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Session 32 - Stars in Various Throes of Stellar Evolution.
Oral session, Monday, January 13
Piers 2/3,

[32.05] Driving g-modes in ZZ Ceti stars

Y. Wu (Caltech)

A number of different types of white dwarfs show g-mode pulsations, each within a narrow temperature range (instability strip). For DA white dwarfs, this is called the ZZ Ceti instability strip . What drives the g-modes has been an important question for white dwarf seismology. A major obstacle to our understanding comes from the presence of convection in the upper atmosphere of these white dwarfs.

In contrast to earlier work, we assume that the convection reacts to the pulsation instantaneously. This is justified, since the convection turn-over time is much shorter than the pulsation period. This assumption allows us to analyze the stability of g-modes in ZZ Ceti stars self-consistently, using an analytical formulation. We find overstability for some low-order g-modes in models for the observed ZZ Ceti variables. We derive the blue edge and the red edge for the ZZ Ceti instability strip, giving a width of \sim\!800\,K, comparable to what has been observed. The red edge of the instability strip results from the decrease in visibility of the driven modes with decreasing temperature. The location of the instability strip is found to depend on the mass of the white dwarf, as well as the adopted mixing length ratio.

Program listing for Monday