RR Lyrae Period-Amplitude Diagrams: From Bailey to Today

TL;DR

This paper reviews the historical and current understanding of RR Lyrae period-amplitude diagrams, emphasizing the influence of Oosterhoff type and metallicity, and discusses peculiar cases like NGC 6388 and NGC 6441.

Contribution

It clarifies the relationship between Oosterhoff types, metallicity, and period-amplitude diagrams, and explores atypical globular clusters with unusual properties.

Findings

Oosterhoff type significantly influences period-amplitude relations.

Metallicity correlates with Oosterhoff classification but is not the sole factor.

Certain clusters defy typical patterns, indicating complex underlying factors.

Abstract

More than a century ago, Solon Bailey's pioneering investigations of the variable stars in globular clusters allowed the first period-amplitude diagrams to be constructed for their RR Lyrae stars. These diagrams differ from cluster to cluster, and there has been debate as to whether these differences are correlated mainly with [Fe/H] or with Oosterhoff type. It is clear now that a cluster's Oosterhoff type plays an important role in determining its period-amplitude relation, although the Oosterhoff dichotomy itself is correlated with metallicity. Not all clusters follow the usual patterns, however. The globular clusters NGC 6388 and NGC 6441 have period-amplitude diagrams similar to those of metal-poor Oosterhoff type II globular clusters, but they themselves are comparatively metal-rich. The period-amplitude diagrams of Oosterhoff-intermediate systems are discussed.