Our understanding of how fundamental stellar properties, such as magnetic fields, rotation, chemical abundances, and asteroseismic behavior, evolve over time remains incomplete. A major challenge lies in the difficulty of determining precise stellar ages. Open clusters provide powerful laboratories for addressing this problem: their well-constrained ages, distances, and metallicities enable systematic studies of their member stars, making them pivotal environments for calibrating the relationships among stellar parameters and their evolution.
Tracing these parameters across clusters of different ages reveals the underlying physics of angular momentum loss, internal mixing, magnetic dynamo action, and chemical abundances (e.g., Li I 6708Å, [C/N], [Y/Mg]), all of which are expected to change as stars evolve. Such insights are essential not only for stellar astrophysics but also for understanding how galactic chemical evolution affects abundance patterns over time, as well as how stellar parameters shape the evolution of planetary systems.
Session Goals and Description
This splinter session brings together observational and theoretical studies of cool stars in open clusters to connect rotation, magnetic activity, chemical evolution, and asteroseismology across a wide age range.
1. Rotational Evolution
Stars in the main-sequence phase lose their angular momentum over time, leading to spin-down, but the progression of this spin evolution remains poorly constrained. Recent gyrochronology models based on open cluster data suggest a mass- and age-dependent slowdown, with possible plateaus in rotation evolution at specific epochs. These findings challenge classical gyrochronology and highlight the need to incorporate additional physics, such as magnetic braking saturation and core-envelope decoupling, to achieve more precise age calibrations.
2. Stellar Magnetic Activity
Magnetic activity, powered by the stellar dynamo, manifests in photospheric, chromospheric, and coronal emissions, all of which evolve as stars spin down. The advent of medium and high-resolution spectroscopic surveys (e.g., Gaia-ESO, GALAH, LAMOST, and upcoming 4MOST) has enabled direct measurements of chromospheric activity in cluster stars across a wide range of masses and ages. These datasets open new opportunities to understand the evolution of stellar activity, which will provide critical constraints on dynamo models, age-dating, and exoplanetary environments.
3. Chemical Abundances
Lithium, easily destroyed at relatively low temperatures, is a sensitive probe of stellar mixing and evolution. Recent large-scale surveys (e.g., Gaia-ESO) have revealed complex lithium-depletion patterns in young open clusters, highlighting the roles of rotation and magnetic activity in driving internal mixing. Additionally, the [C/N] ratio traces mass- and age-dependent dredge-up in red giants, while the [Y/Mg] ratio serves as an age indicator reflecting the delayed enrichment of s-process yttrium relative to α-element magnesium.
4. Asteroseismology
Asteroseismic analyses, enabled by Kepler and TESS (and, from 2027, PLATO), provide the most precise stellar ages for evolved stars. Recent studies of cluster members have demonstrated the power of seismic constraints in calibrating stellar ages and internal structures. The future scope of extending such analyses to cool main-sequence stars will bridge the gap between gyrochronology, activity, and chemical evolution studies.