Problems with the Pulsation Mode Selection Mechanism in the Lower Instability Strip (Observations and Therory)

We examine the severe disagreement between the number of predicted and observed pulsation modes for Delta Scuti stars. The selection of nonradial modes trapped in the outer envelope is considered on the basis of kinetic energy arguments. The trapped l=1 modes for the star 4 CVn are in good, but not perfect agreement with the observations. The trapping of the l=2 modes is weaker, so that this simple rule of mode selection may apply to l=1, and possibly not to l=2 modes.