TRANSIENT PHENOMENA AND MODAL STABILITY IN RR LYRAE AND BL HERCULIS VARIABLE-STARS

This Letter investigates the appearance of transient phenomena such as mode switching, pulsation quenching, and pulsation driving in RR Lyrae and BL Herculis variable stars. We intend to provide a theoretical scenario to be compared with the new homogeneous, accurate, and large photometric databases on light and temperature curves of radial pulsators (MACHO, EROS, and OGLE) which are becoming available. The nonlinear and convective models investigated in this survey present five different types of instability. The location of the transition lines which separate regions of different modal stability inside the instability strip (IS) were thoroughly evaluated. An observational test on the appearance of first-overtone pulsators among BL Herculis stars based on the topology of the instability regions outlined in the present investigation was also suggested.