The age and metallicity dependence of the near-infrared absolute magnitude and colour of red clump stars

TL;DR

The paper investigates how the near-infrared absolute magnitude and colour of red clump stars depend on age and metallicity by analyzing RCs in SMC, MW, and LMC clusters, expanding beyond prior LMC-only studies. Using CMD-based RC identification and a set of regression models, the authors show that, for RCs aged ~1–4 Gyr, age drives a measurable brightening while metallicity has a weak effect on absolute magnitude; colours follow model expectations with stronger age and metallicity impacts on certain colours, notably J−K_S. Comparisons with BaSTI, PARSEC, and MIST indicate general agreement in trends, with subtle offsets in absolute magnitudes and some colour indices. The results validate RCs as a population-sensitive standard candle in the near-infrared, with J−K_S particularly effective for covering wide age-metallicity ranges, though precise distances, ages, and metallicities remain crucial for accurate calibration. The study highlights the value of combining extragalactic clusters and solar neighbourhood data, and suggests asteroseismology and expanded data for tighter population corrections.

Abstract

Understanding the age and metallicity dependence of the absolute magnitude and colour of red clump (RC) stars is crucial for validating the accuracy of stellar evolution models and enhancing their reliability as a standard candle. However, this dependence has previously been investigated in the near-infrared across multiple bands only for -1.05 $\leq$ [Fe/H] $\leq$ 0.40, a range accessible through the star clusters in the Large Magellanic Cloud. Therefore, we used star clusters in the Small Magellanic Cloud and the Milky Way Galaxy to investigate the age and metallicity dependence of the absolute magnitude and colour of RC stars in the near-infrared for a broader parameter space (0.45 $\leq$ Age (Gyr) $\leq$ 10.5, -1.65 $\leq$ [Fe/H] $\leq$ 0.32). Comparison of our results with three isochronous models BaSTI, PARSEC, and MIST reveals that the age dependence of the absolute magnitude for young RC stars aligns well with theoretical predictions, within the fitting errors of the multiple regression analysis. Additionally, the observed colour shows good agreement with the theoretical models. Notably, the $J - K_{S}$ colour, which spans a wide parameter space, reproduces the distribution expected from the theoretical model.

Figures (20)

• Figure 1: $J - K_{S}$ versus $K_{S}$ colour magnitude diagrams of our target star clusters of the SMC. The black rectangles represent the boundary to select the stars for fitting.
• Figure 2: Continued from Fig. \ref{['fig:CMD']}.
• Figure 3: The luminosity functions of the RC stars as a function of their magnitude in the $Y$- (top), $J-$ (centre), and $K_{S}$-bands (bottom) for Lindsay 1 and ESO 28-15. (left) The luminosity functions of the cluster region. (centre) The luminosity functions of the field region. (right) The luminosity functions of the star cluster. The luminosity functions of the field region are subtracted from that of the cluster region. The fitting results with the equation (\ref{['eq:fitting']}) are shown by red lines.
• Figure 4: The age and metallicity of star clusters used to derive the absolute magnitude or colour of RC stars. Black circles are our SMC samples, black squares are our MW samples, and grey triangles are the samples of OIN2019.
• Figure 5: The same figure as Fig. \ref{['fig:CMD']} but for Milky Way star clusters.