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NASA NTRS1987

The details of the convective evaporation of dense and dilute clusters of drops

A global model describing the convective evaporation of dense and dilute clusters of drops has been formulated starting from first principles. The volume of the cluster and the number of drops in a given cluster are fixed, and the drops do not move with respect to each other. The model has been tested for three different drag models and shows less than 10-percent sensitivity in the prediction of the droplet lifetime. The initial relative velocity between drops and gases is a weak control parameter in the 40-1000-cm/s regime.

Bellan, J.↗
NASA NTRS1986

X-ray observations of clusters - Physical implications

The X-ray properties of clusters of galaxies and elliptical galaxies are reviewed, with an emphasis on cooling flows and their effects on the structure and energetics of radio sources. It is found that the radial profiles of the hot (10 to the 8th K) diffuse (n = about 0.001/cu cm) gas in clusters do not require heating of the intracluster medium after the formation of the cluster. Consideration is given to theoretical formulas for cooling rates, the possible reasons for the observed correlation between cooling flows and the presence of a dominant central galaxy, cooling-flow models of elliptical galaxies, and the significant shear in the velocity field of a cooling flow.

Sarazin, Craig L.↗
NASA NTRS1988

IRAS observations of the globular cluster 47 Tucanae

Integrated and individual-star observations of 47 Tuc obtained at 12, 25, 60, and 100 microns by IRAS are reported. The data are presented in tables and brightness contour maps and analyzed. Features discussed include integrated light dominated by stellar photospheric emission, 12- and 25-micron excesses (associated with red giants in the outer region of the cluster), and a small excess at 100 microns (attributed to integrated-light heating of about 0.0003 solar mass of silicate dust within 6 arcmin of the cluster center). It is pointed out that this is much less dust than predicted by theoretical models of accumulation since the cluster passed through the Galactic plane about 30 Myr ago, and possible dust-removal mechanisms are considered.

Gillett, F. G.↗
NASA NTRS1988

The problem of clustering in laboratory studies of cometary dust

In trying to understand the nature of interstellar or interplanetary dust grains it is tempting to simply rely on direct lab measurements of grain properties such as extinction. However, until agreement is achieved between directly measured extinction and the same property calculated from measured optical constants of the solid, application to astronomical observations is questionable. There are two major problems which are discussed: the problem of what optical constants to use in the calculation, and the almost unavoidable problem of particle clustering in lab studies. Regarding small particle measurements, a problem afflicting almost all lab measurements is the clustering of particles. Having failed in efforts to isolate particles, the solution to the problem was attempted with a simple theory which seems to adequately describe the extinction by small, clustered particles in many cases. The theory is briefly explained.

Huffman, Donald R.↗
NASA NTRS1989

Commentary on interstellar matter associated with 18 open clusters

Information supplementary to that contained in Section 4 of an article entitled, A CO Survey of Regions Around 34 Open Clusters, (Leisawitz, Bash, and Thaddeus) published in the Astrophysical Journal Supplement Series, Volume 70, Number 4, August 1989 is summarized. The information presented here, which describes the interstellar environments of young clusters and some cluster physical characteristics, comes from observations published in the astronomical literature and the author's carbon monoxide (CO) emission line survey, and may help clarify our understanding of the interaction of massive stars with the interstellar medium.

Leisawitz, David↗
NASA NTRS1989

Millisecond radio pulsars in globular clusters

It is shown that the number of millisecond radio pulsars, in globular clusters, should be larger than 100, applying the standard scenario that all the pulsars descend from low-mass X-ray binaries. Moreover, most of the pulsars are located in a small number of clusters. The prediction that Teran 5 and Liller 1 contain at least about a dozen millisecond radio pulsars each is made. The observations of millisecond radio pulsars in globular clusters to date, in particular the discovery of two millisecond radio pulsars in 47 Tuc, are in agreement with the standard scenario, in which the neutron star is spun up during the mass transfer phase.

Verbunt, Frank↗
NASA NTRS1988

LMC clusters - Age calibration and age distribution revisited

The empirical age relation for star clusters in the Large Magellanic Cloud presented by Elson and Fall (1985) are reexamined using ages based only on main-sequence turnoffs. The present sample includes 57 clusters, 24 of which have color-magnitude diagrams published since 1985. The new calibration is very similar to that found previously, and the scatter in the relation corresponds to uncertainties of about a factor of 2 in age. The age distribution derived from the new calibration does not differ significantly from that derived in earlier work. It is compared with age distributions estimated by other authors for different samples of clusters, and the results are discussed.

Elson, Rebecca A.↗
NASA NTRS1988

Cluster influences on the internal dynamics of a galaxy

As part of a study of cluster influences, an attempt is made to map out damage to a galaxy under several different kinds of buffeting a galaxy suffers as it sweeps along its orbit through a cluster. It is shown that a cluster's observational characteristics are determined by the shape of its gravitational potential. It is noted the model galaxy must have full freedom to do whatever the physical galaxy wants to do.

Miller, R. H.↗
NASA NTRS1988

On the evolution of globular cluster systems. I - Present characteristics and rate of destruction in our Galaxy

The current dynamic state and rate of destruction for the system of globular clusters in the Galaxy are modeled. It is found that gravitational shocks due to the central bulge of the Galaxy are very efficient in destroying clusters on highly radial orbits. This effect, extrapolated at the present unweighted destruction rate over 10 to the 10th yr, can account for the observed difference between the velocity distribution of globular clusters and other tracers of the halo population.

Aguilar, Luis↗
NASA NTRS1989

Luminous arcs in clusters of galaxies

Observations are reported of what appears to be a new class of spatially coherent extragalactic features having, in the two most compelling known examples, the following joint properties: location in clusters of galaxies, narrow arclike shape, enormous apparent length, and situation of center of curvature toward both a cD galaxy and the apparent center of gravity of the cluster. The principal available facts concerning the arcs are presented and a variety of interpretations are briefly discussed. The weight of evidence seems to favor the interpretation that these features are images of more distant objects produced by the gravitational field of the intervening clusters.

Lynds, Roger↗
NASA NTRS1989

The stellar content of rich young clusters in the Large Magellanic Cloud

Initial mass functions (IMFs) of six young rich clusters in the LMC have been determined from star counts on photographic plates with different limiting magnitudes. Over the range of stellar masses from 1.5 to 6 solar masses, very flat IMF slopes between -0.2 and 0.8 are found. Possible explanations for the discrepancy between these results and those of Mateo (1988), who found step IMFs for a different sample of clusters in the Magellanic Clouds, are considered, and no obvious ones are found. The flat IMFs imply smaller mass-to-light ratios than previously assumed and reinforce previous conclusions that the clusters have unbound halos. Flat IMFs also imply that stellar winds would have been very important in restructuring and expelling the gas from a protocluster and that there would have been many supernovae early in the history of a supercluster.

Elson, Rebecca A. W.↗
NASA NTRS1989

The properties of the disk system of globular clusters

A large refined data sample is used to study the properties and origin of the disk system of globular clusters. A scale height for the disk cluster system of 800-1500 pc is found which is consistent with scale-height determinations for samples of field stars identified with the Galactic thick disk. A rotational velocity of 193 + or - 29 km/s and a line-of-sight velocity dispersion of 59 + or - 14 km/s have been found for the metal-rich clusters.

Armandroff, Taft E.↗
NASA NTRS1989

The fragmentation of proto-globular clusters. I - Thermal instabilities

The metal abundances among the stars within a typical globular cluster are remarkably homogeneous. This indicates that star formation in these systems was a globally coordinated event which occurred over a time span less than or comparable to the collapse time scale of the cluster. This issue is addressed by assuming that the fragmentation of a proto-globular cluster cloud proceeded in two steps. In the first step, thermal instability led to the rapid growth of initial fluctuations. This led to a large contrast in the dynamical time scales between the perturbations and the parent cloud, and the perturbations then underwent gravitational instabilities on short time scales. This process is modeled using one-dimensional hydrodynamic simulations of clouds both with and without external heat sources and self-gravity. The models include the effects of a non-equilibrium H2 abundance. The results indicate that fragmentation can occur on time scales significantly less than the dynamical time scale of the parent cloud.

Murray, Stephen D.↗
NASA NTRS1989

Dynamics of super cloud clusters, westerly wind bursts, 30-60 day oscillations and ENSO - An unified view

Results are presented on numerical experiments that were carried out to investigate the mechanisms of the observed variabilities in wind and convection associated with supercloud clusters (SCCs), westerly wind bursts, and 30-60 day oscillations in the western Pacific region. It is shown that the generation of a 30-60 day eastward propagating precipitation pattern in the Lau and Peng (1987) model, which can be identified as SCC, is accompanied by convective clusters coming in opposite direction to that of the SCC itself. The results suggest that the westward propagating cloud clusters are produced at the initial stage of the 30-60 day disturbance due to mutual adjustment of the large-scale flow and heating.

Lau, K.-M.↗
NASA NTRS1990

More About Evaporation In Clusters Of Drops

Report presents theoretical study of evaporation in clusters of spray drops in liquid fuel. Related to reports described in "Turbulence and Evaporation in Clusters of Drops" (NPO-17323) and "Effects of Turbulence on Ignition" (NPO-17335). Purpose of study to improve theoretical description of transport of molecular species, mass, and heat between cluster and its surroundings.

Bellan, Josette↗
NASA NTRS1988

How to cluster in parallel with neural networks

Partitioning a set of N patterns in a d-dimensional metric space into K clusters - in a way that those in a given cluster are more similar to each other than the rest - is a problem of interest in astrophysics, image analysis and other fields. As there are approximately K(N)/K (factorial) possible ways of partitioning the patterns among K clusters, finding the best solution is beyond exhaustive search when N is large. Researchers show that this problem can be formulated as an optimization problem for which very good, but not necessarily optimal solutions can be found by using a neural network. To do this the network must start from many randomly selected initial states. The network is simulated on the MPP (a 128 x 128 SIMD array machine), where researchers use the massive parallelism not only in solving the differential equations that govern the evolution of the network, but also by starting the network from many initial states at once, thus obtaining many solutions in one run. Researchers obtain speedups of two to three orders of magnitude over serial implementations and the promise through Analog VLSI implementations of speedups comensurate with human perceptual abilities.

Kamgar-Parsi, Behzad↗
NASA NTRS1989

Theoretical investigations of the structures and binding energies of Be(sub n) and Mg(sub n) (n = 3-5) clusters

Researchers determined the equilibrium geometries and binding energies of Be and Mg trimers, tetramers and pentamers using single and double excitation coupled cluster (CCSD) and complete active space self-consistent-field (CASSCF) multireference configuration interaction (MRCI) wave functions in conjunction with extended atomic basis sets. Best estimates of the cluster binding energies are 24, 83 and 110 kcal/mole for Be3, Be4 and Be5; and 9, 31 and 41 kcal/mole for Mg3, Mg4 and Mg5, respectively. A comparison of the MRCI and CCSD results shows that even the best single-reference approach (limited to single and double excitations) is not capable of quantitative accuracy in determining the binding energies of Be and Mg clusters.

Lee, Timothy J.↗
NASA NTRS1989

Rotational velocities of newly discovered, low-mass members of the Alpha Persei cluster

About 30 new, low-mass members of the young open cluster Alpha Persei are identified via a proper-motion study and subsequent photometric and spectroscopic observations. Membership in the cluster is confirmed for a number of the fainter proper-motion candidates from Heckman, Dieckvoss, and Kox (1956). Coordinates, finding charts, BVRI photometry, and rotational velocities are provided for most of the stars. At least two of the stars show peculiar H-alpha emission profiles, with weak but very broad emission wings, and relatively narrow absorption reversals. The rotational velocity distribution for low-mass stars in the Alpha Per cluster are compared with recently derived rotational velocity distributions for T Tauri stars, placing strong constraints on the mechanisms for angular momentum loss during pre-main-sequence evolution.

Stauffer, John R.↗