On the Nature of Stochastic Excited Oscillations: Subdwarf B Star PG 1605+072 as a Case Study

It is well known that the near discontinuities of the background state thermodynamic quantities, like those encountered in the upper layers of some stars, reflect acoustic-gravity waves and thereby partially contain them within the stellar interior. These reflections help to establish resonant cavities in which oscillations of discrete characteristic frequencies propagate. These oscillations, when excited, produce variations on the light curve. We present an investigation of the observed light curves of variable stars using a theoretical time series calculated on the assumption that the observed light curve is the superposition of sinusoidal waves of a given frequency, each one representing a stellar pulsation. By analyzing different simulated light curves, we show that the nature of the excitation of pulsating stars can be gathered from the time dependence of the observed mode's amplitudes, in spite of the limited information contained in some observational data. This analysis is based on the temporal variation of the oscillation amplitudes during several consecutive nights. It is expected that if an oscillation is excited and damped by a mechanism in stochastic equilibrium, the probability distribution of its amplitude, A, will be well defined. An interesting property is that the ratio of the standard deviation σ_A over the mean value μ_A is of the order of 0.52. Furthermore, we show from observational light curves that there is no stochastic mechanism exciting the oscillations in the subdwarf B star PG 1605+072. This new independent observational result solidifies the case that in subdwarf B stars, pulsations are originated by the excitation of intrinsically overstable modes.