Variable Stars in Metal-Rich Globular Clusters: NGC 6304 and NGC 6569

Abstract

It has recently become evident that the RR Lyrae in metal-rich globular clusters ([Fe/H]> -1.0) have properties different from those of metal-rich RR Lyrae found in the field. In particular, they have longer periods than field RR Lyrae of similar metallicities, implying that the cluster RR Lyrae are more luminous than their field counterparts. Because RR Lyrae stars are major Population II standard candles, this finding has consequences for the ages of globular clusters and the galactic and extragalactic distance scales. We propose observations with the YALO telescope to obtain B,V CCD photometry of the RR Lyrae in the metal-rich globular clusters NGC 6304 and NGC 6569. These data will be used to obtain accurate periods and light curves for the RR Lyrae and will be compared to those in other systems to determine their relative luminosities. Direct comparisons will also be made to the other metal-rich globular clusters known to contain RR Lyrae.