THE RR LYRAE FUNDAMENTAL RED EDGE

The dependence of the pulsational red edge on mass and helium abundance was investigated on the basis of sequences of nonlinear, nonlocal, and time-dependent convective models of RR Lyrae stars. The radial pulsating envelope models were computed for two different luminosity levels and for a wide range of effective temperatures. At variance with previous investigations, it was found that at fixed mass and luminosity level an increase in the helium abundance moves the location of the red edge of the instability strip slightly toward higher effective temperatures. The discrepancy between the present investigation and previous ones is connected with the increased spatial resolution of the models which causes a substantial increase in the pulsation driving in the hydrogen ionization region. As a consequence, the red edge shows a weaker dependence on helium than on hydrogen abundance. Moreover, the sequences of models computed at fixed luminosity level and chemical composition show quite clearly that an increase in the stellar mass causes a shift of the red edge toward higher effective temperatures. Finally, the comparison between theoretical red edges and transition line of two metal-poor globular clusters (M15, M68) is also discussed.