Evolution of Stellar Activity and Habitable Zone. IV. Calcium II Infrared Triplet Activity from Open Clusters and White Dwarf Binaries

Abstract Recent high-resolution spectroscopic observations from the Gaia mission provide a great opportunity to study calcium II infrared triplet (Ca ii IRT) activity. In this study, we present the first investigation of Ca ii IRT activity evolution using Gaia observations, together with age estimates derived from open clusters and white dwarf–main-sequence (WD-MS) binaries. We find that R IRT ′ serves as a reliable age proxy, declining with age in a manner consistent with other activity indicators. Notably, R IRT ′ exhibits a steeper decline and a larger decay amplitude for lower-mass stars. Furthermore, unlike the steady decay observed for F and G stars, late K and early M stars display a period of constant saturated activity before 100 Myr (consistent with the pre-main-sequence stage), after which R IRT ′ drops rapidly by about 1 dex between 100 Myr and 1 Gyr. However, due to the limited sample size and the scatter in the activity of WD-MS binaries, it remains unclear whether the activity continues to decline or flattens to a constant level at old ages (≳2 Gyr). More Ca ii IRT observations from the upcoming Gaia DR4, together with more accurate age estimates from various methods, will be key to revealing the true behavior of the activity–age relation at late stages.