Modes Of Disk Oscillations Research Articles

The Radial Electric Field Excited Circular Disk Piezoceramic Acoustic Resonator and Its Properties.

A new type of piezoceramic acoustic resonator in the form of a circular disk with a radial exciting electric field is presented. The advantage of this type of resonator is the localization of the electrodes at one end of the disk, which leaves the second end free for the contact of the piezoelectric material with the surrounding medium. This makes it possible to use such a resonator as a sensor base for analyzing the properties of this medium. The problem of exciting such a resonator by an electric field of a given frequency is solved using a two-dimensional finite element method. The method for solving the inverse problem for determining the characteristics of a piezomaterial from the broadband frequency dependence of the electrical impedance of a single resonator is proposed. The acoustic and electric field inside the resonator is calculated, and it is shown that this location of electrodes makes it possible to excite radial, flexural, and thickness extensional modes of disk oscillations. The dependences of the frequencies of parallel and series resonances, the quality factor, and the electromechanical coupling coefficient on the size of the electrodes and the gap between them are calculated.

Effect of Be-disk evolution on the global one-armed oscillations

AbstractWe consider the effect of density distribution evolution on the global one-armed oscillation modes in disks around Be stars. Previous studies of global oscillations in Be disks assumed a power-law density distribution of the disk. However, observational results show that some Be stars exhibit evidence of formation and dissipation of the equatorial disk. This causes the disk density distribution can be far from a power-law form. Performing calculations for several times in the disk formation and dissipation stages, we find one-armed modes confined to the inner part of the disk in both stages. In the disk formation stage, the oscillation frequency stays approximately constant after the disk is fully developed. In the dissipation stage of the Be disk, the local precession frequency is, in general, higher than in the disk formation stage. Thus, we expect that V/R periods become shorter as the innermost part of the disk starts to accrete.

Global Disk Oscillations in Binary Be Stars

Abstract We consider the effects of the tidal interaction with the companion, via orbital separation and the binary mass ratio, on the global one-armed oscillation modes in disks around binary Be stars. Our model takes into account a three-dimensional effect that contributes to mode confinement, which was recently found by Ogilvie (2008). We find that the one-armed oscillations are well confined in systems with disks larger than a few tens of stellar radii. In such systems, the oscillation period depends little on the binary parameters. On the other hand, in systems with smaller disks, where mode confinement is incomplete, the oscillation period increases with increasing orbital separation and/or decreasing binary mass ratio. The eigenmode is insensitive to the spectral type of the central star. Our results suggest that the dependence of the V$/$R oscillation period on the orbital separation and binary mass ratio should be observed only in short-period binary systems, and that, for systems with a similar orbital period, those with higher mass ratios will show shorter V$/$R variations.

Quasi-Periodic Oscillations Resonantly Induced on Spin-Induced Deformed-Disks of Neutron Stars

We consider the situation where a spin-induced wavy perturbation is present on disks of neutron-star low-mass X-ray binaries. By non-linearly coupling with the perturbation, some modes of disk oscillations resonantly interact with the disks. We examine the resonant conditions under the assumption that the disks are vertically isothermal. The results show that when the spin frequency has some particular values, resonant coupling occurs at some radii. The modes and frequencies of these resonant oscillations are summarized. Some of these resonant oscillations seem to explain the frequencies of the observed kHz QPOs in objects of which the spin frequency is known.

Nonlinear Resonance in the Accretion Disk of a Millisecond Pulsar

Two simultaneous frequencies of quasi-periodic millisecond modulation of the X-ray flux (twin kilohertz quasi-periodic oscillations) have recently been detected in an accreting 2.5 ms X-ray pulsar. Their difference, equal to about of the neutron star spin rate, clearly indicates that resonant oscillations of the accretion disk have been observed. Similar nonlinear resonances may be spontaneously excited in the accretion disk in the absence of a pulsar, e.g., in black holes. We identify modes of disk oscillations whose frequencies are in agreement with the two observed ones when the rotating neutron star is modeled with realistic equations of state.