The Role of Long Period Variable Stars in Observational Astrophysics

TL;DR

This review synthesizes the observational landscape of long-period variable stars (LPVs), emphasizing Miras, SRVs, and OSARGs as a multi-sequence, multi-epoch census of pulsating red giants. It highlights how period–luminosity and period–age relations across these classes, aided by large time-domain surveys, anchor distance measurements and reveal Galactic structure and star-formation histories. The work synthesizes empirical PL relations, with a focus on infrared tightness, metallicity effects, and Wesenheit corrections, while discussing LSPs and potential exoplanet tracers. It also surveys the observational data infrastructure, including OGLE and Gaia catalogs, and outlines the future prospects with LSST, Gaia DR4/DR5, and 4MOST to enrich LPV studies and tighten their role in the distance ladder and galactic archaeology.

Abstract

Long-period variables (LPVs) are evolved red giant and supergiant stars whose pulsations provide unique insights into late stages of stellar evolution and serve as essential tools in modern astrophysics. Their period-luminosity and period-age relations make them valuable distance and age indicators, while their light curve morphology, amplitudes, and multiperiodicity reveal the underlying physics of stellar interiors and mass-loss. In this review, we provide an overview of the current status of LPV studies, focusing on their observational properties and applications, including: - Modern classification of LPVs into Miras, semiregular variables (SRVs), and OGLE small-amplitude red giants (OSARGs), which occupy multiple period-luminosity sequences associated with different pulsation modes, chemical compositions, and evolutionary stages - Mira variables as reliable distance indicators across diverse stellar environments and their increasing role as standard candles - The increasing role of SRVs and OSARGs - Long secondary period (LSP) variables as potential tracers of exoplanets Together with advances in theoretical modeling, these developments establish LPVs as valuable tracers of Galactic structure, stellar populations, and the extragalactic distance scale.

Figures (9)

• Figure 1: The modern classification of LPVs into Miras (red), SRVs (orange), and OSARGs (blue), showing proportions among these LPV subclasses. Object counts are based on OGLE-III catalogs of LPVs in the Galactic bulge Soszynski2013, the Large Magellanic Cloud Soszynski2009, and the Small Magellanic Cloud Soszynski2011. For the LMC and SMC, the outer circle shows a fraction of O-rich (cyan) and C-rich (grey) stars, while the inner circle indicates a fraction of RGB (dark red) and AGB (brown) stars, within each class. Figure courtesy of Jan Skowron.
• Figure 2: I-band light curves of selected O-rich Miras obtained between 2010 and 2020 during OGLE-IV survey. Left panels show unfolded time-series photometry, while right panels show the same data folded with the pulsation periods. The stars are ordered by pulsation period (given in the upper right corner).
• Figure 3: I-band light curves of selected C-rich Miras obtained between 2010 and 2020 during OGLE-IV survey. Left panels show unfolded time-series photometry, while right panels show the same data folded with the pulsation periods. The stars are ordered by pulsation period (given in the upper right corner).
• Figure 4: Typical light curves of SRVs. Left panels show unfolded light curves collected during the first 5 years of the OGLE-IV survey (2010-2015). Right panels show the same light curves folded with the primary (the largest-amplitude) period. The stars are ordered by pulsation period (given in the upper right corner).
• Figure 5: A sample of OSARG light curves obtained during the first five years of the OGLE-IV project (2010–2015). The left panels display unfolded time-series, while the right panels show the same data folded with the primary pulsation period. The stars are ordered by pulsation period (given in the upper right corner). Note that the magnitude scale is the same for all stars, to emphasize the variety of OSARG amplitudes.