the symbol error probability as well as the probability of false synchronization that indicate the existence of a rather severe performance floor, which can easily be the limiting factor in the overall system performance. The performance floor is inherent in the PPM format and random data and becomes more serious as the PPM alphabet size Q is increased. A way to eliminate the performance floor is suggested by inserting special PPM symbols in the random data stream.

Georghiades, Costas N.↗
NASA NTRS1989

Pre-main-sequence stars in the young cluster IC 2391

Seven or eight new, late-type members of the poor open cluster IC 2391 are identified, and membership is confirmed for two other stars. The new members fall approximately along a 3 x 10 to the 7th yr isochrone, which is the age estimated for the cluster on the basis of it super main-seqence turnoff. Echelle spectra were obtained for the most probable cluster members. Most show H-alpha in emission and a strong Li 6707 A absorption line, and a few are rapid rotators. The Li abundances for cluster stars cooler than the sun are considerably less than the primordial Li abundance, providing the first direct evidence for substantial premain-sequence Li burning. The rotational velocities show a range from about 15 to 150 km/s, with a distribution of rotational velocities not significantly different from that observed for low-mass stars in the Pleiades.

Stauffer, John↗
NASA NTRS1989

Laser gain on 3p-3d and 3s-3p transitions and X-ray line ratios for the nitrogen isoelectronic sequence

Results are presented on calculations of the 72 levels belonging to the 2s(2)2p(3), 2s2p(4), 2p(5), 2s(2)2p(2)3s, 2s(2)2p(2)3p, and 2s(2)2p(2)3d configurations of the N I isoelectronic sequence for the ions Ar XII, Ti XVI, Fe XX, Zn XXIV, and Kr XXX, for electron densities up to 10 to the 24th/cu cm. It was found that large population inversions and gain occur between levels in the 2s(2)2p(2)3p configuration and levels in the 2s(2)2p(2)3d configuration that cannot decay to the ground configuration by an electric dipole transition. For increasing electron densities, the intensities of the X-ray transitions from the 2s(2)2p(2)3p configuration to the ground configuration decrease relative to the transitions from the 2s(2)2p(2)3s and 2s(2)2p(2)3d configurations to the ground configuration. The density dependence of these X-ray line ratios is presented.

Feldman, U.↗
NASA NTRS1987

Capacity for patterns and sequences in Kanerva's SDM as compared to other associative memory models

The information capacity of Kanerva's Sparse Distributed Memory (SDM) and Hopfield-type neural networks is investigated. Under the approximations used, it is shown that the total information stored in these systems is proportional to the number connections in the network. The proportionality constant is the same for the SDM and Hopfield-type models independent of the particular model, or the order of the model. The approximations are checked numerically. This same analysis can be used to show that the SDM can store sequences of spatiotemporal patterns, and the addition of time-delayed connections allows the retrieval of context dependent temporal patterns. A minor modification of the SDM can be used to store correlated patterns.

Keeler, James D.↗
NASA NTRS1989

Precedence relationship representations of mechanical assembly sequences

Two types of precedence relationship representations for mechanical assembly sequences are presented: precedence relationships between the establishment of one connection between two parts and the establishment of another connection, and precedence relationships between the establishment of one connection and states of the assembly process. Precedence relationship representations have the advantage of being very compact. The problem with these representations was how to guarantee their correctness and completeness. Two theorems are presented each of which leads to the generation of one type of precedence relationship representation guaranteeing its correctness and completeness for a class of assemblies.

Homendemello, L. S.↗
NASA NTRS1989

Active phenomena in the pre-main sequence Herbig Ae star HD 163296

Observations by IUE of the short-term variability of the Mg II and Ca II resonance lines in the Herbig Ae star HD 163296 are presented. Evidence that these lines show a phenomenon of rotational modulation, similar to the one observed in AB Aur, another Herbig Ae star is found. The variations in the spectrum of HD 163296 are even more conspicuous than in the spectrum of AB Aur. Magnetically structured winds may thus be a widespread phenomenon among the pre-main sequence Herbig Ae/Be stars.

Catala, C.↗
NASA NTRS1989

Automatic sequencing and control of Space Station airlock operations

Procedures that have been developed as part of the NASA JSC-sponsored pre-prototype Checkout, Servicing and Maintenance (COSM) program for pre- and post-EVA airlock operations are described. This paper addresses the accompanying pressure changes in the airlock and in the Advanced Extravehicular Mobility Unit (EMU). Additionally, the paper focuses on the components that are checked out, and includes the step-by-step sequences to be followed by the crew, the required screen displays and prompts that accompany each step, and a description of the automated processes that occur.

Himel, Victor↗
NASA NTRS1990

Evolution and detectability of comet clouds during post-main-sequence stellar evolution

The destruction of volatile-rich comet disks and Oort-type clouds around luminous post-main-sequence stars is modeled. The models are in agreement with several aspects of existing observations of water and complex molecules in the envelopes of giant and supergiant stars. If confirmed, these results would establish the common existence of Oort-type clouds around other stars and would constitute indirect evidence for sites of past planetary formation.

Stern, S. Alan↗