THE DELTA-SCUTI INSTABILITY STRIP - EFFECTS OF OPACITY ENHANCEMENT AND WAVE REFLECTIONS

Theoretical blue edges of the delta Scuti instability strip generally lie at a lower effective temperature than the observed one. It was hoped that the recent opacity enhancement found by the OPAL project could provide more excitation to the pulsations and bring the theoretical prediction to a better agreement with the observations. However, calculations in the present research shatter this hope. Instead of this, the position of the outer pulsation boundary was found to have a strong effect on the location of the blue edge; the blue edge becomes hotter when the outer boundary is at a place of smaller optical depth. Reflections of the pulsation waves at the outer boundary and at a steep density-inversion zone, which is a result of strong ionization of hydrogen, are suggested to be responsible for this relationship. When the outer boundary is at tau = 0.01, the resulting blue edges enclose all observed delta Scuti stars within the instability strip.