Wyniki wyszukiwania - BazTech

Ograniczanie wyników

2 Acta Astronomica

Znaleziono wyników: 2

Liczba wyników na stronie

Wyniki wyszukiwania

Sortuj według:

Ogranicz wyniki do:

Multimode Resonant Coupling in Pulsating Stars

Nowakowski R. M.

Acta Astronomica

2005

Vol. 55, No. 1

1--41

We consider evolution of an unstable acoustic mode interacting with an ensemble of stable g-modes. We show that the static multimode solution does not exist. We then find the condition for the stability of the statistical equilibrium. Performing numerical integration of amplitude equations for a simplified system we find that the acoustic mode amplitude exhibits a large irregular variability on the timescale given by the inverse of the growth rate. The g-mode pairs are excited in significantly wider range of detuning parameters than it is implied by the parametric instability criterion applied to the average amplitude. However, the number of interacting g-mode pairs is reduced because the pairs differing in the detuning parameter by less than their damping rates are synchronized and effectively act as a single pair. We apply the multimode resonant coupling theory to a realistic stellar model. We choose a seismic model of a &delta Sct star XX Pyx. Although for some l=2 modes we find amplitudes of the order of a few millimagnitudes, the typical amplitudes of low-degree modes are much higher. Taking into account the rotational splitting results in decrease of amplitudes by a factor of few which is not enough to obtain consistency with observations. We conclude that in this star and likely in all evolved &delta Sct stars, the resonant mode coupling cannot be the dominant amplitude limiting effect. The nonresonant saturation of the driving effect must play the role.

Resonant Excitation of Nonradial Modes in RR Lyr Stars

Nowakowski R. M., Dziembowski W. A.

Acta Astronomica

2001

Vol. 51, No. 1

5--47

We study a nonlinear development of radial pulsation instability to a resonant excitation of nonradial modes. Our theory covers the cases of axisymmetric (m=0) modes as well as (m,-m) pairs. Adopting a simplified treatment of the radial and nonradial mode coupling we find that the asymptotic state is a pulsation with constant amplitudes and we evaluate the relative amplitude of the nonradial component. Observable consequence of the m=0 mode excitation is a small period change and a more significant amplitude change, especially in the case of a dipole mode (l=1). Such a mode has a fairly large excitation probability. Significant amplitude and phase modulation is predicted in the case of excitation of a m=±1 pair. We suggest that this may explain Blazhko-type modulation in RR Lyr stars. If this model is correct, the modulation period is determined by the rotation rate and the Brunt-Väisälä frequency in the deepest part of the radiative interior.