Searching for signatures of stochastic excitation in stellar pulsations: a look at γ Doradus stars

Abstract

In stellar oscillations, the temporal variation in the amplitude of a given mode can yield some information about the interior of the star. In what concerns the excitation mechanism, it has been shown that for oscillations that are excited and damped by a physical process in stochastic equilibrium, the ratio of the standard deviation of the amplitude σA over the amplitude mean value μA is of the order of 0.52. This "statistical signature" is a general property of stochastically excited oscillations, so it can be used with any type of star - making it a powerful tool. Although the method is simple, its reliability and robustness have yet to be proven. With the help of simulations, these points are addressed in the present work, and the observational requirements for this method to work are more clearly defined. We show that a number of amplitude measurements of about 102 are required to get a good estimate of σ_A/μ_A. The method is also tested against the complication of having a time series crowded with many modes (unresolvable in short time series), and results show that the theoretical relation of σ_A/μA still holds true. Some of the motivation behind the new tests of the method involves applying it to γ Doradus stars. The open question of what drives oscillations in these stars makes this statistical method very interesting when applied to γ Doradus stars. A test of the method is performed using γ Doradus observations. As expected, the results are inconclusive and demonstrate how ground-based observations of γ Doradus stars hardly fulfill the applicability requirements of the method. We show, however, how a possible scenario of stochastic excitation in these stars could be detected with the COROT space mission. Based on observations obtained at Sierra Nevada Observatory ("Instituto de Astrofísica de Andalucía", CSIC), Granada, Spain.