Multimode Resonant Coupling in Pulsating Stars

TL;DR

This paper investigates the complex interactions of acoustic and g-modes in pulsating stars, revealing irregular amplitude variability and challenging the role of resonant mode coupling as the primary amplitude limiter in Delta Scuti stars.

Contribution

It develops a multimode resonant coupling theory for pulsating stars and applies it to a realistic stellar model, showing limitations of resonant coupling in explaining observed amplitudes.

Findings

Acoustic mode amplitude shows irregular variability.

G-mode pairs are excited over a wider parameter range.

Resonant coupling is insufficient to explain amplitude limits.

Abstract

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 the Delta Scuti star XX Pyxidis. Although for some l=2 modes we find amplitudes of the order of a few milimagnitudes, 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 Scuti stars, the resonant mode coupling cannot be the dominant amplitude limiting effect. The nonresonant saturation of the driving effect must play the role.