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

Description

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 sigma_A over the amplitude mean value mu_A 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 sigma_A/mu_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 sigma_A/mu_A still holds true. Some of the motivation behind the new tests of the method involves applying it to gamma Doradus stars. The open question of what drives oscillations in these stars makes this statistical method very interesting when applied to gamma Doradus stars. A test of the method is performed using gamma Doradus observations. As expected, the results are inconclusive and demonstrate how ground-based observations of gamma 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.

Abstract

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