On the determination of Galactic globular cluster metallicities from the periods and V-band amplitudes of their fundamental-mode RR Lyrae variables

Abstract

ABSTRACT There are primarily two relations currently being used in the literature to calculate the metal abundance of ab-type RR Lyrae variables from their periods andV-band amplitudes. They are the one from Alcock et al., which utilizes both the stars’ period and amplitude and the one from Sarajedini et al., which makes use of solely the stars’ period of modulation. In this work, we are seeking to test which of these relations is most accurate in reproducing the spectroscopic metal abundances of Galactic globular clusters containing ab-type RR Lyraes to which we can apply these relations. For our sample of 600 ab-type RR Lyrae variables in 33 Galactic globular clusters, we find very similar results for both equations as regards the accuracy of the mean metallicity calculated for each globular cluster as compared with spectroscopic abundance measurements for these clusters. This implies that knowledge of only the periods of ab-type RR Lyrae variables in a globular cluster is sufficient to allow a determination of the mean metallicity of that cluster. In particular, once corrected for the adopted metallicity scale, both of these equations are able to reproduce the spectroscopic metal abundance of each cluster with an accuracy of ±0.28 dex.