THE CEPHEID INSTABILITY STRIP AND THE CALIBRATION OF THE PRIMARY DISTANCE SCALE

This study examines the possibility of galaxy-to-galaxy differences in the long-period Cepheid distributions of external galaxies. A simple theoretical framework is created and linear pulsation calculations are performed to model these distributions. The sturdy nature of the Ceph eid period-luminosity (P-L) relation is affirmed, but both analytic ar guments and the linear model grids point to potential systematic error s reaching up to a few tenths of a magnitude if the Cepheids in the ca librating and target galaxies have different distributions. We also po int out some difficulties posed for stellar pulsation and evolution th eory by the long-period Cepheids we have studied: the theoretical blue edge seems too hot and/or the inferred masses too large to account fo r the observed stars. Preliminary observational evidence is presented which marginally indicates the existence of two somewhat different typ es of distribution of long-period Cepheids in external galaxies, but f urther data are needed before this can be confirmed.