Period Change Similarities among the RR Lyrae Variables in Oosterhoff I and Oosterhoff II Globular Systems

TL;DR

This study analyzes period change rates of RR Lyrae stars in different globular clusters, revealing larger-than-expected changes and questioning the link between period changes and Oosterhoff classifications.

Contribution

It provides new measurements of period change rates in RR Lyrae stars and challenges existing theories connecting these changes to cluster evolution and Oosterhoff types.

Findings

Observed period change rates are much larger than theoretical predictions.

No correlation between period change rates and Oosterhoff group classifications.

Most RR Lyrae stars show increasing periods, regardless of cluster type.

Abstract

We present period change rates (dP/dt) for 42 RR Lyrae variables in the globular cluster IC$\,$4499. Despite clear evidence of these period increases or decreases, the observed period change rates are an order of magnitude larger than predicted from theoretical models of this cluster. We find there is a preference for increasing periods, a phenomenon observed in most RR Lyrae stars in Milky Way globular clusters. The period-change rates as a function of position in the period-amplitude plane are used to examine possible evolutionary effects in OoI clusters, OoII clusters, field RR Lyrae stars and the mixed-population cluster $\omega$~ Centauri. It is found that there is no correlation between the period change rate and the typical definition of Oosterhoff groups. If the RR Lyrae period changes correspond with evolutionary effects, this would be in contrast to the hypothesis that RR Lyrae variables in OoII systems are evolved HB stars that spent their ZAHB phase on the blue side of the instability strip. This may suggest that age may not be the primary explanation for the Oosterhoff types.