The Oosterhoff period groups and the age of globular clusters. I Photometry of cluster variables in M15

Abstract

Three-color photometry is used to study distributions of color, amplitudes, light-curve shapes, and periods within the instability strip of 60 RR Lyrae stars in M15. Period-amplitude, period-rise time, period-color, and color-amplitude relations are compared with data for M3, and correlations of parameters involving period are shifted by delta log P = 0.055 to longer periods in M15. An M15 horizontal branch brighter than that in M3 by delta log L(HB) = 0.090 is found to be the only explanation consistent with the Ritter condition. The abundance is shown to differ between the clusters by delta Y(M15 minus M3) = +0.05 + or - 0.02 using zero-age horizontal branch models, and the ages of M3 and M15 are shown the same within 0.5 sigma of the combined error. The mean age is found using Iben-Rood and Yale models, and HB luminosity and metallicity is found to be the same as required for main-sequence turnoff luminosity for two clusters of same age and different metallicities.