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See detailKepler observations: Light shed on the hybrid γ Doradus - δ Scuti pulsation phenomenon
Grigahcène, A.; Uytterhoeven, K.; Antoci, V. et al

in Astronomische Nachrichten (2010), 331

Through the observational study of stellar pulsations, the internal structure of stars can be probed and theoretical models can be tested. The main sequence γ Doradus (Dor) and δ Scuti (Sct) stars with ... [more ▼]

Through the observational study of stellar pulsations, the internal structure of stars can be probed and theoretical models can be tested. The main sequence γ Doradus (Dor) and δ Scuti (Sct) stars with masses 1.2-2.5 M[SUB]ȯ[/SUB] are particularly interesting for asteroseismic study. The γ Dor stars pulsate in high-order gravity (g) modes, with pulsational periods of order of one day. The δ Sct stars, on the other hand, show low-order g and pressure (p) modes with periods of order of 2 hours. Theory predicts the existence of `hybrid' stars, i.e. stars pulsating in both types of modes, in an overlap region between the instability strips of γ Dor and δ Sct stars in the Hertzsprung-Russell diagram. Hybrid stars are particularly interesting as the two types of modes probe different regions of the stellar interior and hence provide complementary model constraints. Before the advent of Kepler, only a few hybrid stars had been confirmed. The {{Kepler}} satellite is providing a true revolution in the study of and search for hybrid stars. Analysis of the first 50 days of {{Kepler}} data of hundreds of γ Dor and δ Sct candidates reveals extremely rich frequency spectra, with most stars showing frequencies in both the δ Sct and γ Dor frequency range. As these results show that there are practically no pure δ Sct or γ Dor pulsators, a new observational classification scheme is proposed by \cite{Grig10}. We present their results and characterize 234 stars in terms of δ Sct, γ Dor, δ Sct/γ Dor or γ Dor/δ Sct hybrids. [less ▲]

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See detailTheoretical Aspects of g-mode Pulsations in gamma Doradus Stars
Dupret, Marc-Antoine ULg; Grigahcène, A.; Garrido, R. et al

in Memorie della Società Astronomica Italiana : Journal of the Italian Astronomical Society (2006), 77

gamma Dor stars are main sequence variable A-F stars whose long periods (between 0.35 and 3 days) correspond to high-order gravity modes pulsation. Most of them are multiperiodic. We will concentrate here ... [more ▼]

gamma Dor stars are main sequence variable A-F stars whose long periods (between 0.35 and 3 days) correspond to high-order gravity modes pulsation. Most of them are multiperiodic. We will concentrate here on two theoretical aspects of these stars. First, an analysis of the driving mechanism of the gamma Dor g-modes is presented, using the linear Time-Dependent Convection (TDC) treatment of Gabriel \cite{Gabriel1996} and Grigahcène et al. \cite{Grigahcene}. This driving is due to a periodic flux blocking mechanism at the base of their convective envelope. The location of the blue and red edges of their instability strip as well as the periods range of their observed modes is explained by the balance between this driving mechanism and radiative damping in the g-mode cavity. Secondly, the multi-color photometric amplitude ratios and the phase differences between the light and velocity curves are considered. It is shown that the agreement between theory and observations obtained with TDC models is much better than with Frozen Convection (FC) models. The theoretical analysis of these observables makes the photometric identification of the degree l of the modes possible and gives constraints on the characteristics of the convective envelope of these stars. Finally, the attractive potential of gamma Dor stars as targets for asteroseismology is considered. [less ▲]

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