Disentangling the Core from the Envelope in Pulsating White Dwarf Stars

Abstract

Asteroseismology is the study of the internal structure of stars through their pulsation frequencies. It is well known both analytically and numerically that the separation between the periods of consecutive radial overtones tends to a constant for high-overtone g-modes, so one might think that these periods do not actually contain information concerning the internal structure of the star. The resolution of this apparent paradox is that in general there are sharp features in the Brunt-Väisälä frequency for which the modes may not be considered to be in the asymptotic limit, and the presence of these features perturbs the periods so that the period spacings are no longer uniform, an effect which is commonly referred to as “mode trapping”. Thus, these deviations from a uniform period spacing contain information about sharp features in the Brunt-Väisälä frequency, and can be used to discern internal structure in the star, such as the locations of composition transition zones, for example. To date, almost all analyses of observed mode trapping in white dwarf pulsators have focussed on deter mining the thicknesses of the various chemically pure envelope layers.