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Search Results: 1 - 10 of 625149 matches for " M. S. Warren "
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A boundary value problem for minimal Lagrangian graphs
S. Brendle,M. Warren
Mathematics , 2008,
Abstract: Let \Omega and \tilde{\Omega} be uniformly convex domains in \mathbb{R}^n with smooth boundary. We show that there exists a diffeomorphism f: \Omega \to \tilde{\Omega} such that the graph \Sigma = \{(x,f(x)): x \in \Omega\} is a minimal Lagrangian submanifold of \mathbb{R}^n \times \mathbb{R}^n.
Modeling Core-Collapse Supernovae in 3-Dimensions
C. L. Fryer,M. S. Warren
Physics , 2002, DOI: 10.1086/342258
Abstract: We present the first complete 3-dimensional simulations of the core-collapse of a massive star from the onset of collapse to the resultant supernova explosion. We compare the structure of the convective instabilities that occur in 3-dimensional models with those of past 2-dimensional simulations. Although the convective instabilities are clearly 3-dimensional in nature, we find that both the size-scale of the flows and the net enhancement to neutrino heating does not differ greatly between 2- and 3-dimensional models. The explosion energy, explosion timescale, and remnant mass does not differ by more than 10% between 2- and 3-dimensional simulations.
NICMOS imaging search for damped Lya galaxies
S. J. Warren,P. Moller,S. M. Fall,P. Jakobsen
Physics , 2001, DOI: 10.1046/j.1365-8711.2001.04629.x
Abstract: We are engaged in a programme of imaging with the STIS and NICMOS (NIC2) instruments aboard the Hubble Space Telescope (HST), to search for the galaxy counterparts of 18 high-redshift z>1.75 damped Lya absorption lines and 5 Lyman-limit systems seen in the spectra of 16 target quasars. This paper presents the results of the imaging campaign with the NIC2 camera. We describe the steps followed in reducing the data and combining in mosaics, and the methods used for subtracting the image of the quasar in each field, and for constructing error frames that include the systematic errors associated with the psf subtraction. To identify candidate counterparts, that are either compact or diffuse, we convolved the image and variance frames with circular top-hat filters of diameter 0.45 and 0.90 arcsec respectively, to create frames of summed S/N within the aperture. For each target quasar we provide catalogues listing positions and aperture magnitudes of all sources within a square of side 7.5 arcsec centred on the quasar, detected at S/N>6. We find a total of 41 candidates of which three have already been confirmed spectroscopically as the counterparts. We provide the aperture magnitude detection limits as a function of impact parameter, for both detection filters, for each field. The average detection limit for compact (diffuse) sources is H(AB)=25.0 (24.4) at an angular separation of 0.56 arcsec (0.79 arcsec) from the quasar, improving to H(AB)=25.5 (24.8) at large angular separations. For the brighter sources we have measured the half-light radius and the n parameter of the best-fit deconvolved Sersic-law surface-brightness profile, and the ellipticity and orientation.
Computational Studies of Light Shift in Raman-Ramsey Interference-Based Atomic Clock
G. S. Pati,Z. Warren,N. Yu,M. S. Shahriar
Physics , 2014, DOI: 10.1364/JOSAB.32.000388
Abstract: Determining light shift in Raman-Ramsey interference is important for the development of atomic frequency standards based on a vapor cell. We have accurately calculated light shift in Raman-Ramsey interference using the density-matrix equations for a three-level system without invoking the adiabatic approximation. Specifically, phase shifts associated with coherent density-matrix terms are studied as they are relevant to the detection of Raman-Ramsey interference in transmission (or absorption) through the medium. For the single-velocity case, the numerically computed results are compared with the analytical results obtained using the adiabatic approximation. The result shows light shift suppression in conformity with the closed-form analytic solutions. The computational studies have also been extended to investigate Raman-Ramsey interference for a Doppler-broadened vapor medium. Importantly, a velocity-induced frequency shift is found at the fringe center as an additional source of frequency error for a vapor cell Raman clock.
Estimating Omega from Galaxy Redshifts: Linear Flow Distortions and Nonlinear Clustering
B. C. Bromley,M. S. Warren,W. H. Zurek
Physics , 1997, DOI: 10.1086/303552
Abstract: We propose a method to determine the cosmic mass density Omega from redshift-space distortions induced by large-scale flows in the presence of nonlinear clustering. Nonlinear structures in redshift space such as fingers of God can contaminate distortions from linear flows on scales as large as several times the small-scale pairwise velocity dispersion sigma_v. Following Peacock & Dodds (1994), we work in the Fourier domain and propose a model to describe the anisotropy in the redshift-space power spectrum; tests with high-resolution numerical data demonstrate that the model is robust for both mass and biased galaxy halos on translinear scales and above. On the basis of this model, we propose an estimator of the linear growth parameter beta = Omega^0.6/b, where b measures bias, derived from sampling functions which are tuned to eliminate distortions from nonlinear clustering. The measure is tested on the numerical data and found to recover the true value of beta to within ~10%. An analysis of the IRAS 1.2Jy galaxies yields beta = 0.8+0.4/-0.3 at a scale of 1,000 km/s which is close to optimal given the shot noise and the finite survey volume. This measurement is consistent with dynamical estimates of beta derived from both real-space and redshift-space information. The importance of the method presented here is that nonlinear clustering effects are removed to enable linear correlation anisotropy measurements on scales approaching the translinear regime. We discuss implications for analyses of forthcoming optical redshift surveys in which the dispersion is more than a factor of two greater than in the IRAS data.
Gamma-Ray Lines from Asymmetric Supernovae
A. L. Hungerford,C. L. Fryer,M. S. Warren
Physics , 2003, DOI: 10.1086/376776
Abstract: We present 3-dimensional SPH simulations of supernova explosions from 100 seconds to 1 year after core-bounce. By extending our modelling efforts to a 3-dimensional hydrodynamics treatment, we are able to investigate the effects of explosion asymmetries on mixing and gamma-ray line emergence in supernovae. A series of initial explosion conditions are implemented, including jet-like and equatorial asymmetries of varying degree. For comparison, symmetric explosion models are also calculated. A series of time slices from the explosion evolution are further analyzed using a 3-dimensional Monte Carlo gamma-ray transport code. The emergent hard X- and gamma-ray spectra are calculated as a function of both viewing angle and time, including trends in the gamma-ray line profiles. We find significant differences in the velocity distribution of radioactive nickel between the symmetric and asymmetric explosion models. The effects of this spatial distribution change are reflected in the overall high energy spectrum, as well as in the individual gamma-ray line profiles.
A small area faint KX redshift survey for QSOs in the ESO Imaging Survey Chandra Deep Field South
Scott M. Croom,S. J. Warren,K. Glazebrook
Physics , 2001, DOI: 10.1046/j.1365-8711.2001.04846.x
Abstract: In this paper we present preliminary spectroscopic results from a small area faint K-excess (KX) survey, and compare KX selection against UVX selection. The aim of the KX method is to produce complete samples of QSOs flux-limited in the K band, in order to minimize any selection bias in samples of QSOs from the effects of reddening and extinction. Using the photometric catalogue of the ESO Imaging Survey Chandra Deep Field South (48 arcmin^2) we have identified compact objects with J-K colours redder than the stellar sequence, that are brighter than K=19.5. We have obtained spectra of 33 candidates, using the LDSS++ spectrograph on the AAT. Amongst the 11 bluer candidates, with V-J<3, three are confirmed as QSOs. Identification of the 22 redder candidates with V-J>3 is substantially incomplete, but so far no reddened QSOs have been found. Near-infrared spectroscopy will be more effective in identifying some of these targets. Only two UVX (U-B<-0.2) sources brighter than K=19.5 are found which are not also KX selected. These are both identified as galactic stars. Thus KX selection appears to select all UVX QSOs. The surface density of QSOs in the blue subsample (V-J<3) at K<19.5 is 325^+316_-177 deg^-2. Because identification of the red subsample (V-J>3) is substantially incomplete, the 2sigma upper limit on the density of reddened QSO is large, <1150 deg^-2. As anticipated, at these faint magnitudes the KX sample includes several compact galaxies. Of the 14 with measured redshifts, there are roughly equal numbers of early and late type objects. Nearly all the early type galaxies are found in a single structure at z=0.66.
Are high-redshift DLA galxies Lyman-break galaxies?
P. M?ller,S. J. Warren,S. M. Fall,J. U. Fynbo,P. Jakobsen
Physics , 2002, DOI: 10.1086/340934
Abstract: We use deep HST STIS and NICMOS images of three spectroscopically confirmed galaxy counterparts of high-redshift damped Ly-alpha (DLA) absorbers (one of which is a new discovery) to test the hypothesis that high-redshift DLA galaxies are Lyman-break galaxies. If this hypothesis is correct the emission properties of DLA galaxies must lie within the range of emission properties measured for Lyman-break galaxies of similar absolute magnitude. This will be true regardless of selection biases in the sample of detected DLA galaxies. We test this prediction using several emission properties: half-light radius, radial profile (Sersic n parameter), optical-to-near-infrared colour, morphology, Ly alpha emission equivalent width, and Ly alpha emission velocity structure. In all cases the measured values for the DLA galaxies lie within the range measured for the population of Lyman-break galaxies. None of the measurements is in conflict with the prediction. We conclude that the measured emission properties of the three DLA galaxies studied here are consistent with the conjecture that high-redshift DLA galaxies are Lyman-break galaxies. We show that this result does not conflict with the observation that the few high-redshift DLA galaxies discovered are mostly fainter than spectroscopically confirmed L* Lyman-break galaxies.
Experimental evidence for the ancestry of allotetraploid Trifolium repens and creation of synthetic forms with value for plant breeding
Warren M Williams, Nicholas W Ellison, Helal A Ansari, Isabelle M Verry, S Hussain
BMC Plant Biology , 2012, DOI: 10.1186/1471-2229-12-55
Abstract: T. pallescens plants were identified with chloroplast trnL intron DNA sequences identical to those of white clover. Similarly, T. occidentale plants with nuclear ITS sequences identical to white clover were also identified. Reciprocal GISH experiments, alternately using labeled genomic DNA probes from each of the putative ancestral species on the same white clover cells, showed that half of the chromosomes hybridized with each probe. F1 hybrids were generated by embryo rescue and these showed strong interspecific chromosome pairing and produced a significant frequency of unreduced gametes, indicating the likely mode of polyploidization. The F1 hybrids are inter-fertile with white clover and function as synthetic white clovers, a valuable new resource for the re-incorporation of ancestral genomes into modern white clover for future plant breeding.Evidence from DNA sequence analyses, molecular cytogenetics, interspecific hybridization and breeding experiments supports the hypothesis that a diploid alpine species (T. pallescens) hybridized with a diploid coastal species (T. occidentale) to generate tetraploid T. repens. The coming together of these two narrowly adapted species (one alpine and the other maritime), along with allotetraploidy, has led to a transgressive hybrid with a broad adaptive range.White clover, an allotetraploid (2n=4x=32) stoloniferous herb, is naturally distributed through the grasslands of Europe, W Asia and N Africa, from low to high latitudes and altitudes and, because of its broad adaptation, has become the most extensively used legume of grazed pasture world-wide. Its origin has been long debated [1-4]. The identity of the ancestors has remained elusive and, despite many attempts [e.g. [3,5-7] there has been no successful re-synthesis. A phylogenetic analysis of Trifolium based on the nuclear internal transcribed spacer region of 18 S–26S rDNA (ITS) and chloroplast trnL intron DNA (cpDNA) sequences [8] suggested that the closest extant diplo
Sequence and structure of naturally-occurring tRNA transcripts and site-directed variants are significant barriers to forming oligomers beyond dimers  [PDF]
Harold S. Bernhardt, Warren P. Tate
Advances in Bioscience and Biotechnology (ABB) , 2013, DOI: 10.4236/abb.2013.45A001
Abstract:

Dimers of tRNAs can form through quasi self-complementary anticodon-anticodon interactions, for example at neutral pH in yeast tRNAAsp(GUC) and at pH4.5 inEscherichia coli tRNAGly(GCC) through a partially protonated interaction. The requirements for tRNA oligomerization, and the factors that prevented higher orders of structures forming were examined with unmodified wild-type and variant E. coli tRNAsGly(GCC). Non-denaturing agarose gel electrophoresis was used as a rapid screening method. A number of tRNAGly(GCC) variants with nucleotide substitutions in the loop regions formed dimers, but surprisingly there was no evidence that distinct higher oligomers formed in any of the variants tested. The dimer interfaces of two of the variants were delineated by competitive inhibition with complementary DNA oligonucleotides. Components of an oligomerization facilitating buffer, containing monovalent, divalent and multivalent cations (magnesium and sodium ions and spermine), were tested separately and in combination, to optimize oligomerization and its detection using agarose gel electrophoresis. A rationale for the requirement for magnesium for dimerization is suggested from its role in RNA loop-loop interactions. Sequence specific variant tRNAs that can rapidly form heterodimers with damaging infectious RNA are potential therapeutic agents against viral mechanisms by acting as base pairing inhibitors.

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