Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “clusters”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

X-ray illumination of globular cluster puzzles

Globular clusters are thought to be among the oldest objects in the Galaxy, and provide, in this connection, important clues for determining the age and process of formation of the Galaxy. The present investigation is concerned with puzzles relating to the X-ray emission of globular clusters, taking into account questions regarding the location of X-ray emitting clusters (XEGC) unusually near the galactic plane and/or galactic center. An adopted model is discussed for the nature, formation, and lifetime of X-ray sources in globular clusters. An analysis of the available data is conducted in connection with a search for correlations between binary formation time scales, central relaxation times, galactic locations, and X-ray emission. The positive correlation found between distance from galactic center and two-body binary formation time for globular clusters, explanations for this correlation, and the hypothesis that X-ray sources in globular clusters require binary star systems provide a possible explanation of the considered puzzles.

Lightman, A. P.↗
NASA NTRS1982

An approximately 300 Mpc void of rich clusters of galaxies

Kirshner et al. (1981) have reported an apparent absence of relatively bright galaxies over an enormous volume in the direction of Bootes. This result, if statistically significant, has important implications for theories of galaxy clustering and formation. In connection with the study of the correlation function of rich clusters of galaxies, Bahcall and Soneira (1982) have noticed a large area in the sky, extending approximately 100 deg across, which is completely void of nearby, but not distant, rich Abell clusters. This apparent absence of rich clusters may represent a physical void. If this interpretation is correct, the void extends approximately 150 Mpc/h along the line of sight. The sample studied is Abell's (1958) statistical sample of clusters of galaxies. The distribution of rich clusters is discussed, taking into account observed versus expected characteristics. Attention is also given to aspects of absorption and galaxy counts in the considered region.

Bahcall, N. A.↗
NASA NTRS1982

X-ray diagnostics of globular clusters

The presence of compact X-ray sources in globular clusters allows diagnostic studies of both the X-ray sources themselves and the globular clusters to be carried out. A review of much of this work, primarily based on Einstein X-ray observations and supporting studies of globular clusters at radio through UV wavelengths, is presented. The compact X-ray sources in globular clusters are found to be compact binaries containing neutron stars and - in a separate lower luminosity component of an apparently bimodal luminosity function - possibly white dwarfs. Implications for the formation and evolution of compact binary X-ray sources in globular clusters and in the galactic bulge are discussed. In particular, new evidence is presented that the galactic bulge sources may be compact binaries in the remnants of disrupted globular clusters.

Grindlay, J. E.↗
NASA NTRS1984

A proposal for the proper use of pseudopotentials in molecular orbital cluster model studies of chemisorption

The interaction of CO with Cu5, Ni5, and Al4 are treated as model systems for molecular adsorption on metal surfaces. The effect of the use of pseudopotentials for the metal atoms is studied by considering three types of clusters. In the first case, all of the metal electrons are explicitly included in the wave function; an all electron (AE) treatment. In the second case, the metal atom which directly interacts with the CO is described by AE but the remaining metal atoms include a pseudopotential for their core electrons. Finally, in the third case, all of the metal atoms in the cluster have a pseudopotential treatment for the core electrons. The AE cluster results are taken as reference values for the two pseudopotential treatments. The mixed cluster results are in excellent agreement with those of the all AE clusters; however, the results for the all pseudopotential cluster of Ni5CO or of Cu5CO are qualitatively different. The pseudopotential treatment for all of the metal atoms often leads to results that contain serious errors and it is not a reliable approach.

Bagus, P. S.↗
NASA NTRS1985

X-ray observations of possible binary clusters of galaxies

Many studies of superclusters of galaxies have been conducted, taking into account also superclusters representing candidate binary cluster systems. The present investigation is concerned with further studies of potential binary cluster systems, giving attention to a sample of six cluster pairs, in which the redshifts and X-ray luminosities of each member of a pair have been measured. One of the objectives of the investigation was related to a search for X-ray evidence that the clusters interact in these potentially binary systems. A second objective was to provide a measure of the mass of hot gas in the clusters. Two new systems in which the two clusters may have a physical association were found.

Ulmer, M. P.↗
NASA NTRS1985

The young SMC cluster NGC 330

A color-magnitude diagram has been obtained for the young SMC cluster NGC 330. The diagram shows a well-defined main sequence, a group of blue supergiants, a group of red supergiants between B-V = 1.2 m and 1.6 m about one magnitude fainter, and an empty Hertzsprung gap. The surrounding field is a composite of a very gold population resembling galactic globular clusters and a very young population. DDO and infrared photometry strongly suggest that the cluster is metal-poor, but a definitive measure could not be made because of calibration difficulties. The cluster's age is estimated at 12 million years, with the surrounding field about 50 percent older. The cluster will prove very useful in testing stellar evolution models for young, metal-poor stars if the cluster's metallicity can be established via high-resolution spectroscopy.

Carney, B. W.↗
NASA NTRS1985

The magnitude-redshift relation for 561 Abell clusters

The Hubble diagram for the 561 Abell clusters with measured redshifts has been examined using Abell's (1958) corrected photo-red magnitudes for the tenth-ranked cluster member (m10). After correction for the Scott effect and K dimming, the data are in good agreement with a linear magnitude-redshift relation with a slope of 0.2 out to z = 0.1. New redshift data are also presented for 20 Abell clusters. Abell's m10 is suitable for redshift estimation for clusters with m10 of no more than 16.5. At fainter m10, the number of foreground galaxies expected within an Abell radius is large enough to make identification of the tenth-ranked galaxy difficult. Interlopers bias the estimated redshift toward low values at high redshift. Leir and van den Bergh's (1977) redshift estimates suffer from this same bias but to a smaller degree because of the use of multiple cluster parameters. Constraints on deviations of cluster velocities from the mean cosmological flow require greater photometric accuracy than is provided by Abell's m10 magnitudes.

Postman, M.↗
NASA NTRS1985

Galactic bulge X-ray burst sources from disrupted globular clusters?

The origin of the bright galactic bulge X-ray sources, or GX sources, is unclear despite intensive study for the past 15 years. It is suggested that the fact that many (or most) of the GX sources are X-ray burst sources (GXRBS) and are otherwise apparently identical to the luminous X-ray sources found in globular cluster cores implies that they too may have a globular cluster origin. The possibility that the compact X-ray binaries found in globulars are ejected is constrained by observations of CVs in and out of clusters. The GXRBS are instead hypothesized to have been formed by capture processes in globular clusters which have now largely been disrupted by repeated tidal stripping and shocking in the galactic plane. A statistical analysis of the 12 GXRBS which have precise positions from Einstein and/or optical (or radio) observations indicate that it is probably significant that a bright, of less than about 19, G or K star is found within the error circle (3 arcmin radius) in four cases. These may be surviving giants in a disrupted globular cluster core. Implications for globular cluster evolution and the GXRBS themselves are discussed.

Grindlay, J. E.↗
NASA NTRS1985

Monte Carlo study of reversible growth of clusters on a surface

The effect of binding energy on the growth of clusters on a two-dimensional square lattice was studied by a means of a Monte Carlo simulation. The results indicate that for binding energies higher than 0.5kT, clusters grow relative to the initial random distribution, while for binding energies lower than 0.5 kT, clusters decompose. It was also found that at low binding energies the equilibrium percentage of atoms that are bound in clusters is linearly dependent on the binding energy. The cluster density behaved in a similar manner. This similarity reflects the fact that at low binding energies the growth proceeds mainly by the formation of new clusters rather than by an increase in the size of existing ones.

Salik, J.↗
NASA NTRS1986

Spectroscopy of the globular clusters in M87

With a velocity dispersion of 370 + or - 50 km/sec the globular cluster system of M87 is kinematically hotter than the stars in the giant elliptical itself. This is consistent with the clusters' shallower density distribution for isotropic orbits. The mean metallicity of the 27 clusters in the sample analyzed here is no more than a factor of 2 more metal rich than the cluster system of the Milky Way, but considerably more metal poowr than the integrated starlight in the field at a radius of 1' from the center of M87. There is no evidence for the existence of young clusters in the system. The mass-radius relation between 1' and 5' required to contain the globular clusters joins on to that required to contain the hot gas around M87.

Mould, J. R.↗
NASA NTRS1986

Three-color CCD photometry of the distant cluster of galaxies Cl 0024 + 1654

Photometry of 265 galaxies in rich cluster Cl 0024 + 1654 (z = 0.39), based on CCD imaging in three filters, is presented. The luminosity of this system ranks it among the richest known clusters, and it has a blue mass-to-light ratio of about 210. The color-magnitude diagram has a prominent E/S0 sequence. There are essentially no blue cluster members brighter than r equals about 19.3, and only very red galaxies inhabit the central 30 arcsec of the cluster. The blue cluster members have sizes and surface brightnesses similar to spiral galaxies in nearby clusters.

Schneider, D. P.↗
NASA NTRS1987

Spectroscopy of the globular clusters in M87

With a velocity dispersion of 370 + or - 50 km/sec the globular cluster system of M87 is kinematically hotter than the stars in the giant elliptical itself. This is consistent with the clusters' shallower density distribution for isotropic orbits. The mean metallicity of the 27 clusters in the sample analyzed here is no more than a factor of 2 more metal rich than the cluster system of the Milky Way, but considerably more metal poor than the integrated starlight in the field at a radius of 1' from the center of M87. There is no evidence for the existence of young clusters in the system. The mass-radius relation between 1' and 5' required to contain the globular clusters joins on to that required to contain the hot gas around M87.

Mould, J. R.↗
NASA NTRS1987

The nearby Abell clusters. II - Luminosity and spatial distribution of galaxies in A2175, A2256, and A2384

The luminosity and spatial distributions of galaxies in the nearby Abell clusters A2175, A2256, and A2384 have been studied using an automated object detection, classification, and photometry system. Schecter functions have been fit to the observed luminosity functions of these clusters. Values of the characteristic magnitude Mr(asterisk) at the 'break' in the luminosity function (LF) are found that are within about 0.25 mag of the so-called 'universal' value of Mr(asterisk) for A2256 and A2384. The faint ends of the LFs of these two clusters are fairly flat, with a value of -1.6 that is steeper than the universal value of -1.25. The spatial distributions of galaxies in all three clusters seem to be described fairly well by power laws, with no evidence for constant-density cores, when the cluster center is assumed to be the central D or cD galaxy. These results indicate a strong correlation between the density distribution of galaxies and the presence of a dominant D or cD galaxy in the cluster.

Oegerle, William R.↗
NASA NTRS1987

The AXAF high resolution camera (HRC) and its use for observations of distant clusters of galaxies

The High Resolution Camera being developed for AXAF is briefly described, comparing it with its predecessor, the Einstein HRI, and providing preliminary performance data. The HRC is a 10 cm x 10 cm detector with CsI coated microchannel plates providing a large field of view device with high X-ray quantum efficiency. The overall sensitivity of the HRC on AXAF is about 50 times that of the Einstein HRI. An example of the power of this instrument is discussed in the context of studies of distant clusters of galaxies. The HRC can detect individual galaxies in a cluster of redshift 1, if they are similar to the early-type galaxies in nearby clusters that have been observed by the Einstein instruments. The HRC will be used to measure the luminosity function for clusters of galaxies as a function of their distance up to a redshift of 1. This will allow observation of evolutionary effects in clusters as a whole, while the detection of individual galaxies will permit studies of galaxy evolution in a cluster environment.

Murray, S. S.↗
NASA NTRS1986

The creation of local clusters in arbitrarily given grids

A method is presented to smoothly insert pointwise clusters into any given grid regardless of its origin, its topology, or its dimensionality. The process amounts to a local movement of the given coordinate curves or surfaces to more highly resolve an object. The object about which clustering is created can be a point, a curve, a surface, or segments of a curve or surface. The basic clustering capability is established by forming a grid operator for a single cluster. With a view toward multiple clusters being created about various objects, the basic operator is seen as an elementary operator. An algorithm is presented to execute the general elementary operation in three dimensions. In FORTRAN, this assumes the form of a subroutine which is fully operational and is presented to serve as a basic model for any such elementary clustering operation.

Eiseman, Peter R.↗
NASA NTRS1988

Color-magnitude diagrams for six metal-rich, low-latitude globular clusters

Colors and magnitudes for stars on CCD frames for six metal-rich, low-latitude, previously unstudied globular clusters and one well-studied, metal-rich cluster (47 Tuc) have been derived and color-magnitude diagrams have been constructed. The photometry for stars in 47 Tuc are in good agreement with previous studies, while the V magnitudes of the horizontal-branch stars in the six program clusters do not agree with estimates based on secondary methods. The distances to these clusters are different from prior estimates. Redding values are derived for each program cluster. The horizontal branches of the program clusters all appear to lie entirely redwards of the red edge of the instability strip, as is normal for their metallicities.

Armandroff, Taft E.↗
NASA NTRS1988

Simulation of GaAs cluster formation on GaAs(00-1), AlAs(00-1), Si(001), and As1/Si(001) surfaces

Recently developed semiempirical potential energy functions for the Ga-As-Si and Ga-As-Al systems have been applied here to determine the excess formation energy for GaAs clusters on GaAs(00-1), AlAs(00-1), Si(001), and one atomic layer As-covered Si(001) substrates as a function of cluster size and cluster shape by the Monte Carlo technique. Pyramidal type ledges on the GaAs clusters are found to be the favored ledge for the first three layers while an inverted-pyramidal type ledge is also favored in certain cases for the As1/Si(001) substrate. Cluster formation at ledges is compared with cluster formation on a flat terrace for the Si(001) and the As1/Si(001) substrates.

Choi, D. K.↗
NASA NTRS1988

The globular cluster system of M31

Previous data on globular cluster candidates in M31 have been studied in order to identify a large sample of clusters with reliable UBV photometry and independent reddening estimates. The distribution of the intrinsic colors of the M31 globular clusters is similar to that of the Galactic globulars. While the flattening and kinematics of the system of metal-rich clusters suggest a disk, the more metal-poor clusters reveal little evidence for rotation. It is noted that the flattening of the system of metal-poor and intermediate clusters resembles that of the M31 spheroid, and that the effective radius of the intermediate system also resembles that of the spheroid.

Elson, Rebecca A.↗