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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 457; no. 3; pp. 3163 - 3177
• Main Authors: Xiong, D. R., Deng, L., Zhang, C., Wang, K.
• Format: Journal Article
• Language: English
• Published: Oxford University Press 11-04-2016
• Subjects: Excitation; Instability; Mathematical models; Oscillations; Stability; Stars; Strip; Turbulence; convection; stars: variables: δ Scuti; stars: oscillations; stars: interiors; stars: variables: γ Doradus
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw047

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.