On Metallicity And RR Lyrae Light Curves

Fourier decompositions are performed for the light curves of six RRab and 3 RRc field stars. These are added to previous work to form a sample of 61 RRab and 12 RRc pulsators. The RRab stars are divided into two groups-a short-period (SP) group with P < 0~575 and a long-period (LP) group with P > 0~575. The Fourier phase parameter CP21 is found to fall off very regularly with decreasing metallicity among the SP stars, but this relationship disappears abruptly when one enters the LP group. A small sample of RRab stars from the globular cluster w Cen shows hints of a similar CP2cmetallicity relation. No trend of CP21 with metallicity is found for the RRc field stars. We look for a kinematic difference between the SP and LP RRab samples, but none is apparent. Finally, we use published one-zone models and new linear nonadiabatic pulsation calculations to tentatively identify the CP21-metallicity relation as an opacity effect. However, no explanation is found for the absence of such a relation among the long-period RRab stars. Subject headings: stars: abundances stars: pulsation stars: RR Lyrae