Excitation Of Density Waves Research Articles

Resonant excitation of density waves in Saturn's rings: the effect of spatial inhomogeneity

The investigation of the first papers of the present series (Griv, Planet. Space Sci. 44, 579–589, 1996; Griv and Yuan, Planet. Space Sci. 44, 1185–1190, 1996) is extended to include the effect of spatial inhomogeneity, i.e. the surface density gradient of Saturn's ring disk of mutual-gravitating identical particles. The model of an infinitely thin inhomogeneous disk with rare physical (inelastic) collisions between particles is considered. Similar to the investigation of the first papers, the well-elaborated mathematical methods of plasma linear kinetic theory are utilized. The influence of disk inhomogeneity on the oscillation spectrum of different small-amplitude oscillations in the Saturnian ring system is studied. Estimates are made for the frequencies and the growth rate of the oscillations. It is shown that, as a rule, in Saturn's A ring the spatial inhomogeneity reinforces the growth rate of the oscillating instability (or the resonant self-excitation of density waves), and weakens the growth rate both in the C ring and in the inner portions of the B ring. In addition, the investigation of waves and their instabilities in the plane of Saturn's rings is extended by taking into account the density wave excitation/absorption at Lindblad's resonances and the weak effect of the finite but not large ring disk thickness.

Resonant excitation of density waves in Saturn's rings: the effect of interparticle collisions

Kinetic theory with the phenomenological Bhatnagar-Gross-Krook binary collision integral is used to study the small amplitude oscillations and their stability of a collisional planetary ring system of identical particles. A differentially rotating, spatially homogeneous disk is considered, with the property that both the epicyclic frequency and the angular velocity of differential rotation exceeds the frequency of inelastic (physical) collisions between particles, that is, a disk with rare collisions is considered. The stabilizing influence of collisions on the growth rate of the oscillating resonant type instability, studied in the first paper of the series, is shown. Also it is shown that in low optical depth regions of Saturn's rings rare collisions cannot suppress the resonant excitation of spiral density waves investigated by Griv ( Planet. Space Sci. 44, 579–587, 1996) in the first paper of the present series.

Resonant excitation of density waves in Saturn's rings

The dynamics of regions in the Saturnian ring system with rare collisions between particles, that is, Ω 2⪢ν c 2 , where Ω is the orbital angular frequency and ν c the collision frequency, is considered. According to observations, such low optical depth regions can be found in the C ring, the inner portions of the B ring and the A ring. Kinetic theory with the Vlasov and Poisson equations is used to obtain the eigen-frequencies of oscillations propagating in the plane of the system. In the considered case of rare collisions the resulting kinetic equation for the perturbed distribution function can be solved by successive approximations, neglecting the effect of binary particle collisions in the zeroth-order approximation. An oscillating instability of the kinetic type is discussed. This instability of a particulate disk is similar to the magneto-drift instability first discovered by Krall and Rosenbluth ( Physics Fluids 6, 254–265, 1963) in a nonuniform magnetic plasma, and belongs to the class of microinstabilities of an inhomogeneous plasma. The cause of the oscillating instability in Saturn's rings is a resonant interaction of drifting particles with nonaxisymmetric Jeans-stable waves at the corotation. The waves that may be produced by the corotation-resonance interaction represent non-radial normal modes of the gravitationally stable disk modified by a particle drift. It is shown that density waves are effectively excited at this resonance: the growth rate of the mode of maximum instability is large, Im ω∗∼Ω. The resonant excitation of density waves investigated in the present paper may be proposed as the cause of the irregular, small-scale ∼ 100 m structure in regions of low optical depth in Saturn's rings. It is suggested that Cassini spacecraft high-resolution images of low optical depth regions will show this kind of structure.

Ferromagnetic and merohedral ordering in powder and crystalline tetrakis (dimethylamino) ethylene (TDAE)-C 60

A.c. susceptometry and electron spin resonance (ESR) measurements on crystals and powders show direct evidence of coupling between spin and molecular merohedral degrees of freedom in tetrakis(dimethylamino)ethylene (TDAE)-C 60 (16 < T c < 26 K). Ordering the spins using an external field in the magnetic phase is shown to imprint a merohedral order. Conversely, merohedrally ordering the C 60 molecules at high temperature influences the low-temperature magnetic transition temperature. The merohedral disorder gives rise to a distribution of π-electron exchange interactions between spins on neighbouring C 60 molecules, suggesting a microscopic origin for the observed spin-glass-like behaviour in the magnetic state. ESR measurements on single crystals of TDAE-C 60 show a frequency shift which is maximum when the absolute value of applied field ¦ H ext¦ is along the crystallographic a axis. On the basis of the present data we suggest the material to be a ferromagnet, with the magnetic axis along a and with the dipole-dipole interactions being responsible for the large ESR line shifts and line broadening in the magnetic phase. New IR reflectivity data suggest a low-frequency excitation unobserved to date, whose origin could be a Drude peak, indicating a metallic ground state or a possible low-frequency charge density wave (CDW) excitation.

Orbital Evolution of Bodies Crossing Disks Due to Density and Bending Wave Excitation

Orbital Evolution of Bodies Crossing Disks Due to Density and Bending Wave Excitation

On the Wave Excitation and a Generalized Torque Formula for Lindblad Resonances Excited by External Potential

The existing theory of density wave excitation in two-dimensional gaseous disks by imposed gravitational potentials makes approximations which prevent a direct analytical description of the shifts in effective positions of Lindblad resonances with respect to radii satisfying orbital commensurability requirement, and of the so-called torque cutoff arising at large azimuthal numbers m of the perturbing potential's pattern. Both phenomena are related to the azimuthal forces acting on gas, neglected in the WKB approximation commonly used in the past. We extend previous theories of Lindblad resonance wherever necessary for a consistent analytical treatment of the problem, particularly for large m

Star trapping and metallicity enrichment in quasars and active galactic nuclei

view Abstract Citations (130) References (94) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Star Trapping and Metallicity Enrichment in Quasars and Active Galactic Nuclei Artymowicz, Pawel ; Lin, D. N. C. ; Wampler, E. J. Abstract Recent observational evidence suggests that the metallicity in quasars within a wide range of redshifts, in particular in gas flowing out of the nuclear regions, may be approximately redshift-independent and comparable with or larger than solar. It is plausible that the nuclear metallicity can be internally generated and maintained at approximately time- stationary values in quasars. We identify and estimate efficiency of a mechanism for rapid metallicity enrichment of quasar nuclear gas (in general, in active galactic nuclei) based on star-gas interactions and equivalent to an unusual mode of massive star formation. The mechanism involves capture of low-mass stars from the host galaxy's nucleus by the assemblages of clouds or by accretion disks orbiting the central massive objects (e.g., black holes). Trapping of stars within gaseous disks/clouds occurs through resonant density and bending wave excitation, as well as by hydrodynamical drag. The time scale for trapping stars with total mass equal to that of disk fragment/cloud is of order Hubble time and is remarkably model-independent. The trapped stars, acting as seeds for gas accretion, allow for efficient conversion of nuclear gas, too hot to form stars via cloud collapse, into massive stars (up to ~10^2^ M_sun_). Such stars evolve rapidly toward the supernova stage (Type II, in the case of metal-poor stars). Supernovae ejecta provide heavy element contamination for both the disk gas orbiting the central black hole and for that part of ejecta which escapes from the gravitational potential of an active nucleus. Our results show that the described mechanism can produce features suggested by observations, for example, the (super) solar gas metallicity in the nucleus. Thus the observed metallicities in high-redshift quasars do not necessarily imply that global star formation and efficient chemical changes have occurred in their host galaxies at very early cosmological epochs. We concentrate on the first stages of the mechanisim-trapping, accretion, and radial migration of the stars- and outline prospects for future work related to the population of supernova remnants in and around the disk. Publication: The Astrophysical Journal Pub Date: June 1993 DOI: 10.1086/172690 Bibcode: 1993ApJ...409..592A Keywords: Active Galactic Nuclei; Interstellar Gas; Metallicity; Quasars; Stellar Motions; Trapping; Accretion Disks; Red Shift; Supernova Remnants; Astrophysics; GALAXIES: ABUNDANCES; GALAXIES: ACTIVE; GALAXIES: QUASARS: GENERAL; GALAXIES: STELLAR CONTENT; ISM: SUPERNOVA REMNANTS full text sources ADS | data products SIMBAD (3)

Near-resonant excitation and propagation of eccentric density waves by external forcing

An overview is presented of the astronomical evidence that relatively massive, distended, gaseous disks form as a natural by-product of the process of star formation, and also the numerical evidence that SLING-amplified eccentric modes in the outer parts of such disks can drive one-armed spiral density waves in the inner parts by near-resonant excitation and propagation. An ordinary differential equation (ODE) of the second order that approximately governs the nonlocalized forcing of waves in a disk satisfying Lindblad resonance almost everywhere is derived. When transformed and appended with an extra model term, this ODE implies, for free waves, the usual asymptotic results of the WKBJ dispersion relationship and the propagation Goldreich-Tremaine (1978) formula for the resonant torque exerted on a localized Lindblad resonance. An analytical solution is given for the rate of energy and angular momentum transfer by nonlocalized near-resonant forcing in the case when the disk has power-law dependences on the radius of the surface density and temperature.

Resonance excitation of spiral density waves in a gaseous disk. II - A nonlinear theory and application to the 3 kiloparsec arm

The present nonlinear theory of spiral density waves in a thin, viscous, self-gravitating gaseous disk views the waves as generated near the Lindblad resonance by periodic disturbances through an excitation mechanism. The suggestion of Yuan (1984), that either a minor oval distortion or an uneven distribution of mass in the center can excite a spiral density wave whose radial velocity and mass concentration are in excellent agreement with observations of the 3 kpc arm of the Galaxy, is confirmed. Reliable results are obtained for nonlinear density waves either in a gaseous disk or in the gas components of a galactic disk.

Resonance excitation of spiral density waves in a gaseous disk. I - A linear theory

The wave propagation in a gaseous disk which is self-gravitating and viscous is studied. Waves are generated by an external disturbance and are maintained in motion among forces due to rotation, self-gravity, and pressure. A linear theory is developed for the general case in which all factors are taken into account. The general solution can be applied to the photostellar disk of the solar nebula and the circumstellar disk of a binary in which the effects of self-gravity and pressure are equally important. The theory confirms in great detail that a long wave is excited at one of the Lindblad resonances gravitationally by the external periodic potential, propagates toward the corotation, is reflected before reaching there at the Q-barrier, and finally propagates in the reverse direction toward and past the Lindblad resonance as a short wave.

Charge density waves in a relaxation semiconductor

The response of a relaxation inhomogeneous semiconductor (specimen with an injecting contact and/or an inversion layer) containing an impurity with positive correlation energy, to a sudden increase in the electrical field and/or bias lighting is investigated. Under definite conditions the charge-carrier plasma turns out to be unstable relative to charge density wave excitation; their frequencies and damping decrements are found. Under the influence of waves of spatial overcharging of centers with positive correlation energy an increase is possible in the imaginary part of the impedance, as is the appearance of an oscillating frequency dependence of the imaginary and real parts of the impedance. The time dependence of the response is examined, and both exponential and power-law damping is possible depending on the kind of charge carrier forming the wave.

Pinning of charge density waves by irradiation induced defects in orthorhombic TaS 3

Effects of electron irradiation induced damage on dc conductivity, threshold field and complex dielectric constant at 9 GHz are investigated in a wide temperature range. Whereas the single particle gap in the low temperature phase is not influenced by irradiation, transport properties associated with collective excitation of charge density waves (CDW) are sensitive to the presence of new pinning centres. The results are discussed in the framework of the semiclassical model of Grüner, Zawadowski and Chaikin. The pinning frequency determined from the threshold field agrees well with that of calculated from the complex dielectric constant.

Mechanism of induced transition radiation in fields of regular waves

The elementary effect of induced transition interaction of a moving charge crossing the boundary between two dielectric half-spaces with the field of a monochromatic wave scattered at this boundary is studied. The quantitative relation between this effect and the corresponding collective process of excitation of charge density waves at the same nonuniformity, i.e., the modulation of the flux of charged particles by the field of a counter-propagating wave, is found. The contribution of the physical phenomena under investigation to the development of beam instabilities and electromagnetic wave generation is discussed.

The excitation of density waves at the Lindblad and corotation resonances by an external potential

We calculate the linear response of a differentially rotating two-dimensional gas disk to a rigidly rotating external potential. The main assumptions are that the sound speed is much smaller than the orbital velocity and that the external potential varies on the scale of the disk radius. We investigate disks both with and without self-gravity. The external potential exerts torques on the disk only at the Lindblad and corotation resonances. The torque is positive at the outer Lindblad resonance and negative at the inner Lindblad resonance; at corotation the torque has the sign of the radial gradient of vorticity per unit surface density. The torques are of the same order of magnitude at both types of resonance and are independent of the sound speed in the disk. The external potential also excites density waves in the vicinity of the Lindblad and corotation resonances. The long trailing wave is excited at a Lindblad resonance. It transports away from the resonance all of the angular momentum which is deposited there by the external torque. Short trailing waves are excited at the corotation resonance. The amplitudes of the excited waves are the same on both sides of the resonance and are small unless the disk is almost gravitationally unstable. No net angular momentum is transported away from the corotation region by the waves. Thus the angular momentum deposited there by the external torque accumulates in the gas. We briefly discuss the behavior of particle disks and prove that the external torques on particle disks are identical to those on gas disks.

The excitation and evolution of density waves

We study the linear oscillations of a thin self-gravitating gas sheet. The unperturbed velocity field of the sheet is a parallel shear flow. A Coriolis acceleration is included to simulate the effects of rotation. The sheet exhibits Lindblad resonances, and it can sustain both short and long wavelength density waves. We derive equations which govern the excitation and evolution of density waves in all regions of space, including the Lindblad resonances and the forbidden region around corotation. These equations are solved in the tight winding limit. An initial disturbance in the form of a wave packet of short leading waves evolves as follows. The packet propagates toward corotation, is reflected at the boundary of the forbidden region, and becomes a packet of long leading waves. It then travels back to the Lindblad resonance, where it is reflected and becomes a packet of long trailing waves. Subsequently, this packet moves toward corotation and is reflected again at the boundary of the forbidden region. The packet is now made up of short trailing waves and propagates away from corotation indefinitely. For sufficiently stable disks, the forbidden region around corotation is wide and density waves are almost completely reflected at its boundaries. For marginally stable disks, some of the incident wave tunnels through the forbidden region and the reflected wave is amplified. The excitation of density waves by an arbitrary external potential is considered. In our model sheet, the sole effect of a barlike potential is to excite the long trailing wave at the Lindblad resonances. The amplitude of the excited wave is calculated.

Electronic properties of TTF-TCNQ : an NMR approach

This paper presents the frequency dependance of the proton spin-lattice relaxation time T1 at several temperatures and pressures in TTF-TCNQ(D4) and TTF(D4)-TCNQ. It is shown that only backward (q = 2 kF) and forward (q = 0) scatterings contribute to the nuclear relaxation induced by the modulation of the hyperfine field in these one-dimensional conductors. At medium fields, H 0 ~ 30 kOe, the frequency dependence of T1 originates from the diffuse character of the spin density wave excitations around q = 0, leading to T1-1 αH0- 1/2 . The enhancement of T1 -1, is at low fields, limited by the existence of a finite interchain coupling (tunnelling type). We find, within a RPA analysis, close correlations between the pressure and temperature dependences of the spin excitations diffusion constant and the collision time derived from the longitudinal conductivity. The interpretation of the NMR data in terms of a Hubbard model excludes both big U and small U pictures. However, we point out the importance of the electron-electron interactions on the relaxation rate of TTF-TCNQ. We derive a ratio U/4 t II ~ 0.9 for the TCNQ chain. We also assume that besides charge density waves fluctuations existing between 300 K and the phase transition at 53 K, electron-electron interactions make an important contribution to the temperature dependence of the spin susceptibility. Finally, we give a unified description of quasi one dimensional conductors in which the various systems are classified according to the transverse tunnelling coupling and the electron lifetime. It follows from this description that for TTF-TCNQ and its derivatives, transverse couplings (tunnelling and Coulomb) are large enough to justify the use of a mean-field theory.

Softening of charge density wave excitations at the superstructure transition in 2H-TaSe 2

Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe 2 reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm −1 of E 2 g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm −1 of A 1 g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm −1 of E 2 g shows no temperature dependence and is clearly due to the rigid-layer vibration.

Spiral structure and hydrodynamic gravitational resonance

A persistent and large scale galactic spiral structure might be the result of resonant excitation of density waves, forced in the low viscosity interstellar gas, by the gravitational field of an asymmetric stellar structure (galactic nucleus, condensation, companion galaxy, etc.) rotating like a rigid body around the galactic centre. This paper deals with this problem in the simple case of a homogeneous plane viscous medium of infinite extent, in a state of rigid body rotation. Numerical estimates are given in the case of density waves excited by the gravitational field of a central dumb-bell. They show that the mechanism of hydrodynamic gravitational resonance is worthy of a further, more realistic, treatment.

Parametric Excitation of Plasma Instabilities in Semiconductors

The parametric excitation of density waves in semiconducting plasmas is considered. A new nonlinear mechanism for the direct conversion of photons into plasmons is presented, resulting from the nonparabolic momentum-energy relations for the single electron. Applications to InSb yield threshold intensities typically in the range ${10}^{4}$-${10}^{6}$ ${\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$.

Excitation of Density Waves in a Condensed Boson System

By the use of the ·generalized Thomas-Fermi method a dispersion relation for the density waves in a condensed boson system is derived. Then a discussion is given of a critical phenomenon involving the excitation of these waves by particles injected with a given velocity (v) into the system. It is shown that, provided the imaginary part of a certain admittance constant is very small, a criti,cal velocity (vc) can be defined in the same way as was discussed first by Landau; above this value of velocity the system exerts a strong resistive force (F) on the injected particles. It is also shown that, for O<x= (v-vc)fvc~1, F is proportional to v'x, if the Landau spectrum is used for the density waves ; and it is pro­ portional to x in the plane wave approximation.