December 1999 arXiv papers — page 26

M. A. Boselli, I. C. da Cunha Lima, A. Ghazali

The magnetic order in the diluted magnetic semiconductor barrier of double AlAs/GaAs: Mn quantum well structures is investigated by Monte Carlo simulations. A confinement adapted RKKY mechanism is implemented for indirect exchange between Mn ions mediated by holes. It is shown that, depending on the barrier width and the hole concentration a ferromagnetic or

D. N Argyriou, H. N. Bordallo1, B. J. Campbell, A. K. Cheetham

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr2Mn2O7 by Raman spectroscopy as high as 340 K and with decreasing temperature they become static and form a charge ordered (CO) phase below TCO=210 K. In the static regime, superlattice reflections are observed through neutron and x-ray diffraction with a propagation vector (h+1

Vladimir L. Safonov, H. Neal Bertram

The interaction of coherent magnetization rotation with a system of two-level impurities is studied. Two different, but not contradictory mechanisms, the `slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system of integro-differential equations for the magnetization. In the case that the impurity relaxation rate is much greater

Esteban Moro, Rodolfo Cuerno

We study analytically the equilibrium and near-equilibrium properties of a model of surfaces relaxing via linear surface diffusion and subject to a lattice potential. We employ the variational mean field formalism introduced by Saito for the study of the sine Gordon model. In equilibrium, our variational theory predicts a first order roughening transition be

Xiao-Guang Wang, Ning-Ning Liu, Shao-Hua Pan, Guo-Zhen Yang

We consider a finite ferroelectric superlattice in which the elementary unit cell is made up of l atomic layers of type A and n atomic layers of type B. Based on the transverse Ising model we examine the phase transition properties of the ferroelectric superlattice. Using the transfer matrix method we derive the equation for the Curie temperature of the supe

D. Y. Oberli, F. Vouilloz, R. Ambigapathy, B. Deveaud

We report on time-resolved and steady-state photoluminescence studies of GaAs/AlGaAs V-groove quantum wire structures. Steady-state photoluminescence experiments are performed in the temperature range from 8K to 200K. We evaluate the relation between photoluminescence excitation and absorption and determine experimentally an optical density in order to analy

D. V. Dmitriev, V. Ya. Krivnov, A. A. Ovchinnikov

We have found the exact ground state for two frustrated quantum spin-1/2 models on a linear chain. The first model describes ferromagnet- antiferromagnet transition point. The singlet state at this point has double-spiral ordering. The second model is equivalent to special case of the spin-1/2 ladder. It has non-degenerate singlet ground state with exponenti

L. Sirigu, D. Y. Oberli, L. Degiorgi, A. Rudra

Direct experimental evidences for excitonic lasing is obtained in optically pumped V-groove quantum wire structures. We demonstrate that laser emission at a temperature of 10 K arises from a population inversion of localized excitons within the inhomogenously-broadened luminescence line. At the lasing threshold, we estimate a maximum exciton density of about

Giovanni Bonanno, Fabrizio Lillo, Rosario N. Mantegna

We perform a parallel analysis of the spectral density of (i) the logarithm of price and (ii) the daily number of trades of a set of stocks traded in the New York Stock Exchange. The stocks are selected to be representative of a wide range of stock capitalization. The observed spectral densities show a different power-law behavior. We confirm the $1/f^2$ beh

Henk W. J. Blöte, Lev N. Shchur, Andrei L. Talapov

We present a progress report on the Cluster Processor, a special-purpose computer system for the Wolff simulation of the three-dimensional Ising model, including an analysis of simulation results obtained thus far. These results allow, within narrow error margins, a determination of the parameters describing the phase transition of the simple-cubic Ising mod

G. I. Harus, A. N. Ignatenkov, A. I. Ponomarev, L. D. Sabirzyanova

A systematic study of the resistivity and Hall effect in single crystal Nd{2-x}Ce{x}CuO{4-d} films (0.12 < x < 0.20) is presented, with special emphasis on the low-temperature dependence of the normal state conductance. Two-dimensional weak localization effects are found both in a normally conducting underdoped sample (x = 0.12) and in situ superconducting o

R. Prozorov, R. W. Giannetta, P. Fournier, R. L. Greene

The in-plane penetration depth λ(T) is measured in electron-doped single crystals Nd1.85Ce0.15CuO4-x (NCCO) and Pr1.85Ce0.15CuO4-x (PCCO) using a 11 MHz LC resonator. In NCCO, λ(T) exhibits a minimum at 3.8 K and a pronounced upturn down to 0.4 K due to the paramagnetic contribution of Nd3+ ions. The London penetration depth contribution is linear in T. The

A. Meszaros, Z. Bagoly, R. Vavrek

This articles is concerned primarily with the intrinsic anisotropy in the angular distribution of 2281 gamma-ray bursts (GRBs) collected in Current BATSE Gamma-Ray Burst Catalog until the end of year 1998, and, second, with intrinsic anisotropies of three subclasses (&#34;short&#34;, &#34;intermediate&#34;, &#34;long&#34;) of GRBs. Testing based on spherical

Jason A. Taylor

This dissertation presents synthetic spectra and response functions of the red giant stellar line emission model of active galactic nuclei. Our results agree with the fundamental line emission characteristics of active galactic nuclei within the model uncertainties if the following additional assumptions are made: 1) the mean stellar mass loss rates decrease

Patrick S. Wojdowski, George W. Clark, Timothy R. Kallman

We have investigated the composition and distribution of the wind of Sk 160, the supergiant companion of the X-ray star SMC X-1, by comparing an X-ray spectrum of the source, obtained with the ASCA observatory, during an eclipse with the computed spectra of reprocessed radiation from circumstellar matter with various density distributions. We show that the m

Karl D. Gordon, Geoffrey C. Clayton, Adolf N. Witt, K. A. Misselt

The presence of dust in starburst galaxies complicates the study of their stellar populations as the dust&#39;s effects are similar to those associated with changes in the galaxies&#39; stellar age and metallicity. This degeneracy can be overcome for starburst galaxies if UV/optical/near-infrared observations are combined with far-infrared observations. We p

Lifan Wang, D. Andrew Howell, Peter Hoeflich, J. Craig Wheeler

We discuss the optical spectropolarimetry of several core-collapse supernovae, SN 1996cb (Type IIB), SN 1997X (Type Ic), and SN 1998S (Type IIn). The data show polarization evolution of several spectral features at levels from 0.5% to above 4%. The observed line polarization is intrinsic to the supernovae and not of interstellar origin. These data suggest th

Scott C. Chapman, Simon L. Morris, Gordon A. H. Walker

We present high spatial resolution, near-IR images in J, H, K of the nucleus of NGC3227, obtained with the Adaptive Optics bonnette on CFHT. The ~0.15 arcsec (17pc) resolution allows structures to be probed in the core region. Dust obscuration becomes significantly less pronounced at longer wavelengths, revealing the true geometry of the core region. We are

D. R. Garnett, H. A. Kobulnicky

The age-metallicity relation for F and G dwarf stars in the solar neighborhood, based on the stellar metallicity data of Edvardsson et al. (1993), shows an apparent scatter that is larger than expected considering the uncertainties in metallicities and ages. A number of theoretical models have been put forward to explain the large scatter. However, we presen

Timothy M. Heckman

Starbursts are episodes of intense star-formation in the central regions of galaxies, and are the sites of roughly 25% of the high-mass star-formation in the local universe. In this contribution I review the role starbursts play in the formation and evolution of galaxies, the intergalactic medium, and active galactic nuclei. Four major conclusions are drawn.

S. W. Dieters, T. Belloni, E. Kuulkers, P. Woods

We report on the evolution of the timing of 4U1630-47 during its 1998 outburst using data obtained with the Rossi X-ray Timing Explorer (RXTE). The count rate and position in hardness-intensity, color-color diagrams and simple spectral fits are used to track the concurrent spectral changes. The source showed seven distinct types of timing behavior, most of w

Thomas M. Brown, Charles W. Bowers, Randy A. Kimble, Henry C. Ferguson

The restframe UV-to-optical flux ratio, characterizing the ``UV upturn&#39;&#39; phenomenon, is potentially the most sensitive tracer of age in elliptical galaxies; models predict that it may change by orders of magnitude over the course of a few Gyr. In order to trace the evolution of the UV upturn as a function of redshift, we have used the far-UV camera o

Karl Gebhardt, Douglas Richstone, John Kormendy, Tod Lauer

We fit axisymmetric 3-integral dynamical models to NGC3379 using the line-of-sight velocity distribution obtained from HST/FOS spectra of the galaxy center and ground-based long-slit spectroscopy along four position angles, with the light distribution constrained by WFPC2 and ground-based images. We have fitted models with inclinations from 29 (intrinsic gal

Carlos Abia, Jordi Isern

Abundances of lithium, heavy elements and carbon isotope ratios have been derived in 12 J-type galactic carbon stars. The abundance analysis shows that in these stars the abundances of s-process elements with respect to the metallicity are nearly solar. Tc is not present in most of them. The Rb abundances, obtained from the resonance 7800 Å RbI line, are sur

G. G. Pavlov, V. E. Zavlin, B. Aschenbach, J. Truemper

The central pointlike X-ray source of the Cas A supernova remnant was discovered in the Chandra First Light Observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. The power-law fit yields the photon index 2.6-4.1, and luminosity (2-60)e34 erg/s, for

Simon Portegies Zwart, Stephen McMillan

Mergers of black-hole binaries are expected to release large amounts of energy in the form of gravitational radiation. However, binary evolution models predict merger rates too low to be of observational interest. In this paper we explore the possibility that black holes become members of close binaries via dynamical interactions with other stars in dense st

Doris Folini, Rolf Walder

The colliding winds interaction zone in WR+O binaries is a highly complex environment. In this review we summarize the progress made towards its theoretical understanding during the last years. We review the effect of different physical processes on the interaction zone, among them geometry and orbital motion, radiative forces, thermal conduction, instabilit

Giovanna Pugliese, Heino Falcke, Yiping Wang, Peter L. Biermann

We consider a jet-disk symbiosis model to explain Gamma Ray Bursts and their afterglows. It is proposed that GRBs are created inside a pre-existing jet from a neutron star in a binary system which collapses to a black hole due to accretion. In our model we assume that a fraction of the initial energy due to this transition is deposited in the jet by magnetic

A. Misiriotis, N. D. Kylafis, J. Papamastorakis, E. M. Xilouris

We investigate how significant the spiral structure is on calculations concerning radiative transfer in dusty spiral galaxies seen edge-on. The widely adopted exponential disk model (i.e. both the stars and the dust are distributed exponentially in the radial direction and also perpendicular to the plane of the disk) is now subject to a detailed comparison w

A. Pasquali, A. Nota, N. Langer, R. E. Schulte-Ladbeck

We present new, NTT longslit spectroscopy of the B[e] supergiant in the binary system R4 in the Small Magellanic Cloud. The data show extended, forbidden N and S emissions which are typical signatures of circumstellar matter. Their extension along the space axis of the slit defines an angular size of 8.6&#39;&#39; which translates into a linear size of 2.4 p

R. Della Ceca, V. Braito, I. Cagnoni, T. Maccacaro

We present here a status update on the ASCA Hard Serendipitous Survey (HSS), a survey program conducted in the 2-10 keV energy band. In particular we discuss the number-flux relationship, the 2-10 keV spectral properties of the sources and of the spectroscopically identified objects.

Noam Soker

I study some effects of aspherical mass loss during the last stages of the asymptotic giant branch (AGB) on the appearance of proto planetary nebulae (PNs) and young PNs. The aspherical mass loss can be small scale inhomogeneities, and/or axially symmetric mass loss geometry. I first examine the role of the dust opacity in the optical band on the appearance

S. Bianchi, J. Davies, P. Alton, M. Gerin

We present sub-millimetre images of the spiral galaxy NGC 6946 at 450 and 850 um. The observations have been carried out using the scan/map imaging mode of the Sub-millimetre Common User Bolometer Array (SCUBA) at the James Clark Maxwell Telescope (JCMT). The comparison with a 12CO(2-1) line emission map from IRAM and with an U-band image, clearly shows that

F. R. Pearce, H. M. P. Couchman, P. A. Thomas, A. C. Edge

In this paper we use state-of-the-art N-body hydrodynamic simulations of a cosmological volume of side 100Mpc to produce many galaxy clusters simultaneously in both the standard cold dark matter (SCDM) cosmology and a cosmology with a positive cosmological constant (LCDM). We have performed simulations of the same volume both with and without the effects of

Helmut Jerjen, Ken C. Freeman, Bruno Binggeli

We have obtained deep B and R-band CCD photometry for 5 dE galaxies that were previously identified on Schmidt films covering the Cen A group region. From a Fourier analysis of the R-band CCD images we determined the surface brightness fluctuation (SBF) magnitude for each stellar system. All magnitudes are similar and suggest that these low surface brightnes

W. L. Holzapfel, J. E. Carlstrom, L. Grego, G. Holder

We have used the Berkeley-Illinois-Maryland-Association (BIMA) millimeter array outfitted with sensitive cm-wave receivers to search for Cosmic Microwave Background (CMB) anisotropies on arcminute scales. The interferometer was placed in a compact configuration which produces high brightness sensitivity, while providing discrimination against point sources.

W. L. Holzapfel, J. E. Carlstrom, L. Grego, M. Joy

Recently, Jones et al. (1997) used the Ryle telescope, operating at a frequency of 15 GHz, to detect a flux decrement in the direction of the quasar pair PC1643+461A,B. They interpreted this signal as the Sunyaev-Zel&#39;dovich effect (SZE) produced by a distant cluster of galaxies. In the course of an effort to measure CMB anisotropies using the VLA at 8.4

Philippe Brax, Jerome Martin

It is demonstrated that any realistic model of quintessence should be based on Supergravity since, when the scalar field is on tracks today, $Q\approx m_{\rm Pl}$. This improves the agreement between theoretical predictions and the current observations. In particular, a generic property is that ratio $ω_{\rm Q}\equiv p_{\rm Q}/ρ_{\rm Q}$ is pushed towards -1

Frank C. van den Bosch, Julianne J. Dalcanton

We present detailed semi-analytical models for the formation of disk galaxies both in a Universe dominated by dark matter (DM), and in one for which the force law is given by modified Newtonian dynamics (MOND). We tune the models to fit the observed near-infrared Tully-Fisher (TF) relation, and compare numerous predictions of the resulting models with observ

Bryan C. Dunne, Robert A. Gruendl, You-Hua Chu

The ultraluminous supernova remnant (SNR) in NGC 6946 is the brightest known SNR in X-rays, ~1000 times brighter than Cas A. To probe the nature of this remnant and its progenitor, we have obtained high-dispersion optical echelle spectra. The echelle spectra detect H-alpha, [N II], and [O III] lines, and resolve these lines into a narrow (FWHM ~20--40 km/s)

Justin H. Howell, Puragra Guhathakurta, Amy Tan

It has long been known that the post-core-collapse globular cluster M30 has a bluer-inward color gradient, and recent work suggests that the central deficiency of bright red giants does not fully account for this gradient. This study uses Hubble Space Telescope WFPC2 images in the F439W and F555W bands, along with ground-based CCD images with a wider field o

C. S. Reynolds

We use the method of Press, Rybicki & Hewitt (1992) to search for time lags and time leads between different energy bands of the RXTE data for MCG-6-30-15. We tailor our search in order to probe any reverberation signatures of the fluorescent iron Kalpha line that is thought to arise from the inner regions of the black hole accretion disk. In essence, an opt

D. R. Lorimer

Pulsar astronomy is currently enjoying one of the most productive phases in its history. In this review, I outline some of the basic observational aspects and summarise some of the latest results of searches for pulsars in the disk of our Galaxy and its globular cluster system.

O. E. Kalashev, V. A. Kuzmin, D. V. Semikoz

We developed numerical code for calculation of the extragalactic component of the spectra of leptons, nucleons and $γ$-rays resulting from ``top-down&#39;&#39; (non-acceleration) models for the case of uniform and isotropic source distribution. We explored two different classes of ``top-down&#39;&#39; scenarios: the wide earlier investigated class of X parti