Speaker: Anwesha Biswas, Iowa State University

Host: Jake Simon

Title: The red-end of the DA instability strip 

Abstract: Pulsating white dwarfs occupy narrow temperature ranges where non-radial g-modes drive observable variability. For pulsating DA white dwarfs, theoretical models accurately reproduce the blue edge of the instability strip, showing a  strong dependence on the convective mixing-length parameter -- which we demonstrate with standard linear pulsation models. The cause of the red edge remains uncertain.  Near it, long-period pulsations show less coherent variability than at shorter periods. The stars also occasionally display flare-like outbursts lasting hours. 

These phenomena are symptomatic of nonlinear energy transport processes in the subsurface convection zone.

Linear pulsation modes remain unstable at cooler temperatures and longer periods than observed. Nonlinearity can allow otherwise independent g-modes to exchange energy via parametric instability, triggered by steep thermal and pressure gradient in the outer convection zone.

Here, we investigate nonlinear mode coupling in white dwarf models.  We consider the simplest coupling, involving an l=1 coupled to two damped modes of different l. These nonlinear interactions redistribute pulsation energy among the modes and the convection zone, altering mode amplitudes on time scales of days to weeks. We conclude that nonlinearity can explain the disappearance of driven modes, and perhaps the observed outbursts, at the observed temperatures.