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
⊙
. The results of our study predict theoretical instability strips for δ Scuti and γ Doradus stars, which overlap with each other. The strip of γ Doradus is slightly redder in colour than that of δ Scuti. We have paid great attention to the excitation and stabilization mechanisms for these two types of oscillations, and we conclude that radiative κ mechanism plays a major role in the excitation of warm δ Scuti and γ 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 δ Scuti and γ 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 δ Scuti and γ Doradus stars are both due to the combination of κ 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 δ Scuti–γ Doradus instability strip, most of the pulsating variables are very likely hybrids that are excited in both p and g modes.