Turbulent convection and pulsation stability of stars - II. Theoretical instability strip for δ Scuti and γ Doradus stars

By using a non-local and time-dependent convection theory, we have calculated radial and low-degree non-radial oscillations for stellar evolutionary models with M = 1.4-3.0 M-circle dot. The results of our study predict theoretical instability strips for delta Scuti and gamma Doradus stars, which overlap with each other. The strip of gamma Doradus is slightly redder in colour than that of delta Scuti. We have paid great attention to the excitation and stabilization mechanisms for these two types of oscillations, and we conclude that radiative K mechanism plays a major role in the excitation of warm delta Scuti and gamma Doradus stars, while the coupling between convection and oscillations is responsible for excitation and stabilization in cool stars. Generally speaking, turbulent pressure is an excitation of oscillations, especially in cool delta Scuti and gamma Doradus stars and all cool Cepheid- and Mira-like stars. Turbulent thermal convection, on the other hand, is a damping mechanism against oscillations that actually plays the major role in giving rise to the red edge of the instability strip. Our study shows that oscillations of delta Scuti and gamma Doradus stars are both due to the combination of kappa mechanism and the coupling between convection and oscillations, and they belong to the same class of variables at the low-luminosity part of the Cepheid instability strip. Within the delta Scuti-gamma Doradus instability strip, most of the pulsating variables are very likely hybrids that are excited in both p and g modes.