Limits on mode coherence due to a non-static convection zone
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The standard theory of pulsations deals with the frequencies and growth rates of infinitesimal perturbations in a stellar model. Modes which are calculated to be linearly driven should increase their amplitudes exponentially with time; the fact that nearly constant amplitudes are usually observed is evidence that nonlinear mechanisms inhibit the growth of finite amplitude pulsations. Models predict that the mass of DAV convection zones is very sensitive to temperature (i.e., MCZ is proportional to T -90 eff ), leading to the possibility that even “small amplitude” pulsators may experience significant nonlinear effects. In particular, the outer turning point of finite amplitude g-mode pulsations can vary with the local surface temperature, producing a reflected wave that is slightly out of phase with that required for a standing wave. This can lead to a lack of coherence of the mode and a reduction in its global amplitude. We compute the size of this effect for specific examples and discuss the results in the context of Kepler and K2 observations.
The 21st European Workshop on White Dwarfs was held in Austin, TX from July 23rd to 27th of 2018