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At least 1,549 records · Page 86
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NASA NTRS1988

Accretion-disk modeling of the UV spectrum of quasars

The thin accretion disk spectrum, including the effect of electron scattering, is calculated for a grid of the accretion parameters. It is shown that the luminosity and spectral slope in the UV and optical bands are uniquely determined by these parameters and depend only weakly on the viscosity parameter. When the model calculations are compared with observations of a sample of QSOs and Seyfert 1 nuclei, the black hole mass is in the range 10 to the 8th to 10 to the 9.5 solar for the QSOs and 10 to the 7.5 to 10 to the 8.5 solar for the Seyferts and for low-luminosity objects. The correlation between the spectral slope and luminosity of QSOs in the UV can be explained by evolution along curves of constant black hole mass and decreasing accretion rate. The relation between the spectrum and the accretion parameters can be used to constrain the cosmological evolution of the objects and imposes severe constraints on the pure luminosity evolution interpretation function for QSOs and Seyferts.

Wandel, Amri↗
NASA NTRS1988

On convection-induced viscosity in accretion disks in cataclysmic variables

The vertical structure of accretion disks in cataclysmic variable systems is examined on the assumption that the convection is solely responsible for inducing the viscous heating. The viscous heating is distributed over the entire vertical structure so that purely radiative zones are able to carry a radial mass flux. The convective flux is computed by two different methods in separate calculations and the results from each method are compared. The values of the viscosity obtained are compared with those values inferred from the observed time scales of dwarf nova eruptions. Sequences of equilibrium solutions for constant radius and varying mass accretion rate are examined, the relations between viscosity and surface density are discussed in the context of recent models for the eruptions of dwarf novae.

Cannizzo, John K.↗
NASA NTRS1988

Slim accretion disks around black holes

Black hole accretion disk models at moderately high accretion rates have been constructed, taking correctly into account dissipation and heat transport in the transonic part of the flow. The present models differ from those of Shakura and Sunyaev (1973) in many important respects. The surface density at a given radius relates to the accretion rate through an S-shaped function.

Szuszkiewicz, E.↗
NASA NTRS1988

Discovery of very high velocity outflow in V Hydra - Wind from an accretion disk in a binary?

High-resolution observations of lines from the CO v = 1-0 vibration-rotation band at 4.6 microns, taken with the FTS/KPNO 4-m telescope, are reported for the carbon-rich red giant V Hydra, which is surrounded by an extended expanding molecular envelope resulting from extensive mass loss. The spectrum shows, in addition to the expected absorption at the outflow velocity of the envelope, absorption extending up to 120 km/s bluewards of the stellar velocity. A comparison of the spectrum observed at two epochs shows that the high-velocity absorption features change with time. It is suggested that the observed high-velocity features in V Hydra arise in a high-velocity polar outflow from an accretion disk in a binary system, as proposed in the mass-loss model for bipolar envelopes by Morris (1988).

Sahai, R.↗
NASA NTRS1989

Advanced Engraving Of Angle-Encoder Disks

Precision increased over mechanical engravers. Laser pulses timed according to rotational speed of rotor electrostatically suspended in vacuum marks angular-encoder disk mounted on lower end of rotor. Precision of timing circuitry and low rotational damping (damping time about 700 years in high vacuum) results in highly-precise angular intervals. System conceived to meet need for increased precision of angular measurements in advanced scientific instruments.

Polstorff, Walter K.↗
NASA NTRS1989

Forging Long Shafts On Disks

Isothermal-forging apparatus produces long shafts integral with disks. Equipment based on modification of conventional isothermal-forging equipment, required stroke cut by more than half. Enables forging of shafts as long as 48 in. (122 cm) on typical modified conventional forging press, otherwise limited to making shafts no longer than 18 in. (46cm). Removable punch, in which forged material cools after plastic deformation, essential novel feature of forging apparatus. Technology used to improve such products as components of gas turbines and turbopumps and of other shaft/disk parts for powerplants, drive trains, or static structures.

Tilghman, Chris↗
NASA NTRS1989

Development of a software interface for optical disk archival storage for a new life sciences flight experiments computer

The current Life Sciences Laboratory Equipment (LSLE) microcomputer for life sciences experiment data acquisition is now obsolete. Among the weaknesses of the current microcomputer are small memory size, relatively slow analog data sampling rates, and the lack of a bulk data storage device. While life science investigators normally prefer data to be transmitted to Earth as it is taken, this is not always possible. No down-link exists for experiments performed in the Shuttle middeck region. One important aspect of a replacement microcomputer is provision for in-flight storage of experimental data. The Write Once, Read Many (WORM) optical disk was studied because of its high storage density, data integrity, and the availability of a space-qualified unit. In keeping with the goals for a replacement microcomputer based upon commercially available components and standard interfaces, the system studied includes a Small Computer System Interface (SCSI) for interfacing the WORM drive. The system itself is designed around the STD bus, using readily available boards. Configurations examined were: (1) master processor board and slave processor board with the SCSI interface; (2) master processor with SCSI interface; (3) master processor with SCSI and Direct Memory Access (DMA); (4) master processor controlling a separate STD bus SCSI board; and (5) master processor controlling a separate STD bus SCSI board with DMA.

Bartram, Peter N.↗
NASA NTRS1988

On the outburst recurrence time for the accretion disk limit cycle mechanism in dwarf novae

An attempt is made here to elucidate the dwarf nova outburst time scale in terms of basic physics, and to place limits on the model by using the observed recurrence times. A simple analytical model is developed to describe the limit cycle process, and an expression is obtained for the interval between eruptions. It is shown that the smallness of the amount of mass accreted onto the central white dwarf during eruption can be explained in terms of the propagation of heating and cooling fronts. The dependency of the fraction of the disk mass accreted on the model parameters is derived, and the result is used to construct a semianalytic expression for the recurrence time.

Cannizzo, John K.↗
NASA NTRS1988

Resolution of the circumstellar disk of Beta Pictoris at 10 and 20 microns

Observations at 10 and 20 microns with about 5 arcsec resolution are presented of the central, approximately 13 arcsec diameter region of the A5 dwarf Beta Pictoris. These observations resolve the inner disk and provide important constraints for models of the star and a basis for removing some of their ambiguities. The evidence indicates that submicron-sized grains emit much of the excess 10 and 20 micron radiation from the star. The characteristics of the dust are discussed, and the possibility that comets form the dust reservoir is considered.

Telesco, C. M.↗
NASA NTRS1988

Magnetic bearings for an optical disk read/write head

A program is described whose aim is to establish the technical feasibility of developing magnetic bearings for an optical disk buffer for use in a spacecraft environment. The approach taken was to develop the specifications for the magnetic bearings and define the hardware configuration, controller characteristics, position sensors, and electronic functions required.

Hockney, Richard↗
NASA NTRS1989

On the stream-accretion disk interaction - Response to increased mass transfer rate

The time-dependent interaction between the stream of mass from the inner Lagrangian point and the accretion disk, resulting from an increasing mass transfer rate is calculated. The calculation is fully three-dimensional, using a pseudoparticle description of the hydrodynamics. It is demonstrated that the results of such calculations, when combined with specific observations, have the potential of both determining essential parameters, such as the viscosity parameter alpha, and can distinguish between different models of dwarf nova eruptions.

Dgani, Ruth↗
NASA NTRS1989

Disk-integrated photometry of Neptune at methane-band and continuum wavelengths

The present disk-integrated photometry of Neptune, which was obtained in 1986 and 1987, is used to study the diurnal and short-term variability; the fact that this photometry was obtained from high-resolution CCD images allows the discrete cloud features' causing of the rotational lightcurve seen at methane-band wavelengths to be definitively demonstrated. A quiescent state of the Neptune atmosphere is suggested by the planet's 8900 A reflectivity in the absence of bright clouds, appears to have remained nearly constant over the observational period.

Hammel, H. B.↗
NASA NTRS1989

Off disk-center potential field calculations using vector magnetograms

A potential field calculation for off disk-center vector magnetograms that uses all the three components of the measured field is investigated. There is neither any need for interpolation of grid points between the image plane and the heliographic plane nor for an extension or a truncation to a heliographic rectangle. Hence, the method provides the maximum information content from the photospheric field as well as the most consistent potential field independent of the viewing angle. The introduction of polarimetric noise produces a less tolerant extrapolation procedure than using the line-of-sight extrapolation, but the resultant standard deviation is still small enough for the practical utility of this method.

Venkatakrishnan, P.↗
NASA NTRS1989

Monte Carlo models and analysis of galactic disk gamma-ray burst distributions

Gamma-ray bursts are transient astronomical phenomena which have no quiescent counterparts in any region of the electromagnetic spectrum. Although temporal and spectral properties indicate that these events are likely energetic, their unknown spatial distribution complicates astrophysical interpretation. Monte Carlo samples of gamma-ray burst sources are created which belong to Galactic disk populations. Spatial analysis techniques are used to compare these samples to the observed distribution. From this, both quantitative and qualitative conclusions are drawn concerning allowed luminosity and spatial distributions of the actual sample. Although the Burst and Transient Source Experiment (BATSE) experiment on Gamma Ray Observatory (GRO) will significantly improve knowledge of the gamma-ray burst source spatial characteristics within only a few months of launch, the analysis techniques described herein will not be superceded. Rather, they may be used with BATSE results to obtain detailed information about both the luminosity and spatial distributions of the sources.

Hakkila, Jon↗
NASA NTRS1989

Numerical and experimental analysis of a thin liquid film on a rotating disk related to development of a spacecraft absorption cooling system

The numerical and experimental analysis of a thin liquid film on a rotating and a stationary disk related to the development of an absorber unit for a high capacity spacecraft absorption cooling system, is described. The creation of artificial gravity by the use of a centrifugal field was focused upon in this report. Areas covered include: (1) One-dimensional computation of thin liquid film flows; (2) Experimental measurement of film height and visualization of flow; (3) Two-dimensional computation of the free surface flow of a thin liquid film using a pressure optimization method; (4) Computation of heat transfer in two-dimensional thin film flow; (5) Development of a new computational methodology for the free surface flows using a permeable wall; (6) Analysis of fluid flow and heat transfer in a thin film in the presence and absence of gravity; and (7) Comparison of theoretical prediction and experimental data. The basic phenomena related to fluid flow and heat transfer on rotating systems reported here can also be applied to other areas of space systems.

Faghri, Amir↗
NASA NTRS1990

Probabilistic analysis of bladed turbine disks and the effect of mistuning

Probabilistic assessment of the maximum blade response on a mistuned rotor disk is performed using the computer code NESSUS. The uncertainties in natural frequency, excitation frequency, amplitude of excitation and damping are included to obtain the cumulative distribution function (CDF) of blade responses. Advanced mean value first order analysis is used to compute CDF. The sensitivities of different random variables are identified. Effect of the number of blades on a rotor on mistuning is evaluated. It is shown that the uncertainties associated with the forcing function parameters have significant effect on the response distribution of the bladed rotor.

Shah, A. R.↗
NASA NTRS1989

Analysis of reactively loaded microstrip disk antenna

The moment method solution to the problem of a reactively loaded circular patch is presented. Using the reaction integral equation in conjuction with the method of moments, parameters of the Thevenin's equivalent network for the loaded patch are obtained. From the equivalent network parameters an expression for the imput impedance of the loaded patch is derived. A design procedure for a circularly polarized disk antenna is presented. Computed results are compared with the experimental data.

Deshpande, M. D.↗
NASA NTRS1989

Three very cool degenerate stars in Luyten common proper motion binaries - Implications for the age of the galactic disk

During the course of a spectroscopic study of Luyten common proper motion (CPM) stars, spectrophotometric observations have been obtained of three binaries containing degenerate stars with estimated absolute magnitudes M(V) of about 16. Each of the three pairs consists of a yellow degenerate star primary and a DC 13 + secondary 1.4-2.3 mag fainter. One of the primary stars is spectral class DC 7, another is a sharp-lined DA 8, and the third shows peculiar broad absorption features which we interpret as pressure-shifted C2 Swan bands. The LP 701 - 69/70 system has survived for over 8 billion years without disruption by passing stars, despite its 1500 a.u. orbital major axis. The three cool degenerate companions nearly double the available sample of stars at the low-luminosity terminus of the white dwarf cooling sequence. These findings appear consistent with the conclusion that degenerate stars in the old disk population have not had time to evolve to a luminosity fainter than M(V) about 16.2.

Hintzen, Paul↗