Abstract
Globular clusters are both primary fossils of galactic evolution and formation and are ideal laboratories for constraining the evolution of low-mass and metal-poor stars. RR Lyrae and type II Cepheid variables are low-mass, radially pulsating stars that trace old-age stellar populations. These stellar standard candles in globular clusters are crucial for measuring their precise distances and, in turn, absolute ages, and for the calibration of the extragalactic distance scale. Herein, the evolutionary stages of RR Lyrae and type II Cepheids are discussed, and their pulsation properties, including the light curves, color–magnitude and period–amplitude diagrams, and period–luminosity relations in globular clusters at optical and infrared wavelengths are presented. The RR Lyrae visual magnitude–metallicity relation and the multiband period–luminosity–metallicity relations in globular clusters covering a wide metallicity range are also discussed in detail for their application to the RR Lyrae-based distance scale.
Highlights
Galactic globular clusters (GCs) have traditionally been used as isolated laboratories for our understanding of many aspects of stellar structure, evolution, and dynamics, and for constraining models of stellar populations
The article provides a review of recent advancements in the studies of the optical and NIR pulsation properties of RR Lyrae (RRL) and T2Cs in GCs
While the modern time-domain surveys provide a vast amount of data at optical wavelengths, the JHKs time-series is still limited to only a few GCs, covering a statistically significant sample of RRL
Summary
Galactic globular clusters (GCs) have traditionally been used as isolated laboratories for our understanding of many aspects of stellar structure, evolution, and dynamics, and for constraining models of stellar populations These remarkable astronomical objects are compact, gravitationally bound systems of up to a million stars and host some of the oldest stars. This review summarizes recent progress in studies of RRL and T2Cs in GCs based on multiband photometric data. It is beyond the scope of this article to provide a comprehensive overview of these variables in GCs, and it will focus on a comparative study of optical and infrared pulsation properties of population II variables.
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