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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Properties of the SN 1987A circumstellar ring and the distance to the Large Magellanic Cloud

The distance to SN 1987A was determined by comparing the angular size of its circumstellar ring, measured from an HST image obtained in a narrow forbidden O III 5007 A filter, with its absolute size derived from an analysis of the light curves of narrow UV lines measured with the IUE. The analysis confirms that the observed elliptical structure is a circular ring seen at a mean inclination of i = 42.8 + or - 2.6 deg and provides a determination of its absolute diameter of (1.27 + or - 0.07) x 10 to the 18th cm. Its ratio to the angular diameter of 1.66 + or - 0.03 arcsec gives an accurate determination of the distance to SN 1987A, i.e., d(1987A) = 51.2 + or - 3.1 kpc.

Panagia, N.↗
NASA NTRS1992

Halo common proper motion stars - Subdwarf distance scale, halo binary fraction, and UBVRI colors

Common proper motion stars identified by Luyten (1979, 1980) were selected according to reduced proper motion and photometric criteria to produce a list of candidate halo objects. UBVRI photometry was obtained to identify genuine halo parts. About 20 percent of the stars with (Fe/H) less than -1.0 were found to have random errors not less than several x 0.1 mag in distance modulus, in addition to the photometric errors, thought to be due to the existence of close binary companions to some of the common proper motion stars. The implications of these errors for kinematic studies of field Population II stars are considered. Systematic errors of order 16 percent were also detected in distances derived from the U-B,B-V deblanketing technique. It is suggested that Population II field stars did not originate in globular clusters which subsequently disintegrated but formed in less dense environments. Several K subdwarfs were found with very high UV excesses, comparable with those predicted by some synthetic colors.

Ryan, Sean G.↗
NASA NTRS1991

Efficient distance calculation using the spherically-extended polytope (s-tope) model

An object representation scheme which allows for Euclidean distance calculation is presented. The object model extends the polytope model by representing objects as the convex hull of a finite set of spheres. An algorithm for calculating distances between objects is developed which is linear in the total number of spheres specifying the two objects.

Hamlin, Gregory J.↗
NASA NTRS1992

A distance determination for the supernova remnant G27.4+0.0 and its central X-ray source

We have determined the distance to the supernova remnant G27.4+0.0, one of the handful of remnants with a bright central X-ray source. H I absorption data obtained with the VLA toward the remnant and adjacent H II regions allow us to constrain the distance to G27.4+0.0 to lie between 6 and 7.5 kpc. We briefly discuss the implications of this result for the nature of the central source.

Sanbonmatsu, K. Y.↗
NASA NTRS1993

Afterglows from gamma-ray bursters - A way to estimate the burster distance scale?

We calculate the strength of secondary gamma-ray signals that could be produced by positron annihilation in a medium surrounding a gamma-ray burster. We predict that such signals should be observable for a fairly wide range of conditions of this medium for bursts at both Galactic and cosmological distances. Furthermore, the time behavior of such signals together with the redshift measurement could be used to determine the distance scale to bursters.

Fencl, H. S.↗
NASA NTRS1993

A provisional RR Lyrae distance scale

Hydrodynamic pulsation models are matched with observations of globular cluster RRc stars to derive masses, luminosities and temperatures as functions of two observables: pulsation period, P1, and Fourier phase parameter, phi sub 31. We find that mean RRc masses and luminosities increase, and mean temperatures fall, with decreasing cluster metallicity. The Oosterhoff dichotomy is explained as mainly a temperature effect, while hints of a new dichotomy (in RRc mass and luminosity) are found among certain Oo II clusters. A provisional RR Lyrae distance scale emerges for the RRc stars in the form of a relation between luminosity and the two observables, P1 and phi sub 31. Finally, the scheme we propose passes three independent tests: (1) it reproduces the observed hierarchy of relative luminosity among a large sample of RRc stars in Omega Centauri; (2) it yields mean RRc masses which are fully consistent with the RRd masses derived for the clusters M68 and M15; and (3) it gives an LMC distance modulus in agreement with that obtained by other methods.

Simon, Norman R.↗
NASA NTRS1992

Distance between images

Comparing two binary images and assigning a quantitative measure to this comparison finds its purpose in such tasks as image recognition, image compression, and image browsing. This quantitative measurement may be computed by utilizing the Hausdorff distance of the images represented as two-dimensional point sets. In this paper, we review two algorithms that have been proposed to compute this distance, and we present a parallel implementation of one of them on the MasPar parallel processor. We study their complexity and the results obtained by these algorithms for two different types of images: a set of displaced pairs of images of Gaussian densities, and a comparison of a Canny edge image with several edge images from a hierarchical region growing code.

Gualtieri, J. A.↗
NASA NTRS1993

Trace of planetary nebula evolution by distance-independent parameters

Using existing infrared and radio data on a sample of 432 planetary nebulae, we derived a number of distance-independent parameters for comparison with evolutionary models of planetary nebulae. We find that many of the observed properties of planetary nebulae can be explained by current central star evolutionary models, even if the time scales are subject to significant change by a factor of up to an order of magnitude. Specifically, we find that the evolutionary tracks are well separated in the radio surface brightness-central star temperature plane, therefore allowing us to determine the core mass of individual planetary nebulae. We also obtain the luminosity and gravity of the central stars of individual nebulae, from their temperature and core mass, without relying on the distance assumptions. We find that our results of the core mass are in good agreement with those of Mendez et al. (1992) and Tylenda et al. (1991). A systematic, large discrepancy is found between the luminosity found in this work and that found by Gathier and Pottasch (1986).

Zhang, C. Y.↗
NASA NTRS1993

Solar wind conditions in the outer heliosphere and the distance to the termination shock

The Plasma Science experiment on the Voyager 2 spacecraft has measured the properties of solar wind protons from 1 to 40.4 AU. We use these observations to discuss the probable location and motion of the termination shock of the solar wind. Assuming that the interstellar pressure is due to a 5 micro-G magnetic field draped over the upstream face of the heliopause, the radial variation of ram pressure implies that the termination shock will be located at an average distance near 89 AU. This distance scales inversely as the assumed field strength. There are also large variations in ram pressure on time scales of tens of days, due primarily to large variations in solar wind density at a given radius. Such rapid changes in the solar wind ram pressure can cause large perturbations in the location of the termination shock. We study the nonequilibrium location of the termination shock as it responds to these ram pressure changes. The results of this study suggest that the position of the termination shock can vary by as much as 10 AU in a single year, depending on the nature of variations in the ram pressure, and that multiple crossings of the termination shock by a given outer heliosphere spacecraft are likely. After the first crossing, such models of shock motion will be useful for predicting the timing of subsequent crossings.

Belcher, John W.↗
NASA NTRS1994

Flight investigation of the use of a nose gear jump strut to reduce takeoff ground roll distance of STOL aircraft

A series of flight tests was conducted to evaluate the reduction of takeoff ground roll distance obtainable from a rapid extension of the nose gear strut. The NASA Quiet Short-haul Research Aircraft (QSRA) used for this investigation is a transport-size short takeoff and landing (STOL) research vehicle with a slightly swept wing that employs the upper surface blowing (USB) concept to attain the high lift levels required for its low speed, short-field performance. Minor modifications to the conventional nose gear assembly and the addition of a high pressure pneumatic system and a control system provided the extendible nose gear, or 'jump strut,' capability. The limited flight test program explored the effects of thrust-to-weight ratio, storage tank initial pressure, and control valve open time duration on the ground roll distance. The data show that the predicted reduction of takeoff ground roll on the order of 10 percent was achieved with the use of the jump strut. Takeoff performance with the jump strut was also found to be essentially independent of the pneumatic supply pressure and was only slightly affected by control valve open time within the range of the parameters examined.

Eppel, Joseph C.↗
NASA NTRS1994

Intelligent monitoring system applied to super long distance telerobotic tasks

Time delay and small capacity of communication are the primary constraint in super long distance telerobotic systems such as astronautical robotic tasks. Intelligent telerobotics is thought to break this constraint. We aim to realize this super long distance telerobotic system with object handling knowledge base and intelligent monitoring. We will discuss physical and technical factors for this purpose.

Wakita, Yujin↗
NASA NTRS1995

Attenuation distance of low frequency waves upstream of the pre-dawn bow shock: GEOTAIL and ISEE 3 comparison

We have made a statistical study of the spatial distribution of low frequency waves (approx. 0.01-0.1 Hz) in the region upstream of the pre-dawn to dawn side bow shock (-50 Re less than X less than 15 Re) using both GEOTAIL and international sun earth explorer 3 (ISEE-3) magnetometer data. We have found that the wave amplitude dependence on D and X(sub s), where D is the distance from the bow shock and X(sub s) the x-coordinate position of shock foot point of the IMF, can be described by a functional form of A exp (X(sub s)/L(sub X)-D/L(sub D), with the characteristic attenuation distances, L(sub X) = 62 +/- 12 Re and L(sub D) = 59 +/- 38 Re.

Sugiyama, T.↗
NASA NTRS1995

Time-distance helioseismology in the vicinity of sunspots

We use the ray description of acoustic-gravity modes to calculate time-distance diagrams for the quiet Sun and for regions in the vicinity of a sunspot with a monolithic flux-tube structure. Time-distance curves for the quiet Sun match the observations of Duvall et al. In the vicinity of a sunspot these quiet Sun curves split into a family of closely spaced curves. The structure of this bandlike feature is found to be sensitive to the sunspot model and can be a diagnostic of the subsurface geometry of the sunspot flux tube.

D'Silva, Sydney↗
NASA NTRS1995

The distance to the heliospheric VLF emission region

Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983/84 and 1992/3. This higher-frequency component is apparently triggered by solar wind transients with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2/3 power and in the other as time to the 4/5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of approximately 130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at approximately 70 AU when the transient shock collided with it, the true distance to the source region was probably closer to approximately 115 AU.

Mcnutt, R. L., Jr.↗
NASA NTRS1995

Distances to the termination shock and heliopause from a simulation analysis of the 1992-93 heliospheric radio emission event

A new heliospheric radio emission event observed by Voyagers 1 and 2 in mid-1992 is believed to have been produced by the interaction of an interplanetary shock with the heliopause. The shock is thought to have oriented near the Sun during a period of intense solar activity in late-May and early-June, 1991. The observed travel time of the shock to the heliopause is 408 days; the initial speed is estimated to be between 600 and 800 km/s. We use a numerical gasdynamic simulation of an interplanetary shock, propagating through an equilibrium solution of the solar wind/interstellar medium interaction, to compute the distances to the termination shock and the heliopause that are consistent with these observations. For a shock speed of 600 km/s, the termination shock is located at 92 AU, and the heliopause is located at 128 AU. These distances increase to 112 AU and 156 AU when the shock speed is increased to 800 km/s.

Steinolfson, R. S.↗
NASA NTRS1993

The effect of scene content on speed, time, and distance perception

Helicopter flights performed at low-levels place high demands on pilots; they must simultaneously control the vehicle, avoid obstacles, and navigate. Therefore, pilots must correlate cues viewed in the external scene with information on map in order to maintain their geographical orientation. This is a particularly difficult task when helicopter pilots fly through visually unfamiliar terrain without highly detailed maps. As a result, pilots must often use estimates of elapsed time, distance traveled, and/or average speed in order to maintain a flight path indicated on a map during flight segments when these cues are absent. Therefore, the current study is concerned with the perception of speed, time, and distance, which we assume underlies the ability to orient oneself during these types of flight segments.

Awe, Cynthia A.↗
NASA NTRS1995

The Distance to the Heliospheric VLF Emission Region

Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983-1984 and 1992-1993. This higher-frequency component is apparently triggered by solar wind transients with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2/3 power and in the other as time to the 4/5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of approx. 130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at approx. 70 AU when the transient shock collided with it, the true distance to the source region was probably closer to approx. 115 AU.

McNutt, R. L., Jr.↗
NASA NTRS1997

Time-Distance Helioseismology with the MDI Instrument: Initial Results

In time-distance helioseismology, the travel time of acoustic waves is measured between various points on the solar surface. To some approximation, the waves can be considered to follow ray paths that depend only on a mean solar model, with the curvature of the ray paths being caused by the increasing sound speed with depth below the surface. The travel time is effected by various inhomogeneities along the ray path, including flows, temperature inhomogeneities, and magnetic fields. By measuring a large number of times between different locations and using an inversion method, it is possible to construct 3-dimensional maps of the subsurface inhomogeneities. The SOI/MDI experiment on SOHO has several unique capabilities for time-distance helioseismology. The great stability of the images observed without benefit of an intervening atmosphere is quite striking. It his made it possible for us to detect the travel time fo separations of points as small as 2.4 Mm in the high-resolution mode of MDI (0.6 arc sec 1/pixel). This has enabled the detection of the supergranulation flow. Coupled with the inversion technique, we can now study the 3-dimensional evolution of the flows near the solar surface.

Duvall, T. L., Jr.↗