RR Lyrae variables in Galactic globular clusters - I: The observational scenario

TL;DR

This study analyzes RR Lyrae stars in 32 Galactic globular clusters, revealing that their period distribution is largely independent of metallicity and discussing implications for the Oosterhoff dichotomy and stellar evolution.

Contribution

It provides new observational insights into RR Lyrae period distributions across clusters, challenging previous assumptions about metallicity dependence and the origin of the Oosterhoff dichotomy.

Findings

Period range remains constant across metallicities.

Width of the instability strip is largely independent of metallicity.

Constancy of mean fundamentalized period across Oosterhoff types.

Abstract

In this paper we revisit observational data concerning RR Lyrae stars in Galactic globular clusters, presenting frequency histograms of fundamentalized periods for the 32 clusters having more than 12 pulsators with well recognized period and pulsation mode. One finds that the range of fundamentalized periods covered by the variables in a given cluster remains fairly constant in varying the cluster metallicity all over the metallicity range spanned by the cluster sample, with the only two exceptions given by M15 and NGC6441. We conclude that the width in temperature of the RR Lyrae instability strip appears largely independent of the cluster metallicity. At the same time, it appears that the fundamentalized periods are not affected by the predicted variation of pulsators luminosity with metal abundance, indicating the occurrence of a correlated variation in the pulsator mass. We discuss mean periods in a selected sample of statistically significant "RR rich" clusters with no less than 10 RRab and 5 RRc variables. One finds a clear evidence for the well known Oosterhoff dichotomy in the mean period <Pab> of ab-type variables, together with a similarly clear evidence for a constancy of the mean fundamentalized period <Pf> in passing from Oosterhoff type II to type I clusters. On this basis, the origin of the Oosterhoff dichotomy is discussed, presenting evidence against a strong dependence of the RR Lyrae luminosity on the metal content.