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NASA NTRS1992

IRAS observations of novae. III - The related objects

Results are presented on an analysis of IRAS observations of a selected sample of 116 objects that show properties similar to those of classical novae detected and modeled by Harrison and Gehrz (1988, 1991). These objects include dwarf novae, novalike variables, symbiotic stars, and planetary nebulae with close binary central-star systems. A high detection rate of 80 percent was found for this sample, as compared with about 50 percent detection rate found in the earlier examined sample. A reexamination of data showed that, at low galactic latitudes, source confusion can account for many of the detections of novae. The seemingly high detection rates of other objects and the many high-latitude novae are difficult to explain.

Harrison, Tom↗
NASA NTRS1992

Object impedance control for cooperative manipulation - Theory and experimental results

This paper presents the dynamic control module of the Dynamic and Strategic Control of Cooperating Manipulators (DASCCOM) project at Stanford University's Aerospace Robotics Laboratory. First, the cooperative manipulation problem is analyzed from a systems perspective, and the desirable features of a control system for cooperative manipulation are discussed. Next, a control policy is developed that enforces a controlled impedance not of the individual arm endpoints, but of the manipulated object itself. A parallel implementation for a multiprocessor system is presented. The controller fully compensates for the system dynamics and directly controls the object internal forces. Most importantly, it presents a simple, powerful, intuitive interface to higher level strategic control modules. Experimental results from a dual two-link-arm robotic system are used to compare the object impedance controller with other strategies, both for free-motion slews and environmental contact.

Schneider, Stanley A.↗
NASA NTRS1992

Model scattering envelopes of young stellar objects. I - Method and application to circumstellar disks

We describe a Monte Carlo code that accurately treats multiple scattering, absorption, and polarization by dust, and use this code to calculate images of dusty disks around young stellar objects. We present some approximate analytic results that describe the behavior of the Monte Carlo calculations. A geometrically thin disk illuminated by a central T Tauri star scatters very little light at distances of many AU from the star. Viewed at any inclination, the flux scattered by such a disk at the distance to the nearest star-forming region will be overwhelmed by the stellar image. An optically thick disk that has a flaring surface may be observable, especially if viewed nearly edge-on so that the stellar source becomes occulted. An optically thin disk with a finite opening angle, similar to the one surrounding beta Pictoris, is about as observable as the typical flared optically thick disk at a similar distance from the earth. The polarization position angle is perpendicular to the disk plane in all of the models, in contrast to observations of many young stellar objects which have the position angle oriented parallel to the presumed disk plane. We suggest that the scattered light structures observed around many premain-sequence objects are dusty envelopes rather than disks.

Whitney, Barbara A.↗
NASA NTRS1992

A survey of commercial object-oriented database management systems

The object-oriented data model is the culmination of over thirty years of database research. Initially, database research focused on the need to provide information in a consistent and efficient manner to the business community. Early data models such as the hierarchical model and the network model met the goal of consistent and efficient access to data and were substantial improvements over simple file mechanisms for storing and accessing data. However, these models required highly skilled programmers to provide access to the data. Consequently, in the early 70's E.F. Codd, an IBM research computer scientists, proposed a new data model based on the simple mathematical notion of the relation. This model is known as the Relational Model. In the relational model, data is represented in flat tables (or relations) which have no physical or internal links between them. The simplicity of this model fostered the development of powerful but relatively simple query languages that now made data directly accessible to the general database user. Except for large, multi-user database systems, a database professional was in general no longer necessary. Database professionals found that traditional data in the form of character data, dates, and numeric data were easily represented and managed via the relational model. Commercial relational database management systems proliferated and performance of relational databases improved dramatically. However, there was a growing community of potential database users whose needs were not met by the relational model. These users needed to store data with data types not available in the relational model and who required a far richer modelling environment than that provided by the relational model. Indeed, the complexity of the objects to be represented in the model mandated a new approach to database technology. The Object-Oriented Model was the result.

Atkins, John↗
NASA NTRS1992

Method and apparatus for evaluating multilayer objects for imperfections

A multilayer object where the layers are arranged in a stacking direction is evaluated for imperfections such as voids, delaminations, and microcracks. First, an acoustic wave is transmitted into the object in the stacking direction via an appropriate transducer/waveguide combination. The wave propagates through the multilayer object and is received by another transducer/waveguide combination preferably located on the same surface as the transmitting combination. The received acoustic wave is correlated with the presence or absence of imperfections by generating pulse echo signals indicative of the received acoustic wave, wherein the successive signals form distinct groups over time. The respective peak amplitudes of each group are sampled and fitted to an exponential curve, wherein a substantial fit of approximately 80-90 percent indicates the absence of imperfections. Alternatively, the time interval between distinct groups can be measured, wherein equal intervals indicate the absence of imperfections and unequal intervals indicate the presence of imperfections.

Heyman, Joseph S.↗
NASA NTRS1992

International Ultraviolet Explorer (IUE) ultraviolet spectral atlas of selected astronomical objects

The IUE Ultraviolet Spectral Atlas of Selected Astronomical Objects (or 'the Atlas'), is based on the data that were available in the IUE archive in 1986, and is intended to be a quick reference for the ultraviolet spectra of many categories of astronomical objects. It shows reflected sunlight from the Moon, planets, and asteroids, and also shows emission from comets. Comprehensive compilations of UV spectra for main sequence, subgiant, giant, bright giant, and supergiant stars are published elsewhere. This Atlas contains the spectra for objects occupying other areas of the Hertzsprung-Russell diagram: pre-main sequence stars, chemically peculiar stars, pulsating variables, subluminous stars, and Wolf-Rayet stars. This Atlas also presents phenomena such as the chromospheric and transition region emissions from late-type stars; composite spectra of stars, gas streams, accretion disks and gas envelopes of binary systems; the behavior of gas ejecta shortly after the outburst of novac and supernovac; and the H II regions, planetary nebulae, and supernova remnants. Population 2 stars, globular clusters, and luminous stars in the Magellanic Clouds, M31, and M33, are also included in this publication. Finally, the Atlas gives the ultraviolet spectra of galaxies of different Hubble types and of active galaxies.

Wu, Chi-Chao↗
NASA NTRS1992

Multi-axis transient vibration testing of space objects: Test philosophy, test facility, and control strategy

IABG has been using various servohydraulic test facilities for many years for the reproduction of service loads and environmental loads on all kinds of test objects. For more than 15 years, a multi-axis vibration test facility has been under service, originally designed for earthquake simulation but being upgraded to the demands of space testing. First tests with the DFS/STM showed good reproduction accuracy and demonstrated the feasibility of transient vibration testing of space objects on a multi-axis hydraulic shaker. An approach to structural qualification is possible by using this test philosophy. It will be outlined and its obvious advantages over the state-of-the-art single-axis test will be demonstrated by example results. The new test technique has some special requirements to the test facility exceeding those of earthquake testing. Most important is the high reproduction accuracy demanded for a sophisticated control system. The state-of-the-art approach of analog closed-loop control circuits for each actuator combined with a static decoupling network and an off-line iterative waveform control is not able to meet all the demands. Therefore, the future over-all control system is implemented as hierarchical full digital closed-loop system on a highly parallel transputer network. The innermost layer is the digital actuator controller, the second one is the MDOF-control of the table movement. The outermost layer would be the off-line iterative waveform control, which is dedicated only to deal with the interaction of test table and test object or non-linear effects. The outline of the system will be presented.

Lachenmayr, Georg↗
NASA NTRS1993

Gas turbine system simulation: An object-oriented approach

A prototype gas turbine engine simulation has been developed that offers a generalized framework for the simulation of engines subject to steady-state and transient operating conditions. The prototype is in preliminary form, but it successfully demonstrates the viability of an object-oriented approach for generalized simulation applications. Although object oriented programming languages are-relative to FORTRAN-somewhat austere, it is proposed that gas turbine simulations of an interdisciplinary nature will benefit significantly in terms of code reliability, maintainability, and manageability. This report elucidates specific gas turbine simulation obstacles that an object-oriented framework can overcome and describes the opportunity for interdisciplinary simulation that the approach offers.

Drummond, Colin K.↗
NASA NTRS1993

A model-based approach for detection of objects in low resolution passive millimeter wave images

A model-based vision system to assist the pilots in landing maneuvers under restricted visibility conditions is described. The system was designed to analyze image sequences obtained from a Passive Millimeter Wave (PMMW) imaging system mounted on the aircraft to delineate runways/taxiways, buildings, and other objects on or near runways. PMMW sensors have good response in a foggy atmosphere, but their spatial resolution is very low. However, additional data such as airport model and approximate position and orientation of aircraft are available. These data are exploited to guide our model-based system to locate objects in the low resolution image and generate warning signals to alert the pilots. Also analytical expressions were derived from the accuracy of the camera position estimate obtained by detecting the position of known objects in the image.

Kasturi, Rangachar↗
NASA NTRS1992

Herbig-Haro objects in the receding lobe of the L 1551 outflow

A spectrum has been obtained of two Herbig-Haro objects which are seen against the receding lobe of the bipolar outflow within the dark cloud Lynds 1551. Positive heliocentric velocities up to 90 km/s have been measured from the H-alpha line which point to an association of these emission knots with the embedded infrared source L 1551-IRS 5 rather than with other young stellar objects in this part of the sky. There is a velocity range of 50-100 km/s within each object. (S II) lambda 6716 is also detected at a strength of about 50 pct of H-alpha. Along the entire length of the slit there is broad H-alpha emission with strength about four times that normally seen in emission from the night sky. This feature partially resolves into two components, one of which we suggest is from the general Galactic field, and the other from extended bow-shock emission.

Graham, J. A.↗
NASA NTRS1993

Ultraviolet polarimetry and spectroscopy of the BL Lacertae object PKS 2155-304

HST and Faint Object Spectrograph spectropolarimetry is presented for the BL Lac object PKS 2155-304; attention is given to the wavelength dependence of the UV linear polarization in a BL Lac object. The UV polarimetry yields evidence that the UV polarized flux is generated by the synchrotron mechanism responsible for optical polarization. Both the UV and optical emission are produced in the same region of the source.

Allen, Richard G.↗
NASA NTRS1993

Ultraviolet spectra of QSOs, BL Lacertae objects, and Seyfert galaxies

Motivated by the possibility of detecting strong absorption features toward extragalactic background objects, we present optimally extracted and co-added IUE spectra of QSOs, BL Lac objects, and Seyfert galaxies. These spectra together with those presented (in a similar work) by Kinney et al. form an essentially complete sample of the SWP, LWP, and LWR observations of QSOs, BL Lac objects, and Seyfert galaxies with redshifts z greater than 0.05 available in the IUE merged observation log as of 1991 December.

Lanzetta, Kenneth M.↗
NASA NTRS1992

Time-dependent inhomogeneous jet models for BL Lac objects

Relativistic beaming can explain many of the observed properties of BL Lac objects (e.g., rapid variability, high polarization, etc.). In particular, the broadband radio through X-ray spectra are well modeled by synchrotron-self Compton emission from an inhomogeneous relativistic jet. We have done a uniform analysis on several BL Lac objects using a simple but plausible inhomogeneous jet model. For all objects, we found that the assumed power-law distribution of the magnetic field and the electron density can be adjusted to match the observed BL Lac spectrum. While such models are typically unconstrained, consideration of spectral variability strongly restricts the allowed parameters, although to date the sampling has generally been too sparse to constrain the current models effectively. We investigate the time evolution of the inhomogeneous jet model for a simple perturbation propagating along the jet. The implications of this time evolution model and its relevance to observed data are discussed.

Marlowe, A. T.↗
NASA NTRS1993

Mission objectives and comparison of strategies for Mars exploration

Over the past several years, a number of candidate scenarios for the human exploration of Mars have been advanced. These have had a range of mission objectives, scope, scale, complexity and probable cost. The Exploration Programs Office (ExPO) has developed a reference Mars exploration program and a means of comparing it to other proposed Mars programs. The reference program was initiated in a workshop of Mars exploration advocates which defined two objectives of equal importance for early Mars exploration - understanding Mars and understanding the potential of Mars to support humans. These goals were used to define a set of transportation and surface elements that could carry out a robust exploration program. The approach to comparing alternate architectures has three principal parts: (1) Bringing the architectures into rough commonality in terms of surface mission objectives and hardware capabilities; (2) Providing a common level of human support for flights to and from Mars; and (3) Comparing the complexity of the elements needed to carry out the program and using partial redundancy to approximate the same statistical probability of mission success. This top-level approach has been applied to the ExPO reference program, the 'Mars Directs strategy (Zubrin, 1991) and the Stanford International Mars Mission (Lusignan, 1992).

Duke, Michael B.↗
NASA NTRS1993

BL Lacertae objects, Fanaroff-Riley type I radio sources, and cluster cooling flows

The correctness of the view that the Fanaroff-Riley type I (F-R I) radio galaxies and the BL Lac objects are intrinsically the same and that the strong emission seen in BL Lac may be the result of beaming of radiation from the galactic nucleus is examined. Expressions are derived for the surface brightness of scattered beams of polarized or unpolarized radiation, and the expected scattered light surface is calculated assuming the beams have spectra and fluxes consistent with those observed for BL Lac objects. The models are compared to recent observations of lobes of excess, polarized blue light in Cy-A, and observations of lobes of UV light in the central regions of the cD galaxies in A1795 and A2597. It was found that the models predict scattered surface brightnesses in reasonable agreement with those observed. If these lobes are indeed scattered light, this would indicate that F-R I radio galaxies do possess beams of radiation with apparent luminosities similar to BL Lac objects and that the radiation is actively beamed and not merely obscured by a torus of dust around the nucleus.

Sarazin, Craig L.↗
NASA NTRS1993

The generation and use of numerical shape models for irregular Solar System objects

We describe a procedure that allows the efficient generation of numerical shape models for irregular Solar System objects, where a numerical model is simply a table of evenly spaced body-centered latitudes and longitudes and their associated radii. This modeling technique uses a combination of data from limbs, terminators, and control points, and produces shape models that have some important advantages over analytical shape models. Accurate numerical shape models make it feasible to study irregular objects with a wide range of standard scientific analysis techniques. These applications include the determination of moments of inertia and surface gravity, the mapping of surface locations and structural orientations, photometric measurement and analysis, the reprojection and mosaicking of digital images, and the generation of albedo maps. The capabilities of our modeling procedure are illustrated through the development of an accurate numerical shape model for Phobos and the production of a global, high-resolution, high-pass-filtered digital image mosaic of this Martian moon. Other irregular objects that have been modeled, or are being modeled, include the asteroid Gaspra and the satellites Deimos, Amalthea, Epimetheus, Janus, Hyperion, and Proteus.

Simonelli, Damon P.↗
NASA NTRS1993

Explosions of small Spacewatch objects in the Earth's atmosphere

It is shown here that explosions due to Spacewatch objects with diameters less than 50 m typically occur too high in the atmosphere to cause substantial surface damage. Exclusive of relatively rare iron objects, no comet or asteroid with an energy below about 2 megatons threatens the Earth's surface. The high flux of small Earth-crossing objects identified by Spacewatch therefore does not imply a greater terrestrial hazard.

Chyba, Christopher F.↗
NASA NTRS1993

Lick slit spectra of thirty-eight objective prism quasar candidates and low metallicity halo stars

Lick Observatory slit spectra of 38 objects which were claimed to have pronounced UV excess and emission lines are presented. Eleven QSOs, four galaxies at z of about 0.1, 22 stars, and one unidentified object with a low S/N spectrum were found. Of 11 objects which Zhan and Chen (1987, 1989) suggested were QSO with z(prism) not greater than 2.8; eight are QSOs. Six of the QSOs show absorption systems, including Q0000+027A with a relatively strong associated C IV absorption system, and Q0008+008 with a damped Ly-alpha system with an H I column density of 10 exp 21/sq cm. The equivalent widths of the Ca II K line, the G band, and the Balmer lines in 10 stars with the best spectra are measured, and metallicities are derived. Seven of them are in the range -2.5 to -1.7, while the others are less metal-poor.

Tytler, David↗