Type II Cepheids: Period-Luminosity-Metallicity relations for the Population II distance scale

Abstract

Type II Cepheids are a class of pulsating variable stars that play a critical role in our understanding of stellar evolution, distance measurement and tracing the structure and kinematics of old stars in nearby galaxies. This review provides a comprehensive summary of the current state of research on Type II Cepheids, including their observed properties, pulsation mechanisms and their distinction from other variable stars. These pulsating variable stars, found primarily in older stellar populations, exhibit well-defined period-luminosity relations but with an added advantage that they exhibit weak or negligible dependence on metallicity. We explore their relevance in the context of their role as distance indicators and potential calibrators of the first rung of the extragalactic distance ladder. Finally, the review highlights recent advancements in theoretical models, observations across different wavelengths and ongoing debates concerning their classification.

Figures (1)

• Figure 1: Empirical (in circles) and theoretical (in star-shaped symbols) estimates of the T2CEP metallicity dependence $\gamma$ from the literature across different passbands. The top three panels present results from the individual T2CEP subclasses- BL Her, W Vir and RV Tau stars. The bottom two panels demonstrate the effect of metallicity when different subclasses are combined- BL Her+W Vir and T2CEP (BL Her+W Vir+RV Tau), respectively. Passbands include the bolometric (Bol), Johnson-Cousins-Glass ($UBVRIJHKLMW_{VI}W_{JK}$), ZTF ($gri$), Gaia ($GG_{BP}G_{RP}W_{gaia}$) and the Rubin--LSST ($W_{gi}, W_{gy}, W_{iz}$) filters. The x-axis is in increasing order of the central effective wavelengths ($\lambda_{\rm eff}$) of the respective passbands as provided by the SVO Filter Profile Service rodrigo2012rodrigo2020.