Gravitational Theories of Spiral Structure

1970 ◽  
pp. 303-316 ◽  
Author(s):  
G. Contopoulos
Keyword(s):  
Spiral Structure ◽  
Gravitational Theories

scholarly journals C. Spiral Structure

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.

scholarly journals Gravitational theories of spiral structure

1970 ◽  
Vol 38 ◽  
pp. 303-316 ◽  
Author(s):  
G. Contopoulos
Keyword(s):  
Linear Problem ◽  
Spiral Structure ◽  
Initial Value ◽  
Gravitational Theories ◽  
Non Linear ◽  
Recent Developments ◽  
Linear Effects ◽  
Basic Ideas ◽  
Lindblad Resonance ◽  
Inner Lindblad Resonance

The basic ideas and some of the most important recent developments of the gravitational theories of spiral structure are described. A separation between linear and non linear effects is made. The linear self consistent problem consists of the problem of modes and of the initial value problem, which is discussed here in some detail. More emphasis is put on the non linear problem near resonances and in particular the inner Lindblad resonance. The linear density response to a slightly growing spiral potential (trailing or leading) near the inner Lindblad resonance is always trailing, while non linear effects form a density distribution with a roughly quadruple symmetry.

Density waves in disk galaxies

1967 ◽  
Vol 31 ◽  
pp. 313-317 ◽  
Author(s):  
C. C. Lin ◽  
F. H. Shu
Keyword(s):  
Mathematical Analysis ◽  
Spiral Structure ◽  
Stellar System ◽  
Collective Modes ◽  
Disk Galaxies ◽  
Spiral Pattern ◽  
Density Waves ◽  
The Galaxy ◽  
Detailed Mathematical Analysis

Density waves in the nature of those proposed by B. Lindblad are described by detailed mathematical analysis of collective modes in a disk-like stellar system. The treatment is centered around a hypothesis of quasi-stationary spiral structure. We examine (a) the mechanism for the maintenance of this spiral pattern, and (b) its consequences on the observable features of the galaxy.

scholarly journals Non-Riemannian gravity actions from double field theory

2021 ◽  
Vol 2021 (6) ◽  
Author(s):  
A. D. Gallegos ◽  
U. Gürsoy ◽  
S. Verma ◽  
N. Zinnato
Keyword(s):  
Field Theory ◽  
Quantum Gravity ◽  
Condensed Matter ◽  
Equations Of Motion ◽  
Technical Note ◽  
Action Principle ◽  
Relativistic Symmetry ◽  
Gravitational Theories ◽  
Double Field Theory

Abstract Non-Riemannian gravitational theories suggest alternative avenues to understand properties of quantum gravity and provide a concrete setting to study condensed matter systems with non-relativistic symmetry. Derivation of an action principle for these theories generally proved challenging for various reasons. In this technical note, we employ the formulation of double field theory to construct actions for a variety of such theories. This formulation helps removing ambiguities in the corresponding equations of motion. In particular, we embed Torsional Newton-Cartan gravity, Carrollian gravity and String Newton-Cartan gravity in double field theory, derive their actions and compare with the previously obtained results in literature.

scholarly journals Entanglement entropy in cubic gravitational theories

2021 ◽  
Vol 2021 (5) ◽  
Author(s):  
Elena Cáceres ◽  
Rodrigo Castillo Vásquez ◽  
Alejandro Vilar López
Keyword(s):  
Entanglement Entropy ◽  
Gravitational Theory ◽  
Riemann Tensor ◽  
Holographic Entanglement Entropy ◽  
Gravitational Theories ◽  
Entropy Functional ◽  
Wide Range ◽  
Splitting Problem ◽  
Range Of Values

Abstract We derive the holographic entanglement entropy functional for a generic gravitational theory whose action contains terms up to cubic order in the Riemann tensor, and in any dimension. This is the simplest case for which the so-called splitting problem manifests itself, and we explicitly show that the two common splittings present in the literature — minimal and non-minimal — produce different functionals. We apply our results to the particular examples of a boundary disk and a boundary strip in a state dual to 4- dimensional Poincaré AdS in Einsteinian Cubic Gravity, obtaining the bulk entanglement surface for both functionals and finding that causal wedge inclusion is respected for both splittings and a wide range of values of the cubic coupling.

Magnetic Field Spectroheliograms from the San Fernando Observatory

1971 ◽  
Vol 43 ◽  
pp. 329-339 ◽  
Author(s):  
Dale Vrabec
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Spiral Structure ◽  
Longitudinal Component ◽  
Solar Telescope ◽  
Solar Magnetic Fields ◽  
San Fernando ◽  
Field Lines ◽  
The Magnetic Field ◽  
Field Network

Zeeman spectroheliograms of photospheric magnetic fields (longitudinal component) in the CaI 6102.7 Å line are being obtained with the new 61-cm vacuum solar telescope and spectroheliograph, using the Leighton technique. The structure of the magnetic field network appears identical to the bright photospheric network visible in the cores of many Fraunhofer lines and in CN spectroheliograms, with the exception that polarities are distinguished. This supports the evolving concept that solar magnetic fields outside of sunspots exist in small concentrations of essentially vertically oriented field, roughly clumped to form a network imbedded in the otherwise field-free photosphere. A timelapse spectroheliogram movie sequence spanning 6 hr revealed changes in the magnetic fields, including a systematic outward streaming of small magnetic knots of both polarities within annular areas surrounding several sunspots. The photospheric magnetic fields and a series of filtergrams taken at various wavelengths in the Hα profile starting in the far wing are intercompared in an effort to demonstrate that the dark strands of arch filament systems (AFS) and fibrils map magnetic field lines in the chromosphere. An example of an active region in which the magnetic fields assume a distinct spiral structure is presented.

scholarly journals On the maintenance of spiral structure

1979 ◽  
Vol 84 ◽  
pp. 151-153
Author(s):  
James W-K. Mark ◽  
Linda Sugiyama ◽  
Robert H. Berman ◽  
Giuseppe Bertin
Keyword(s):  
Spiral Structure ◽  
Travelling Waves ◽  
Integral Condition ◽  
Stimulated Emission ◽  
Stellar Disk ◽  
Wave System ◽  
Wave Amplification ◽  
Corotation Radius ◽  
The Galaxy ◽  
Central Regions

A concentrated nuclear bulge with about 30% of the galaxy mass is sufficient (Lin, 1975; Berman and Mark, 1978) to eliminate strong bar-forming instabilities which dominate the dynamics of the stellar disk. Weak bar-like or oval distortions might remain depending on the model. In such systems self-excited discrete modes give rise to global spiral patterns which are maintained in the presence of differential rotation and dissipation (cf. especially the spiral patterns in Bertin et al., 1977, 1978). These spiral modes are standing waves that are physically analyzable (Mark, 1977) into a superposition of two travelling waves propagating in opposite directions back and forth between galactic central regions and corotation (a resonator). Only a few discrete pattern frequencies are allowed. An interpretation is that the central regions and corotation radius must be sufficiently far apart so that a Bohr-Sommerfeld type of phase-integral condition is satisfied for the wave system of each mode. The temporal growth of these modes is mostly due to an effect of Wave Amplification by Stimulated Emission (of Rotating Spirals, abbrev. WASERS, cf. Mark 1976) which occurs in the vicinity of corotation. In some galaxies one mode might be predominent while other galaxies could exhibit more complicated spiral structure because several modes are present. Weak barlike or oval distortions hardly interfere with the structure of these modes. But they might nevertheless contribute partially towards strengthening the growth of one mode relative to another, as well as affecting the kinematics of the gaseous component.

scholarly journals An overview of the structure and kinematics of the Magellanic Clouds

1991 ◽  
Vol 148 ◽  
pp. 15-23 ◽  
Author(s):  
B. E. Westerlund
Keyword(s):  
Star Formation ◽  
Observational Data ◽  
Spiral Structure ◽  
Large Magellanic Cloud ◽  
Magellanic Cloud ◽  
Magellanic Clouds ◽  
Line Of Sight ◽  
Small Magellanic Cloud ◽  
Complex Velocity ◽  
The Past

A vast amount of observational data concerning the structure and kinematics of the Magellanic Clouds is now available. Many basic quantities (e.g. distances and geometry) are, however, not yet sufficiently well determined. Interactions between the Small Magellanic Cloud (SMC), the Large Magellanic Cloud (LMC) and our Galaxy have dominated the evolution of the Clouds, causing bursts of star formation which, together with stochastic self-propagating star formation, produced the observed structures. In the youngest generation in the LMC it is seen as an intricate pattern imitating a fragmented spiral structure. In the SMC much of the fragmentation is along the line of sight complicating the reconstruction of its history. The violent events in the past are also recognizable in complex velocity patterns which make the analysis of the kinematics of the Clouds difficult.

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