Mode power and stability in radial post-AGB pulsations

We examine the relation between mode power and excitation rate for radial pulsations along a sequence of high-luminosity post-AGB envelope models. The net thermodynamic exchange power, including boundary contributions, is compared with the excitation rate and the inertia, compression and horizontal area deformation norms entering the balance relation. We find that positive net exchange power can accompany damping, while negative net exchange power can accompany excitation. Along an individual mode sequence, the power may reverse sign repeatedly without a change of stability. These reversals are associated with changes in the sign of the effective norm. Including the compression contribution in the pressure-force power does not remove this behaviour and can extend the intervals of sign disagreement. Thus, although spatial exchange integrals identify positive and negative contributions to the modal power, the sign of the global power alone does not generally determine mode stability.