2,674 research outputs found

    True Airspeed Measurement by Ionization-Tracer Technique

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    Ion bundles produced in a pulse-excited corona discharge are used as tracers with a radar-like pulse transit-time measuring instrument in order to provide a measurement of airspeed that is independent of all variables except time and distance. The resulting instrumentation need not project into the air stream and, therefore, will not cause any interference in supersonic flow. The instrument was tested at Mach numbers ranging from 0.3 to 3.8. Use of the proper instrumentation and technique results in accuracy of the order of 1 percent

    Three-point Green function of the stress-energy tensor in the AdS/CFT correspondence

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    We compute the 3-point function of the stress-energy tensor in the d-dimensional CFT from the AdS_{d+1} gravity. For d=4 the coefficients of the three linearly independent conformally covariant forms entering the 3-point function are exactly the same as given by the free field computations in the N=4{\cal N}=4 SYM just as expected from the known renormalization theorems. For d=3 and d=6 our results give the value of the corresponding 3-point function in the theories of strongly coupled N=8{\cal N}=8 superconformal scalar and (2,0) tensor multiplets respectively.Comment: Latex, 13 pages, eq. (2.10) is correcte

    Some Cubic Couplings in Type IIB Supergravity on AdS5×S5AdS_5\times S^5 and Three-point Functions in SYM_4 at Large N

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    All cubic couplings in type IIB supergravity on AdS5×S5AdS_5\times S^5 that involve two scalar fields sIs^I that are mixtures of the five form field strength on S5S^5 and the trace of the graviton on S5S^5 are derived by using the covariant equations of motion and the quadratic action for type IIB supergravity on AdS5×S5AdS_5\times S^5. All corresponding three-point functions in SYM4_4 are calculated in the supergravity approximation. It is pointed out that the scalars sIs^I correspond not to the chiral primary operators in the N=4{\cal N}=4 SYM but rather to a proper extension of the operators.Comment: Latex, 24p, misprints are correcte

    A proposal for a new type of thin-film field-emission display by edge breakdown of MIS structure

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    A new type of field emission display(FED) based on an edge-enhance electron emission from metal-insulator-semiconductor (MIS) thin film structure is proposed. The electrons produced by an avalanche breakdown in the semiconductor near the edge of a top metal electrode are initially injected to the thin film of an insulator with a negative electron affinity (NEA), and then are injected into vacuum in proximity to the top electrode edge. The condition for the deep-depletition breakdown near the edge of the top metal electrode is analytically found in terms of ratio of the insulator thickness to the maximum (breakdown) width of the semiconductor depletition region: this ratio should be less than 2/(3 \pi - 2) = 0.27. The influence of a neighboring metal electrode and an electrode thickness on this condition are analyzed. Different practical schemes of the proposed display with a special reference to M/CaF_2/Si structure are considered.Comment: 11 pages, 5 figure

    Four-point Functions of Lowest Weight CPOs in N=4 SYM_4 in Supergravity Approximation

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    We show that the recently found quartic action for the scalars from the massless graviton multiplet of type IIB supergravity compactified on AdS_5\times S^5 background coincides with the relevant part of the action of the gauged N=8 5d supergravity on AdS_5. We then use this action to compute the 4-point function of the lowest weight chiral primary operators \tr(\phi^{(i}\phi^{j)}) in N=4 SYM_4 at large NN and at strong `t Hooft coupling.Comment: Latex, 21p, misprints are correcte

    The Age Difference between the Globular Cluster Sub-populations in NGC 4472

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    The age difference between the two main globular cluster sub-populations in the Virgo giant elliptical galaxy, NGC 4472 (M 49), has been determined using HST WFPC2 images in the F555W and F814W filters. Accurate photometry has been obtained for several hundred globular clusters in each of the two main sub-populations, down to more than one magnitude below the turn-over of their luminosity functions. This allows precise determinations of both the mean colors and the turn-over magnitudes of the two main sub-populations. By comparing the data with various population synthesis models, the age-metallicity pairs that fit both the observed colors and magnitudes have been identified. The metal-poor and the metal-rich globular clusters are found to be coeval within the errors (∌3\sim 3 Gyr). If one accepts the validity of our assumptions, these errors are dominated by model uncertainties. A systematic error of up to 4 Gyr could affect this result if the blue and the red clusters have significantly different mass distributions. However, that one sub-population is half as old as the other is excluded at the 99% confidence level. The different globular cluster populations are assumed to trace the galaxy's major star-formation episodes. Consequently, the vast majority of globular clusters, and by implication the majority of stars, in NGC 4472 formed at high redshifts but by two distinct mechanisms or in two episodes.Comment: 32 pages, including 12 postscript figures, accepted for publication in the Astronomical Journal, December 1999 issu

    Dissipative quantum mechanics and Kondo-like impurities on noncommutative two-tori

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    In a recent paper, by exploiting the notion of Morita equivalence for field theories on noncommutative tori and choosing rational values of the noncommutativity parameter Ξ\theta (in appropriate units), a general one-to-one correspondence between the mm-reduced conformal field theory (CFT) describing a quantum Hall fluid (QHF) at paired states fillings Μ=mpm+2\nu =% \frac{m}{pm+2} and an Abelian noncommutative field theory (NCFT) has been established . That allowed us to add new evidence to the relationship between noncommutativity and quantum Hall fluids\cite% {ncmanybody}. On the other hand, the mm-reduced CFT is equivalent to a system of two massless scalar bosons with a magnetic boundary interaction as introduced by Callan et al., at the so called ``magic''\ points. We are then able to describe, within such a framework, the dissipative quantum mechanics of a particle confined to a plane and subject to an external magnetic field normal to it. Here we develop such a point of view by focusing on the case m=2m=2 which corresponds to a quantum Hall bilayer. The key role of a localized impurity which couples the two layers is emphasized and the effect of noncommutativity in terms of generalized magnetic translations (GMT) is fully exploited. As a result, general GMT operators are introduced, in the form of a tensor product, which act on the QHF and defect space respectively, and a comprehensive study of their rich structure is performed.Comment: 18 pages, 1 figure, accepted for publication in International Journal of Modern Physics

    The M31 Globular Cluster Luminosity Function

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    We combine our compilation of photometry of M31 globular cluster and probable cluster candidates with new near-infrared photometry for 30 objects. Using these data we determine the globular cluster luminosity function (GCLF) in multiple filters for the M31 halo clusters. We find a GCLF peak and dispersion of V_0^0=16.84 +/-0.11, sigma_t=0.93 +/- 0.13 (Gaussian sigma = 1.20 +/- 0.14), consistent with previous results. The halo GCLF peak colors (e.g., B^0_0 - V^0_0) are consistent with the average cluster colors. We also measure V-band GCLF parameters for several other subsamples of the M31 globular cluster population. The inner third of the clusters have a GCLF peak significantly brigher than that of the outer clusters (delta V =~ 0.5mag). Dividing the sample by both galacticentric distance and metallicity, we find that the GCLF also varies with metallicity, as the metal-poor clusters are on average 0.36 mag fainter than the metal-rich clusters. Our modeling of the catalog selection effects suggests that they are not the cause of the measured differences, but a more complete, less-contaminated M31 cluster catalog is required for confirmation. Our results imply that dynamical destruction is not the only factor causing variation in the M31 GCLF: metallicity, age, and cluster initial mass function may also be important.Comment: AJ, in press. 36 pages, including 7 figure

    Holographic Normal Ordering and Multi-particle States in the AdS/CFT Correspondence

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    The general correlator of composite operators of N=4 supersymmetric gauge field theory is divergent. We introduce a means for renormalizing these correlators by adding a boundary theory on the AdS space correcting for the divergences. Such renormalizations are not equivalent to the standard normal ordering of current algebras in two dimensions. The correlators contain contact terms that contribute to the OPE; we relate them diagrammatically to correlation functions of compound composite operators dual to multi-particle states.Comment: 18 pages, one equation corr., further comments and refs. adde

    An HST/WFPC2 survey of bright young clusters in M31. I. VdB0, a massive star cluster seen at t ≃ 25 Myr

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    Aims. We introduce our imaging survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). We obtained shallow (to B ~ 25) photometry of individual stars in 20 candidate clusters. We present here details of the data reduction pipeline that is being applied to all the survey data and describe its application to the brightest among our targets, van den Bergh 0 (VdB0), taken as a test case. Methods. Point spread function fitting photometry of individual stars was obtained for all the WFPC2 images of VdB0 and the completeness of the final samples was estimated using an extensive set of artificial stars experiments. The reddening, the age and the metallicity of the cluster were estimated by comparing the observed color magnitude diagram (CMD) with theoretical isochrones. Structural parameters were obtained from model-fitting to the intensity profiles measured within circular apertures on the WFPC2 images. Results. Under the most conservative assumptions, the stellar mass of VdB0 is M> 2.4 x 10^4 M_☉ , but our best estimates lie in the range ≃4-9 x 10^4 M_☉. The CMD of VdB0 is best reproduced by models having solar metallicity and age ≃25 Myr. Ages less than ≃12 Myr and greater than ≃60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of red super giants (≳18) and a CMD very similar to Large Magellanic Cloud clusters usually classified as young globulars such as NGC 1850, for example. Conclusions. VdB0 is significantly brighter (≳1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius ((r_h = 7.4 pc) are more typical of faint globular clusters than of open clusters. However, given its position within the disk of M31, it is expected to be destroyed by dynamical effects, in particular by encounters with giant molecular clouds, within the next ~4 Gyr
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