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Cluster Formation (cluster + formation)
Selected AbstractsReversible Cluster Formation of Colloidal Nanospheres by Interparticle Photodimerization,ADVANCED FUNCTIONAL MATERIALS, Issue 5 2004X. Yuan Abstract Crosslinked spherical nanoparticles based on trimethoxysilane monomers have been prepared by polycondensation in aqueous emulsion. These particles have been labeled chemically at their surface region with two different types of organic dye molecules (cinnamate, coumarin), which both are well known for their ability to undergo a reversible photodimerization if irradiated with light of a suitable wavelength. Upon irradiation of dilute solutions of these nanoparticles with UV light, the photodimerization of labels belonging to different colloidal nanoparticles caused the formation of large colloidal clusters consisting of chemically bound individual nanospheres. This process has been quantitatively investigated using light scattering and atomic force microscopy. Importantly, utilizing the reversibility of the photoreaction, the clusters could be broken up by irradiation of the sample with UV light of shorter wavelengths than the light used for their formation [source] Spectroscopic Elucidation of First Steps of Supported Bimetallic Cluster Formation,ANGEWANDTE CHEMIE, Issue 51 2009Apoorva Kulkarni Die ersten Stufen der Bildung von Ru-Os-Clustern auf einer MgO-Oberfläche in Gegenwart von H2 wurden anhand von EXAFS- und IR-Spektren analysiert. Der Bildung der Ru-Os-Bindungen geht die Decarbonylierung von Ru3 - und dann, bei höheren Temperaturen, von Os3 -Clustern voraus (siehe Schema). [source] Cluster Formation as a Measure of Interpretability in Multiple TestingBIOMETRICAL JOURNAL, Issue 5 2008Juliet Popper Shaffer Abstract Multiple test procedures are usually compared on various aspects of error control and power. Power is measured as some function of the number of false hypotheses correctly identified as false. However, given equal numbers of rejected false hypotheses, the pattern of rejections, i.e. the particular set of false hypotheses identified, may be crucial in interpreting the results for potential application. In an important area of application, comparisons among a set of treatments based on random samples from populations, two different approaches, cluster analysis and model selection, deal implicitly with such patterns, while traditional multiple testing procedures generally focus on the outcomes of subset and pairwise equality hypothesis tests, without considering the overall pattern of results in comparing methods. An important feature involving the pattern of rejections is their relevance for dividing the treatments into distinct subsets based on some parameter of interest, for example their means. This paper introduces some new measures relating to the potential of methods for achieving such divisions. Following Hartley (1955), sets of treatments with equal parameter values will be called clusters. Because it is necessary to distinguish between clusters in the populations and clustering in sample outcomes, the population clusters will be referred to as P -clusters; any related concepts defined in terms of the sample outcome will be referred to with the prefix outcome. Outcomes of multiple comparison procedures will be studied in terms of their probabilities of leading to separation of treatments into outcome clusters, with various measures relating to the number of such outcome clusters and the proportion of true vs. false outcome clusters. The definitions of true and false outcome clusters and related concepts, and the approach taken here, is in the tradition of hypothesis testing with attention to overall error control and power, but with added consideration of cluster separation potential. The pattern approach will be illustrated by comparing two methods with apparent FDR control but with different ways of ordering outcomes for potential significance: The original Benjamini,Hochberg (1995) procedure (BH), and the Newman,Keuls (Newman, 1939; Keuls, 1952) procedure (NK). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Distributed end-host multicast algorithms for the Knowledge GridCONCURRENCY AND COMPUTATION: PRACTICE & EXPERIENCE, Issue 15 2007Wanqing Tu Abstract The Knowledge Grid built on top of the peer-to-peer (P2P) network has been studied to implement scalable, available and sematic-based querying. In order to improve the efficiency and scalability of querying, this paper studies the problem of multicasting queries in the Knowledge Grid. An m -dimensional irregular mesh is a popular overlay topology of P2P networks. We present a set of novel distributed algorithms on top of an m -dimensional irregular mesh overlay for the short delay and low network resource consumption end-host multicast services. Our end-host multicast fully utilizes the advantages of an m -dimensional mesh to construct a two-layer architecture. Compared to previous approaches, the novelty and contribution here are: (1) cluster formation that partitions the group members into clusters in the lower layer where cluster consists of a small number of members; (2) cluster core selection that searches a core with the minimum sum of overlay hops to all other cluster members for each cluster; (3) weighted shortest path tree construction that guarantees the minimum number of shortest paths to be occupied by the multicast traffic; (4) distributed multicast routing that directs the multicast messages to be efficiently distributed along the two-layer multicast architecture in parallel, without a global control; the routing scheme enables the packets to be transmitted to the remote end hosts within short delays through some common shortest paths; and (5) multicast path maintenance that restores the normal communication once the membership alteration appears. Simulation results show that our end-host multicast can distributively achieve a shorter delay and lower network resource consumption multicast services as compared with some well-known end-host multicast systems. Copyright © 2006 John Wiley & Sons, Ltd. [source] Fine-needle aspiration cytology of pleomorphic hyalinized angiectatic tumor: A case reportDIAGNOSTIC CYTOPATHOLOGY, Issue 4 2005Oscar Lin M.D., Ph.D. Abstract Pleomorphic hyalinized angiectatic tumor (PHAT) of soft parts is a neoplasm characterized by spindle and pleomorphic cells associated with an angiectatic vasculature. We describe the cytological findings of a fine-needle aspiration biopsy (FNAB) from the right medial knee of a 45-yr-old woman. The aspirate material was entirely submitted in Cytolit solution. The specimen was moderately cellular and was comprised of spindle cells in a background of fibrinous material. The cells varied from small, bland spindle cells with a fine chromatin pattern and inconspicuous nucleoli to larger pleomorphic cells with coarser chromatin and occasional intranuclear inclusions. Most of the cells were arranged singly with sporadic small cluster formation with indistinct cell borders. Rare mononuclear inflammatory cells morphologically compatible with mast cells were identified. The differential diagnosis include solitary fibrous tumor (SFT) and ancient schwannoma, which also shows fibrous-like material and spindle cells that may have intranuclear inclusions. Diagn. Cytopathol. 2005;32:238,242. © 2005 Wiley-Liss, Inc. [source] Sustained LFA-1 cluster formation in the immune synapse requires the combined activities of L -plastin and calmodulinEUROPEAN JOURNAL OF IMMUNOLOGY, Issue 9 2010Guido H. Wabnitz Abstract Formation of immune synapses (IS) between T cells and APC requires multiple rearrangements in the actin cytoskeleton and selective receptor accumulation in supramolecular activation clusters (SMAC). The inner cluster (central SMAC) contains the TCR/CD3 complex. The outer cluster (peripheral SMAC) contains the integrin LFA-1 and Talin. Molecular mechanisms selectively stabilizing receptors in the IS remained largely unknown. Here, we demonstrate that sustained LFA-1 clustering in the IS is a consequence of the combined activities of the actin-bundling protein L -plastin (LPL) and calmodulin. Thus, upon antigen-recognition of T cells, LPL accumulated predominantly in the peripheral SMAC. siRNA-mediated knock-down of LPL led to a failure of LFA-1 and Talin redistribution , however, not TCR/CD3 relocalization , into the IS. As a result of this LPL knock-down, the T-cell/APC interface became smaller over time and T-cell proliferation was inhibited. Importantly, binding of calmodulin to LPL was required for the maintenance of LPL in the IS and consequently inhibition of calmodulin also prevented stable accumulation of LFA-1 and Talin, but not CD3, in the IS. [source] Bacterial IscU is a well folded and functional single domain proteinFEBS JOURNAL, Issue 11 2004Salvatore Adinolfi Iron,sulfur clusters are widely represented in most organisms, but the mechanism of their formation is not fully understood. Of the two main proteins involved in cluster formation, NifS/IscS and NifU/IscU, only the former has been well studied from a structural point of view. Here we report an extensive structural characterization of Escherichia coli IscU. We show by a variety of physico-chemical techniques that E. coli IscU construct can be expressed to high purity as a monomeric protein, characterized by an ,, fold with high ,-helix content. The high melting temperature and the reversibility of the thermal unfolding curve (as measured by CD spectroscopy) hint at a well ordered stable fold. The excellent dispersion of cross peaks in the 1H- 15N correlation spectrum is consistent with these observations. Monomeric E. coli IscU is able to provide a scaffold for Iron,sulfur cluster assembly, but has no direct interaction with either Fe(II) or Fe(III) ions, suggesting the need of further partners to achieve a stable interaction. [source] Structural patterns in coarse gravelriver beds: typology, survey and assessment of the roles of grain size and river regimeGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 1 2002Lea Wittenberg The concept of river-bed stability as indexed by the occurrence of stable bed forms was examined in humid-temperate perennial streams and in Mediterranean ephemeral streams. The study examined the structural patterns of bed forms and their spatial distribution between temperate-humid and Mediterranean streams. Study sites in Northumberland, UK, and Mt. Carmel, Israel, were selected for their morphometric similarity, despite the contrast in climate, vegetation and hydrological regime. Fieldwork was based on a large number of Wolman grain size distributions and structure measurements along cross-sections at seven sites; Differences in mean grain size of bed structures were estimated using the general linear model (GLM) procedure and Duncan's multiple range test. Based on field evidence, river-bed configurations were divided into structural categories, according to the depositional setting of each measured particle on the river bed. Statistical analysis confirmed former qualitative descriptions of small-scale bed forms. The study identified spatial segregation in bed form distribution. In general, 30,40%of the bed material in the surveyed perennial streams was clustered, in contrast to approximately 10%in the ephemeral counterparts. The sorting index revealed higher values for the perennial streams, namely 2.39,3.59 compared with 1.73,2.07 for the ephemeral counterparts. It is suggested that the degree of sediment sorting and the proportion of clusters are strongly related. Sediment sorting, sediment supply and the hydrological regime explain the mechanism of cluster formation. It is assumed that climate shifts or human interference within river basins might affect the regional characteristic flood hydrograph, and consequently alter the sedimentary character of the river bed. In the case where river bed stability is reduced owing to changes in cluster bed form distribution, rivers that normally do not yield a significant amount of sediment might be subject to notable sedimentation problems. [source] Exploiting Context-Awareness in Cluster-Based Wireless Sensor NetworksIEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING, Issue 5 2009Md Enamul Haque Non-member Abstract Most of the wireless sensor network's (WSN's) applications require getting accurate information as well as uninterrupted, prolonged service life. Among the constituting elements of WSN, an efficient routing protocol plays a significant role in attaining such service requirements. In this article, an energy efficient routing protocol, context-aware clustering hierarchy (CACH), is proposed where cluster formation is based on the context of the environment. Moreover, a technique is utilized to avoid similar data traffic across the network. The performance in the simulation shows energy saving which ensures prolonged service life. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [source] Autonomic self-organization architecture for wireless sensor communicationsINTERNATIONAL JOURNAL OF NETWORK MANAGEMENT, Issue 3 2007Jiann-Liang Chen Wireless sensor nodes may be spread over large areas and long distances, and require multi-hop communications between nodes, making direct management numerous wireless sensor nodes inefficient. Hierarchical management can be adopted to control several nodes. Effectively controlling the top-level nodes can decrease the costs of managing nodes and of the communication among them. The lower-level nodes are controlled and organized with the higher-level nodes. This study presents an algorithm for self-organization mechanism of higher-level nodes, contesting member nodes by multi-hop to form hierarchical clusters, and applying the ,20/80 rule' to determine the ratio of headers to member nodes. Furthermore, the broadcast tree is constructed with the minimum number of hops. Simulation results indicate that the mechanism has a 6,22% lower cover loss than other approaches. The average delay of the minimum hop count approach is 0.22,1.57ms less than that of free hop count approach. The simulation also reveals the influence of 20/80 rule on cluster formation between sensor nodes. Copyright © 2006 John Wiley & Sons, Ltd. [source] The two-Yukawa model and its applications: the cases of charged proteins and copolymer micellar solutionsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2007Sow-Hsin Chen Charged and uncharged colloidal systems are known from experiment to display an extremely rich phase behavior, which is ultimately determined by the effective pair potential between particles in solution. As a confirmation, the recent striking observation of an equilibrium cluster phase in charged globular protein solutions [Stradner, Sedgwick, Cardinaux, Poon, Egelhaaf & Schurtenberger (2004). Nature, 432, 492,495] has been interpreted as the effect of competing short-range attractive and long-range repulsive interactions. The `two-Yukawa (2Y) fluid' model assumes an interparticle potential consisting of a hard core plus an attractive and a repulsive Yukawa tail. We show that this rather simple model can indeed explain satisfactorily the structural properties of diverse colloidal materials, measured in small-angle neutron scattering (SANS) experiments, including the cases of equilibrium cluster formation and soft-core repulsion. We apply this model to the analysis of SANS data from horse-heart cytochrome c protein solutions (whose effective potential can be modeled as a hard-sphere part plus a short-range attraction and a weaker screened electrostatic repulsion) and micellar solutions of a triblock copolymer (whose effective potential can be modeled as a hard-sphere part plus a repulsive shoulder and a short-range attraction). The accuracy of the 2Y model predictions is successfully tested against Monte Carlo simulations in both cases. [source] Critical amino acid residues of the ,4 subunit for ,4,7 integrin functionJOURNAL OF CELLULAR BIOCHEMISTRY, Issue 2 2001Yvonka Zeller Abstract A characteristic feature of integrin,ligand interactions is the requirement for divalent cations. Putative cation binding sites have been identified in the , and , subunit of the ,4 integrins, ,4,1 and ,4,7, and within their ligands which display the tripeptide LDV in fibronectin and homologous motifs in VCAM-1 and MAdCAM-1. The extracellular domain of the murine and human ,4-subunit contains three conserved LDV motifs, designated LDV-1 to -3. Using site directed mutagenesis and transfection studies, we now examined the functional relevance of the LDV motifs for ,4,7 integrins. We present evidence that LDV-1 mutants (D489N) behave like ,4 wt cells, but LDV-3 mutants (D811N) are impaired in ,4,7 integrin-triggered homotypic cell aggregation and in adhesion and spreading on ,4 specific ligands. Further characterization of LDV-3 mutants revealed a defect in mAb-induced ,4,7-cell surface cluster formation. Mutation of the LDV-2 motif (D698N) caused loss of ,4,7 integrin cell surface expression. Our results indicate: (i) that LDV-3, located proximal to the cell membrane, is important for ,4,7 integrin-triggered functions and for lateral clustering and (ii) that LDV-2 affects ,4,7 heterodimer stability. J. Cell. Biochem. 83: 304,319, 2001. © 2001 Wiley-Liss, Inc. [source] The effect of temperature on the stability of compounds used as UV-MALDI-MS matrix: 2,5-dihydroxybenzoic acid, 2,4,6-trihydroxyacetophenone, ,-cyano-4-hydroxycinnamic acid, 3,5-dimethoxy-4-hydroxycinnamic acid, nor-harmane and harmaneJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 2 2009Olga I. Tarzi Abstract The thermal stability of several commonly used crystalline matrix-assisted ultraviolet laser desorption/ionization mass spectrometry (UV-MALDI-MS) matrices, 2,5-dihydroxybenzoic acid (gentisic acid; GA), 2,4,6-trihydroxyacetophenone (THA), ,-cyano-4-hydroxycinnamic acid (CHC), 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid; SA), 9H-pirido[3,4-b]indole (nor-harmane; nor-Ho), 1-methyl-9H-pirido[3,4-b]indole (harmane; Ho), perchlorate of nor-harmanonium ([nor-Ho + H]+) and perchlorate of harmanonium ([Ho + H]+) was studied by heating them at their melting point and characterizing the remaining material by using different MS techniques [electron ionization mass spectrometry (EI-MS), ultraviolet laserdesorption/ionization-time-of-flight-mass spectrometry (UV-LDI-TOF-MS) and electrospray ionization-time-of-flight-mass spectrometry (ESI-TOF-MS)] as well as by thin layer chromatography analysis (TLC), electronic spectroscopy (UV-absorption, fluorescence emission and excitation spectrosco y) and 1H nuclear magnetic resonance spectroscopy (1H-NMR). In general, all compounds, except for CHC and SA, remained unchanged after fusion. CHC showed loss of CO2, yielding the trans-/cis -4-hydroxyphenylacrilonitrile mixture. This mixture was unambiguously characterized by MS and 1H-NMR spectroscopy, and its sublimation capability was demonstrated. These results explain the well-known cluster formation, fading (vanishing) and further recovering of CHC when used as a matrix in UV-MALDI-MS. Commercial SA (SA 98%; trans -SA/cis -SA 5 : 1) showed mainly cis- to- trans thermal isomerization and, with very poor yield, loss of CO2, yielding (3,,5,-dimethoxy-4,-hydroxyphenyl)-1-ethene as the decarboxilated product. These thermal conversions would not drastically affect its behavior as a UV-MALDI matrix as happens in the case of CHC. Complementary studies of the photochemical stability of these matrices in solid state were also conducted. Copyright © 2008 John Wiley & Sons, Ltd. [source] Overexpression of nPKC , is inhibitory for agrin-induced nicotinic acetylcholine receptor clustering in C2C12 myotubesJOURNAL OF NEUROSCIENCE RESEARCH, Issue 2 2003Kathryn Miles Abstract Protein kinase C (PKC) activity has been implicated in nicotinic acetylcholine receptor (nAChR) cluster disruption but the specific PKC isoforms involved have not been identified. We first tested whether phorbol esters, which activate PKCs, regulate agrin-induced nAChR clustering in C2C12 cells. We found that extended phorbol ester treatment (6 hr) increased nAChR clustering by two-fold. This increase correlated in time with downregulation of PKCs, as indicated by the disappearance of cPKC ,, suggesting that the presence of PKCs is inhibitory for maximal nAChR clustering. To address the question whether nPKC ,, a specific PKC isoform restricted in expression to skeletal muscle and localized to neuromuscular junctions, regulates agrin-induced nAChR cluster formation we overexpressed an nPKC , -green fluorescent protein (GFP) fusion protein in C2C12 myotubes. The number of nAChR clusters was significantly reduced in nPKC ,-GFP compared to GFP overexpressing myotubes at less-than-maximal clustering concentrations of agrin. These data indicate that nPKC , activity inhibits nAChR cluster formation. To examine whether nPKC , activation by phorbol esters regulates agrin-induced nAChR clustering, we treated overexpressing myotubes overnight with maximal agrin concentrations followed by phorbol esters for 1 hr. Phorbol ester treatment reduced preexisting nAChR cluster numbers in nPKC ,-GFP compared to GFP-overexpressing myotubes, suggesting that stimulating nPKC , activity disrupts nAChR clusters in the presence of maximal clustering concentrations of agrin. Together these findings, that nPKC , activity inhibits agrin-induced nAChR cluster formation and disrupts preexisting agrin-induced nAChR clusters, suggest that nPKC , activity is inhibitory for agrin function. © 2002 Wiley-Liss, Inc. [source] Nonstoichiometry in A2B2O7 PyrochloresJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2002Christopher R. Stanek Energies associated with deviations from stoichiometry for an extensive series of A2B2O7 pyrochlores have been predicted. A3+ cations range in size from Lu to La and B4+ cations from Ti to Pb. Results are presented in the form of contour maps as a means of conveying large quantities of data as well as predicting characteristics for pyrochlore compounds not explicitly modeled. These contour maps indicate that the BO2 excess nonstoichiometry in the pyrochlore structure is distinct from solid-solution fluorite. Within the limitations of this methodology, the contour maps provide a means to understand and predict distinct compositional variations. Defect cluster formation is discussed. [source] Staphylococcal NreB: an O2 -sensing histidine protein kinase with an O2 -labile iron,sulphur cluster of the FNR typeMOLECULAR MICROBIOLOGY, Issue 3 2004Annegret Kamps Summary The nreABC (nitrogen regulation) operon encodes a new staphylococcal two-component regulatory system that controls dissimilatory nitrate/nitrite reduction in response to oxygen. Unlike other two-component sensors NreB is a cytosolic protein with four N-terminal cysteine residues. It was shown that both the NreB,cysteine cluster and Fe ions are required for function. Isolated NreB was converted to the active form by incubation with cysteine desulphurase, ferrous ions and cysteine. This activation is typical for FeS-containing proteins and was reversed by oxygen. During reconstitution an absorption band at 420 nm and a yellow-brownish colour (typical for an FNR-type iron,sulphur cluster formation) developed. After alkylation of thiol groups in NreB and in the cysteine mutant NreB(C62S) almost no iron,sulphur cluster was incorporated; both findings corroborated the importance of the cysteine residues. Comparison of the kinase activity of (i) the reconstituted (ii) the unreconstituted, and (iii) the unreconstituted and deferrated NreB,His indicated that NreB kinase activity depended on iron availability and was greatly enhanced by reconstitution. NreB is the first direct oxygen-sensing protein described in staphylococci so far. Reconstituted NreB contains 4,8 acid-labile Fe and sulphide ions per NreB which is in agreement with the presence of 1,2 iron,sulphur [4Fe-4S]2+ clusters of the FNR-type. Unlike FNR, NreB does not act directly as transcriptional activator, but transfers the phosphoryl group to the response regulator NreC. [source] A comprehensive set of simulations studying the influence of gas expulsion on star cluster evolutionMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007H. Baumgardt ABSTRACT We have carried out a large set of N -body simulations studying the effect of residual-gas expulsion on the survival rate, and final properties of star clusters. We have varied the star formation efficiency (SFE), gas expulsion time-scale and strength of the external tidal field, obtaining a three-dimensional grid of models which can be used to predict the evolution of individual star clusters or whole star cluster systems by interpolating between our runs. The complete data of these simulations are made available on the internet. Our simulations show that cluster sizes, bound mass fraction and velocity profile are strongly influenced by the details of the gas expulsion. Although star clusters can survive SFEs as low as 10 per cent if the tidal field is weak and the gas is removed only slowly, our simulations indicate that most star clusters are destroyed or suffer dramatic loss of stars during the gas removal phase. Surviving clusters have typically expanded by a factor of 3 or 4 due to gas removal, implying that star clusters formed more concentrated than as we see them today. Maximum expansion factors seen in our runs are around 10. If gas is removed on time-scales smaller than the initial crossing time, star clusters acquire strongly radially anisotropic velocity dispersions outside their half-mass radii. Observed velocity profiles of star clusters can therefore be used as a constraint on the physics of cluster formation. [source] The evolution of the cluster X-ray scaling relations in the Wide Angle ROSAT Pointed Survey sample at 0.6 < z < 1.0MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006B. J. Maughan ABSTRACT The X-ray properties of a sample of 11 high-redshift (0.6 < z < 1.0) clusters observed with Chandra and/or XMM,Newton are used to investigate the evolution of the cluster scaling relations. The observed evolution in the normalization of the L,T, M,T, Mg,T and M,L relations is consistent with simple self-similar predictions, in which the properties of clusters reflect the properties of the Universe at their redshift of observation. Under the assumption that the model of self-similar evolution is correct and that the local systems formed via a single spherical collapse, the high-redshift L,T relation is consistent with the high- z clusters having virialized at a significantly higher redshift than the local systems. The data are also consistent with the more realistic scenario of clusters forming via the continuous accretion of material. The slope of the L,T relation at high redshift (B= 3.32 ± 0.37) is consistent with the local relation, and significantly steeper than the self-similar prediction of B= 2. This suggests that the same non-gravitational processes are responsible for steepening the local and high- z relations, possibly occurring universally at z, 1 or in the early stages of the cluster formation, prior to their observation. The properties of the intracluster medium at high redshift are found to be similar to those in the local Universe. The mean surface-brightness profile slope for the sample is ,= 0.66 ± 0.05, the mean gas mass fractions within R2500(z) and R200(z) are 0.069 ± 0.012 and 0.11 ± 0.02, respectively, and the mean metallicity of the sample is 0.28 ± 0.11 Z,. [source] Turbulent gas motions in galaxy cluster simulations: the role of smoothed particle hydrodynamics viscosityMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005K. Dolag ABSTRACT Smoothed particle hydrodynamics (SPH) employs an artificial viscosity to properly capture hydrodynamic shock waves. In its original formulation, the resulting numerical viscosity is large enough to suppress structure in the velocity field on scales well above the nominal resolution limit, and to damp the generation of turbulence by fluid instabilities. This could artificially suppress random gas motions in the intracluster medium (ICM), which are driven by infalling structures during the hierarchical structure formation process. We show that this is indeed the case by analysing results obtained with an SPH formulation where an individual, time-variable viscosity is used for each particle, following a suggestion by Morris & Monaghan. Using test calculations involving strong shocks, we demonstrate that this scheme captures shocks as well as the original formulation of SPH, but, in regions away from shocks, the numerical viscosity is much smaller. In a set of nine high-resolution simulations of cosmological galaxy cluster formation, we find that this low-viscosity formulation of SPH produces substantially higher levels of turbulent gas motions in the ICM, reaching a kinetic energy content in random gas motions (measured within a 1-Mpc cube) of up to 5,30 per cent of the thermal energy content, depending on cluster mass. This also has significant effects on radial gas profiles and bulk cluster properties. We find a central flattening of the entropy profile and a reduction of the central gas density in the low-viscosity scheme. As a consequence, the bolometric X-ray luminosity is decreased by about a factor of 2. However, the cluster temperature profile remains essentially unchanged. Interestingly, this tends to reduce the differences seen in SPH and adaptive mesh refinement simulations of cluster formation. Finally, invoking a model for particle acceleration by magnetohydrodynamics waves driven by turbulence, we find that efficient electron acceleration and thus diffuse radio emission can be powered in the clusters simulated with the low-viscosity scheme provided that more than 5,10 per cent of the turbulent energy density is associated with fast magneto-sonic modes. [source] MSX mid-infrared imaging of massive star birth environments , II.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2004Giant H ii regions ABSTRACT We conduct a Galactic census of giant H ii (GH ii) regions, based on the all-sky 6-cm data set of Kuchar & Clark, plus the kinematic distances obtained by Russeil. From an inspection of mid-infrared (MIR) Mid-course Space Experiment (MSX) and far-IR IRAS Sky Survey Atlas images, we identify a total of 56 GH ii regions in the Milky Way, of which 15 per cent (65 per cent) can be seen at optical (near-IR) wavelengths. The mid to far-IR fluxes from each GH ii region are measured, and sample the thermal emission from the ubiquitous dust present within the exciting clusters of OB stars, arising from the integrated luminosity of the hot stars heating the cluster dust, for which we obtain log L(IR) = 5.5,7.3 L,. The MIR 21-,m spatial morphology is presented for each GH ii region, and often indicates multiple emission sources, suggesting complicated cluster formation. IR colour,colour diagrams are presented, providing information concerning the temperature distribution and the optical depth of the dust. For the clusters of our study, the dust is not optically thick to all stellar radiation, thus the measured infrared luminosity is lower than Lbol. As the dust environment of a cluster begins to dissipate, the thermal emission and its optical depth ought to decrease even before the stars evolve appreciably. We see evidence of this in our empirical relationship between the integrated IR and Lyman continuum luminosities. [source] High-resolution simulations of galaxy mergers: resolving globular cluster formationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008F. Bournaud ABSTRACT Massive star clusters observed in galaxy mergers are often suggested to be progenitors of globular clusters. To study this hypothesis, we performed the highest resolution simulation of a gas-rich galaxy merger so far. The formation of massive star clusters of 105 to 107 M,, triggered by the galaxy interaction, is directly resolved in this model. We show that these clusters are tightly bound structures with little net rotation, due to evolve into compact long-lived stellar systems. Massive clusters formed in galaxy mergers are thus robust candidates for progenitors of long-lived globular clusters. The simulated cluster mass spectrum is consistent with theory and observations. Tidal dwarf galaxies of 108,9 M, can form at the same time, and appear to be part of a different class of objects, being more extended and rotating. [source] Self-consistent simulations of star cluster formation from gas clouds under the influence of galaxy-scale tidal fieldsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008Jarrod R. Hurley ABSTRACT We present the first results of a project aimed at following the formation and long-term dynamical evolution of star clusters within the potential of a host galaxy. Here, we focus on a model evolved within a simplified potential representing the Large Magellanic Cloud. This demonstrates for the first time the self-consistent formation of a bound star cluster from a giant molecular cloud. The model cluster reproduces the density profiles and structural characteristics of observed star clusters. [source] Cross-sectional AFM study of etching kinetics of oxidized porous siliconPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 4 2004J. C. Poler Abstract We have studied the morphology and etching kinetics of partially oxidized porous silicon films. Film morphology and thickness was measured using cross-sectional atomic force microscopy. Air and UV-ozone oxidized porous silicon films were etched in buffered hydrofluoric acid. The observed etching rate was much faster than expected for a bulk film. These data consist of: direct visualization of cluster formation, enhanced etching rates and subsequent analysis of the liberated nanoparticles, and consistent with a mesoporous morphology of undulating Si nanowires. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Landing distance in a synchronic North American fireflyPHYSIOLOGICAL ENTOMOLOGY, Issue 2 2008JONATHAN COPELAND Abstract The flash communication system of Photinus carolinus in Great Smoky Mountain National Park (Tennessee) is characterized by male firefly synchrony. Photinus carolinus males signal a conspecific female with synchronic trains of flashes. A solitary responding female attracts several males, which is not common in North American rover fireflies. The female and the group of males that she attracts are called a cluster. It is hypothesized that the first male attracted to the female would land closer to the female than would additional males because there would be less tendency for visual confusion. This hypothesis is explored under controlled conditions by replacing the responsive female with an appropriately flashing light-emitting diode (LED) located in the center of a flat target area. When infra-red videography is used to measure the first male and the additional males' landing distances from the counterfeit female (LED), most fireflies land within 15 cm of the target LED, and the first male does not land closer than the additional males. It is suggested that cluster formation is a by-product of male synchrony and is facilitated by the tendency of males to land near, but not on, females. [source] Characterization of ammonium chloride derivatives by salt clustering in electrospray mass spectrometryRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 22 2003Laurence Charles Clustering of ammonium chloride salts was studied using an electrospray source to characterize the form, mono- vs. dihydrochloride, of organic compounds by mass spectrometry. This new way of taking advantage of cluster formation is applied to aminomethylacridines as their storage requires the synthesis of such derivatives. Both positive and negative cluster mass spectra were obtained and allowed the determination of the nature of the hydrochloride forms as the salt mass is straightforwardly derived from the mass of the cluster monomer, calculated from singly and multiply charged ion distributions. The identity of the counterion is confirmed from the mass of the ionic moieties in the clusters. Copyright © 2003 John Wiley & Sons, Ltd. [source] Systematic Reactions of [Pt(PF3)4]CHEMISTRY - A EUROPEAN JOURNAL, Issue 14 2008Thomas Drews Abstract Tetrahedral [Pt(PF3)4] reacts with H+ to form trigonal bipyramidal [Pt(PF3)4H]+. This in turn looses PF3 to form square-planar [Pt(PF3)3H]+. The complex [Pt(PF3)4] can be oxidized with AsF5 to form the square-planar complex, [Pt(PF3)4]2+, which can be more conveniently obtained from PtF4 and PF3 in HF/SbF5 solution. [Pt(PF3)4]2+ reacts with F, in HF under cluster formation to [Pt4(PF3)8H]+. [source] Structural changes in Si crystals exposed to chemical etching and ion implantationPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 8 2009I. Fodchuk Abstract Results of X-ray diffraction studies on structural changes in Si crystals caused by chemical etching for 40,s in HF, HNO3 solution and phosphorus-ion implantation with the energy E,=,100,keV and dose D,=,8,×,1014,cm,2 are represented. Two- and three-crystal X-ray diffractometry methods are used for a quantitative assessment of structural changes occurring in the near-surface crystal layers. Analysis of experimental rocking curves was made with the use of different approaches developed on the basis of kinematic and dynamic theory of X-ray scattering in the imperfect crystals. A model of possible system of structural defects in the near-surface silicon layers modified by chemical etching and ion implantation is proposed. The model accounts for the availability of respective sizes and concentrations of spherical and disc-shaped cluster formations, dislocation loops. [source] | ||||||||||||||||||||||||||||