Comments on Nodal Quenching and on Convective Quenching in RR Lyrae Stars

Abstract

Linear nonadiabatic pulsation theory has been used to examine recent hypotheses concerning a transition edge in the color-magnitude diagram for cluster variables. It is found that the occurrence of a transition edge cannot readily be interpreted in terms of a nodal quenching phenomenon as introduced by Cox and King and by Cox, Castor, and King. In a separate investigation it is shown that, when convective flux is treated in the manner of Baker and Kippenhahn and limited in the fashion suggested by Christy, envelope structure changes abruptly and stability against radial pulsation in the fundamental mode increases abruptly when surface temperature passes a critical, low value. Thereby strengthened is the suggestion that the red edge of the instability strip is due to the onset of strong convective currents in the hydrogen and helium ionization zones. Subject headings: convection - interiors, stellar - pulsation - RR Lyrae stars