Researchers from Yunnan Observatories of the Chinese Academy of Sciences have addressed the long-standing astronomical puzzle known as the Oosterhoff dichotomy. Dr. LI Linjia and Prof. ZHU Liying, from the Binaries & Variables Group, have developed a comprehensive model based on stellar evolution that explains the mechanisms behind this phenomenon. Their findings were recently published in Monthly Notices of the Royal Astronomical Society.

The research team combined theoretical evolutionary models with observational data. They established empirical relationships between pulsation parameters, such as period and amplitude, and stellar physical parameters, including effective temperature, luminosity, mass, and metallicity. This allowed them to map horizontal branch (HB) evolutionary tracks onto the Bailey diagram for direct comparison between theory and observation.

The study demonstrated that all zero-age horizontal branch (ZAHB) RR Lyrae stars are located within the Oosterhoff I (OoI) region. In contrast, Oosterhoff II (OoII) stars were identified as evolved HB stars with lower masses than OoI stars at the same metallicity. These stars begin their HB evolution blueward of the instability strip and enter it at a later stage, after an increase in brightness, which results in longer periods. The findings indicate that the Oosterhoff type corresponds to different evolutionary stages on the HB.

The research also confirmed the role of metallicity as the "first parameter" in shaping HB morphology. Clusters with lower metallicity were shown to have bluer HBs. These blue HB stars evolve into the instability strip later in their evolution, becoming OoII-type variables. This results in the cluster statistically exhibiting OoII characteristics, such as longer mean periods.

This work provides a theoretical framework connecting stellar evolution effects to population-level phenomena. The team plans to use data from surveys including LAMOST, Gaia, and TESS to further validate the relationships between pulsation and physical parameters and to investigate other factors, such as helium enhancement and binarity.

Contact:
LI Linjia
Yunnan Observatories, CAS
E-mail: lipk@ynao.ac.cn