Pressure induced structural and dynamical changes in liquid Si. An ab-initio study

The static and dynamic properties of liquid Si at high-pressure have been studied using the orbital free ab-initio molecular dynamics method. Four thermodynamic states at pressures 4, 8, 14 and 23 GPa are considered. The calculated static structure shows qualitative agreement with the available experimental data. We analize the remarkable structural changes occurring between 8 and 14 GPa along with its effect on several dynamic properties.Comment: 10 pages, 11 figures. Accepted for publication in Journal of Physics: Condensed Matte

Collective dynamics of liquid aluminum probed by Inelastic X-ray Scattering

An inelastic X-ray scattering experiment has been performed in liquid aluminum with the purpose of studying the collective excitations at wavevectors below the first sharp diffraction peak. The high instrumental resolution (up to 1.5 meV) allows an accurate investigation of the dynamical processes in this liquid metal on the basis of a generalized hydrodynamics framework. The outcoming results confirm the presence of a viscosity relaxation scenario ruled by a two timescale mechanism, as recently found in liquid lithium.Comment: 8 pages, 7 figure

Eddy genesis and manipulation in plane laminar shear flow

Eddy formation and presence in a plane laminar shear flow configuration consisting of two infinitely long plates orientated parallel to each other is investigated theoretically. The upper plate, which is planar, drives the flow; the lower one has a sinusoidal profile and is fixed. The governing equations are solved via a full finite element formulation for the general case and semi-analytically at the Stokes flow limit. The effects of varying geometry (involving changes in the mean plate separation or the amplitude and wavelength of the lower plate) and inertia are explored separately. For Stokes flow and varying geometry, excellent agreement between the two methods of solution is found. Of particular interest with regard to the flow structure is the importance of the clearance that exists between the upper plate and the tops of the corrugations forming the lower one. When the clearance is large, an eddy is only present at sufficiently large amplitudes or small wavelengths. However, as the plate clearance is reduced, a critical value is found which triggers the formation of an eddy in an otherwise fully attached flow for any finite amplitude and arbitrarily large wavelength. This is a precursor to the primary eddy to be expected in the lid-driven cavity flow which is formed in the limit of zero clearance between the plates. The influence of the flow driving mechanism is assessed by comparison with corresponding solutions for the case of gravity-driven fluid films flowing over an undulating substrate. When inertia is present, the flow generally becomes asymmetrical. However, it is found that for large mean plate separations the flow local to the lower plate becomes effectively decoupled from the inertia dominated overlying flow if the wavelength of the lower plate is sufficiently small. In such cases the local flow retains its symmetry. A local Reynolds number based on the wavelength is shown to be useful in characterising these large-gap flows. As the mean plate separation is reduced, the form of the asymmetry caused by inertia changes, and becomes strongly dependent on the plate separation. For lower plate wavelengths which do not exhibit a cinematically induced secondary eddy, an inertially induced secondary eddy can be created if the mean plate separation is sufficiently small and the global Reynolds number sufficiently large

The Case for Optically-Thick High Velocity Broad Line Region Gas in Active Galactic Nuclei

A combined analysis of the profiles of the main broad quasar emission lines in both Hubble Space Telescope and optical spectra shows that while the profiles of the strong UV lines are quite similar, there is frequently a strong increase in the Ly-alpha/H-alpha ratio in the high-velocity gas. We show that the suggestion that the high velocity gas is optically-thin presents many problems. We show that the relative strengths of the high velocity wings arise naturally in an optically-thick BLR component. An optically-thick model successfully explains the equivalent widths of the lines, the Ly-alpha/H-alpha ratios and flatter Balmer decrements in the line wings, the strengths of CIII] and the lambda 1400 blend, and the strong variability of high-velocity, high-ionization lines (especially HeII and HeI).Comment: 34 pages in AASTeX, including 10 pages of figures. Submitted to Astrophysical Journa

The End of the Lines for OX 169: No Binary Broad-Line Region

We show that unusual Balmer emission line profiles of the quasar OX 169, frequently described as either self-absorbed or double peaked, are actually neither. The effect is an illusion resulting from two coincidences. First, the forbidden lines are quite strong and broad. Consequently, the [N II]6583 line and the associated narrow-line component of H-alpha present the appearance of twin H-alpha peaks. Second, the redshift of 0.2110 brings H-beta into coincidence with Na I D at zero redshift, and ISM absorption in Na I D divides the H-beta emission line. In spectra obtained over the past decade, we see no substantial change in the character of the line profiles, and no indication of intrinsic double-peaked structure. The H-gamma, Mg II, and Ly-alpha emission lines are single peaked, and all of the emission-line redshifts are consistent once they are correctly attributed to their permitted and forbidden-line identifications. A systematic shift of up to 700 km/s between broad and narrow lines is seen, but such differences are common, and could be due to gravitational and transverse redshift in a low-inclination disk. Stockton & Farnham (1991) had called attention to an apparent tidal tail in the host galaxy of OX 169, and speculated that a recent merger had supplied the nucleus with a coalescing pair of black holes which was now revealing its existence in the form of two physically distinct broad-line regions. Although there is no longer any evidence for two broad emission-line regions in OX 169, binary black holes should form frequently in galaxy mergers, and it is still worthwhile to monitor the radial velocities of emission lines which could supply evidence of their existence in certain objects.Comment: 19 pages, 5 figures, accepted for publication in Ap.

The Nuclear Reddening Curve for Active Galactic Nuclei and the Shape of the Infra-Red to X-Ray Spectral Energy Distribution

We present extinction curves derived from the broad emission lines and continua of large samples of both radio-loud and radio-quiet AGNs. The curves are significantly flatter in the UV than are curves for the local ISM. The reddening curves for the radio-quiet LBQS quasars are slightly steeper than those of the radio-loud quasars in the UV, probably because of additional reddening by dust further out in the host galaxies of the former. The UV extinction curves for the radio-loud AGNs are very flat. This is explicable with slight modifications to standard MRN dust models: there is a relative lack of small grains in the nuclear dust. Our continuum and broad-emission line reddening curves agree in both shape and amplitude, confirming that the continuum shape is indeed profoundly affected by reddening for all but the bluest AGNs. With correction by our generic extinction curve, all of the radio-loud AGNs have continuous optical-UV spectra consistent with a single shape. We show that radio-quiet AGNs have very similar intrinsic UV to optical shape over orders of magnitude in luminosity. We also argue that radio-loud and radio-quiet AGNs probably share the same underlying continuum shape and that most of the systematic differences between their observed continuum shapes are due to higher nuclear reddening in radio-selected AGNs, and additional reddening from dust further out in the host galaxies in radio-quiet AGNs. Our conclusions have important implications for the modelling of quasar continua and the analysis of quasar demographics.Comment: 41 pages, including 6 figures and 3 tables. To appear in ApJ vol. 614, October 20 issue. Some slight wording changes. Some additional references added. Small changes in the model fit in section 6.2, to the analytical fit in the Appendix, and to the tabulated reddening curve in the Appendi

Path integral Monte Carlo simulation of charged particles in traps

This chapter is devoted to the computation of equilibrium (thermodynamic) properties of quantum systems. In particular, we will be interested in the situation where the interaction between particles is so strong that it cannot be treated as a small perturbation. For weakly coupled systems many efficient theoretical and computational techniques do exist. However, for strongly interacting systems such as nonideal gases or plasmas, strongly correlated electrons and so on, perturbation methods fail and alternative approaches are needed. Among them, an extremely successful one is the Monte Carlo (MC) method which we are going to consider in this chapter.Comment: 18 pages, based on talks on Hareaus school on computational methods, Greifswald, September 200

On some nonlinear extensions of the angular momentum algebra

Deformations of the Lie algebras so(4), so(3,1), and e(3) that leave their so(3) subalgebra undeformed and preserve their coset structure are considered. It is shown that such deformed algebras are associative for any choice of the deformation parameters. Their Casimir operators are obtained and some of their unitary irreducible representations are constructed. For vanishing deformation, the latter go over into those of the corresponding Lie algebras that contain each of the so(3) unitary irreducible representations at most once. It is also proved that similar deformations of the Lie algebras su(3), sl(3,R), and of the semidirect sum of an abelian algebra t(5) and so(3) do not lead to associative algebras.Comment: 22 pages, plain TeX + preprint.sty, no figures, to appear in J.Phys.