Exponential Density Profile Research Articles

Onset of Turbulence and Profile Resilience in the Helimak Configuration

An experimental study of the onset of drift wave and flute interchange instabilities in the Helimak configuration is presented. It is shown that the Helimak offers the opportunity to separate the regions where these instabilities are active and to assess their relative role in cross-field anomalous transport and in the self-organization of exponential plasma density profiles with resilient scale length. Some results indicating a period doubling route to turbulence are also presented.

Charge Displacement in Ultraintense Laser-Plasma Interaction

The distribution of optical field and charge density in the interaction between ultraintense ultrashort pulse laser and plasma is studied by numerical computation. The plasma considered has an exponential density profile, which corresponds to isothermal expanding. Our calculation shows that electrons are pushed forward by the incident laser, but ions, due to their much greater inertia, remain stationary. The resulting charge displacement forms a strong electrostatic field in the plasma. After the interaction of laser pulse and plasma, electrostatic energy still exists even after the laser pulse and will be absorbed by the plasma finally. This serves as an explanation to the mechanism of laser energy deposited into plasma.

Conjugate flows for waves with trapped cores

Previous numerical simulations have shown that in a bounded fluid, internal solitary waves with trapped cores become horizontally uniform in their center and have a maximum amplitude that is dependent on the details of the recirculating flow inside the core [K. G. Lamb, “A numerical investigation of solitary internal waves with trapped cores formed via shoaling,” J. Fluid Mech. 451, 109 (2002)]. The flow in the horizontally uniform center of these waves is called a conjugate flow. Here a theoretical model for conjugate flows with a trapped core (also called a vortex core) is presented. The flow in the core is assumed to be steady with constant density. General expressions for arbitrary nonuniform core vorticity are developed which depend on the surface velocity and the mean squared horizontal velocity in the core. Analytic results for cores with constant vorticity are presented for a two-layer fluid with constant density in the lower layer and constant buoyancy frequency in the upper layer. Comparisons of the theory with time-dependent numerical simulations of waves with flat-centered trapped cores in exponential density profiles are made. The theoretical results are shown to accurately predict the amplitude and core thickness of the conjugate flows when the values of the surface velocity and mean squared velocity are taken from the simulations.

The Influence of Supershells and Galactic Outflows on the Escape of Ionizing Radiation from Dwarf Starburst Galaxies

We study the escape of Lyman continuum radiation from the disks of dwarf starburst galaxies, with and without supershells, by solving the radiation transfer problem of stellar radiation through them. We model disks with Md=10^8-10Msun, with exponential surface density profiles as a function of redshift, and model the of repeated supernova explosions driving supershells out of the disks, using the hydrodynamic simulation code, ZEUS-3D. The amount of star formation is assumed proportional to mass above some density threshold. We vary the threshold to explore the range of star formation efficiencies, f*=0.006, 0.06, and 0.6. We find that the interstellar gas swept up in dense supershells can effectively trap the ionizing photons, before the supershells blow out of the disks. The blow-outs then create galactic outflows, chimneys which allow the photons to escape directly to the intergalactic medium. Our results are consistent with escape fractions of less than 0.1 measured in local dwarf starburst galaxies, because they are likely observed while the starbursts are young, before blow-out. We suggest that high-redshift dwarf starburst galaxies may make a substantial contribution to the UV background radiation with total escape fractions >0.2, as expected if star formation efficiencies >0.06.

Are Compact High‐Velocity Clouds Extragalactic Objects?

Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the Lo- cal Group model and, at d � 1 Mpc, they have HI volume densities of � 3×10 −4 cm −3 . Clouds with these volume densities and the observed column densities NHI � 10 19 cm −2 will be largely ionized, even if exposed only to the extragalactic ionizing radiation field. Here we examine the implications of this process for models of CHVCs. We have mod- eled the ionization structure of spherical clouds (with and without dark matter halos) for a large range of densities and sizes, appropriate to CHVCs over the range of sug- gested distances, exposed to an extragalactic ionizing photon fluxi � 10 4 phot cm −2 s −1 . Constant-density cloud models in which the CHVCs are at Local Group distances have total (ionized plus neutral) gas masses � 20 30 times larger than the neutral gas masses, implying that the gas mass alone of the observed population of CHVCs is � 4 × 10 10 M⊙. With a realistic (10:1) dark matter to gas mass ratio, the total mass in such CHVCs is a significant fraction of the dynamical mass of the Local Group, and their line widths would greatly exceed the observedV . Self-consistent models of gas in dark matter halos fare even more poorly; they must lie within approximately 200 kpc of the Galaxy, and (for a given distance) are much more massive than the correspond- ing uniform density models. We also show that exponential neutral hydrogen column density profiles are a natural consequence of an external source of ionizing photons, and argue that these profiles cannot be used to derive model-independent distances to the CHVCs. These results argue strongly that the CHVCs are not cosmological objects, and are instead associated with the Galactic halo. Subject headings: ISM: clouds — ISM: H I — Galaxy: halo — Local Group

Mechanisms of the vertical secular heating of a stellar disk

We investigate the nonlinear growth stages of the bending instability in stellar disks with exponential radial density profiles. We found that the unstable modes are global (the wavelengths are larger than the disk scale lengths) and that the instability saturation level is much higher than that following from a linear criterion. The instability saturation time scales are of the order of one billion years or more. For this reason, the bending instability can play an important role in the secular heating of a stellar disk in the z direction. In an extensive series of numerical N-body simulations with a high spatial resolution, we were able to scan in detail the space of key parameters (the initial disk thickness z 0, the Toomre parameter Q, and the ratio of dark halo mass to disk mass M h/M d). We revealed three distinct mechanisms of disk heating in the z direction: bending instability of the entire disk, bending instability of the bar, and heating on vertical inhomogeneities in the distribution of stellar matter.

Cyclic plasma shearing interferometry for temporal characterization of a laser-produced plasma

A cyclic shearing interferometer has been employed to characterize a laser-produced plasma with 180 ps resolution. Counterpropagation maintains an equal path length for the probe and reference beams, and the shear is provided solely by the plasma, which appears within the circuit after the reference beam has passed the laser focal spot. The background is virtually fringe free because of the overlapping of the reference and probe beams so that analysis is simplified. The plasma, which is formed by a line focus, is seen to expand in a cylindrical manner away from the line focus with an exponential density profile. In addition, the interferometer shows evidence of a bow shock when an interaction beam is introduced into the plasma parallel to the direction of the line focus.

Instabilities and Clumping in Type Ia Supernova Remnants

We present two-dimensional high-resolution hydrodynamical simulations in spherical polar coordinates of a Type Ia supernova interacting with a constant-density interstellar medium. The ejecta are assumed to be freely expanding with an exponential density profile. The interaction gives rise to a double-shocked structure susceptible to hydrodynamical instabilities. The Rayleigh-Taylor instability initially grows, but the Kelvin-Helmholtz instability takes over, producing vortex rings. Provided the simulation is initiated early in the evolution, with a perturbation 1%, the instability reaches its fully developed nonlinear strength within 5 doubling times. The further nonlinear evolution does not depend on the initial conditions. Considering the small initial radii of Type Ia supernovae, they are likely to reach this fully developed phase. The nonlinear instability initially evolves toward longer wavelengths and eventually fades away when the reverse shock front is in the flatter part of the supernova density distribution. On the basis of observations of X-ray knots and the protrusion in the southeast outline of Tycho's supernova remnant, we include clumping in the ejecta because these features cannot be explained by instabilities growing from small perturbations. The clump interaction with the reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the clump surface that facilitate fragmentation. In order to survive crushing and to have a bulging effect on the forward shock, the clump's initial density ratio to the surrounding ejecta must be at least 100 for the conditions in Tycho's remnant. The 56Ni bubble effect may be important for the development of clumpiness in the ejecta. The observed presence of an Fe clump would then require a nonradioactive origin for this Fe, possibly 54Fe. The large radial distance of the X-ray-emitting Si and S ejecta from the remnant center indicates that they were initially in clumps.

Interaction of Type Ia Supernovae with Their Surroundings: The Exponential Profile in Two Dimensions

The evolution of Type Ia supernovae in the surrounding medium is studied using two-dimensional numerical hydrodynamic simulations. The ejecta are assumed to be described by an exponential density profile, following the work of Dwarkadas & Chevalier. The case of a circumstellar region formed by mass loss from the progenitor or a companion star is also considered. The decelerating contact discontinuity is found to be Rayleigh-Taylor (R-T) unstable, as expected, and the nature of the instability is studied in detail for two cases: (1) a constant density ambient medium, and (2) a circumstellar medium whose density scales as r-2. The nature of the instability is found to be different in each case. In the case of a circumstellar medium the instability is much better resolved, and a fractal-like structure is seen. In the case of a constant density medium the extent of growth is less, and the R-T fingers are found to be limited by the presence of Kelvin-Helmholtz mushroom caps at the tips of the fingers. The unstable region is far enough away from the reverse shock that the latter is not affected by the mixing taking place in the interaction region. In contrast, the reverse shock in the case of a circumstellar medium is found to be rippled due to the formation of instabilities. In neither case is the outer shock front affected. These results are consistent with similar studies of power-law ejecta profiles conducted by Chevalier, Blondin, & Emmering. Our results are then applied to Tycho's supernova remnant. We conclude that it is unlikely that the instabilities seen in recent radio images of the remnant are similar to those studied herein, although we do not discount the possibility of initial conditions different from those studied herein leading to a much larger growth than we see in our simulations. We suggest that such instabilities may be better observed at X-ray wavelengths which probe the high-density shocked ejecta region.

The Self‐regulated Evolution of Dwarf Galaxies

A model of the interstellar medium (ISM) is developed that takes into account its multiphase structure, in particular the existence of a system of cold clouds embedded in a hot intercloud medium. The model considers star formation, kinetic energy input into the cloud system from stars, and kinetic energy loss through cloud-cloud collisions. These processes result in a characteristic equilibrium state of the ISM with properties that are in good agreement with observations. Analytical solutions for the global equilibrium states of dwarf galaxies are derived for simplified scenarios. First results of three-dimensional numerical simulations are presented. The self-regulated evolution within a confining dark halo leads to a model galaxy characterized by exponential density profiles for the gaseous and stellar components, as observed in dwarf galaxies. No significant mass loss of cold gas occurs. The characteristic global properties of the simulated galaxies are in excellent agreement with the observations.

SMM analysis of reflection, absorption, and transmission from nonuniform magnetized plasma slab

An new analytical technique for obtaining a complete set of reflection, absorption, and transmission coefficients for a stratified plasma slab is presented, named the scattering matrix method (SMM). The nonuniform magnetized plasma slab is modeled by a number of subslabs. Each subslab has a fixed electron density. The overall density profile across the whole slab follows any practical distribution function. Because the field in each subslab can be represented by the sum of the reflected and incident components, the partial reflection and transmission coefficients can be obtained by successively matching boundary conditions at all interfaces. The partial and total reflected, absorbed, and transmitted powers as the functions of the electron density, collision frequency, and electron cyclotron frequency for typical parabolic and exponential density profiles are investigated.

Exact solutions for matter-enhanced neutrino oscillations

The analogy between supersymmetric quantum mechanics and matter-enhanced neutrino oscillations is exploited to obtain exact solutions for a class of electron density profiles. This integrability condition is analogous to the shape-invariance in supersymmetric quantum mechanics. This method seems to be the most direct way to obtain the exact survival probabilities for a number of density profiles of interest, such as linear and exponential density profiles. The resulting neutrino amplitudes can also be utilized as comparison amplitudes for the uniform semiclassical treatment of neutrino propagation in arbitrary electron density profiles.

The formation of galactic discs

We study the population of galactic discs expected in current hierarchical clustering models for structure formation. A rotationally supported disc with exponential surface density profile is assumed to form with a mass and angular momentum which are fixed fractions of those of its surrounding dark halo. We assume that haloes respond adiabatically to disc formation, and that only stable discs can correspond to real systems. With these assumptions the predicted population can match both present-day discs and the damped Lya absorbers in QSO spectra. Good agreement is found provided that: (i) the masses of discs are a few per cent of those of their haloes; (ii) the specific angular momenta of discs are similar to those of their haloes; (iii) present-day discs were assembled recently (at z # 1). In particular, the observed scatter in the size–rotation velocity plane is reproduced, as are the slope and scatter of the Tully–Fisher (TF) relation. The zero-point of the TF relation is matched for a stellar mass-to-light ratio of 1 to 2 h in the I-band, consistent with observational values derived from disc dynamics. High-redshift discs are predicted to be small and dense, and could plausibly merge together to form the observed population of elliptical galaxies. In many (but not all) currently popular cosmogonies, discs with rotation velocities exceeding 200 kms ¹1 can account for a third or more of the observed damped Lya systems at z , 2:5. Half of the lines of sight to such systems are predicted to intersect the absorber at r * 3h ¹1 kpc and about 10 per cent at r > 10h ¹1 kpc. The cross-section for absorption is strongly weighted towards discs with large angular momentum and therefore large size for their mass. The galaxy population associated with damped absorbers should thus be biased towards low surface brightness

Interactions of atmospheric solitary waves of different modes

The interactions of atmospheric solitary waves with different modes are investigated by a perturbation method. The model considered in this paper consists of a lower layer with exponential density profile and an infinitely deep upper layer with constant density. The analysis show that the waves obey the Benjamin-Ono equation before and after interaction, and the main effect of the interaction is the phase shifts for each wave.

Stimulated Raman backscattering with and without optical fiber smoothing technique in 0.53 μm laser-created plasmas

Backward stimulated Raman scattering (SRS) experiments have been performed using the optical fiber smoothing implemented on the high-power Phébus laser facility [Laser Part. Beams 4, 93 (1988)]. The interaction took place in low-Z plasmas presenting either exponential density profiles (solid targets) or Gaussian-type profiles. Raman instability was driven with 1.4 ns duration—0.53 μm laser pulses, containing an energy up to 1.8 kJ. Comparative and absolute measurements of SRS reflectivity are reported as a function of the incident laser intensity, both with and without beam smoothing. Near-backward time-resolved SRS spectra are also presented. Random phase plates are not used in these experiments. With or without smoothing, SRS reflectivities of the order of 10% have been measured. However, smoothing becomes effective in exponential profiles below 1015 W/cm2. Although the optical fiber efficiently reduces the contrast of the energy modulations in the focal spot, the spectral bandwidth may be insufficient to quench SRS development in these conditions. This last assumption is in agreement with theoretical predictions.

Landau-Zener treatments of solar neutrino oscillations.

We present a simple derivation of the Landau-Zener formulas for nonadiabatic level crossings for linear and exponential density profiles. The derivation depends on a single assumption that applies somewhat differently to the exponential and linear cases, providing some insight into the relationship between the respective formulas. We note that one exponential Landau-Zener formula in common use is only approximately correct. Comparisons with exact calculations are made.

Closed-form solution for the transient reflected pressure for a point source above an acoustic half-space with an exponentially stratified density

The reflection problem for a transient point source over an acoustic half-space with an exponentially stratified density is considered. A spatial Hankel transform is used to obtain a one-dimensional wave equation. The transient reflected pressure on the surface in the Hankel-transformed space is expressed by a convolution of a reflection kernel and the transformed incident pressure, and the exact expression for the reflection kernel is obtained by means of a Laplace transform. Closed-form solution of the transient reflected pressure in physical space is obtained through an inverse Hankel transform. Closed-form solution for a generalization of the exponential density profile is also given. Numerical examples are given to illustrate these closed-form solutions.

Effects of density scale length on backward and forward stimulated Raman scattering in 0.53 μm laser–plasma interaction

Forward and backward stimulated Raman scattering (SRS) have been investigated in three different plasma geometries: in exponential density profile, in Gaussian density profile, and in quasihomogeneous preformed plasmas. The experiments were performed with the same irradiation setup at 0.53 μm wavelength and at a laser intensity ∼1015 W/cm2. Time-resolved spectra and energy rates of this instability have been measured in the three experiments. A strong increase of the backward SRS energy rate—up to 1%—with the increasing length of the plasma was observed. From data analysis, backward SRS is explained in terms of absolute instability. On the other hand, forward SRS can be interpreted by considering linear scattering of the incident laser light on the plasma wave driven by backward SRS. This assumption is adequately supported both by the spectral characteristics and by the energy rates of the forward emission light.

Analysis of the photospheric epoch spectra of type 1a supernovae SN 1990N and SN 1991T

This paper presents an LTE analysis of the photospheric epoch optical and IUE UV spectra of the recent Type Ia supernovae SN 1990N and SN 1991T. It is found that SN 1990N has matter moving as fast as 40,000 km/s, while SN 1991T has matter moving as fast as 20,000 km/s. The exponential density profile with e-folding velocity of 3160 km/s suggested by deflation and delayed/late-detonation explosion models is adequate for calculating fits to the observed spectra. Findings on the iron peak elements suggest that in both SNe some nuclear burning continues into the outer ejecta or the newly synthesized elements are mixed into the outer ejecta. The observed spectra of SN 1990N are consistent with a composition of the inner envelope like that of model W7. Intermediate-mass elements with abundances higher than solar are probably in the outer ejecta of both SNe, while silicon, sulfur, and calcium are underabundance in SN 1991T relative to SN 1990N by factors of order 3, 3, and 120, respectively.

Dissipational galaxy formation. II - Effects of star formation

view Abstract Citations (554) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dissipational Galaxy Formation. II. Effects of Star Formation Katz, Neal Abstract We present numerical simulations of hierarchical galaxy formation, including gasdynamics, star formation, and supernova feedback. The simulations model the collapse of isolated constant-density perturbations, initially in solid-body rotation and in Hubble flow The perturbations are made of dark and baryonic matter in a 10:1 ratio. Small-scale power is added using the Zel'dovich approximation, assuming a power-law slope of -2.5. We are successful in making a three-component system that resembles a spiral galaxy: a thin disk made of stars and gas, a spheroidal component made of stars, and a dark matter halo. The disk has a flat rotation curve and an exponential surface density and surface brightness profile. Many stars form in the dissipated gas cores of dark matter clumps that form during the collapse. The remaining gas merges to form a disk, while the dark matter and stars merge to form a spheroidal distribution. Most of the supernova energy is radiated away and does not greatly affect the evolution of the galaxy. The star formation history is combined with population synthesis models to determine the absolute and apparent brightness of the simulated galaxy in many color bands. Simulated observations are also made. The system is brightest during a burst of star formation that occurs as the disk forms. It first reaches its peak I-magnitude of 22 at z = 2 and is fainter than B = 24.5 at z = 1. Publication: The Astrophysical Journal Pub Date: June 1992 DOI: 10.1086/171366 Bibcode: 1992ApJ...391..502K Keywords: Computational Astrophysics; Computerized Simulation; Galactic Evolution; Spiral Galaxies; Star Formation; Baryons; Dark Matter; Gas Dynamics; Supernovae; Astrophysics; GALAXIES: FORMATION; GALAXIES: SPIRAL; HYDRODYNAMICS; METHODS: NUMERICAL full text sources ADS | Related Materials (1) Part 1: 1991ApJ...377..365K