On the Age Calibration of Open Clusters using Red Clump Stars

In this study, we extend the dust-independent Hatzidimitriou (1991) relation between cluster age and d₁-ₑ color difference between red giant branch (RGB) and red clump (RC) to younger cluster ages. We perform membership analysis on twenty-two open clusters using Gaia DR3 astrometry, then compute the difference in color of red giant branch and red clump d₁-ₑ using Gaia photometry. We find that the trend derived from older clusters does not extrapolate to younger ages and becomes double-valued. We confirm that d₁-ₑ is independent of metallicity. Current stellar evolutionary isochrones do not quantitatively reproduce the trend and furthermore predict an increased color gap with a decrease in metallicity that is not echoed in the data. Integrated light models based on current isochrones exaggerate the color change over the -0. 5 < Fe/H < 0 interval at the few-percent level.