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\,$\mathrm{M}_\odot$. 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 $\kappa$ 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.