At the Cross-Roads of Astrophysics and Cosmology (IAU S376) Period-Luminosity Relations in the 2020s Proceedings of the International Astronomical Union Symposia and Colloquia Series

The period?luminosity relation (PLR) is an important relationship between the fundamental physical properties of many types of variable stars. Most recognisable is the PLR of Cepheids, which allows classical Cepheids to be used as primary distance indicators, establishing the traditional cosmic distance scale. Well-defined relationships also exist between the periods and luminosities of many other types of pulsating stars. IAU Symposium 376 discusses recent observational and theoretical results achieved by the continued, detailed scrutiny of the PLR, including its shape, calibration and dependence on stellar parameters and chemical abundances. Intrinsic PLR widths may offer unique insights into the physical processes shaping these relations and the underlying physical properties of the stars contributing to these relationships. This topic is timely, given the wealth of data recently obtained using state-of-the-art ground- and space-based facilities, high-impact Gaia data releases and the initial operations of the James Webb Space Telescope.

Part I. Setting the Scene – The Extragalactic 'Problem': The Cepheid extragalactic distance scale: Past, present and future Wendy Freedman; The local value of H0 Adam Riess; The tip of the Red Giant Branch as a cosmological probe Myung Gyoon Lee; Period–luminosity relations in the Local Group of galaxies Igor Soszyński; Part II. Stellar Pulsation Physics: Theoretical stellar pulsation physics Marcella Marconi; Anomalous Cepheids: Updated theoretical period–luminosity–color and period–wesenheit relations Giulia De Somma; Pulsation of chemically peculiar stars Ernst Paunzen; A study of the stellar photosphere–hydrogen ionization front interaction in δ Scuti stars Mami Deka; A multi-wavelength analysis of BL Her stars: Models vs observations Susmita Das; Part III. Primary Period–Luminosity Relation Calibrators in the Milky Way: Cepheids and RR Lyrae Gisella Clementini; Primary PL-relation calibrators in the Milky Way: Cepheids and RR Lyrae: Physical basis, calibration, and applications Martin Groenewegen; Cepheids with giant companions: A new, abundant source of Cepheid astrophysics Bogumił Pilecki; Part IV. Disentangling the Structural Components of the Milky Way: Implication of the period–magnitude relation for massive AGB stars and its astronomical applications Akiharu Nakagawa; The structure of the Milky Way from period-luminosity relations Dorota Skowron; Granulation in red supergiants: The scaling relations Yi Ren; The distance scale of Type II Cepheids from near infrared observations in the Magellanic Clouds Teresa Sicignano; Binary star masses and distances from interferometry Alexandre Gallenne; Globular clusters metallicity and distance from their RR Lyrae stars light curves Armando Arellano Ferro; The PL diagram for δ Sct stars: back in business as distance estimators Antonio García Hernández; Part V. Period–Luminosity Relations in the Nearby Universe: Period–Luminosity–Metallicity relations for classical pulsators at near-infrared wavelengths Anupam Bhardwaj; A multiphase study of classical Cepheids in the Magellanic Clouds – Models and observations Kerdaris Kurbah; Double-mode RR Lyrae star – robust distance and metallicity indicators Shu Wang; Empirical constraints for the instability strip from the analysis of LMC Cepheids Felipe Espinoza-Arancibia; Part VI. Non-traditional Period–Luminosity Relations: The period-luminosity relations of red supergiants Biwei Jiang; Long-period variables as distance and age indicators in the era of Gaia and LSST Michele Trabucchi; China Space Station Telescope and variable star studies Xiaodian Chen; OH/IR stars and the period–luminosity relation of Mira variables Dieter Engels.