Driving the gravity-mode pulsations in γ Doradus variables

The gamma Doradus stars are a newly discovered class of gravity-mode pulsators that lie just at or beyond the red edge of the delta Scuti instability strip. We present the results of calculations that the predict pulsation instability of high-order g-modes with periods between 0.4 and 3 days, as observed in these stars. The pulsations are driven by the modulation of the radiative flux by convection at the base of a deep envelope convection zone. Pulsation instability is predicted only for models with temperatures at the convection zone base between similar to 200,000 and similar to 480,000 K. The estimated shear dissipation that is due to turbulent viscosity within the convection zone or in an overshoot region below the convection zone can be comparable to or even exceed the predicted driving and is likely to reduce the number of unstable modes or possibly quench the instability. Additional refinements in the pulsation modeling are required to determine the outcome. At least one gamma Doradus star has been observed that also pulsates in delta Scuti-type p-modes, and others have been identified as chemically peculiar. Since our calculated driving region is relatively deep, gamma Doradus pulsations are not necessarily incompatible with surface abundance peculiarities or with delta Scuti p-mode pulsations driven by the H and He ionization K-effect. Such stars will provide useful observational constraints on the proposed gamma Doradus pulsation mechanism.