HYBRID γ DORADUS-δ SCUTI PULSATORS: NEW INSIGHTS INTO THE PHYSICS OF THE OSCILLATIONS FROM KEPLER OBSERVATIONS

Observations of the pulsations of stars can be used to infer their interior structure and test theoretical models. The main-sequence gamma Doradus (Dor) and delta Scuti (Sct) stars with masses 1.2-2.5 M(circle dot) are particularly useful for these studies. The gamma Dor stars pulsate in high-order g-modes with periods of order 1 day, driven by convective blocking at the base of their envelope convection zone. The d Sct stars pulsate in low-order g- and p-modes with periods of order 2 hr, driven by the kappa mechanism operating in the He II ionization zone. Theory predicts an overlap region in the Hertzsprung-Russell diagram between instability regions, where hybrid stars pulsating in both types of modes should exist. The two types of modes with properties governed by different portions of the stellar interior provide complementary model constraints. Among the known gamma Dor and delta Sct stars, only four have been confirmed as hybrids. Now, analysis of combined Quarter 0 and Quarter 1 Kepler data for hundreds of variable stars shows that the frequency spectra are so rich that there are practically no pure delta Sct or gamma Dor pulsators, i.e., essentially all of the stars show frequencies in both the delta Sct and the gamma Dor frequency range. A new observational classification scheme is proposed that takes into account the amplitude as well as the frequency and is applied to categorize 234 stars as delta Sct, gamma Dor, delta Sct/gamma Dor or gamma Dor/delta Sct hybrids.