Statistical Mechanics of Kinks in (1+1)-Dimensions

We investigate the thermal equilibrium properties of kinks in a classical $\phi^4$ field theory in $1+1$ dimensions. The distribution function, kink density, and correlation function are determined from large scale simulations. A dilute gas description of kinks is shown to be valid below a characteristic temperature. A double Gaussian approximation to evaluate the eigenvalues of the transfer operator enables us to extend the theoretical analysis to higher temperatures where the dilute gas approximation fails. This approach accurately predicts the temperature at which the kink description breaks down.Comment: 8 pages, Latex (4 figures available on request), LA-UR-92-399

Optical "fingerprints" of dielectric resonators

The complete picture of the optical properties of resonant structures, along with the frequency, quality factor, and line shape in the scattering spectra, is determined by the electromagnetic field distribution patterns, which are a kind of "fingerprint" of each resonant eigenmode. In this paper, we simultaneously analyze the changes in the spectra and the transformation of the field pattern during the topological transitions from a thin disk to a ring with a gradually increasing thickness and further to a split ring. In addition, we demonstrate characteristic optical fingerprints for well-known interference effects such as bound states in the continuum and Fano resonances.Comment: 11 pages, 5 figure

Statistical Mechanics of Kinks in (1+1)-Dimensions: Numerical Simulations and Double Gaussian Approximation

We investigate the thermal equilibrium properties of kinks in a classical \F^4 field theory in $1+1$ dimensions. From large scale Langevin simulations we identify the temperature below which a dilute gas description of kinks is valid. The standard dilute gas/WKB description is shown to be remarkably accurate below this temperature. At higher, ``intermediate'' temperatures, where kinks still exist, this description breaks down. By introducing a double Gaussian variational ansatz for the eigenfunctions of the statistical transfer operator for the system, we are able to study this region analytically. In particular, our predictions for the number of kinks and the correlation length are in agreement with the simulations. The double Gaussian prediction for the characteristic temperature at which the kink description ultimately breaks down is also in accord with the simulations. We also analytically calculate the internal energy and demonstrate that the peak in the specific heat near the kink characteristic temperature is indeed due to kinks. In the neighborhood of this temperature there appears to be an intricate energy sharing mechanism operating between nonlinear phonons and kinks.Comment: 28 pages (8 Figures not included, hard-copies available), Latex, LA-UR-93-276

New broad 8Be nuclear resonances

Energies, total and partial widths, and reduced width amplitudes of 8Be resonances up to an excitation energy of 26 MeV are extracted from a coupled channel analysis of experimental data. The presence of an extremely broad J^pi = 2^+ ``intruder'' resonance is confirmed, while a new 1^+ and very broad 4^+ resonance are discovered. A previously known 22 MeV 2^+ resonance is likely resolved into two resonances. The experimental J^pi T = 3^(+)? resonance at 22 MeV is determined to be 3^-0, and the experimental 1^-? (at 19 MeV) and 4^-? resonances to be isospin 0.Comment: 16 pages, LaTe

Hot Nucleons in Chiral Soliton Models

Chiral lagrangians as effective field theories of QCD are most suitable for the study of nucleons in a hot pion gas because they contain pions and also baryons as solitons of the same action. The semiclassical treatment of the soliton solutions must be augmented by pionic fluctuations which requires renormalisation to 1-loop, and finite temperatures do not introduce new ultraviolet divergencies and may easily be considered. Alternatively, a renormalisation scheme based on the renormalisation group equation at finite temperature comprises and extends the rigorous results of chiral perturbation theory and renders the low energy constants temperature-dependent which allows the construction of temperature-dependent solitons below the critical temperature. The temperature-dependence of the baryon energy and the pion-nucleon coupling is studied. There is no simple scaling law for the temperature-dependence of these quantities.Comment: 17 pages (RevTeX), 5 figure

Determining the Parameters of Massive Protostellar Clouds via Radiative Transfer Modeling

A one-dimensional method for reconstructing the structure of prestellar and protostellar clouds is presented. The method is based on radiative transfer computations and a comparison of theoretical and observed intensity distributions at both millimeter and infrared wavelengths. The radiative transfer of dust emission is modeled for specified parameters of the density distribution, central star, and external background, and the theoretical distribution of the dust temperature inside the cloud is determined. The intensity distributions at millimeter and IR wavelengths are computed and quantitatively compared with observational data. The best-fit model parameters are determined using a genetic minimization algorithm, which makes it possible to reveal the ranges of parameter degeneracy as well. The method is illustrated by modeling the structure of the two infrared dark clouds IRDC-320.27+029 (P2) and IRDC-321.73+005 (P2). The derived density and temperature distributions can be used to model the chemical structure and spectral maps in molecular lines.Comment: Accepted for publication in Astronomy Report

Intermediate resolution H-beta spectroscopy and photometric monitoring of 3C 390.3 I. Further evidence of a nuclear accretion disk

We have monitored the AGN 3C390.3 between 1995 and 2000.Two large amplitude outbursts, of different duration, in continuum and H beta light were observed ie.: in October 1994 a brighter flare that lasted about 1000 days and in July 1997 another one that lasted about 700 days were detected. The flux in the H beta wings and line core vary simultaneously, a behavior indicative of predominantly circular motions in the BLR.Important changes of the Hbeta emission profiles were detected: at times, we found profiles with prominent asymmetric wings, as those normaly seen in Sy1s, while at other times, we observe profiles with weak almost symmetrical wings, similar to those seen in Sy1.8s. We found that the radial velocity difference between the red and blue bumps is anticorrelated with the light curves of H beta and continuum radiation.e found that the radial velocity difference between the red and blue bumps is anticorrelated with the light curves of H-beta and continuum radiation. Theoretical H-beta profiles were computed for an accretion disk, the observed profiles are best reproduced by an inclined disk (25 deg) whose region of maximum emission is located roughly at 200 Rg. The mass of the black hole in 3C 390.3, estimated from the reverberation analysis is Mrev = 2.1 x 10^9 Msun, ie. 5 times larger than previous estimatesComment: 18 pages, 13 figures, 4 tables. to appear in Astronomy and Astrophysic

Radioactive decays at limits of nuclear stability

The last decades brought an impressive progress in synthesizing and studying properties of nuclides located very far from the beta stability line. Among the most fundamental properties of such exotic nuclides, usually established first, is the half-life, possible radioactive decay modes, and their relative probabilities. When approaching limits of nuclear stability, new decay modes set in. First, beta decays become accompanied by emission of nucleons from highly excited states of daughter nuclei. Second, when the nucleon separation energy becomes negative, nucleons start to be emitted from the ground state. Here, we present a review of the decay modes occurring close to the limits of stability. The experimental methods used to produce, identify and detect new species and their radiation are discussed. The current theoretical understanding of these decay processes is overviewed. The theoretical description of the most recently discovered and most complex radioactive process - the two-proton radioactivity - is discussed in more detail.Comment: Review, 68 pages, 39 figure

Computation of the winding number diffusion rate due to the cosmological sphaleron

A detailed quantitative analysis of the transition process mediated by a sphaleron type non-Abelian gauge field configuration in a static Einstein universe is carried out. By examining spectra of the fluctuation operators and applying the zeta function regularization scheme, a closed analytical expression for the transition rate at the one-loop level is derived. This is a unique example of an exact solution for a sphaleron model in $3+1$ spacetime dimensions.Comment: Some style corrections suggested by the referee are introduced (mainly in Sec.II), one reference added. To appear in Phys.Rev.D 29 pages, LaTeX, 3 Postscript figures, uses epsf.st

Radial HI Profiles at the Periphery of Galactic Disks: The Role of Ionizing Background Radiation

Observations of neutral hydrogen in spiral galaxies reveal a sharp cutoff in the radial density profile at some distance from the center. Using 22 galaxies with known HI distributions as an example, we discuss the question of whether this effect can be associated exclusively with external ionizing radiation, as is commonly assumed. We show that before the surface density reaches $\sigma_{\textrm{HI}}\le 0.5 {\cal M}_\odot/{\textrm {pc}}^2$(the same for galaxies of different types), it is hard to expect the gas to be fully ionized by background radiation. For two of 13 galaxies with a sharp drop in the HI profile, the "steepening" can actually be caused by ionization. At the same time, for the remaining galaxies, the observed cutoff in the radial HI profile is closer to the center than if it was a consequence of ionization by background radiation and, therefore, it should be caused by other factors.Comment: 15 pages, 6 figure