Some Recent Developments in the Theory of Spiral Structure

The particle resonance in a galaxy is of special interest because, according to the recent theory of Lynden-Bell and Kalnajs (1972), it provides the excitation of the density waves. In particular it is of interest to know if the density wave reaches the particle resonance, because the dispersion relation derived by Lin and Shu (1966) has a forbidden region if the parameter where (with α0 the surface density and κ the epicyclic frequency), is larger than one. If Q= 1, Lin and Shu find two waves going through the resonance but the amplitude tends to infinity there.

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C. Spiral Structure

Transactions of the International Astronomical Union ◽

10.1017/s0251107x00006490 ◽

1985 ◽

Vol 19 (1) ◽

pp. 425-427

Keyword(s):

Spiral Structure ◽

Density Wave ◽

Wave Theory ◽

Density Waves ◽

External Perturbations ◽

Gravitational Theories ◽

Recent Developments ◽

Star Formation In Galaxies ◽

Spiral Structures ◽

Density Wave Theory

The spiral structure of galaxies is probably related to density waves, primarily governed by gravitational forces. Density waves may result from an inherent instability of galaxies against spiral perturbations as conceived in the conventional density-wave theory or may be forced by other internal or external perturbations of the gravitational field, such as neighbouring galaxies or oval distortions and bars in the inner regions of the galaxies. Reviews of recent developments on the various aspects of gravitational theories of spiral structure have been given by Ambastha and Varma (30.151.029), Athanassoula (33.151.051, 1984), Contopoulos (32.151.021, 34.151.103), Donner (30.151.085), Hunter (34.151.053), James and Wilkinson (29.151.023), Kalnajs (33.151.024), Kormendy (32.151.049), Lin (32.151.040, 33.151.025, 33.151.071), Lin and Bertin (30.151.068, 1984), Lin and Roberts (30.151.045), Martinet (30.151.043), McElroy (34.157.160), Norman (33. 157.088), Pasha and Tsitsin (34.151.042), Sorensen (29.151.024), Thonnard (31.158. 357), and Toomre (30.151.021). Seiden and Gerola (31.151.084) reviewed the theory of formation of spiral structures by stochastic self-propagating star formation in galaxies.

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Spin density wave – metal (superconductor) competition: A phase segregation scenario

Journal de Physique IV (Proceedings) ◽

10.1051/jp4:20020355 ◽

2002 ◽

Vol 12 (9) ◽

pp. 61-64

Author(s):

C. Pasquier ◽

M. Héritier ◽

D. Jérome

Keyword(s):

Free Energy ◽

Dispersion Relation ◽

Fermi Level ◽

Spin Density ◽

Density Wave ◽

Spin Density Wave ◽

Phase Segregation ◽

Organic Conductor ◽

One Dimensional ◽

Unique Parameter

We present a model comparing the free energy of a phase exhibiting a segregation between spin density wave (SDW) and metallic domains (eventually superconducting domains) and the free energy of homogeneous phases which explains the findings observed recently in (TMTSF)2PF6. The dispersion relation of this quasi-one-dimensional organic conductor is linearized around the Fermi level. Deviations from perfect nesting which stabilizes the SDW state are described by a unique parameter t$'_b$, this parameter can be the pressure as well.

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Dynamical imprint of interstellar gas on persistence of spiral structure in galaxies

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317006494 ◽

2017 ◽

Vol 13 (S334) ◽

pp. 296-297

Author(s):

Soumavo Ghosh ◽

Chanda J. Jog

Keyword(s):

Time Scale ◽

Spiral Structure ◽

Density Wave ◽

Disk Galaxies ◽

Interstellar Gas ◽

Spiral Arms ◽

Pattern Speed

AbstractThe persistence of the spiral structure in disk galaxies has long been debated. In this work, we investigate the dynamical influence of interstellar gas on the persistence of the spiral arms in disk galaxies. We show that the gas helps the spiral arms to survive for longer time-scale (~ a few Gyr). Also, we show that the addition of gas in calculation is necessary for getting a stable density wave corresponding to the observed pattern speed of the spiral arms.

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Galactic Dynamos and Density Wave Theory

Symposium - International Astronomical Union ◽

10.1017/s0074180900189764 ◽

1990 ◽

Vol 140 ◽

pp. 135-135

Author(s):

L Mestel ◽

K Subramanian

Keyword(s):

Growth Rate ◽

Angular Velocity ◽

Numerical Solutions ◽

Density Wave ◽

Wave Theory ◽

Interstellar Gas ◽

Global Magnetic Field ◽

Dynamo Equation ◽

Density Wave Theory ◽

Image Position

A steady density wave in the stellar background of a disk–like galaxy is supposed to force a spiral shock wave in the interstellar gas. The jump in vorticity across the shock leads to a locally enhanced helicity, and so to an α–effect which is steady but azimuth–dependent in the frame rotating with the angular velocity ω of the density wave. This is simulated by the adoption of the form for the local dynamo growth rate arising when the standard kinematic dynamo equation is treated by the thin–disk approximation (Ruzmaikin et al 1988). The global magnetic field is proportional to the function Q satisfying where η is the turbulent resistivity (for simplicity assumed uniform) and is the laminar angular velocity of the gas in the inertial frame. We look for solutions of the form where is a global eigen-value, and the non-vanishing of couples all odd or all the even m-values. Anticipating that the strong differential rotation will ensure that in the modes with the largest growth-rate the higher-m parts are weak, the equations are truncated, leaving just a pair in q1, q-1, to describe a basically bisymmetric (m = 1) mode. Approximate treatment by the WKBJ technique suggests that a corotating growing mode (with Γ real and positive) will differ significantly from zero over the range between the points where Numerical solutions have been found for a set of illustrative parameters with corotation occurring at 6.67 kpc, and the turbulence parameters close to those in the M51 mode studied by Ruzmaikin et al which extends over = 1 kpc. Three growing corotating modes were found, the fastest extending for ~ 3 kpc, the other two for over 4 kpc. The first two grow 2-3 times faster, the third somewhat slower, than the M51 mode.

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Existence of Positive Global Solutions of Mixed Sublinear-Superlinear Problems

Canadian Journal of Mathematics ◽

10.4153/cjm-1988-052-4 ◽

1988 ◽

Vol 40 (5) ◽

pp. 1222-1242

Author(s):

W. Allegretto ◽

Y. X. Huang

Keyword(s):

Positive Solutions ◽

Special Interest ◽

Sufficient Conditions ◽

Global Solutions ◽

Linear Type ◽

Existence Of Positive Solutions ◽

Image Position ◽

Quasilinear Problem

Consider the elliptic quasilinear problem:1in Rn, n ≧ 3, whereWe are interested in establishing sufficient conditions on f for the existence of positive solutions u(x) with specified behaviour at ∞. Of special interest to us are criteria which guarantee that u(x) decays at least as fast as |x|−α for some α ≧ 0, given below, in the case f(x, u, ∇u) contains terms of typeThat is: f is of mixed sublinear-super linear type. Our main result is Theorem 3 below which explicitly states sufficient conditions for the existence of such solutions.

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From the Desk of the Guest Editor: Volume Overview

International Journal of Telerehabilitation ◽

10.5195/ijt.2011.6061 ◽

2011 ◽

Vol 3 (2) ◽

pp. 1-2

Author(s):

Jana Cason

Keyword(s):

Interest Group ◽

Guest Editor ◽

Special Interest ◽

Working Group ◽

Special Interest Group ◽

Original Research ◽

Committee Report ◽

Letter To The Editor ◽

Recent Developments

This issue of the International Journal of Telerehabilitation (IJT) contains original research evaluating the efficacy of telerehabilitation, viewpoint articles, a letter to the editor, a committee report from a working group within the American Telemedicine Association’s Telerehabilitation Special Interest Group, and an announcement from the American Telemedicine Association. It is important to note that legislation, regulation, and policy related to telerehabilitation continues to evolve. Therefore, the information presented in this issue of the IJT is current as of the time of its publication and reflects the most recent developments in the field.

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ELECTRONIC STRUCTURE AND RECONSTRUCTION OF THE Mo(001) SURFACE

Modern Physics Letters B ◽

10.1142/s0217984993000862 ◽

1993 ◽

Vol 07 (13n14) ◽

pp. 865-880 ◽

Cited By ~ 2

Author(s):

J.W. CHUNG ◽

K.S. SHIN ◽

S.C. HONG

Keyword(s):

Electronic Structure ◽

Charge Density Wave ◽

Density Wave ◽

Surface States ◽

Wave Model ◽

Theoretical Models ◽

Driving Mechanism ◽

Surface Electronic Structure ◽

Local Bonding ◽

Recent Developments

Recent developments in understanding the nature of surface electronic structure focused on illuminating the electronic origin of reconstruction of the Mo (001) surface have been reviewed. The long-standing disputes between two competing theoretical models, the charge density wave model and the local bonding model, for the driving mechanism of the reconstructional transition seem to be resolved by recent experimental evidences favoring the Peierls-type 2kF instabilities with significant matrix element effects. Details of recent experimental and theoretical findings for the surface electronic bands are discussed briefly with an emphasis placed on characterizing the surface states and their roles in the reconstruction.

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