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

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 M 15 and NGC 6441. 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 of ab-type variables, together with a similarly clear evidence for a constancy of the mean fundamentalized period 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. On the contrary, i) the continuity of the mean fundamentalized period, ii) the period frequency histograms in the two prototypes M 3 (type I) and M 15 (type II), iii) the relative abundance of first overtone pulsators, and iv) the observed difference between mean fundamental and fundamentalized periods , all agree in suggesting the dominant occurrence of a variation in the pulsation mode in a middle region of the instability strip (the "OR" zone), where variables of Oosterhoff type I and type II clusters are pulsating in the fundamental or first overtone mode, respectively.